Question 5 of 32intermediate🔍 AnalyzeShort Answer2 marks

Write structures for each of the following compounds. Why are the given names incorrect? Write correct IUPAC names. (i) 2-Ethylpentane (ii) 5-Ethyl – 3-methylheptane

(i)
Answer

Structure: CH3−CH(C2H5)−CH2−CH2−CH3\text{CH}_3 - \text{CH}(\text{C}_2\text{H}_5) - \text{CH}_2 - \text{CH}_2 - \text{CH}_3. The given name is incorrect because the longest chain is of six carbon atoms and not that of five. Correct IUPAC name: 3-Methylhexane.

(ii)
Answer

Structure: CH3−CH2−CH(CH3)−CH2−CH(C2H5)−CH2−CH3\text{CH}_3 - \text{CH}_2 - \text{CH}(\text{CH}_3) - \text{CH}_2 - \text{CH}(\text{C}_2\text{H}_5) - \text{CH}_2 - \text{CH}_3. The given name is incorrect because numbering is to be started from the end which gives lower number to ethyl group. Correct IUPAC name: 3-ethyl-5-methylheptane.

Explanation

For part (i), drawing the structure of 2-Ethylpentane reveals that the longest continuous carbon chain contains six carbons, making the parent alkane hexane, not pentane. The ethyl group effectively becomes a methyl substituent on the third carbon. For part (ii), the structure of 5-Ethyl-3-methylheptane shows that numbering from the opposite end gives the substituents lower locants, specifically placing the ethyl group at carbon 3 and the methyl group at carbon 5.

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