Animated Solution for Chemistry - Organic Chemistry: The major product of the following reaction is ${\text{CH}}_3\text{C}\equiv \text{CH}\underset{\text{(ii) DI}}{\overset{\text{(i) DCl (1 equiv.)}}{\to }}$

It is subjected to two sequential electrophilic addition reactions. First, it reacts with one equivalent of deuterium chloride ($DCl$). Then, the resulting intermediate reacts with deuterium iodide ($DI$). Our objective is to determine the structure of the final major product.

Markovnikov's Rule When an unsymmetrical alkyne reacts with a hydrogen halide (or in this case, a deuterium halide), the addition follows Markovnikov's rule. The electrophile, $D^{+}$, will attack the pi bond in a way that generates the most stable carbocation intermediate.

If $D^{+}$ attaches to the central carbon, a primary vinylic carbocation is formed at the terminal carbon. However, if $D^{+}$ attaches to the terminal carbon, a secondary vinylic carbocation is formed at the central carbon. Since secondary carbocations are significantly more stable than primary ones due to the inductive effect of the adjacent methyl group, the $D^{+}$ selectively bonds to the terminal carbon.

Following the formation of this intermediate, the nucleophilic chloride ion ($C{l}^{-}$) attacks the positively charged central carbon, yielding the intermediate alkene: 2-chloro-1-deuteriopropene.

Resonance Stabilization Now, we introduce the second reagent, $DI$. The alkene undergoes another electrophilic addition. Once again, the $D^{+}$ ion must choose between the two carbons of the double bond.

If it attaches to the central carbon, the positive charge lands on the terminal carbon. But if it attaches to the terminal carbon, the positive charge lands on the central carbon, which is already bonded to a chlorine atom.

This is where a crucial stabilizing effect comes into play. The chlorine atom possesses lone pairs of electrons. It can donate a lone pair to the adjacent empty p-orbital of the carbocation through resonance (the +M effect). This resonance stabilization is incredibly powerful, making this carbocation highly favored.

It bonds to the central carbon, completing the reaction.

The final major product has both the chlorine and iodine atoms attached to the central carbon, and two deuterium atoms attached to the terminal carbon. This is a classic example of geminal dihalide formation from a terminal alkyne.