Alcohols (A-level organic chemistry)

Alcohols/Alkanols

These are organic compounds that contain at least one hydroxyl group (-OH) attached to saturated carbon atom.

Classification

(a) According to the number of –OH group

(i) Monohydric alcohols have one hydroxyl (-OH) group

Example

CH₃CH₂OH    ethanol

(ii) Dihydric alcohols or glycols have two hydroxyl groups Examples

(iii) Trihydric alcohols have three hydroxyl groups.

(iv) Polyhydric alcohols or polyol contain more than three hydroxyl groups

 

(b)  Classification of monohydric alcohols

They are classified according to the number of alkyl groups attached to the carbon atom that bears a hydroxyl (OH) group.

(i) Primary alcohols have one alkyl group attached to the carbon atom that carries OH group, i.e. ROH.

Example

CH₃CH₂OH  Ethanol

CH₃CH₂CH₂OH            Propan-1-ol

(ii) Secondary alcohol: have two alkyl groups attached to the carbon atom that bears OH group, i.e. R₂CHOH

Example

(iii) Tertiary alcohols: have three alkyl groups attached to a carbon atom that bears OH group

Nomenclature

(a) Are named by replacing the final “e” in the corresponding alkanes with “ol”

(b) The hydroxyl group is taken as a substituent group, and its position is given by numbering the carbon atoms in the chain from the side nearest to the carbon atom that carries OH group.

Examples

Physical properties

1. Alcohols have high melting and boiling points than corresponding hydrocarbon of similar molecular masses due to intermolecular hydrogen bonds. consequently alcohols ate either liquids or solids.

Example

Propane (44)      Bpt = -42⁰C

Ethanol (46)        Bpt = 78⁰C

2. Lower members are completely soluble in water due to the formation of hydrogen bonds with water. But solubility of alcohols decrease as alkyl group length increase due to increase in “alkane like” character.

Preparation of alcohols

(a) Industrial preparation

(i) From petroleum products,e.g. alkenes

(ii) Fermentation of sugars

Ethanol can be made by fermentation of sugars. Fermentation is usually carried out by adding yeast to a mixutre of sugar and water.

(iii) Methanol is produced by catalytic  reduction of carbon monoxide.

(b) Laboratory preparation alcohols

(i) Hydration of alkenes in presence of acid catalyst

(ii) By hydrolysis of alkyl halides using aqueous alkali

(iii) By reduction of carbonyl compounds (aldehyde and ketones) using lithium aluminium hydride (LiAlH₄) in presence of dry ether.

(iv) Reduction of carboxylic acids using LiAlH₄ in dry ether

Absolute ethanol

All aqueous solutions of ethanol yield, on fractional distillation a constant boiling mixture (azeotrope) of 96% ethanol and 4% water known as rectified spirit.

(i) In the laboratory rectified spirit is stored over quick lime overnight. Quick lime dehydrates the mixture; then pure ethanol called absolute ethanol is distilled.

(ii) In industry, benzene is added to the rectified spirit. Distillation yields three fractions

At 65⁰C a constant boiling mixture of ethanol, benzene and water.

At 68⁰C a constant boiling mixture of ethanol and water

At 78⁰C pure ethanol distills of

Reactions of monohybrid alcohol

1. Cleavage of O-H bond
2. Cleavage od C-O bond
3. Oxidation
4. Dehydration

(a) Cleavage of O-H bond

(i) Behavior as a weak acid

2ROH + 2Na       →   RO^–Na⁺  + H₂(g)

Acidity: primary alcohol> secondary alcohol>tertiary alcohol

(ii) Behavior as a base

ROH + H⁺       →     RO⁺H₂

(iii) Formation of ether

Ethanol reacts the presence of concentrated sulphuric acid at 140⁰C to form diethyl ether.

(iv) Formation of esters from alkylhalides

(a) Alcohols react with carboxylic acids in the presence of a mineral acid (phosphoric or sulphuric acid) to form esters. However, this is not a good method because the reaction is reversible and does not go to completion.

Example

(b) Alcohols react with acid halides to form esters.

(c) Alcohols react with acid anhydride to form esters.

Mechanism

(b) Cleavage of C-O bond

Formation of alkyl halide

Alcohols react with a variety of reagents to yield alkyl halides. The most commonly used reagents are hydrogen halides (HCl, HBr and HI) phosphorus tribromide (PBr₃) and thionyll chloride.

Examples

The order of reactivity of hydrogen halides is HI> HBr> HCl whereas the order of reactivity of alcohols is 3⁰> 2⁰> 1⁰. The reaction of HCl with alcohol is catalyzed by anhydrous zinc chloride. The reaction is used to distinguish between primary, secondary and tertiary alcohols.

Tertiary alcohol react readily with HCl in presence of anhydrous zinc chloride to form an insoluble chloride giving two layers immediately.

Secondary alcohols form two-layer in 5 -10minutes

Primary alcohol does not form layers at room temperature.

Mechanism

Example

(c) Oxidation

(i) Alcohols are burnt in oxygen to produce carbon dioxide, water, and heat. Due to the production of heat on combustion, alcohol is used as fuel.

(ii) Mild oxidizing agents like acidified potassium dichromate oxidize primary alcohols to aldehydes and then, to carboxylic acids. Secondary alcohols are oxidized to ketones.

(iii) Tertiary alcohols are not oxidized by potassium dichromate and therefore acidified potassium dichromate is used to distinguish tertiary alcohols from primary or secondary alcohols. When reacted with primary or secondary alcohol, the color of acidified potassium dichromate changes from orange to green.

(d) Dehydration of alcohol

Alcohols are dehydrated by hot concentrated sulphuric or phosphoric acid to form alkenes. The mechanisms depend on the class of alcohols.

(i) Primary alcohol undergoes elimination bimolecular; E2.

Example

(ii) Tertiary alcohols undergo elimination unimolecular; E1.

Example

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