(1) Boiling point :
(a) Alcohols show increase in boiling point with increase in molecular weight amongst homologues.
(b) Alcohols have higher boiling point than hydrocarbons of the same molecular weight. The reason for higher boiling point is the intermolecular H-bonding present in alcohols.
Intermolecular H bonds in alcohols.
(2) Solubility in water :
As molecular weight increases solubility in water decreases. The lower alcohols are miscible with water. This is due to intermolecular hydrogen bonding between alcohol and water molecules.
Intermolecular H bond between water & alcohol molecules.
Preparation of alcohols:
(1) From alkenes
(a) By acid catalyzed hydration of alkenes: Formation of carbocation intermediate (Markovnikov addition, rearrangement possible).
(b) By Oxymercuration - demercuration process :
(1) Oxymercuration involves an electrophilic attack on the double bond by the positively charged mercury species. The product is a mercurinium ion, an organometallic cation containing a three-membered ring.
(2) In the second step, water from the solvent attacks the mercurinium ion to give (after deprotonation) an organomercurial alcohol.
(3) The third step is demercuration to remove the Hg. Sodium borohydride (NaBH4, a reducing agent) replaces the mercuric acetate fragment with hydrogen.
+ OH- + NaBH4
(c) By Hydroboration-oxidation process : (Forms anti-Markovnikov alcohol, no rearrangement).
General reaction: R-Ch=CH2R-CH2-CH2-OH
Ex. Give the major product of the following reaction:
Ans. Major product is
because 3o carbocation is more stable.
(2) From alkyl halides : By nucleophilic substitution reactions.
(a) By SN2 mechanism (second-order substitution) : It is given by primary (and some secondary) halides.
General reaction: R-CH2-Br R-CH2-OH
e.g. (CH3)2CHCH2CH 2-Br (CH3)2CHCH2CH 2- OH
(b) By SN1 mechanism : It is given by tertiary and some secondary halides.
General reaction :
(3) From Grignard's reagent.
(a) From air:
A Grignard reagent may be used to synthesize an alcohol by treating it with dry oxygen and decomposing the product with acid :
General reaction RMgX RO2MgX 2ROMgX 2ROH
e.g. C2H5MgBrC2H5O2 MgX2C2H5OMgX 2C2H5OH MgBr(OH)
(b) From ethylene oxide:
Addition of Grignard reagent to ethylene oxide gives a primary alcohol (with two carbon atoms added).
General reaction RCH2CH2OMgX RCH2CH2OH MgX(OH)
e.g. C2H5MgBr C2H5CH2CH2 OMgX MgBr(OH)
(c) From carbonyl compounds : Nucleophilic addition to the carbonyl groups by Grignard's reagent.
General reaction Mg(OH)X
(i) Addition of formaldehyde gives a primary alcohol.
General reaction= O RMgX RCH2-OH
(ii) Addition to an aldehyde (other than formaldehyde) gives a secondary alcohol.
(iii) Addition to a ketone gives a tertiary alcohol.
e.g. CH3CH2MgCl +
(iv) Addition to an acid halide or an ester gives a tertiary alcohol.
Esters on treatment with Grignard's reagent first form ketones which then react with second molecule of Grignard's reagent and form tertiary alcohol.
(4) By reduction of carbonyl compounds.
(a) Catalytic hydrogenation of aldehydes and ketones.
e.g. CH3CHO + 2H CH3CH2OH
(b) Lithium aluminium hydride reduction of aldehydes and ketones.
(iii) RCOOH+ 4H RCH2OH + H2O
(iv) RCH2OH+ HCl
(v) R-CH2OH +R'OH
(c) By NaBH4 (sodium borohydride) : It is insoluble in ether and is used in aqueous ethanolic solution to reduce carbonyl compounds. It does not reduce esters and acids.
CH=CHCHO +4H CH3CH=CHCH2OH
(ii) Reduction of a ketone gives a secondary alcohol.
(d) Bouveault-Blanc reduction : The reduction of aldehydes, ketones or esters by means of excess of sodium and ethanol or n-butanol as the reducing agent.
(i) Aldehyde RCHO RCH2OH
(ii) Esters R'CO2R'' R'CH2OH R''OH
(iii) Ketones R2CO R2CHOH
The Bouveault-Blanc reduction is believed to occur in steps involving transfer of one electron at a time.
e.g. CH3CHO +2H CH3CH2OH
Ex. Identify (X) in the following reaction:
Ex. What are the product A, B, C, D and E in the following reactions?
Ans. A : (EtOH)
B : Ester part is not affected by NaBH4
C : Ester part and keto parts are affected by LiAlH4
(5) By reaction of nitrous acid on aliphatic primary amines.
General reaction: R-NH2 + HONO R-OH +N2 + H2O
R-NH2 (R) ROH +N2 +
e.g. (i) C2H5NH2 +HNO2 C2H5OH+ N2 + H2O
(ii) CH3-CH2- +HONO CH3-CH2-+ N2 +H2O
(6) Hydroxylation : Forms vicinal diols (glycols).
Converting an alkene to a glycol requires adding a hydroxy group to each end of the double bond. This addition is called hydroxylation of the double bond.
(a) Syn hydroxylation, using KMnO4 / NaOH or using OsO4/H2O2
General reaction :
(b) Anti hydroxylation, using per acids.
Chemical reactions of alcohols:
1. Reaction with hydrogen halides.
General reaction :
R - OH HX R - X H2O (R may rearrange)
Reactivity of HX : Hl > HBr > HCl
Reactivity of ROH: allyl > benzyl > 3o > 2o > 1o
Mechanism R - OH R- R-X
2. Reaction with Phosphorus trihalides:
(1) Several phosphorus halides are useful for converting alcohols to alkyl halides. PBr3, PCl3, & PCl5 work well and are commercially available.
(2) Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with ter. alcohols. The two phosphorus halides used most often are PBr3 and the P4 / I2 combination.
General reaction :
3R - OH + PX3 3R - X + H3PO3
The mechanism for the reaction involves attachment of the alcohol group on the phosphorus atom, displacing a bromide ion and forming a protonated alkyl dibromophosphite (see following reaction).
In second step a bromide ion acts as nucleophile to displace HOPBr2, a good leaving group due to the electronegative atoms bonded to the phosphorus.
3. Reaction with thionyl chloride
R-Cl SO2 HCl
4. Dehydration of alcohols
H2O (Rearrangement may occur)
Step 1 :
Step 2 :
Step 3 :
Reactivity of ROH : 3º > 2º > 1º
5. Reaction with metals
6. Ester formation
+ + H2O
e.g. CH3CH2O - H + + H2O
7. Oxidation reactions
(a) oxidation of primary alcohols
Oxidation of a primary alcohol initially forms an aldehyde. Obtaining the aldehyde is often difficult, since most oxidizing agents are strong enough to oxidize the adehydes formed. CrO3 acid generally oxidizes a primary alcohol all the way upto the carboxylic acid
(b) oxidation of secondary aldohols-
Sec. alcohols are easily oxidized to give excellent yields of ketones. The chromic acid reagent is often best for laboratory oxidations of secondary alcohols. The active species in the mixture is probably chromic acid, H2CrO4, or the acid chromate ion,
(c) Resistance of tertiary alcohols to oxidation-
Oxidation of ter-alcohol is not an important reaction is organic chemistry. Ter-alcohols have hydrogen atoms on the carbinol carbon atom, so oxidation must take place by breaking C-C bonds. These oxidations require severe conditions and result in mixtures of products.