Aromatic Alcohols
Aromatic alcohols are compounds containing a hydroxyl group in a side- chain, and may be regarded as aryl derivatives of the aliphatic alcohols
Preparation 1-By hydrolyzing benzyl chloride with aqueous sodium hydroxide
CH2Cl NaOH CH2OH NaCl
2-By reducing benzaldehyde with zinc and hydrochloric acid
e, H CHO CH2OH By means of the cannizaro reaction
2 CHO NaOH CH2OH COONa
90 %
It may also be prepared by means of a crossed Cannizzaro reaction
Ar-CHO + HCHO Ar-CH2OH
The reactions of benzyl alcohol are similar to those of the primary aliphatic alcohols; e.g., on oxidation, it forms benzaldehyde and finally benzoic acid
[O] [O] CH2OH CHO COOH
Benzyl alcohol may be catalytically (palladium) reduced to toluene:
Pd/C CH2OH H2 CH3 H2O Aromatic aldehydes
Aromatic aldehydes fall into two groups: those in which the aldehyde group is directly all to the nucleus, and those in which it is attached to the side- chain. The former group comprise the aromatic aldehydes; the latter, which behave as aliphatic aldehydes, are best regarded aryl-substituted aliphatic aldehydes
1- Benzaldehyde (benzenecarbonal)
It is also known as oil of bitter almonds, since it is found in glucoside amygdalin, which occurs in bitter almonds Preparation
CHCl2 CHO
H O+ 1- 3 + 2 HCl (76%) Hydrolysis with aq acid
Benzylidene chloride
H O
CH2Cl CH O N CHO O
2- Pb(NO3)2, + HNO2 CO2 Benzyl chloride
CH3 CH(OCOCH3)2 CHO
+ 3- CrO3 H + 2 CH3COOH (CH3CO)2O Toluene Benzylidene acetate (40-50%)
Hydrolysis with dilute H2SO4 or HCl 4- Gattermann-Koch aldehyde synthesis:
CHO
AlCl + CO + HCl 3 + HCl Cu2Cl2
Mechanism
.. CO + HCl + AlCl3 CH O CH O + AlCl4
Formyl cation 5- Gattermann aldehyde synthesis CHO
AlCl HCl 3 + HCN + + NH3
Mechanism AlCl HCl + HCN 3 NH CHCl (imidoformyl chloride) CH NH
AlCl + NH CHCl 3 + HCl
Aryl imine
H2O
CHO
+ NH3 Benzaldehyde and aromatic aldehydes in general resembles aliphatic aldehydes in the following reactions: (i) It gives the Schiff’s reaction. (ii) It is readily oxidised, i.e., it is a strong reducing agent; e.g., it reduces ammoniacal silver nitrate, itself being oxidised to benzoic acid when exposed to air. (iii) Benzaldehyde forms a bisulphite compound, and may be prepared via this compound: OH
CHO CH SO3Na
+ NaHSO3
(iv) Benzaldehyde forms a cyanohydrin (mandelonitrile)
OH
CHO CH CN
+ HCN
Cyanohydrin (mandelonitrile) CHO NHNH2 HC N NH
+
Benzaldehyde Phenyl hydrazine Phenylhydrazone CHO HC N N CH
+ NH2NH2
Benzaldehyde Hydrazine Benzylideneazine
Benzaldehyde reacts with hydroxylamine to give benzaldoxime which exists in two geometrical isomeric forms.
H OH H CHO C N C N OH
+ NH2OH +
Benzaldoxime (syn-form) Benzaldoxime (anti-form) Benzaldehyde and other aromatic aldehydes differs from aliphatic aldehydes in the following ways:
(i) It does not reduce Fehling’s solution. (ii) It does not readily polymerise; e.g., it does not resinify with sodium hydroxide (Aldol reaction no alpha hydrogen), (iii) but undergoes the Cannizzaro reaction because it is not having an alpha- hydrogen atom (The reaction is limited to aldehydes lacking alpha hydrogen centers. Under ideal conditions the reaction produces only 50% of the alcohol and the carboxylic acid.
