(1) Boiling point :
(a) Alcohols show an increase in boiling point with an increase in molecular weight amongst homologues.
(b) Alcohols have a higher boiling point than hydrocarbons of the same molecular weight. The reason for the higher boiling point is the intermolecular H-bonding present in alcohols.
Ethanol has a higher Boiling Point than Ethane.
Hydrogen Bonding in Ethanol
(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.
Table showing Solubility of various Alcohols
The reaction of Alkenes with Acids follow Markovnikov's Rule
(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) 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
(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.
By nucleophilic substitution reactions.
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
General reaction :
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)
The addition of Grignard reagent to ethylene oxide gives primary alcohol (with two carbon atoms added).
General reaction RCH2CH2OMgX RCH2CH2OH MgX(OH)
e.g. C2H5MgBr C2H5CH2CH2 OMgX MgBr(OH)
General reaction Mg(OH)X
(i) Addition of formaldehyde gives primary alcohol.
General reaction= O RMgX RCH2-OH
(ii) Addition to an aldehyde (other than formaldehyde) gives secondary alcohol.
(iii) Addition to a ketone gives tertiary alcohol.
e.g. CH3CH2MgCl +
(iv) Addition to an acid halide or an ester gives tertiary alcohol.
Esters on treatment with Grignard's reagent first form ketones which then react with the second molecule of Grignard's reagent and form tertiary alcohol.
e.g. CH3CHO + 2H CH3CH2OH
(iii) RCOOH+ 4H RCH2OH + H2O
(iv) RCH2OH+ HCl
(v) R-CH2OH +R'OH
It is insoluble in ether and is used in an 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 secondary alcohol.
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
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
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.
General reaction :
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
(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 the following reaction).
In a second step a bromide ion acts as a nucleophile to displace HOPBr2, a good leaving group due to the electronegative atoms bonded to the phosphorus.
R-Cl SO2 HCl
H2O (Rearrangement may occur)
Step 1 :
Step 2 :
Step 3 :
Reactivity of ROH : 3º > 2º > 1º
+ + H2O
e.g. CH3CH2O - H + + H2O
Oxidation of primary alcohol initially forms an aldehyde. Obtaining the aldehyde is often difficult since most oxidizing agents are strong enough to oxidize the aldehydes formed. CrO3 acid generally oxidizes primary alcohol all the way upto the carboxylic acid
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,
Oxidation of ter-alcohol is not an important reaction in organic chemistry. Ter-alcohols have hydrogen atoms on the carbonyl carbon atom, so oxidation must take place by breaking C-C bonds. These oxidations require severe conditions and result in mixtures of products.
Methanol (CH3OH) and ethanol (C2H5OH) are the simplest members of the primary alcohol family and have a wide range of applications in the fuel industry. Some important uses of methanol and ethanol are listed in this article.