Alcohol

The organic compound which contains -OH functional group is called alcohol.

\underset{Alkane}{R-H}\xrightarrow[-H]{+OH}\underset{Alcohol}{R-OH}

Classification

1. On the basis of number of -OH group:

i. Monohydric alcohol: Containing only one -OH group. eg. CH₃OH, C₂H₅OH, etc.
ii. Dihydric alcohol: Containing two -OH groups. eg. Glycol

iii. Trihydric alcohol: Containing three -OH groups. eg. Glycerol

Alcohols with two or more -OH groups attached with the same carbon atom of the molecule becomes unstable and hence it is decomposed to aldehyde, ketone or carboxylic acid.

2. On the basis of attachment of -OH group

i. Primary (1°) alcohol: Alcohol in which -OH group is attached with primary carbon is called primary alcohol.

ii. Secondary (2°) Alcohol: Alcohol in which -OH group is attached with secondary carbon is called secondary alcohol.

iii. Tertiary (3°) Alcohol: Alcohol in which -OH group is attached with tertiary carbon is called tertiary alcohol.

Nomenclature of alcohol

Isomerism:

1. Chain isomerism:

2. Positional isomerism:

3. Functional isomerism: Alcohol shows functional isomerism with ether having general molecular formula of C_nH_2n+2O. Examples:

• Isomers of C₂H₆O

• Isomers of C₃H₈O

Q. Write all possible isomers of C₄H₁₀O with IUPAC name.

Distinction of primary, secondary, and tertiary alcohol by Victor Meyer method

i. Given alcohol is treated with PI₃ (red P₄ + I₂) to get corresponding Iodoalkane.
ii. The iodoalkane thus formed is treated with an alcoholic AgNO₂ solution to get the corresponding nitroalkane.
iii. The nitroalkane thus formed is treated with nitrous acid and then with aqueous alkali.If a blood-red color is formed, the alcohol is primary.

• If a blood-red color is formed, the alcohol is primary.
• If a blue color is formed, the alcohol is secondary.
• Colourless solution indicates the presence of tertiary alcohol.

Q: How would you distinguish n-propyl alcohol, Iso-propyl alcohol and tert-butyl alcohol by Victor Meyer method?
Q: How would you distinguish Butan-1-ol, Butan-2-ol and 2-methyl propan-2-ol by Victor Meyer method?
Q: Write all possible alcoholic isomers of C₃H₈O and distinguish them by Victor Meyer test.

General methods of preparation of alcohol (monohydric)

1. From haloalkane: Haloalkane reacts with aqueous KOH or NaOH to give alcohol.

\begin{align*} R-X+KOH(aq.)&\rightarrow R-OH+KX\\ CH_{3}CH_{2}Cl + KOH(aq.) &\rightarrow CH_{3}CH_{2}OH + KCl \end{align*}

2. From primary amines: Primary amine reacts with nitrous acid in cold condition to give alcohol.

3. From ester

• By hydrolysis: Alcohols are prepared by hydrolysis of ester.

• By reduction: Alcohols are prepared by the reduction of ester with LiAlH₄ or Na/C₂H₅OH or H₂/Ni or NaBH₄.

Industrial preparation of alcohol

1. Oxo process

Alkene reacts with carbon monoxide and hydrogen in presence of oxo-catalyst (dicobalt octacarbonyl) at high temperature and pressure to give aldehyde which on catalytic hydrogenation with nickel gives alcohol.

2. Fermentation of sugar

It is a slow process in which a complex substance is broken into simpler fragments under the influence of enzymes. This is a very old process for the preparation of ethanol in which alcohol is prepared from carbohydrates like maize, wheat, rice, potato, grapes, apple, etc.

i. Preparation of ethyl alcohol from molasses or sugar: Molasses is a mother liquor left during the crystallization of sugar. It is a dark viscous liquid that contains 30% sucrose and 32% invert sugar (a mixture of glucose and fructose).

\begin{align*} \underset{Sucrose}{C_{12}H_{22}O_{11}}+H_{2}O&\xrightarrow[]{invertase}\underset{Glucose}{C_{6}H_{12}O_{6}}+\underset{Fructose}{C_{6}H_{12}O_{6}}\\ \underset{\substack{Glucose\ or\\fructose\ or\\ both }}{C_{6}H_{12}O_{6}}&\xrightarrow[]{Zymase}\underset{Ethanol}{2C_{2}H_{5}OH}+2CO_{2} \end{align*}

ii. Preparation of ethanol from starch:

\begin{align*} \underset{Starch}{(2C_{6}H_{10}O_{5})_{n}}+nH_{2}O&\xrightarrow[]{diastase}\underset{Maltose}{n(C_{12}H_{22}O_{11})}\\ \underset{Maltose}{C_{12}H_{22}O_{11}}+H_{2}O&\xrightarrow[]{Maltase}\underset{Glucose}{2C_{6}H_{12}O_{6}}\\ \underset{Glucose}{C_{6}H_{12}O_{6}}\xrightarrow[]{Zymase}&\underset{Ethanol}{2C_{2}H_{5}OH}+2CO_{2} \end{align*}

The fermented liquor obtained from the above process contains only 15-18% of ethyl alcohol known as wash. When this wash is subjected to distillation, 95% of ethyl alcohol is obtained which is called rectified spirit. When this rectified spirit is further subjected to distillation in presence of quick lime, 100% pure ethyl alcohol is obtained which is called absolute alcohol.
A mixture of absolute alcohol and petrol in the ratio of 20:80 along with benzene or ether as co-solvent is known as power alcohol.
Ethanol is made available only for industrial purposes by the addition of some poisonous substances like methanol, acetone, pyridine or colouring matter. Such undrinkable ethanol containing poisonous substances especially methyl alcohol is called methylated spirit or denatured alcohol.

