StemCET Logo

Chemistry Question 138 – AP-EAMCET 2026

← Back
Consider the given sequence of reactions CH3−C≡CH→HBrX→HBrY Total number of isomers possible for the product Y is

Understand the mechanism of electrophilic addition of HBr to alkynes and alkenes, specifically Markovnikov's rule.

Step 1: Determine the structure of product X✦ Active

The starting material is propyne (CH3−C≡CH). The addition of the first equivalent of HBr follows Markovnikov's rule. The hydrogen adds to the terminal carbon (CH), and bromine adds to the more substituted carbon (C). This leads to the formation of a vinylic halide.

CH3−C≡CH+HBr→CH3−C(Br)=CH2(X)

Thus, product X is 2-bromopropene.

💡 Teacher's Secret Hint

Remember that Markovnikov's rule dictates that the hydrogen adds to the carbon with more hydrogens, and the halogen adds to the carbon with fewer hydrogens, forming the more stable carbocation intermediate.

Step 2: Determine the structure of product Y○ Expand

Product X (2-bromopropene) then reacts with a second equivalent of HBr. This is an electrophilic addition to an alkene, again following Markovnikov's rule. The hydrogen adds to the carbon with more hydrogens (the CH2 group), and bromine adds to the carbon already bonded to bromine (the more substituted carbon, which forms a more stable carbocation due to resonance and hyperconjugation).

CH3−C(Br)=CH2+HBr→CH3−C(Br)2−CH3(Y)

Thus, product Y is 2,2-dibromopropane.

💡 Teacher's Secret Hint

The presence of an existing bromine atom on the carbon further stabilizes the carbocation formed at that position, reinforcing Markovnikov's regioselectivity for the second addition.

Step 3: Determine the molecular formula of product Y○ Expand

The structure of Y is 2,2-dibromopropane (CH3−C(Br)2−CH3). Counting the atoms, we have 3 carbons, 6 hydrogens (3 from each methyl group), and 2 bromines.

Molecular formula of Y=C3H6Br2
💡 Teacher's Secret Hint

Always double-check the molecular formula by counting all atoms in the determined structure to ensure accuracy for isomer enumeration.

Step 4: Identify all possible structural isomers for the molecular formula C3H6Br2○ Expand

The question asks for the total number of isomers possible for the product Y. Given the options, this typically refers to the number of structural isomers for the molecular formula C3H6Br2. We need to find all unique ways to arrange 3 carbons, 6 hydrogens, and 2 bromines in an acyclic saturated structure. The carbon backbone will be a propane chain.

The possible positions for the two bromine atoms are:

1. **1,1-dibromopropane:** CH3CH2CHBr2

2. **2,2-dibromopropane:** CH3CBr2CH3 (This is the product Y formed in the reaction)

3. **1,2-dibromopropane:** CH3CHBrCH2Br

4. **1,3-dibromopropane:** BrCH2CH2CH2Br

These are 4 distinct structural isomers. While 1,2-dibromopropane has a chiral center (leading to R and S enantiomers), if the question intended to include stereoisomers, the total would be 5. Since 4 is an option and marked correct, the question is asking for the number of structural isomers.

💡 Teacher's Secret Hint

When enumerating isomers, systematically consider all possible positions for substituents on the carbon backbone. For dihaloalkanes, remember to check for chiral centers if stereoisomers are implied, but often 'isomers' without further qualification refers to structural isomers.

✦ STEM Console utilizes AI models to generate step-by-step explanations and math clues. AI can make mistakes.