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Organic Compounds Containing Halogen Flipbook PDF
Organic Compounds Containing Halogen
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Organic Compounds Containing Halogen ALKYL HALIDES General Methods of Preparation 1. From Alcohols By the action of halogen acids Alkyl chlorides are obtained by passing dry hydrogen chloride gas into the alcohol in presence of anhydrous ZnCl2.
The order of rate of reaction is 3° alcohol > 2° alcohol > 1° alcohol.
(b) By the action of phosphorus halides Alkyl chlorides can be prepared by refluxing alcohols with phosphorus pentachloride or phosphorus trichloride.
Alkyl bromides or alkyl iodides are prepared by the action of phosphorus tribromide or triiodides on alcohols.
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By the action of thionyl chloride (SOCl2)- Alkyl chlorides can be prepared by heating alcohol (1° or 2°) and thionyl chloride in the presence of pyridine (base). (Darzen's Reaction)
2.From Alkenes (a)Alkyl halides can be prepared by the addition of halogen acids to alkenes.
3.From Alkanes: Alkyl halides can be prepared by the direct halogenation of alkanes in the presence of light or heating at least at 250 – 300°C or a suitable catalyst, e.g.,
The mechanism is a free radical reaction mechanism. The reactivity of halogen depends upon the nature of halogen used to abstract hydrogen hence F2 > Cl2 > Br2 >I2 The reaction with the iodine being reversible can take place in the presence of an oxidizing agent like iodic acid (HIO3), nitric acid (HNO3) or mercuric oxide (HgO).
Fluorination of alkanes is carried out by heating suitable halo alkanes with inorganic fluorides, such as AsF3, SbF3, Hg2F2 etc.
4 .From silver salts of fatty acids (Borodine - Hundsdiecker Reaction)
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5. By halogen exchange
6.From Alkyl hydrogen sulphate Alkyl iodides can be prepared by treating alkyl hydrogen sulphates with an aqueous solution of potassium iodide.
CHEMICAL REACTIONS Halogen derivatives of alkanes are highly reactive as the halogen atoms are easily replaced. These derivatives especially the alkyl halides are widely used in the synthesis of many organic compounds. The Chemical reactions of alkyl halides may be classified into three types : (A) Nucleophilic substitution reactions. (B) Elimination reactions. (C) Miscellaneous reactions. Nucleophilic substitution reactions
Alkyl halides are highly reactive in nature. This is due to the fact that halogen atom is good leaving group. I is best and F is worst leaving group among halogen. Therefore for a given alkyl group, the order of reactivity for SN reactions is, iodides >
bromides > chlorides. Nucleophilic substitution reaction can occur through two type of mechanisms
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Kinetics of nucleophilic substitution For the given Nucleophilic substitution reactions :
The rate of the first reaction (calculated by experimental methods) is Rate = k[CH3Br] [KOH] i.e. it follows SN2 mechanism In second reaction, the rate is dependent on the concentration of alkyl halide only and is independent of nucleophile concentration Rate = k[(CH3)3 C–Br] i.e, It follows SN1 mechanism. Hence first reaction follows second order kinetics and second reaction follows first order kinetics.
Mechanism of SN2 (Substitution nucleophilic bimolecular) In this the nucleophile (OH¯ ) collides with the reactant (CH3Br) molecule at the face most remote (Back side attack) from the halogen atom and possesses sufficient energy to break the C – Br bond to form C – OH bond. Thus a complete inversion of configuration takes place.
Since this reaction involves the formation of only one transition state and no intermediates between the reactants and the products. So there will only one activation energy of the reaction as shown in the potential energy of the graph. The reactants are shown to be slightly higher in energy than products since the reaction is exothermic.
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Mechanism of SN1 Reaction : It is two steps reaction
Stereochemistry of SN1 reaction : Partial racemisation in case of lesser stable carbocation and complete racemisation in case of more stable carbocation takes place in SN1 reaction.
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Factors affecting the rate of SN1 and SN2 reactions : 1.
Structure of Substrate :
for SN1 : Systems giving stable carbocation or having bulkier groups at position. X > (CH3)3C—X > (CH3)2CH—X > CH3CH2X > CH3X for SN2 : Greater the positive charge on the carbon of C – X bond, lesser is the steric hinderance easier will be the attack of the nucleophile on it. The order of reactivity of RX in SN2 reactions is CH3X > primary alkyl halide > secondary alkyl halide > tertiary alkyl halides > neopentyl halides. 2.
Strength and concentration of nucleophile :
for SN1 : As nucleophile does not participate in rate determining step, therefore there is no effect on rate of concentration and strength of nucleophile. for SN2 : Increasing the concentration of nucleophile increases the rate for SN2, a better nucleophile will yield better products. 3.
Solvent Effect : Polar protic solvent favours SN1 whereas polar aprotic solvent favours SN2 mechanism.
4.
Nature of leaving group : Good leaving group always increases rate of nucleophilic substitution reactions for the same reason the order of reactivity for SN reactions of halides follow the given trend :
RI > RBr > RCl > RF
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Miscellaneous reactions
1. Reaction with magnesium (formation of Grignard's reagent). R – X + Mg RMgX (a) For a given alkyl group, the ease of formation of Grignard’s reagent is of the order : iodide > bromide > chloride 2.
Reduction (formation of alkanes)
Alkyl halides are reduced to alkanes by any of the following reducing agents : (a) H2 in the presence of Ni, Pt or Pd (catalytic hydrogenation). (b) Lithium-aluminium hydride (LiAlH4). (c) Nascent hydrogen obtained from Zn-Cu couple and alcohol or Zn and HCl or Sn and HCl or Na and alcohol. Friedel Craft’s reaction Formation of Alkyl benzene Vinylic halides and aryl halides do not give a silver halide precipitate, when treated with alc. AgNO3 because vinylic and phenyl cations are very unstable and therefore, do not form readily.
