Those atoms or groups of atoms present in organic compounds which mainly determine their functions are called functional groups.
For example -
- -CH3 and -OH groups are present in methyl alcohol (CH3OH). The –OH group takes part in most of the chemical reactions of this compound and the –CH3 group remains unchanged. Hence –OH group is the functional group in this compound.
- Nitro-methane (CH3NO2) and nitro-ethane (C2H5NO2) both have –NO2 group. Even though the second group present in these two compounds is different, the chemical properties of these two compounds are similar. Hence –NO2 group is the functional group in both these compounds.
- –COOH group is present in both acetic acid (CH3COOH) and benzoic acid (C6H5COOH). The -COOH group takes part in many reactions of both these compounds. In these reactions the -COOH group is equally affected. Hence –COOH group is the functional group in both these compounds.
Some of the major functional groups are briefly described below-
1. Carbon-Carbon Double bond - The compounds having carbon-carbon double bond (>C = C<) show addition reactions due to the presence of double bond.
2. Carbon - Carbon triple bond - The compounds in which carbon-carbon triple bond (–C≡C—) are present, exhibit addition reactions due to the presence of triple bond.
The compounds in which carbon-carbon double or triple bonds are present are called unsaturated compounds. The property of compounds to exhibit addition reactions due to the presence of carbon-carbon double bonds and triple bonds is called unsaturation.
3. Alcoholic group -OH group is called hydroxy group. When the —OH group is attached to a saturated carbon atom, it is called an alcoholic group. Saturated carbon atom means such a carbon atom whose remaining three valencies are used to form single bonds. The compounds in which alcoholic group is present are called alcohols. For example-
CH3OH CH2 = CH – CH2OH
Methyl Alcohol Allyl Alcohol
C2H5OH C6H5 – CH2 – OH
Ethyl Alcohol Benzyl Alcohol
It is clear that hydroxy group is present in water (HOH), acetic acid (CH3–CO–OH) and phenol (C6H5–OH) but alcoholic group is not present.
4. Aldehydic group - —CHO group is called formnyl group. When – CHO group is attached to H or C atom, it is called aldehyde group and the related compound is called aldehyde. For example – aldehyde group is present in formaldehyde (H-CHO) and acetaldehyde (CH3-CHO).
5. Keto or Ketonic group - (—CO−) group is called carbonyl group. When it is attached to carbon atoms from both the sides, it is called keto or ketone group and the related compound is called ketone. For example, ketone group is present in acetone (CH3-CO-CH3 ) while ketone group is not present in acetic acid (CH3-CO-OH).
6. Carboxyl group – (—COOH) group is called carboxyl group. The compounds in which carboxyl group is present are called carboxylic acids. For example – formic acid (HCOOH) and acetic acid (CH3COOH) are carboxylic acids.
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