Notes on lesson on Alcohols and Phenols |

Alcohols and Phenols


Alcohol and its types.

Alcohols are hydroxyl derivative of alkanes containing –OH as a functional group.These compounds are obtained from alkanes replacing hydrogen atom by a hydroxyl group.An alcohol containing two -OH group is called dihydric alcohol.

Isomerism of Alcohol

Alcohols exhibits following three types of isomerism. They are Chain isomerism.Position isomerism. Functional isomerism.In the common system of nomenclature,alcohols are named as. $$Alkyl\,+alcohol=Alkyl\,alcohol$$ Structural formula of tertiary butanol is given below and its IUPAC name is 2.methyl propane-2ol

General method of preparation of Alcohol

Haloalkane reacts with aueous alkali or moist AgO to give corresponding alcohols. $$\underbrace{R-H}_{halo\,Alkane}\xrightarrow{aq.alkali\,or\,moist\,Ag_2O}\underbrace{R-OH}_{Alcohol}$$ Aldehydes and ketones from addition product with Grignard reagent. These addition products of hydrolysis in presence of an acid give alcohols. All three types of alcohol )10, 20, and 30) can be prepared by this method.Primary amines react with nitrous acid to give corresponding alcohol. $$\underbrace{R-NH_2}_{Primary\,amine}+HO-NO→\underbrace{R-OH}_{Alcohol}+N_2+H_2O$$

Industrial method of preparation of alcohol

There are three methods to get alcohols, industrially. all these methods utilise raw materials which can be obtained from petroleum, natural gas, coal and biomass. The methods are outlined below.By hydration of alkenes. By oxo process. By fermentation of carbohydrates.

Victor Meyers method of distinguish Primary,Secondary and Tertiary alcohol

Victor Mayer method is one of the most important methods of identification of alcohols. In this methods primary,secondary and tertiary alcohols are subjected to a series of chemical analysis and the colour of resulting solution is observed. A primary alcohol gives blood red colour. secondary alcohol gives the blue colour. A tertiary alcohol does not produce any colour.

Chemical property of Alcohol

The presence of electrons donating group (+I groups) decreases the acidic strength of alcohol. Therefore, alcohols are less acidic than water. $$\underbrace{H-O-H}_{water}$$ $$\underbrace{R-O-H}_{Alcohol}$$ Similarly, acidic strength decreases gradually on moving from primary to secondary to tertiary alcohol. $$R-CH_2-OH>R_2CH-OH>R_3-C-OH$$ Esterification Alcohols react with carboxylic acids in presence of small amount of conc. H2SO4 to produce sweet smelling compound called esters. Thereaction is known as esterification. $$\underbrace{R-COOH}_{Carboxylic\,acid}+\underbrace{R-OH}_{alcohol}\xrightarrow{Conc.H_2SO_4}\underbrace{R-COOR}_{Ester}+H_2O$$

Other reaction of Alcohol

$$\underbrace{CH_3-CH_2-OH}_{Ethanol}\xrightarrow{Conc-H_2SO_4\,160^0-170^0C}\underbrace{CH_2=CH_2}_{Ethene}+H_2O$$ $$\underbrace{CH_3-CHOH-CH_3}_{2-Propanol}\xrightarrow{Conc-H_2SO_4\,160^0-170^0C}\underbrace{CH_2=CH-CH_3}_{Propene}$$ $$\underbrace{CH_3-CH_2-CHOH-CH_3}_{2-butanol}\xrightarrow{Conc-H_2SO_4\,160^0-170^0C}\underbrace{CH_3-CH=CH-CH_3}_{2-butene}+H_2O$$ A primary alcohol is easily oxidised to an aldehyde which gets further oxidised to give a carboxylic acid containing carbon atoms equal to that of alcohols. Tertiary alcohol are not oxidised by neutral or alkaline oxidising agents but acidic oxidising oxidise them. At first, the alkene is formed due to dehydration which is then oxidised to give a mixture of aldehydes, ketones and acids.

Physical property of alcohol

Alcohol can form the hydrogen bond with the water molecule. Therefore, they are soluble in polar solvents like water. Their solubility , however, decreases with increase in a size of an alkyl group.Ethene is oxidised in the air in presence of catalyst and temperature to produce epoxy ethane which on acid hydrolysis gives ethylene glycol.Fats and oils are the esters of long chain fatty acids known as triglycerides. These esters can be hydrolyzed both in acidic and basic medium to manufacture glycerol. Alkali hydrolysis of these esters produces glycerol and soap. This process is known as saponification.