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Alcohols

Alcohols are compounds which has got OH group as their functional group. Alcohols are present naturally in many product like ethanol and methanol.

Alcohols are classified with number of OH groups attached. For example when the alcohol contains only one OH it is called as mono hydric alcohol for example ethanol whose formula is CH3CH2OH, when the compound contains two OH group it is called as di hydric alcohol for example as in ethylene glycol CH2OH-CH2OH. The alcohol is called as tri hydric alcohol when there is three OH group attached in the compound for example as in glycerol CH2OH-CHOH-CH2OH.

Another important way of classifying the alcohol is by the carbon in which the alcohol is attached.

 

What is Alcohol?

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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.

Synthesis of Alcohols

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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.

Hydration of Propene
Oxidation Reaction

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-OH

Similarly ketones on reduction give secondary alcohols. For example ketone on reduction gives 2-propanol.
Acyl chlorides like acetyl chloride on strong reduction give alcohols.

Secondary Alcohols

Acetyl chloride on strong reduction gives ethyl alcohol.

CH3-COCl CH3CHO CH3-CH2-OH

3. By hydrolysis of Grignard reagent

Grignard 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.

Hydrolysis of Grignard Reagent


Nomenclature of Alcohols

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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’ with ‘ol’, 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.

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.

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.

List of Alcohols

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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.

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.

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.

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.

Benzyl Alcohol

7. Dihydric alcohol


The compounds containing two hydroxy group are called as dihydric alcohol.
Ethylene glycol is an example for dihydric alcohol.

Dihydric Alcohol

8. Trihydric alcohol


The compounds containing three hydroxy group are called as trihydric alcohol.
Glycerol is an example for trihydric alcohol.

Trihydric Alcohol

Physical Properties of Alcohols

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  • 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.

Chemical Properties of Alcohols

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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. Reactions involving acidic hydrogen
  2. Reactions involving hydroxyl functional group

Reactions of Alcohols

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1. Reaction with sodium

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 + H2O

3. 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 + HCl

5. 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 + H2O

CH3-NH2 + CH3-OH CH3-NH-CH3 + H2O

CH3-NH-CH3 + CH3-OH CH3-N-(CH3)2 + H2O

6. Dehydration reaction

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
More topics in Alcohols
Primary Alcohol Secondary Alcohol
Tertiary Alcohol Oxidation of Alcohols
Alcohol Dehydration
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