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ARYL HALIDES General methods of Preparation
1.From arenes by direct halogenation
(ii) Preparation of aryl iodides Aryl iodides cannot be prepared by direct iodination because the reaction is reversible and hydriodic acid formed being a strong reducing agent reduces C 6H5I to C6H6. C6H6 + I2 C6H5I + HI To overcome this difficulty, iodination is carried out in the presence of oxidizing agent such as nitric acid, mercuric oxide (HgO) or iodic acid, which oxidizes the hydroiodic acid to iodine and thus the reaction proceeds in the forward direction.
(iii) Aryl fluorides cannot be prepared by this method because fluorine is highly reactive and the reaction is very violent and uncontrolable.
2. Direct Halogenation When calculated amount of chlorine is passed through boiling toluene in presence of sunlight or ultra – violet light and in the absence of halogen carrier, benzyl chloride is formed. educareacademydgl.com
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3.With NBS When toluene is treated with NBS (N-bromosuccinimide) in the presence of peroxides, benzyl bromide is formed.
4. From Diazonium salts (a) Sandmeyer’s reaction (i)
Preparation of chlorobenzene + N2 Cl
Cl CuCl / HCl
+ N2
Benzenediazonium chloride
Chlorobenzene
–
C 6H5 N2 Cl KI C 6H5I HCl N2 Bnzenediazonium chloride
Iodobenzene
c)Preparation of fluorobenzene (Balzschiemann reaction). + N2 Cl
educareacademydgl.com Benzenediazonium chloride
+ N2 BF4
Benzenediazonium fluroborate
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5.By Raschig Process: On commercial scale, chlorobenzene is prepared by passing a mixture of benzene vapours, air and hydrogen chloride gas over cupric chloride (catalyst) at 500 K.
6.By Hunsdiecker Reaction : COOAg + Br2
Br Distillation CCl4, 350 K
+ AgBr + CO2
7.From phenol: C 6H5 OH PCl5 Phenol
C 6H5 Cl POCl3 Chlorobenzene
HCl
Physical Properties (i) They are colourless stable liquids. (ii) Insoluble in water but soluble inorganic solvent. (iii) Boiling and melting points : Their boiling and melting point is higher than alkyl halides. Boiling point increases with increasing size of halogen. F
Cl
C=O). O
O
O
–C–
R–C–H
R–C–R
Carbonyl group
An aldehyde
Ketone
Structure of the carbonyl group: Like the carbon-carbon double bond of alkenes, the carbon-oxygen double bond of the carbonyl group is composed of one s and one p bond.In the carbonyl group, carbon atom is in state of sp2hybridisation. The C–O s bond is produced by overlap of ansp2 orbital of carbon with a p-orbital of oxygen. On the other hand, the C–O p bond is formed by the sideways overlap of p orbitals of carbon and that of p orbital of oxygen. The remaining two sp2 orbitals of carbon form s bonds with the s orbital of hydrogen or sp3 orbital of carbon of the alkyl group. p
p
X C
Y
:
O:
:
C
O:
120°
120° :
C
O:
Y
(a)
X
p +
X
Y (b)
120° (c)
The polar nature of the carbonyl group causes intermolecular attraction (dipole-dipole attraction) in aldehydes and ketones and hence accounts their higher boiling points than that of hydrocarbons and ethers of comparable mol. wt. However, the high values of dipole moments (2.3 - 2.8 D) of aldehydes and ketones can’t be accounted for, only by inductive effect; this can be accounted for if carbonyl group is a resonance hybrid of the following two structures.
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General Methods of Preparation of Aldehydes and Ketones 1. From Alcohols : (i) By Oxidation : Primary alcohols gives aldehyes, while secondary alcohols give ketones. K Cr O dil. H SO
7 4 CH3 .CH2 OH 2 2 2 CH3 .CHO H2 O
Ethyl alcohol (Pr imary Alcohol)
Acetaldehyde
CHO
CH2OH K2Cr2O7 + H2SO4 [O],
Benzyl alcohol
Benzaldehyde
Controlled oxidation of 1°-alcohol and 2°-alcohol with PCC + CH2Cl2 or CrO3 forms aldehyde and ketone respectively.
Ketones in good yield can be prepared by Oppenauer oxidation of secondary alcohols. CH3
R CHOH + R´ Sec. alcohol
C=O CH3 Acetone
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[(CH 3)3CO] 3Al (Aluminium t-butoxide)
CH3
R C=O + R´ Ketone
CHOH CH3 Isopropanol
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(ii)By catalytic dehydrogenation of alcohols :1° Alcohols yield aldehyde in this method. Cu CH3 OH HCHO H2 573K
Methanol
Formaldehyde
Secondary alcohols, on similar treatment, give ketones. CH3 CHOH CH3 Isopropanol
Cu 573 K
CH3 C = O + H2 CH3 Acetone
2. From Fatty Acids : By dry distillation of calcium salts of fatty acids : Pyrolysis (heating) of calcium salts of fatty acid or a mixture of two fatty acids leads to the formation of aldehydes and/or ketones depending upon the nature of the fatty acid. (a) Distillation of calcium formate to formaldehyde H COO
O OCH Ca + Ca
H COO
2HCHO + 2CaCO3 O OCH
Formaldehyde
Calcium formate (2 moles)
Distillation of mixture of Ca(CH3COO)2 and Ca(HCOO)2 CH3 COO
O OCH Ca + Ca
CH3 COO Calcium acetate
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O OCH
2CH3CHO + 2CaCO3 Acetaldehyde
Calcium formate
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3.From gem-dihalides : OH CH3 CHCl2
aq. KOH
CH3 CH
Ethylidene chloride
CH3 CHO + H2 O Acetaldehyde
OH Unstable
gem-Dihalides having two halogen atoms to a non-terminal carbon atom give ketone on alkaline hydrolysis. Cl | aq. alkali CH3–C–CH3 | Cl
OH | CH3–C–CH3 | OH
O || CH3–C–CH3 + H2O Acetone
2, 2-Dichloropropane (Isopropylidene chloride)
4.From Alkynes : 4 CH CH H2 O 2 [CH2 CHOH ] CH3 CHO