CHO CH2OH COONa
2 + NaOH +
(iii) Reduction of benzaldehyde with zinc/HCl or sodium amalgam gives hydrobenzoin as well as benzyl alcohol.
H OH OH
.. . C O . HC C H
2 Zn/HCl H+ Hydrobenzoin
With aliphatic aldehydes it gives only the alcohol (vi) When benzaldehyde chlorinated , benzoyl chloride is produced (no alpha-hydrogen is present
H O
C O C Cl
+ Cl2 + HCl
In aliphatic aldehydes, halogenation occur at alpha hydrogen Condensation reactions of benzaldehyde
1- Claisen reaction benzaldehyde, in the presence of dilute alkali, condense with aliphatic aldehydes or ketones containing α-hydrogen
CHO CH CHCHO
NaOH + CH3CHO + H2O
Cinnamaldehyde
CHO CH CHCOCH3
NaOH + CH3COCH3 + H2O
Benzylideneacetone (65-78%) CHO CH CHCOCH3
NaOH + CH3COCH3 + H2O
Benzylideneacetone (65-78%) CHO
CH CHCOCH CH
Dibenzylideneacetone CHO
CH NaOH CHCO + CH3CO + H2O
(Benzylideneacetophenone, Chalcone) (phenyl styryl ketone) (85%)
CH CHCO CH2 CH2CO
Pt/EtOAc + H 2 Zn/HOAc
(Benzylideneacetophenone, Chalcone) Benzylacetophenone (81-95%) (phenyl styryl ketone) (85%) 2- Perkin reaction
When benzaldehyde (or any other aromatic aldehyde) is heated with the anhydride of an aliphatic acid (containing two α-hydrogen atoms) in the presence of its salt, condensation takes place to form a β-arylacrylic acid Perkin reaction does not usually take place with aliphatic aldehydes.
CHO CH CHCOOH
CH3COONa + (CH3CO)2O
Cinnamic acid arylacrylic acid Mechanism
CH COOCOCH 3 3 + CH3COO CH2COOCOCH3 + CH3COOH
O OH H O - H O CH COOCOCH 2 2 C + 2 3 C CH2COOCOCH3 H H
C CHCOOCOCH H3O C CHCOOH 3 + CH3COOH H H 3- Benzoin condensation The benzoin condensation is a reaction between two aromatic aldehydes, particularly benzaldehyde. The reaction is catalyzed by a nucleophile such as the cyanide anion. The reaction product is benzoin as the parent compound
(aliphatic aldehydes do not undergo the benzoin condensation)
HO
CHO CH CO
2 KCN EtOH/H2O
Benzoin (83%) Mechanism
O O OH C + CN C CN C.. CN H H
CHO OH O O OH
C C C C
CN H CN H
O OH
C C + CN H
OH O
C C HO C C OH
H O OH O H Reduction C C C C
H H
Benzoin Deoxybenzoin (desoxybenzoin) Derivatives of Benzaldehyde
Phenolic aldehydes (Hydroxyaldehydes):
Gattermann’s aldehyde synthesis (prep. of p-hydroxybenzaldehyde
OH OH
AlCl + HCN + HCl 3 + HCl
OH CH NH
ClCH NH H2O + NH3 Imidoformyl chloride
CHO Reimer-Tiemann reaction
(for prep. of o and p-hydroxybenzaldehyde)
OH OH
CHO NaOH + CHCl3 60oC Mechanism
: CHCl3 + OH H2O + CCl3 Cl + : CCl2 Dichlorocarbene
O O O H H
CCl2 C Cl
+ : CCl2 Cl
Dichlorocarbene (Electrophilic reagent)
O O O H H H OH C Cl C Cl C .. OH OH
O OH H H
