See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Stereoisomerism in Butene/Butanoic Acid Compounds **Definition:** Stereoisomerism requires the same molecular formula but different spatial arrangements that cannot be interconverted by single bond rotation. **Analysis of each option:** **(A) 2-methylbutene-1:** $CH_2=C(CH_3)CH_2CH_3$ - The double bond carbons are: $CH_2=C(CH_3)—$ - One carbon has two different groups ($H$ and $CH_3$), the other has two $H$ atoms - This creates a chiral double bond → exhibits **cis-trans (geometric) isomerism** ✓ **(B) 3-methylbutyne:** $HC \equiv C-CH(CH_3)_2$ - Triple bonds cannot exhibit geometric isomerism (no rotation is restricted in a way that matters) - No other stereogenic centers → No stereoisomerism ✗ **(C) 3-methylbutanoic acid:** $CH_3CH_2CH(CH_3)COOH$ - Only one chiral center at $C^*H(CH_3)COOH$ - Shows **enantiomerism** (R/S forms), technically a type of stereoisomerism - However, less common in typical curricula **(D) 2-methylbutanoic acid:** $CH_3CH_2CH(CH_3)COOH$ - Similar structure to (C), also has one chiral center - Same reasoning as (C) **Answer: (A)** is correct because 2-methylbutene-1 exhibits **cis-trans geometric isomerism** due to the disubstituted double bond, which is the most straightforward and commonly tested example of stereoisomerism.