dil. H SO
Acetylene
HgSO4
Vinyl alcohol (Unstable)
Acetaldehyde
O
OH CH3C CH + H2O
H2SO4
CH3.C
HgSO4
CH2
Tautomerisation CH3
Unstable
C
CH3
Acetone
5.From Grignard Reagents : CH3 H – C N + CH3MgI
H – C = NMgI
CH3 H2O
H – C = O + NH2MgI Acetaldehyde
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6.ByReducitveOzonolysis of Alkenes : O C
O3
C
H3C = CH2 + O 3
C
C
O
O O CH2
H2O, Zn
CH2
C
H2O, Zn
O+O
C
+ H2O2
CH2O + CH2O Formaldehyde (2 moles)
O
O
7. Methods giving only Aldehydes : (a)From Acid Chlorides (Rosenmund Reduction) :
O C Cl R Acid chloride
H2/Pd + BaSO4, S Boiling xylene
R
CHO + HCl
O C
CH3
Cl
Acetyl chloride
Pd/BaSO4,S Boiling xylene
CH3 CHO + HCl Acetaldehyde
From nitriles (Stephen's reduction) :
CH3.C N
1. SnCl2 + HCl 2. OH
Methyl cyanide
–
CH3.CH = NH
H2O/OH
–
CH3CHO + NH3
Aldimine
8.Methods for Aromatic Aldehydes and Ketones : (a) Aromatic Aldehydes :
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CH3
CH(OCOCH3)2 CrO3
alkaline
(CH3CO)3O
hydrolysis
CHO + CH3COONa + H2O
b)Aromatic Ketones : (Friedal-craft's acylation) anhy. C 6H6 CH3 COCl C 6H5 COCH 3 HCl Benzene
AlCl3
Acetophenone
Here instead of acid chloride we can use anhydrides also anhy. C 6H6 C 6H 5COCl C 6H5 COC 6H 5 HCl AlCl3
Benzophenone
Physical Properties Methanal is gas at room temperature, ethanal is liquid at room temperature. Other carbonyl compounds are liquids or solids at room temperature. Lower members have sharp pungent odours. As the size of the molecule increases, the odour becomes less pungent and more fragrant. They can form H-bonding with water that's why lower members are miscible with water. With the increase in the size of the alkyl group their solubility in water decreases. However higher members are soluble in non polar organic solvents. Their boiling point is greater than comparable molecular weight of hydrocarbon or ether because of their polarity but less than alcohol because alcohol has H-bonding.
Trend of Boiling Point : educareacademydgl.com
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O || CH3–CH3–OH > CH3CH2CHO > CH3CCH3 > CH3CH2CHO > CH 3–O–CH 2–CH 3 > CH 3–CH 2–CH 2–CH 3
Chemical Properties Both aldehydes and ketones contain a carbonyl group in the structure and hence show marked similarity in their chemical behaviour.
(a) Nucleophilic addition reactions Aldehydes are more reactive than ketones because greater the alkyl group (as in ketone) more will be electron density and hindrance hence lesser will be chance for the attack of nucleophile and hence lesser will be ease of nucleophilic addition.
(1) Addition of hydrogen cyanide : Aldheydes and ketones react with hydrogen cyanide to form cyanohydrins.
CH3
OH
CH3 C = O + HCN
C H
H Acetaldehyde
CH3
Acetaldehyde cyanohydrin
OH
CH3 C = O + HCN
CH3 Acetone
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CH
C CH3
CN
Acetone cyanohydrin
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C6H5
C6H5 C = O + HCN
OH C
CH3
CH3 Acetophenone
CN
Acetophenone cyanohydrine
Benzophenone does not react with hydrogen cyanide because of steric hindrance. On the other hand, aromatic aldehydes (e.g., C6H5CHO) when refluxed with alcoholic potassium cyanide solution undergo dimerization to form benzoin.
OH O CN–
2C6H5CHO
ethanol
C6H5 – CH – C – C6H5
Benzaldehyde
Benzoin
Above reaction is known as benzoin condensation. (2) Addition of sodium bisulphite : Aldehydes and methyl ketones react with a saturated aqueous solution of bisulphite to form crystalline sodium bisulphite derivatives. CH3
OH
CH3 C = O + NaHSO3 H
H Acetaldehyde
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C
Sod. bisulphite
SO3Na
Acetaldehyde sod. bisulphite
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OH
C6H5
C6H5 C = O + NaHSO3
C
H
H
Benzaldehyde
SO3Na
Benzaldehyde sod. bisulphite
Aromatic ketones and aliphatic ketones having higher alkyl groups do not react with sodium bisulphite. This is due to the fact that the large ion cannot attack the carbonyl carbon atom when it is surrounded by larger substituents (steric hindrance). Thus C2H5COC2H5, C6H5COCH3, C6H5COC6H5 do not react with sodium bisulphite. Methyl ketones give this reaction. (3) Addition of Grignard reagents : H
CH3
H C = O + CH3MgI
H Formaldehyde
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C H
OMgI
H2O
CH3
H C H
OH
Ethyl alcohol (primary alcohol)
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CH3
CH3 C = O + CH3MgI
CH3 C
H
H3C
CH3 C
OMgI
H
H
Acetaldehyde
H3C
CH3
H2O
OH
Isopropyl alcohol (Sec. alcohol)
H3C
C = O + CH3MgI
C
OMgI
H3C
Acetone
CH 3
CH3 |
CH3 C OH H2O
|
CH3 ter. butyl alcohol(ter. alcohol)
(3) Addition of alcohols (Acetal formation) : Aldehydes (not Ketones) react with alcohols in presence of dry HCl gas to form hemi-acetals (hemi means half) which being unstable immediately react with another molecule of alcohol to form stable acetals. For example, C=O + C2H5OH
dry HCl gas
OC2H5
CH3 C
H
H
OH
C
dry HCl (–H2O)
H
Acetaldehyde hemiacetal (1-Ethoxyethanol)
C = O + R Ethanol
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HO — CH2
R
HO— CH2
R
Ethylene glycol
OC2H5
Acetaldehyde acetal (1, 1-diethoxyethane, Gem diether)
3 2
C
—
R
OC2H5
CH3
C2H5OH
O — CH2 1
—
CH3
O — CH2
Ethylene glycol ketal
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(5) Reduction by metal hydrides such as lithium aluminium hydride
CH3 C=O
LiAlH4
H
CH3
H
C
or CH3CH2OH OH
H
Acetaldehyde
Ethyl alcohol
Similar product are also formed by NaBH4 , H2 — Pt, H2 — Ni or metal-acid.
