Organic Chemistry

Lesson 4. IMPORTANT PROPERTIES OF MONO, DI AND TRIHYDRIC ALCOHOLS (GLYCOL AND GLYCEROL)

Module 2. Alcohols, aldehydes and ketones

Lesson 4
IMPORTANT PROPERTIES OF MONO, DI AND TRIHYDRIC ALCOHOLS (GLYCOL AND GLYCEROL)

4.1 Introduction

• Organic hydroxyl compounds of the general formula R-OH, where ‘R’ is an alkyl or substituted alkyl group
• For example (Fig. 4.1)

• Contain hydroxyl (-OH) group as the functional group - determines characteristics and properties of this family.

4.2 Classification

Classified on different basis

• Based on number of hydroxyl groups

1. According to number of hydroxyl groups content
2. As mono-, di-, tri- and polyhydroxy (Fig. 4.2)

• Based on kind of carbon atom/ degree of carbon atom to which –OH is attached.

1. According to the kind of carbon atom that bears hydroxyl group
2. A carbon atom may be classified as primary, secondary and tertiary
3. According to number of carbon atoms attached it (Fig. 4.3)

• Aliphatic alcohols and Aromatic alcohols

1. Alcohols of different classes differ in rate and/or mechanism of reaction
2. Substitution may also affect the reactivity and physical constants.

4.2.1 Physical properties

• Alcohols are considerably different from hydrocarbons due to presence of polar

–OH group – polar compounds

• Form intermolecular hydrogen bonds – as shown already

Therefore;

• Their boiling points are much higher
• Lower members (methanol, ethanol and propanol-1) are miscible with water
• Ethylene glycol is used as an antifreeze in automobiles.

4.2.2 Chemical properties

• Determined by its functional group- hydroxyl group → -OH
• Reactions of an alcohol can involve the breaking of either of two bonds =

1. C-OH bond - leads to removal of -OH group or
2. O-H bond - leads to removal of -H

• In either kind substitution or elimination takes place.
• The substitution replaces –OH or –H
• The elimination forms double bond

4.2.3 Industrial sources

• There are two principal ways to get the simple alcohols = by hydration of alkenes and by fermentation of carbohydrates

4.2.3.1 Hydration of alkenes

• Alkene - obtained by cracking of petroleum

- converted into alcohol by addition of water (Fig. 4.4)

4.2.3.2 Fermentation of carbohydrates

• Fermentation of sugars by yeast is used for manufacture of ethyl alcohol

Fig. 4.5 Fermentation reactions

• The sugars come from a variety of sources - mostly from molasses from sugar cane or starch from grains (maize, peas, potatoes etc).

a) Ethyl alcohol

Widely used

• As a solvent for lacquers, varnishes, perfumes and flavoring
• As a medium for chemical reactions and in recrystallizations
• As an important raw material for synthesis of aliphatic compounds – alkenes, alkyl halides, ethers, aldehydes, ketones, acids, esters etc

If nothing is mentioned about type of alcohol

• To be considered as ethyl alcohol

b) 95% ethyl alcohol (an azeotrope = constant boiling point mixture)

• Generally ethyl alcohol used is a mixture of 95% alcohol and 5% water
• After production alcohol is purified and concentrated by fractional distillation
• In distillation first material to distill is the one which has the highest volatility i.e. lowest boiling point
• In a mixture of ethyl alcohol and water the lowest boiling component is not water (boiling point 100°C) or ethyl alcohol (boiling point 72.30°C) but a mixture of alcohol and water (95:5) since its boiling point is 78.15°C therefore ethyl alcohol is concentrated upto 95%
• A liquid mixture that has the peculiar property of giving a vapour of the same composition is called azeotrope or constant-boiling mixture

c) Absolute alcohol

• 100% ethyl alcohol is called absolute alcohol
• Obtained by taking advantage of the another azeotrope
• When a mixture containing 150 g of 95% alcohol and 74 g benzene is distilled – a ternary azeotrope consisting of 7.5% water, 18.5% alcohol and 74% benzene distilled off first due to its lower boiling point (64.90°C), leaving pure (anhydrous) alcohol behind
• Traces of water from the absolute alcohol can be removed by treatment with metallic magnesium
• Water is converted into Mg(OH)₂ from which alcohol is distilled.