See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen adds to the carbon bearing more hydrogens (or the more electron-rich carbon), and X adds to the carbon bearing fewer hydrogens (more substituted or more stable carbocation). Anti-Markovnikov (violation) occurs when the addition goes opposite to this rule, typically when the double bond carbon is attached to a strongly electron-withdrawing group. Step 1: Analyze each substituent attached to the vinyl group (CH=CH2). - Option (a): CH3—O— is an electron-donating group (EDG) via lone pair donation (+M effect). The oxygen donates electrons into the double bond, making the alpha carbon (CH=) more electron-rich. HBr adds in Markovnikov fashion: H goes to CH2, Br goes to CH bearing O. This follows Markovnikov's rule. - Option (b): CH3—NH— is also an electron-donating group (EDG) via lone pair donation (+M effect). Similar reasoning as (a). The nitrogen donates electrons, making the alpha carbon more electron-rich. Markovnikov addition occurs. This follows Markovnikov's rule. - Option (c): CH3—S— is an electron-donating group (EDG) via lone pair donation (+M effect, though weaker than O or N). The sulfur still donates electrons into the double bond. Markovnikov addition occurs. This follows Markovnikov's rule. - Option (d): O2N— (nitro group) is a strongly electron-withdrawing group (EWG) by both inductive (-I) and resonance (-M) effects. The nitro group withdraws electron density from the double bond, particularly from the alpha carbon (CH=). This makes the beta carbon (=CH2) relatively more electron-rich. When HBr adds, H goes to the carbon bearing the nitro group (alpha carbon) and Br goes to the terminal CH2 (beta carbon). This is opposite to normal Markovnikov addition, hence Markovnikov's rule is violated. Step 2: EDG groups (O, N, S attached directly) activate the alpha carbon, leading to Markovnikov addition. EWG groups (like NO2) deactivate the alpha carbon and reverse regioselectivity, leading to anti-Markovnikov addition. Step 3: Options (a), (b), and (c) all have electron-donating heteroatoms that direct addition in a Markovnikov sense. Only option (d) has a strongly electron-withdrawing nitro group that causes anti-Markovnikov (violation) addition. Therefore, the correct answer is D.