Consider the following organic compounds:
(i) H–C–C–C=O with three H atoms on top bonded to carbons and H atoms below the first two carbons (i.e. CH₃–CH₂–CHO type structure).
(ii) H–C–C=O with two H on top and one H below the first carbon (i.e. CH₃–CHO type structure).
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Model Answer
(a) The functional group present in both compounds is the aldehyde group (–CHO), i.e., $\text{–C}\overset{\displaystyle O}{\overset{\|}{}}H$.
(b) The general formula for aldehydes is: $C_nH_{2n}O$ (or $C_nH_{2n+1}CHO$).
(c) The two compounds are homologs — they belong to the same homologous series (alkanals). Compound (i) is propanal (CH₃CH₂CHO) and compound (ii) is ethanal (CH₃CHO); they differ by one –CH₂– unit.
Structure of another aldehyde — Methanal (HCHO):
$$H–\overset{\displaystyle O}{\overset{\|}{C}}–H \quad \text{(HCHO)}$$
Source: Carbon and its Compounds, sections 4.2.3 and 4.2.5
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Explanation
- (a) The –CHO group (C double-bonded to O, with H on the same C) is the aldehyde functional group. Examiners expect the name and the structural formula of the group.
- (b) CBSE accepts $C_nH_{2n}O$ or the form $C_nH_{2n+1}CHO$ for aldehydes.
- (c) "Relationship" means they are homologs — same functional group, differ by –CH₂–. You must state this explicitly and draw any one aldehyde (methanal is simplest). Drawing propanal or butanal is equally acceptable.