Chapter 4 · Question 7

What is a functional group? List the main functional groups in organic chemistry, giving the formula and the class of compounds for each. Explain why compounds with the same functional group show similar chemical properties.

Answer Revealed

Direct Answer:

A functional group is a heteroatom (such as Cl, Br, O, N, S) or a group of atoms containing heteroatoms that, when attached to a carbon chain, confers specific chemical properties to the compound, regardless of the chain length. Main functional groups: (i) Halo (

\!-\!\text{Cl}

\!-\!\text{Br}

) → haloalkanes; (ii) Alcohol (

\!-\!\text{OH}

); (iii) Aldehyde (

\!-\!\text{CHO}

); (iv) Ketone (

\!-\!\text{CO} \!-\!

); (v) Carboxylic acid (

\!-\!\text{COOH}

). Compounds with the same functional group show similar chemical properties because the chemical reactivity is determined primarily by the functional group, not the carbon chain length.

Simple Explanation

A functional group is the 'active part' of an organic molecule — like a key attachment that determines how the molecule behaves chemically. The alcohol group (

\!-\!\text{OH}

) makes all alcohols behave similarly: methanol, ethanol, propanol, and butanol all react with sodium to produce hydrogen gas. The aldehyde group (

\!-\!\text{CHO}

) makes aldehydes reduce Tollens' reagent. The carboxylic acid group (

\!-\!\text{COOH}

) makes carboxylic acids turn blue litmus red. Different chain lengths change physical properties (boiling point, melting point) but the functional group decides the chemistry.

Exam-Ready Structure

Functional groups are the cornerstone of organic chemistry classification: 1. Definition: In a hydrocarbon chain, one or more hydrogen atoms can be replaced by atoms of other elements (heteroatoms — such as halogens, oxygen, nitrogen, and sulphur) or groups of atoms containing heteroatoms. These heteroatoms or groups are called functional groups because they impart specific and characteristic properties to the compound, irrespective of the length and nature of the carbon chain to which they are attached. 2. The free valency or valencies of the functional group are shown by the single line (dash). The functional group attaches to the carbon chain through this valency by replacing one hydrogen atom. 3. Main functional groups (from NCERT Table 4.3): (a) Haloalkane:

\!-\!\text{Cl}

(chloro),

\!-\!\text{Br}

(bromo) — the halogen atom replaces one H atom. (b) Alcohol:

\!-\!\text{OH}

(hydroxyl group) — oxygen bonded to hydrogen. (c) Aldehyde:

\!-\!\text{CHO}

— carbon double-bonded to O and single-bonded to H, with one free valency for the chain. (d) Ketone:

\!-\!\text{CO} \!-\!

— carbon double-bonded to O with two free valencies (the carbonyl group is flanked by two carbon chains, unlike aldehydes). (e) Carboxylic acid:

\!-\!\text{COOH}

— carbon double-bonded to O and single-bonded to OH, with one free valency. 4. Chemical similarity: All members of a homologous series (same functional group but different chain lengths) exhibit very similar chemical properties. The functional group is the reactive site — the rest of the molecule is essentially a framework. For example, all carboxylic acids (

\text{HCOOH}

\text{CH}_3\text{COOH}

\text{C}_2\text{H}_5\text{COOH}

) react with carbonates to produce

\text{CO}_2

and react with alcohols to form esters, regardless of chain length.

Key Points

Functional group: a heteroatom or group of atoms that gives characteristic properties to a carbon compound
Halo ( $\!-\!\text{Cl}$ , $\!-\!\text{Br}$ ) → haloalkanes; alcohol ( $\!-\!\text{OH}$ ) → alcohols
Aldehyde ( $\!-\!\text{CHO}$ ); Ketone ( $\!-\!\text{CO} \!-\!$ ); Carboxylic acid ( $\!-\!\text{COOH}$ )
Aldehydes and ketones both contain the carbonyl ( $\text{C} \!=\! \text{O}$ ) group
Chemical similarity: same functional group → same chemical reactions (regardless of chain length)

What is a functional group? List the main functional groups in organic chemistry, giving the formula and the class of compounds for each. Explain why compounds with the same functional group show similar chemical properties.