See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Analysis of Optical Isomerism in Lactic Acid **Structure of Lactic Acid:** $$CH_3-\underbrace{CHOH}_{*}-COOH$$ The central carbon atom (marked with *) is bonded to: - A methyl group ($-CH_3$) - A hydroxyl group ($-OH$) - A carboxylic acid group ($-COOH$) - A hydrogen atom ($-H$) **Why Optical Isomerism Occurs at the Central Carbon:** 1. **Tetrahedral geometry:** The central carbon is $sp^3$ hybridized with tetrahedral geometry (109.5°) 2. **Four different groups:** The central carbon has four *distinct* substituents attached, making it a **chiral center** 3. **Stereoisomerism:** These four different groups can be arranged in two non-superimposable mirror-image configurations (D- and L-lactic acid), producing optical isomers 4. **Why NOT at other positions:** - The methyl carbon has two identical H atoms (not a chiral center) - The carboxylic acid carbon has a double bond, not tetrahedral - Hydroxyl oxygen is not carbon (not a chiral center) **The correct answer is (1) Central carbon atom** — it is the only site with four different groups in tetrahedral arrangement, creating the asymmetric carbon required for optical isomerism.