See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The addition of HCl to an alkene proceeds via an electrophilic addition mechanism. The first step involves the pi electrons of the alkene acting as a nucleophile attacking the electrophilic proton (H) of HCl, with simultaneous heterolytic cleavage of the H-Cl bond, generating a carbocation intermediate and a chloride ion. Step 1 - Identify the correct mechanism type: HCl addition to alkenes is ionic (electrophilic addition), not radical. This eliminates option (a), which shows homolytic cleavage producing a carbon radical and a chlorine radical. Step 2 - Identify the correct regiochemistry (Markovnikov's rule): When HCl adds to 1-butene (CH2=CH-CH2-CH3), the proton adds to the terminal carbon (C1), generating the more stable secondary carbocation at C2 (but-2-yl cation). This is consistent with Markovnikov's rule. Step 3 - Evaluate option (b): Shows curved arrows from the pi bond of 1-butene attacking H of HCl, and the H-Cl bond electrons going to Cl, producing a secondary carbocation (positive charge at C2) and Cl minus. This correctly depicts: (i) heterolytic bond cleavage, (ii) pi electrons attacking H (the electrophile), (iii) Markovnikov regioselectivity giving the more stable 2° carbocation, (iv) Cl departing as chloride ion. Step 4 - Why option (c) fails: Option (c) shows formation of a primary carbocation (positive charge at C1), which violates Markovnikov's rule and gives the less stable carbocation. Step 5 - Why option (d) fails: Option (d) shows the pi electrons attacking Cl rather than H, and produces H minus, which is incorrect; Cl is not the electrophilic atom in HCl, and H minus is not a viable leaving group under these conditions. Therefore, the correct answer is B.