Alcoholic beverages: They are liquor for drinking purposes. They contain, ethanol, water, colouring material and flavouring material. They may be distilled (eg. vodka, gin, whisky, brandy, etc) or undistilled (eg. beer, wine, etc.)

3. Hydroboration of ethene

Ethene is reacted with diborane to give triethyl borane which on oxidation with hydrogen peroxide gives ethanol.

Physical properties:

1. State, colour, odour: The lower members are colourless liquid having alcoholic odour whereas higher members are colourless and odourless solid.
2. Solubility: Lower members are soluble in water due to formation of an intermolecular hydrogen bond with water.

Higher members are insoluble in water due to an increase in the insoluble hydrocarbon part.

3. Boiling point: The boiling point of alcohol is higher than the corresponding alkane, alkyl halide, and ethers of comparable molecular masses due to the formation of an intermolecular hydrogen bond.

Chemical properties:

1. Action with HX, PCl₃, PCl₅,SOCl₂:

\begin{align*}R-OH + HCl &\xrightarrow[]{anhy.\ ZnCl_{2}} R-Cl + H_{2}O\\ R-OH + PCl_{3} &\rightarrow R-Cl + H_{3}PO_{3}\\ R-OH + PCl_{5} &\rightarrow R-Cl + POCl_{3} + HCl\\ R-OH + SOCl_{2} &\xrightarrow[]{Pyridine} R-Cl + SO_{2} + HCl\\ \end{align*}

2. Action with sodium and potassium: Alcohols are treated with active metals like Na, K, etc. to give metal alkoxides.

\begin{align*}\underset{Alcohol}{R-OH}+Na&\rightarrow \underset{\substack{Sodium\\ alkoxide}}{R-ONa}+H_{2}\\ \underset{Ethanol}{C_{2}H_{5}OH} + Na&\rightarrow \underset{\substack{Sodium\\ ethoxide}}{C_{2}H_{5}ONa} + H_{2}\\ \underset{Ethanol}{C_{2}H_{5}OH} + K&\rightarrow \underset{\substack{Potassium\\ ethoxide}}{C_{2}H_{5}OK} + H_{2} \end{align*}

3. Dehydration of alcohols: Common dehydrating agents are conc. H₂SO₄, conc. H₃PO₄, alumina (Al₂O₃).

\begin{align*}\underset{Ethanol}{CH_{3}CH_{2}OH}&\xrightarrow[110\degree C]{conc.\ H_{2}SO_{4}} \underset{\substack{Ethyl\ hydrogen\\ sulphate}}{CH_{3}CH_{2}OSO_{3}H}+H_{2}O\\ \underset{Ethanol}{CH_{3}CH_{2}OH}&\xrightarrow[140\degree C]{conc.\ H_{2}SO_{4}} \underset{\substack{Ethoxy\ ethane}}{CH_{3}CH_{2}OCH_{2}CH_{3}}+H_{2}O\\ \underset{Ethanol}{CH_{3}CH_{2}OH}&\xrightarrow[170\degree C]{conc.\ H_{2}SO_{4}} \underset{Ethene}{CH_{2}=CH_{2}}+H_{2}O\\ \underset{Ethanol}{CH_{3}C_{2}OH}&\xrightarrow[250\degree C]{Al_{2}O_{3}} \underset{Ethoxy\ ethane}{CH_{3}CH_{2}OCH_{2}CH_{3}}+H_{2}O\\ \underset{Ethanol}{CH_{3}CH_{2}OH}&\xrightarrow[350\degree C]{Al_{2}O_{3}} \underset{Ethene}{CH_{2}=CH_{2}}+H_{2}O\end{align*}

Dehydration of alcohol to alkene takes place by carbocation intermediate. The alcohol that forms stable carbocation undergoe dehydration easily.
The order of dehydration is: tertiary alcohol > secondary alcohol > primary alcohol

4. Esterification reaction: A mixture of alcohol and carboxylic acid is heated in presence of conc. sulphuric acid to give an ester having fruity smell. This reaction is called esterification test.

5. Oxidation of primary, secondary and tertiary alcohol using K₂Cr₂O₇/H⁺ or KMnO₄/H⁺

i. Primary alcohol is oxidized first to aldehyde which on further oxidation gives carboxylic acid.

ii. Secondary alcohol is oxidized to ketone.

iii. Tertiary alcohol is not oxidized at normal conditions. However, it oxidizes under drastic conditions first to alkene and then into carboxylic acid having less number of carbon atoms than parent alcohol.

*During the drastic oxidation of unsymmetrical ketone, carbonyl group remain in the smaller alkyl group to form carboxylic acid. This is called Popoff’s rule.

6. Catalytic dehydrogenation:

i. Primary alcohol is dehydrogenated into aldehyde.

ii. Secondary alcohol is dehydrogenated into ketone.

Laboratory test of ethanol:

1. Esterification:

2. Iodoform test: