Which of the following best represents the major organic product formed when a terminal alkyne reacts with one equivalent of $H_{\mathrm{Cl}}$ under standard conditions?

• A. An alkane with a chlorine atom attached to one of the terminal carbons
• B. A geminal dichloride, where two chlorine atoms add to the same carbon
• C. A vinyl chloride, where the chlorine atom adds to the more substituted carbon of the double bond (Markovnikov addition)
• D. An alkene with the chlorine atom on the less substituted carbon (anti-Markovnikov addition)

Answer: (C)

Verified step by step guidance

1. Identify the type of reaction: The addition of HCl to an alkyne is an electrophilic addition reaction where the pi bond of the alkyne is broken and new sigma bonds are formed.
2. Determine the regioselectivity: According to Markovnikov's rule, the proton (H⁺) adds to the carbon with more hydrogens (less substituted carbon), and the chloride ion (Cl⁻) adds to the more substituted carbon.
3. Consider the intermediate formed: The addition of one equivalent of HCl to a terminal alkyne forms a vinyl carbocation intermediate, which is stabilized by the more substituted carbon, leading to Markovnikov addition.
4. Predict the product after one equivalent of HCl: The product is a vinyl chloride, where the chlorine atom is attached to the more substituted carbon of the newly formed double bond.
5. Note that further addition (with excess HCl) could lead to geminal dichloride formation, but with only one equivalent, the major product is the vinyl chloride.