C C O H O Salicylaldehyde (o-hydroxybenzaldehyde) Preparation
OSO2 OSO2 OH
CH3 CHO CHO + MnO2 H
H2SO4 o-Methylphenyl benzenesulphonate
Properties
OH OH OH
CH2OH CHO COOH Reduction Oxidation
o-Hydroxybenzyl alcohol Salicylaldehyde o-Hydroxybenzoic acid oxidation with alkaline H2O2 OH
OH
Catechol m-Hydroxybenzaldehyde
CHO CHO CHO
(i) Sn-HCl H2O
(ii) NaNO2- HCl boil (51-56%) NO2 OH N2 Cl
Vanillin OH OH
OCH3 OCH3 CHCl3 NaOH
CHO
Guaiacol Vanillin Aromatic Ketones
Aromatic ketones may be either alkaryl ketones or diaryl ketones Acetophenone (methyl phenyl ketone, acetylbenzene) Preparation: (1) From calcium or iron salt of benzoic acid:
COO COCH3
2 Ca + (CH3COO)2Ca + 2 CaCO3 2 Calcium benzoate Calcium acetate acetophenone
(2) Friedel-Crafts acylation of benzene: COCH3
AlCl3 + CH3COCl + HCl (50%)
acetophenone (3) Oxidation of ethylbenzene
CH2CH3 COCH3
(O) 126oC acetophenone Ethylbenzene Chemical reactions:-
(1) Reduction:- OH
COCH3 HC CH3
Na/EtOH
acetophenone Phenylmethylmethanol
COCH3 H2C CH3
Zn/HCl
Or LiAlH4 acetophenone ethylbenzene (2) Oxidation:-
O COOH COCH3 C COOH
KMnO4 KMnO4 acetophenone Phenylglyoxylic acid Benzoic acid (benzoyl formic acid)
COCH3 COCHO
+ SeO2 + Se + H2O + H2O (72-96%)
Phenylglyoxal CO2OH
COCH3 OCOCH3
(63%)
Phenyl acetate (3) Halogenation
COCH COCH Br 3 2
Br2 + HBr (64-66%) Ether at 0oC
Phenacyl bromide
Acetophenone forms an oxime, phenylhydrazone and cyanohydrin
The acetyl group is m-orienting, and so when acetophenone undergoes substitution, the main product is the m-derivative. Condensation reaction of acetophenone
- Acetophenone condenses with acetic anhydride in the presence of BF3 to form benzoylacetone.
COCH3 COCH2COCH3
BF3 + (CH3CO)2O + CH3COOH (83%)
Benzoylacetone
CHO CH CHCOOH
CH3COONa + (CH3CO)2O
Cinnamic acid arylacrylic acid Acetophenone condenses with ethyl benzoate in the presence of sodium ethoxide
COOC H COCH3 2 5 COCH2CO
C H ONa + 2 5 (62-71%)
ethylbenzoate dibenzoylmethane
+ C2H5OH Acetophenone undergoes the Mannich reaction:-
COCH3 COCH2CH2N(CH3)2. HCl
+ HCHO + (CH3)2NH. HCl (60%) Benzophenone (diphenyl ketone) O Preparation C
(1) By heating of Calcium benzoate
COO CO
Ca + CaCO3 (30%) 2 (2) By Friedel-Crafts condensation between benzoyl chloride and benzene
COCl CO (1)
AlCl + 3 + HCl (80%)
(2)
AlCl3 2 + COCl2 + 2 HCl Carbonyl chloride CO Chemical reactions Benzophenone forms an oxime, but does not form a cyanohydrin or bisulphate addition compound
1- Reduction:
OH Zn, CH COOH HC Zn, KOH C 3 C C
OH O OH
Benzhydrol (diphenylcarbinol) (90%) Benzopinacol (90%) CO CH2
LiAlH4/AlCl3
diphenylmethane
2- Fusion with KOH
CO COOK
KOH +
H2O