(b) Nucleophillic Substitution (1) Replacement of Carbonyl Oxygen : (i) Reaction with Ammonia Derivatives : Aldehydes and ketones react with a number of ammonia derivatives likeNH2OH, NH2NH2, C6H5NHNH2 etc. in weakly acidic medium. Such reactions take place in slightly acidic medium and involvesnucleophilic addition of the ammonia derivative followed by dehydration.
H —C = O + :N
H Z
H Ammonia der.
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+ —C—N—Z
–O
–H2O
–C=NZ
H
Addition product
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Ammonia derivatives H2N — OH Hydroxylamine
Final products Oximes
H2N — NH2 Hydrazine Hydrazones H2N — NHC6H5Phenylhydrazine Phenylhydrazones H2N — NHCONH2
NH2—NH—
—NO2
NO2
2, 4 DNP
SemicarbazideSemicarbazones
C=N–NH
(also known as Brady's reagent)
NO2
NO2
(ii) Reaction with thioalcohols (mercaptans) : Aldehydes and ketones react with thioalcohols and form thioacetals (mercaptals) and thioketals (mercaptals) respectively. (2) Reaction involving alkyl as well as carbonyl group (condensation reactions) : (a) Aldol condensation between acetaldehyde molecules : H | CH3–C + HCH2–CHO || O
dil alkali
CH3–CH–CH2CHO | OH -hydroxybutyraldehyde (Aldol)
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(b)Aldol condensation between acetone molecules :
CH3 C=O + HCH2–COCH3
Ba(OH)2
CH3
CH3
CCH2COCH3
CH3 Acetone(2 molecules)
Diacetone alcohol (Ketol)
Mechanism : (i) Abstraction of acidic hydrogen by base
H
O
O –
CH2—C—H + OH
CH2—C—H + H2 (Enolate ion which is stabilised by resonance)
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Nucleophilic attack of enolate ion O O CH3—C—H + CH2—C—H
O–
O
CH3—CH—CH2—CH + H2O OH CH3—CH—CH2CHO + OH (-hydroxy carbonyl compound)
(iii) Crossed AldolCondensation : When mixture of two carbonyl compounds having ahydrogen on at least one of them is treated with dilute alkali the mixture of products is formed and this reaction is called as crossed aldol condensation. For example when mixture of CH3CHO and CH3– CO–CH3 is treated with dilute alkali then four products are formed.
OH OH OH O OH O | | || | | || CH3–CH–CH2–CHO, CH3–CH–CH2–C–CH3, CH3–C–CH2CHO CH3–C–CH–C–CH3 | | CH3 CH3
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However if one of them does not have -hydrogen then the number of products formed will be two
CH3–CHO + HCHO
OH
–
OH OH | | CH3–CH–CH2CHO + CH2–CH2–CHO
(iv) Perkin reaction : Condensation of an aromatic aldehyde with acid anhydride in presence of base (sodium salt of the acid from which the anhydride is derived) to form , -unsaturated acid as known as Perkin reaction. For example, 3 C 6H5 CHO (CH3 CO )2 O C 6H5 CH CHCOOH
CH COONa
Benzaldehyde
Acetic anhydride
Cinnamic acid
Aldehydes are oxidised not only by strong oxidising agents like KMnO4 and K2Cr2O7 but also by much milder oxidising agents like bromine water, Tollen’s reagent, Fehling’s solution and Benedict’s solution. Tollen’sreagent : Tollen's reagent is Ammoniacal silver nitrate solution –
R.CHO 2[Ag(NH 3 )2 ]OH RCO ONH4 2Ag H2O 3NH 3 (Tollen's reagent)
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Silver mirror
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Fehling solution : [Alkaline solution of copper sulphate containing sodium potassium tartarate (Roschelle salt)] R–CHO + 2Cu2+ + 5OH– R–COO–+ + 3H2O Benedict Solution : Its a solution of CuSO4, sodium citrate and sodium carbonate. When heated with aldehyde it gives reddish brown ppt. of Cu2O. Note : Benzaldehyde although reduces Tollen’s reagent, it does not reduce Fehling and Benedict solutions Ketones are not oxidised by mild oxidising agents. Oxidation in drastic condition Oxidation of mixed ketones is governed by Popoff’s rule according to which the carbonyl group of the ketone goes with the smaller alkyl group. Thus in the above case 'b' type of cleavage will decide major products. (c) Reaction with Ammonia : OH CH3CH = O + HNH2
CH3CH NH2
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–H2O
CH3—CH = NH Acetaldimine
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6HCHO 4NH 3 (CH2 )6 N4 6H2 O Hexamethylene tetramine (Urotropine)
(c) Cannizzaro reaction : This reaction is preferentially given by those aldehydes which do not contain -hydrogen. In Cannizzaro reaction, one molecule of the aldehyde is oxidised to acid at the expense of the other which is reduced to alcohol i.e., disproportionation reaction takes place. The reaction occurs in the presence of concentrated solution of any base.
2 HCHO Formaldehyde
NaOH HCOONa CH3 OH Sod. formate
Methyl alcohol
(e) Reaction given only by Ketones : (1) Reduction in Neutral or Alkaline Medium : To form Pinacolwhich undergoes pinacol - Pinacolone rearrangement in acidic medium CH3 | H H C C C CH3 3 CH3 || | CH O 3 CH3
CH3
C=O+O=C
CH3
CH3 CH3
Mg — Hg/H 2O
CH3 CH3
C—C | | OH OH
Pinacolone
Ketones can be reduced to secondary alcohols with aluminum isopropoxide in 2-propanol solution (Meerwein - PonndorfVerley reduction).
