In alcohols, -OH group is the functional group. Therefore, the chemical properties of alcohols generally involve the reactions of -OH group. These can undergo substitution as well as elimination reaction. Alcohols can react both as nucleophiles and electrophiles. The bond between O-H is broken when they react as nucleophiles. The bond between C-O is broken when they react as electrophiles. Protonated alcohols react in this manner.
(protonated alcohols as electrophiles)
Some common reactions of alcohols are as follows:
Alcohols are very weak acids (Ka = 10-16- 10-18), ever’ feeble than water (Ka = 10-14) . They do not turn blue litmus-1 red but when treated with active metals like sodium, potassium and aluminium these liberate hydrogen along with the formation of alkoxides.
Alcohols behave as acids because of the presence of polar 0-H group. Due to greater electronegativity of oxygen atom the shared pair between 0 and H is drawn.
The order of acidic strength among various types of alkanols is
primary > secondary > tertiary
The order can also be explained in terms of electron releasing inductive effect of alkyl groups. Greater is the number of alkyl groups attached to the carbon carrying the -OH group lesser shall be the polarity of OH group and hence lesser will be the acidic character. In tertiary alcohols, the carbon atom carrying -OH is attached to three alkyl groups, due to which electron density on oxygen is maximum in their case and hence the loss of H+ is most difficult. Thus, tertiary alcohols are least acidic, whereas primary alcohols (with only one alkyl group) are most acidic. The secondary alcohols have intermediate acidic character.
Alkanols are weaker acids than water. This can be illustrated by the reaction of water with an alkoxide
This reaction shows that water is a better proton donor and hence stronger acid than alkanols. Also, in the above reaction; we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which shows that alkoxides are stronger bases. Thus, sodium ethoxide is a stronger base than sodium hydroxide.
Due to the presence of lone pairs of electrons on oxygen, alkanols can also act as proton acceptors and hence behave as Bronsted bases
REACTION WITH MONOCARBOXYLIC ACIDS (Esterification)
Alkanols react with monocarboxylic acids in the presence of cone. tetraoxosulphate(VI) acid to form esters. The function of cone. H2SO4 is to act as a protonating agent as well as a dehydrating agent. The reaction is called esterification.
This reaction is reversible in nature and the equilibrium can be shifted to the right by removing” water as soon as it is formed. During esterification water molecule is formed by combination of -OH of carboxylic acid and H from alkanol molecule.
REACTION WITH HYDROGEN HALIDES
Hydrogen halides react with alcohols to form alkyl halides. The alkanol may be primary, secondary or tertiary and the halogen acid may be HCl, HBr or HI.
The order of reactivity of various alcohols towards this reaction is
tertiary > secondary > primary
The order of reactivity of various hydrogen halides is in the order
HI >HBr >HCl
ACIDIC DEHYDRATION (Formation of Alkenes)
When heated with cone. H2SO4 or H3PO 4 alkanols undergo dehydration to form alkenes. The reaction with cone. H2SO4 is carried at 443 K, whereas H3PO4 reacts at higher temperature.
Alcohols undergo oxidation with oxidising agents (in neutral or acidic or alkaline medium) such as chromic anhydride or chromium(VI) oxide (CrO3), K2Cr2O7, KMnO4.
(l) A primary alkanol on oxidation with acidified K2Cr2O7 first gives an aldehyde which on further oxidation gives an acid with the same number of carbon atoms as the original alcohol.
The orange colour of the reagent changes into green due to formation of Cr3+ ions during the reaction.
(ii) A secondary alkanol on oxidation, first gives a ketone with the same number of carbon atoms as the original alcohol.
Ketones are quite resistant to further oxidation
(iii) Tertiary alkanols are not oxidised with mild oxidizing agents in aqueous, alkaline or under neutral conditions. However, when oxidation is carried out under acidic conditions, tertiary alkanols undergo dehydration to form alkenes.
Primary alkanols can be oxidized to alkanols with chromic anhydride (CrO3) in an anhydrous medium.
DEHYDROGENATION (Reaction with Hot Reduced Copper)
Primary, secondary and tertiary alkanols give different products when their vapours are passed through a tube packed with reduced copper at 575 K.
(i) A primary alkanol is dehydrogenated to aldehyde.
(ii) A secondary alkanol is dehydrogenated to ketone.
(iii) A tertiaty alkanols does not dehydrogenate due to absence of a.-hydrogen. However, it gets dehydrated to form an alkene.