Alcohol is an organic compound containing hydroxyl functional group. Ethanol a main constituent of beverages and medicines and glycol common antifreeze are some examples of alcohols.
The general formula of alcohol is R-OH, where R can be alkyl group. Depending on the nature of alkyl group (R) the alcohol are classified as primary, secondary, tertiary, vinyl, allyl and benzyl alcohol.
Alcohols can be synthesized by any of the following ways.1. By the hydration of alkenes
Alkenes on hydration in the presence of dilute acids give alcohols. For example ethylene on hydration gives ethyl alcohol.
CH2=CH2 + H2O → CH3-CH2-OH
Hydration of propene gives 2-propanol in according to Markovnikov's rule. The reaction involves protonation of alkene followed by addition of water.2. By reduction of aldehydes, ketones, acyl chlorides
Alcohols are synthesized by reduction of aldehyde with lithium aluminum hydride. Reduction of aldehyde generally gives primary alcohols. For example acetaldehyde on reduction gives ethyl alcohol.
CH3-CHO → CH3-CH2-OHSimilarly ketones on reduction give secondary alcohols. For example ketone on reduction gives 2-propanol.
Acyl chlorides like acetyl chloride on strong reduction give alcohols.
Acetyl chloride on strong reduction gives ethyl alcohol.
CH3-COCl → CH3CHO → CH3-CH2-OH3. By hydrolysis of Grignard reagentGrignard reagent like methyl magnesium chloride reacts with formaldehyde to give a primary alcohol. Here methyl magnesium chloride reacts with formaldehyde which on further hydrolysis gives ethanol. If we want to change the carbon chain length in primary alcohol, the same should be changed in alkyl part of Grignard reagent.
CH3-Mg-Cl + HCHO → CH3-CH2-OMgCl
CH3-CH2-OMgCl + H2O → CH3-CH2-OH + MgCl(OH)
Similarly Grignard reagent on reacting with acetaldehyde followed by hydrolysis gives secondary alcohol. For example methyl magnesium chloride reacts with acetaldehyde followed by hydrolysis gives 2-propanol. Grignard reagent on reacting with acetone followed by hydrolysis gives tertiary alcohol.
Alcohols are named after the parent alkane chain by replacing ‘ane’
in the parent alkane chain with ‘ol’
. For example in CH3
–OH there is one carbon atom and so the parent alkane name is methane. If we replace ‘ane’
, we will get methanol. Hence, the compound name is methanol.
If the compound contains branches then the longest chain is selected as parent chain and other alkyl groups are treated as substituents. For example in the following compound the longest chain contains five carbon atoms and hence the compound name is 2 methyl Butanol.
If the compound contains more than two carbon atoms then the position of attachment of OH group should be indicated. For example the following compound is named as 2-Butanol.
In case of compounds containing more than one functional group, the order of priority should be followed. In this halogens will have least priority than alcohols and other functional groups like acids will have more priority than alcohol.
1. Primary alcohol
Here the functional group is attached to a primary carbon atom ( a carbon atom which is connected to exactly one carbon atom). Ethanol is an example for primary alcohol.
2. Secondary alcohol
Here the functional group is attached to a secondary carbon atom which is exactly connected to two carbon atoms. 2-propanol is an secondary alcohol.
3. Tertiary alcohol
Here the functional group is attached to a tertiary carbon atom which is exactly connected to three carbon atoms. 2-methyl-2-propanol is an example for tertiary alcohol.
4. Vinyl alcohol
Iit is an alcohol where the functional group is directly attached to a carbon containing double bond. Prop-1-ene-1-ol is an example for vinyl alcohol.
5. Allyl alcohol
It is an alcohol where the functional group is directly attached to a carbon which is connected to unsaturated carbon. Prop-2-ene-1-ol is an example for allyl alcohol.
6. Benzyl alcohol
Here the functional group is attached to a carbon chain containing the benzene ring. The structure of benzyl alcohol is given below.
7. Dihydric alcohol
The compounds containing two hydroxy group are called as dihydric alcohol.
Ethylene glycol is an example for dihydric alcohol.
8. Trihydric alcohol
The compounds containing three hydroxy group are called as trihydric alcohol.
Glycerol is an example for trihydric alcohol.
- Alcohols are colorless liquids with characteristic smell.
- They are high boiling liquids due to presence of hydrogen bonding.
- They are soluble in water.
- Their boiling point increases with increase in the length of carbon chain and increase in the number of hydroxyl groups.
- So, ethylene glycol will have more boiling point that ethanol.
Alcohol are polar molecules due to the presence of OH functional group. The OH functional group can release proton in solution and hence alcohol are slightly acidic.
In the second case the OH functional group can altogether be replaced. So the reaction of alcohols are classified as
1. Reaction with sodium
- Reactions involving acidic hydrogen
- Reactions involving hydroxyl functional group
Alcohols react with sodium to give sodium alkoxide and hydrogen. This is the characteristic reaction of alcohol and is often used in organic analysis to identify the alcohol.
2CH3-CH2-OH + 2Na → 2CH3-CH2-ONa + H2
2. Reaction with acids (Esterification reaction)
Alcohols condense with acids in the presence of concentrated sulfuric acid to give ester. For example ethyl alcohol condense with acetic acid to give ethyl acetate.
CH3-CH2-OH + CH3-COOH → CH3-CH2-O-CO-CH3 + H2O3. Reaction with acidified potassium permanganate (oxidation reaction)
Primary alcohols on oxidation give aldehyde which easily undergoes oxidation to give carboxylic acid with same number of carbon atoms. This is characteristic reaction of primary alcohol as secondary alcohols on oxidation gives ketone which is difficult to oxidize to further.
CH3-CH2-OH → CH3-CHO → CH3-COOH
4. Reaction with PCl5, PCl3, SOCl2
Alcohols react with chlorinating agents like phosphorus pentachloride, phosphorus trichloride and thionyl chloride to give chloro alkanes.
CH3-CH2-OH + PCl5 → CH3-CH2-Cl + POCl3 + HCl
CH3-OH + SOCl2 → CH3-Cl + SO2 + HCl5. Reaction with ammonia
Alcohols react with ammonia to give a mixture of amines. The reaction yields depends on the concentration of ammonia and alcohol.
CH3-OH + NH3 → CH3-NH2 + H2O6. Dehydration reaction
CH3-NH2 + CH3-OH → CH3-NH-CH3 + H2O
CH3-NH-CH3 + CH3-OH → CH3-N-(CH3)2 + H2O
Alcohols undergo intra-molecular dehydration with concentrated sulfuric acid to give alkene. For example, ethanol on dehydration gives ethene and propanol on dehydration give propene.
CH3-CH2-OH → CH2=CH2 + H2O
CH3-CH2-CH2-OH → CH3-CH=CH2 + H2O
At elevated temperature alcohols undergo intermolecular dehydration to give ethers.
CH3OH + CH3OH → CH3-O-CH3 + H2O