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` 2 3 R 2 C O R 2 CHOH
[Me CHO ] Al Me2 CHOH
(Meerwein - PonndorfVerley reduction). 2 3 R 2 C O R 2 CHOH
[Me CHO ] Al Me2 CHOH
Condensation with chloroform : CH3 C = O + CHCl3
CH3
NaOH
OH C
CH3
CH3
CCl3
1, 1, 1-Trichloro-2-methylpropanol-2 (Chloretone) used as hypnotic drug
Haloformreaction : Methyl ketones and acetaldehyde react rapidly with halogens (Cl2, Br2 or I2) in the presence of alkali to form haloform. O R
C
e.g.,
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CH3 + 3Br2 + 4NaOH
heat
RCOONa + CHBr3 + 3H2O + 3NaBr Bromoform (reddish brown ppt.)
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(f) Special Reactions of Aromatic Aldehydes and Ketones : (i) Reaction with Ammonia :
C6H5CH=OH2NH
C6H5CH=N
+ O=CHC6H5
C6H5CH=OH2NH
C6H5CH=N
CHC6H5
Hydrobenzamide
(ii) Reaction with amines :
C 6H5 CH O H2NC 6H5 C 6H5 CH NC 6H5 H2 O Benzylidene aniline. (Benzal aniline)
(iii)
Reaction of benzene nucleus
CHO
CHO conc. HNO3 conc. H2SO4 Benzaldehyde
NO2 m-Nitrobenzaldehyde
CHO
CHO conc. H2SO3
SO3H Benzaldehyde
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3-Benzaldehyde sulphonic acid
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COCH3
COCH3
COCH3
NO2
conc. HNO3
Br 2
conc. H2SO4
AlCl3
3-Nitroacetophenone
Acetophenone
COCH3
Br 3-Bromoacetophenone
COCH2Br Br2/OH
–
273K Acetophenone
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-Bromoacetophenone
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Alcohols , Phenols & Ethers ALCOHOLS Molecules containing –OH group are termed as alcohols. Classification of alcohols they are classified as primary, secondary or tertiary alcohol according to the carbon that is bonded with –OH. Again when any molecule contain 1, 2 or 3 –OH groups then it is called mono, di or tri hydric alcohols respectively. (as in case of alkyl halides) OH
OH
OH
OH
OH
CH3 – CH2OH
CH2 – CH2
CH2 – CH – CH2
Ethyl alcohol (monohydric)
Ethylene glycol (dihydric)
Glycerol (trihydric)
GENERAL METHODS OF PREPARATION
1. From Alkenes : (i) By direct hydrolysis : OH CH3 – CH = CH2 + H2O
H2SO4
CH3 – CH – CH3
(ii)Oxymercurationdemercuration :
CH3 – CH = CH2 + H2O
(iii)
Hg (OAc)2
OH CH2 – CH – CH2
THF
NaBH4 OH–
OH CH3 – CH – CH3
Hg(OAc)
Hydroboration oxidation :
6CH3 – CH = CH2
B2H6
2(CH3 – CH2 – CH2 –)3B
H2O2/OH
OH 6CH3 – CH2 – CH2 + 2H3BO3
Overall result of above reaction is anti Markwonikoff addition of water and with no rearrangement.
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iv) Oxo process followed by hydrogenation : O CH3 – CH = CH2 + CO + H2
CH3 – CH2 – CH2 – C – H
[Co(CO)4]2
H 2/Pd
high temperature and high pressure
CH3 – CH2 – CH2 – CH2 – OH
Product has one more carbon.
2. From Alkyl Halides : When alkyl halides are treated with aq. KOH or aq. NaOH or moist Ag2O, alcohols are formed.
R X O H R OH X –
3. Reduction of Carbonyl Compounds, Carboxylic Acids and their Derivatives : Table : Reducing nature of different reagents O R–C–H
red. agent
R – CH2OH
O R – C – R
OH red. agent
O R – C – OR
red. agent
R – CH2OH + R – OH
O R–C–X O
red. agent
O
R–C–O–C–R
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R – CH2OH red. agent
2R – CH2OH
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Table : Reducing nature of different reagents Group O
NaBH4/C2H5OH
LiAlH4/H2O
Product
B2H6/THF
H2/Pt
– CH2 – OH
Yes
Yes
Yes
Yes
>C=O
> CH – OH
Yes
Yes
Yes
Yes
– COOH
– CH2OH
Yes
No
Yes
Yes
– CH2OH
Yes
Yes
No
Yes
R – CH2OH
Yes
No
Yes
Yes
– C – OR
– CH2OH + R – OH
Yes
No
Yes
Yes
>C=C
CH – CH
2° > 3° (iii)Solubility : The extent of solubility of any alcohol in water depends upon the capability of its molecules to form hydrogen bonds with water molecule. (iv)Alcohols are lighter than water however, the density increases with the increase in molecular mass.
Chemical Properties 1. Reactions involving cleavage of O – H Bond Alcohols are acidic in nature but they are less acidic than water hence they do not give H+ in aqueous solution. They do not change the colour of litmus paper. Their acidic strength increases by increasing–I strength of the groups attached and decreases by increasing +I strength of the groups. (i) Alcohols do not react with aqueous alkali, as it does not give H + in aqueous solution. (ii) Action of active metal :When alcohols are treated with active metal they form alkoxides with the liberation of H2 gas.
2ROH 2Na 2R O N a H2 (iii)Esterification : When carboxylic acid is treated with alcohols in the presence of acid as catalyst, esters are formed.
O R – C – OH + H – O – R
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H+
O R – C – OR + HOH
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Note : (a) OH is removed from carboxylic acid and H is removed from alcohol. (b) Overall reaction is SN2 in which alcohol acts as nucleophile. (iv) Reaction with Grignard Reagent :When Grignard reagents are treated with alcohol (or any proton donor) they form alkanes. R – MgX + H – OR R– H + R– OMgX Other proton donors can be carboxylic acids, phenols, alkynes, H2O, Amines, NH3 etc. 2. Reactions Involving Cleavage of C– O Bond (i) Reaction with HX :Most alcohols undergo SN1. 2 R OH HCl (g) R Cl H2O
Anhyd. ZnCl
(a)
Note :HCl + anhyd. ZnCl2 is called Lucas reagent. (c)
H / H2SO4 R OH HIReactivity R isI H2O order of HX conc .
HI >HBr>HCl (ii) Dehydration : Alkyl chlorides can also be prepared by following methods : R – OH + PCl5
R – Cl + POCl3 + HCl
3R – OH + PCl3
3R – Cl + H3PO3
R – OH + SOCl2
R – Cl + SO2 + HCl (Darzen's process)
Darzen’s process is the best method as the other products are gases. 3.Reduction : Alcohols are reduced to alkanes when they are treated with Zn-dust or red P + HI. Zn dust R OH R H ZnO
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4. Oxidation :
O CH3OH
[O]
O [O]
H–C–H
H – C – OH
[O]
CO2
3°-alcohols can’t be oxidised. (i) Strong oxidising agent like KMnO4 or K2Cr2O7 cause maximum oxidation as above. (ii) If 1°-alcohol has to be converted into aldehyde PCC + CH2Cl2 or CrO3 should be used among which PCC + CH2Cl2 is the best. (iii)2°-alcohol can converted to ketone best by PCC + CH2Cl2 or CrO3 or H2CrO4 in aq. acetone (Jones reagent). (iv)MnO2 selectively oxidises the –OH group of allylic and benzylic 1° and 2° alcohols to aldehydes and ketones respectively.
4. Action of Heated Copper : (i) O CH3 – CH2 – OH
Cu 573K
CH3 – C – H + H2 (Dehydrogenation)
(ii) OH
O
CH3 – CH – CH 3
Cu 573K
CH 3 – C – CH 3 + H 2 (Dehydrogenation)
(iii)Tertiary alcohols undergo dehydration to give alkene under similar condition.
CH3 CH3 – C – OH CH3
CH2 Cu 573 K
CH3 – C
+ H2O
CH3
Distinction Between 1°, 2° and 3° Alcohols
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1. Lucas Test : Any alcohol is treated with Lucas reagent (HCl + an hyd. ZnCl2) at room temperature if (i) Solution becomes cloudy immediately, alcohol is 3°. (ii) Solution becomes cloudy after 5-min, alcohol is 2°. (iii)In solution cloud does not form at room temperature, alcohol is 1°.
2. Victor Meyer’s Method : R – CH2OH
P + I2
R – CH2I
AgNO2
R – CH2 – NO2 + HNO2
–H2O
R – C – NO2
(1°-alcohol)
N OH (Nitrolic acid) NaOH
blood red colour R2CH – OH
P + I2
R2 – CH – I
AgNO2
R2CH – NO2 + HNO2
–H2O
(2°-alcohol)
R2C – NO2 NO Pseudonitrol NaOH
blue colour
R3 – C – OH
P + I2
R3C – I
AgNO2
R3C – NO2
HNO2
No reaction
NaOH
Colourless
(3°-alcohol)
Note : Rectified Spirit :Azeotropic mixture of 95% C2H5OH and 5% H2O is called rectified spirit. Denatured Spirit :Azeotropic mixture of C2H5OH and CH3OH is called methylated spirit.
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PHENOLS OH
and their derivatives are called phenols. In phenol R– of alcohol is replaced by aryl ring.
Comparison of bond Angles in Phenols, Alcohols and Ethers : ..
109° .. O ..
H–C
108.5°
H
H
.. O
H
..
.. O
H
H–C
H
111.7°
H
H C H
H
Bond angle increases with the increase in hindrance. Method of Preparation 1. From Aryl Sulphonic Acids : When aryl sulphonic acids are fused with NaOH at 570 – 620 K followed by hydrolysis phenols are formed. SO3H + NaOH
SO3Na
ONa
OH +
H /H2O
2. From Haloarenes : (Dow's process) + ONa
Cl + NaOH (aq.)
623 K 320 atm
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OH H+
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Note :
Condition of reaction become less vigorous when –M groups are present at ortho or para or both position to the chlorine atom.
3. From Benzene DiazoniumSalts :
+ N2 Cl
NH2
OH
NaNO2 + HCl
H2 O
0 – 5°C
+ N2 + H–Cl
H2SO4
Note :In the absence of H2SO4diazocoupling will also take place. +
N2 Cl–
OH
OH
+
+ HCl N=N
4. CumeneProcess :
CH3 CH3
H +CH3 – C = CH2 Benzene
Propene
CH3
H
C
H 3PO 4
CH3
+ O2 Cumene
light
C–O–O–H H +/H2O
OH
O
+ CH3—C—CH3
Cumene hydroperoxide
5. Grignard's Synthesis : OH 1 Mg O2 H2O / H 2 C6H5MgBr + C6H5OMgBr C6H5OH +
Br
6. From Salicylic Acid :
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OH
OH COOH
OH COONa + NaOH
+ NaOH
CaO, –Na2CO3
Chemical Properties 1. Acidic Nature : (i) Phenol behave as a weak acid forming phenoxide ion with strong alkalies.
C 6H5 OH Phenol
C 6H5 ONa + NaOHSodiumphenoxide+ H2O
(ii) It also reacts with sodium metal to form sodium phenoxide and hydrogen is evolved
C 6H5 OH Phenol
1 C 6 H5 O N a H 2 2 + Na
(a) Effect of substituents on the acidity of phenols : It should be noted that the presence of electron withdrawing groups like –NO2, – CN, –CHO,–X, –COOH, etc. increases the acidic strength (because of the greater polarity of O–H bond the greater stability of the phenoxide ion by the dispersal of –ve charge, by –R effect). On the other hand, electron-releasing groups like –CH3, –NH2, –OH, etc., tend to destabilize the phenoxide ion by intensifying its –ve charge by +R effect and hence decreases the acidic strength. o - chlorophenol> m - chlorophenol> p - chlorophenol Ka = 7.7 × 10–9 1.6 × 10–9 6.3 × 10–10
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In case of haloarenes –I effect of halogens dominates over it's +M effect. (except for fluorine) p-nitrophenol> o-nitrophenol> m - nitrophenol> phenol (b) Steric Effect : 3, 5 -dimethyl-4-nitrophenol is weaker acid than the isomeric 2, 6-dimethyl - 4 - nitrophenol. 2.
Alkylation or Etherification :When sodium phenoxide is treated with alkyl halides (but not with aryl halides as they are inert) form phenolic ethers. CH I
NaOH 3 C 6H5 OH C 6H5 ONa C 6H5 OCH3 NaI Phenol
H 2 O
Sodium phenoxide
Methyl phenylether (Anisole)
C H Br
NaOH 5 C 6H5 OH C 6H5 ONa 2 C 6H5 OC 2H5 NaI Phenol
Sodium phenoxide
Ethyl phenyl ether (Phenetole)
3. Claisenrearrangement : C 6H5 O CH 2 CH CH 2 NaBr
C6H5ONa + BrCH2 – CH = CH2
Allyl phenyl ether
When aryl allyl ether is heated to 475 K, the allyl group of the ether migrates from ethereal oxygen to the ring carbon at ortho position. Note : Carbon attached with oxygen is not attached with the carbon of benzene ring in the product. 4. Acylation and benzoylation :
O
O
C 6H5 OH Phenol
CH3 – C – Cl
+
Acetyl chloride
Pyridine
C6H5 – O – C – CH 3 Phenyl acetate
5. Fries Rearrangement : When heated with anhydrous aluminium chloride, phenyl esters undergo Fries rearrangement forming a mixture of o- and p-hydroxy ketones.
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O
OH
O – C – CH3
OH O C – CH3
Phenyl acetate
Heat AlCl3
O = C – CH3 o-hydroxyacetophenone + p-Hydroxyacetophenone
The para isomer is formed predominantly at low temperature while at higher temperatures o - isomer is predominant. 6. Reactions due to C–O Bond : (i) Reaction with PCl5 : C6H5OH + PCl5 C6H5Cl + POCl3 + HCl P(OC 6H5 )3 3HCl
3C6H5OH + PCl3
T riphenylphosphate
The yields of C6H5Cl is very poor due to the formation of triaryl phosphate. (ii) Reaction with Ammonia : 2 C6H5 OH NH 3 C6H5NH 2 H2 O
ZnCl
Phenol
573 K
Aniline
(iii)Reaction with Zinc Dust :
Δ C6H5 OH Zn C6H6 Phenol
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Benzene +ZnO
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(iv) Reaction with Neutral FeCl3: ( Test for phenol) (C 6H5 O)3 Fe 3HCl Ferric phenoxide (Violet)
3C6H5OH + FeCl3
7.
Electrophilic Substitution Reaction on the Benzene Ring : From the contributing structure of phenol, it is clear that ortho- and para-position on it become rich in electron density. Thus the electrophilic attack at these positions is facilitated. Again present on the benzene ring is the very powerful ring activator towards electrophilic aromatic substitution.
(i) Bromination : OH Br
Br
OH Br
Phenol + 3Br2 H2O
2,4, 6-Tribromophenol (yellow ppt.) +
3HBr
OH Br
OH
Br
Br SO3H p-Phenolsulphonic acid
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+ 3Br2 (aq.)
2,4, 6-Tribromophenol (yellow ppt.) +
3HBr + H2SO4
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(ii) Nitration : OH
OH NO2
OH
NO2 o-Nitrophenol
293 K (40% yield) + (a) Phenol + dil HNO3
p-Nitrophenol (13% yield)
With concentrated nitric acid and sulphuric acid, it forms 2, 4, 6-trinitrophenol (Picric acid). OH O2N
NO2
OH
NO2 H SO conc. 3HNO 3 2 4 2,4, 6-trinitrophenol
+
(Picric acid)
(conc.)
(iii)Sulphonation :When heated with conc. sulphuric acid, phenol forms hydroxy benzene sulphonic acid. OH
OH
SO3H
H2SO4, 298 K –H2O
373 K OH
H2SO 4, 373 K –H2O SO3H p-Hydroxy benzene sulphonic acid
(iv) Friedel-Crafts Alkylation and Acylation :Phenol undergo both these reaction to form mainly p-isomer.
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OH
OH
OH
CH3
CH3 Phenol + CH3Cl Anhyd. AlCl3
(Major product) p–Cresol
o-Cresol
+
OH OH COCH3
+
OH
COCH3 p-
o-
+ CH3COCl Anhyd. AlCl3
Hydroxy acetophenone
8. Kolbe’s reaction OH ONa
COONa
OH COOH
Sodium salicylate H 398 K, 4 – 7 atm Sodium phenoxide + CO Salicylic acid (Main product) 2
Note :
(i) Methylsalicylate with methanol. OH O C – OCH3
OH COOH
+ CH3OH
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H2 SO4 few drop
Methyl Salicylate
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(ii) Salol (with phenol) : OH
OH COOH +C6H5OH
O C – O – C6H5
H 2SO4
Phenyl salicylate (Salol)
Methyl salicylate is known as oil of winter green. Phenyl salicylate is known as salol. Salol is an intestinal antiseptic. 9. RiemerTiemannReaction ;
(a) On heating with chloroform and alkali phenols are converted to phenolic aldehydes OH
OH
+ CHCl3 + 3NaOH
CHO
333343 ( aq.) H
+ 3NaCl + 2H2O
In this reaction dichlorocarbene is formed as intermediate which attack on benzene ring as electrophile. (b) If instead of chloroform, carbon tetrachloride is used, salicylic acid is formed. Some para isomers is also formed.
ONa
ONa
OH
CCl3
+ CCl4
NaOH 340 K
COONa
3NaOH ( aq.) – 3NaCl
Dil. H 2SO4
OH COOH
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In this reaction is formed as intermediate which attack on benzene ring as electrophile. 10. Coupling with DiazoniumSalts : N=N
C6H5N2Cl + C6H5OH
0 5 C pH 9 10
OH
p-Hydroxy azobenzene (An orange dye)
11. Test for phenol (i) Neutral FeCl3 test Aqueous solution of phenol gives a violet colouration with FeCl3. (ii) Br2 water test Aqueous solution of phenol gives a yellow precipitate of 2, 4, 6tribromophenol with bromine water. (iii)Phenol gives Liebermann's nitroso reaction.
Phenol
NaNO2 NaOH Re d colour excess blue colour
(in conc. H2SO4 ) excess of water
ETHERS Ethers are those organic compounds which contain two alkyl groups attached to an oxygen atom, i.e., R–O–R. They are regarded as dialkyl derivatives of water or anhydrides of alcohols. –2H H – O – H R – O – R R – OH HO – R H O Water
2R
Ether
2
Alcohol ( 2 moles)
Ethers may be of two types : (i) Symmetrical or simple ether are those in which both the alkyl groups are identical and (ii) unsymmetrical or mixed ethers are those in which the two alkyl groups are different. CH3–O–CH3; C6H5–O–C6H5 Symmetrical (simple) ethers educareacademydgl.com
CH3–O–C2H5; CH3–O–C6H5 Unsymmetrical (mixed)ethers Page 66
Like water, ether has two unshared pair of electrons on oxygen atom, yet its angle is greater than normal tetrahedral (109°28´) and different from that in water (105°). This is because of the fact that in ethers the repulsion between lone pairs of electrons is overcome by the repulsion between the bulky alkyl groups.
Preparation of Ethers : 1. By dehydrating excess of alcohols : Simple ethers can be prepared by heating an excess of primary alcohols with conc. H2SO4 at 413K. Alcohol should be taken in excess so as to avoid its dehydration to alkenes. Conc. H2SO4 C 2H5 – OH HO – C 2H5 C 2H5 – O – C 2H5 H2 O Ethanol ( 2 molecules)
413K
Diethyl ether
Dehydration may also be done by passing alcohol vapours over heated catalyst like alumina under high pressure and temperature of 200 – 250°C. 2. By heating alkyl halide with dry silver oxide (only for simple ethers) : Remember that reaction of alkyl halides with moist silver oxide (Ag2O + 2H2O = 2AgOH) gives alcohols C2H5I + Ag2O (moist) C2H5OH + AgI 3. By heating alkyl halide with sod. or pot. alkoxides (Williamson synthesis): C2H5ONa + ICH3 C2H5OCH3 + NaI
ONa + BrCH3 Sod. phenoxide
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OCH3 + NaBr Methoxybenzene (Anisole)
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However
CH3 | + CH3–Cl + NaO–C–CH3 | CH3
CH3 | CH3–O–C–CH3 + NaCl | CH3
If alkyl halide is other than methyl halide and it is treated with tertiary alkoxide ion, Hoffmann elimination takes place instead of Williamson's ether synthesis.
CH3 Cl | | + CH3–CH2–CH–CH3 + NaO–C–CH3 | CH3
CH3–CH2–CH=CH2 + HCl (Major)
4. Methyl ethers can be prepared by treating primary or secondary alcohol or phenol with diazomethane in presence of BF3.
3 C 2H5 OH CH 2N2 C 2H5 OCH3 N2
BF
Ethylmethyl ether
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Chemical Properties : A. Properties due to Alkyl Groups : 1. Halogenation : When ethers are treated with chlorine or bromine in the dark, substitution occurs at the -carbon atom. The extent of substitution depends upon the reaction conditions.
´
´
dark C H3 – C H2 – O – C H2 – C H3 Cl 2 CH3 .CHCl – O – CH2 .CH3 – Chlorodiethyl ether
Cl 2 CH2Cl.CHCl – O – CH2 .CH3 CH3 CHCl – O – CHCl.CH3 , – Dichlorodiethyl ether
, ´– Dichlorodiethyl ether
light CH3 CH 2 – O – CH 2 .CH3 10Cl 2 CCl 3 .CCl 2 – O – CCl 2 .CCl 3 Perchlorodiethyl ether
2. Combustion : C2H5.O.C2H5+ 6O24CO2 + 5H2O B. Properties due to Ethereal Oxygen : 1. Chemical inertness : Since ethers do not have an active group, in their molecules, these do not react with active metals like Na, strong bases like NaOH, reducing or oxidising agents. 2. Formation of peroxide (Autoxidation) : On standing in contact with air and light ethers are converted into unstable peroxides (R2O O) which are highly explosive even in low concentrations. educareacademydgl.com
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3. Basic nature : Owing to the presence of unshared electron pairs on oxygen, ether behave as Lewis bases. Hence they dissolve in strong acids (e.g. conc. HCl, conc. H2SO4) at low temperature to form oxonium salts.
(C 2H5 )2 O H2SO 4 Diethyl ether
[(C 2H5 )2 OH] HSO 4– Diethyloxonium hydrogen sulphate
On account of this property, ether is removed from ethyl bromide by shaking with conc. H2SO4. Being Lewis bases, ethers also form coordination complexes with Lewis acids like BF3, AlCl3, RMgX, etc. If one of the the group around oxygen is aryl group then I – will always attack on the group other than aryl group. C. Properties Due to Benzene Nucleus : Alkoxy group, being o-, p- directing, anisole undergoes substitution in o- and ppositions. However, –OR group is less activating than the phenolic group. (i) Nitration : OCH3
OCH3 conc. HNO3
OCH3 NO2
conc. H2SO4
+
NO2 Methylphenyl ether (Anisole)
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Methyl 2-nitrophenyl ether or o-Nitroanisole
Methyl 4-nitrophenyl ether or p-Nitroanisole
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(ii) Bromination :
OCH3
OCH3 Br
Br
Br2/Fe
Br 2, 4, 6, Tribromoanisole
Anisole OCH3
OCH3
OCH3
+ Br2
Br
CS2
Anisole
+
2-Bromoanisole
Br 4-Bromoanisole
(ii) Sulphonation : OCH3
OCH3
OCH3 SO3H
H2SO4 SO3 SO3H Anisole
p-Methoxybenzene sulphonic acid
o-Methoxybenzene sulphonic acid
It is for this reason that ethers are used as solvent for Grignard reagents.
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D. Properties due to carbon-oxygen bond : 1. Hydrolysis : The hydrolysis may also be effected by boiling the ether with water or by treating it with steam. Note : Ethers can never be hydrolysed in alkaline medium. 2. Action of hydroiodic or hydrobromicacid : In cold, ether react with HI or HBr to give the corresponding alkyl halide and alcohol. In case of mixed ethers, the halogen atom attaches itself to the smaller alkyl group.
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