What is the major product obtained from the reaction of each of the following compounds with excess HCl?
c. CH₃CH₂C☰CCH₂CH₂CH₃

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Identify the functional group in the given compound. The compound contains a terminal alkyne group (C≡C) at the third carbon position.

Understand the reaction mechanism. When a terminal alkyne reacts with excess HCl, the reaction proceeds via Markovnikov's addition of HCl across the triple bond. This means the hydrogen (H) will add to the carbon with more hydrogens, and the chlorine (Cl) will add to the carbon with fewer hydrogens.

In the first step, one equivalent of HCl adds to the alkyne, converting the triple bond into a double bond. This forms a vinyl halide intermediate (CH3CH2CH=CHClCH2CH2CH3).

In the second step, the excess HCl reacts with the vinyl halide intermediate. Another molecule of HCl adds across the double bond, following Markovnikov's rule again. This results in a geminal dihalide (CH3CH2CHCl2CH2CH2CH3), where both chlorine atoms are attached to the same carbon.

Verify the major product. The major product is the geminal dihalide formed after the complete addition of excess HCl to the terminal alkyne.

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Alkyne Reactivity

Alkynes are hydrocarbons containing a carbon-carbon triple bond, which makes them more reactive than alkenes and alkanes. When treated with strong acids like HCl, alkynes can undergo protonation to form more stable carbocations, leading to further reactions such as addition. Understanding the mechanism of alkyne reactivity is crucial for predicting the products formed in acid-catalyzed reactions.

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Alkyne Hydration

Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX (where X is a halogen) to an alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This principle helps predict the regioselectivity of the reaction, guiding the formation of the major product when excess HCl reacts with alkynes.

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The 18 and 16 Electron Rule

Carbocation Stability

Carbocations are positively charged carbon species that can form during reactions involving alkenes and alkynes. The stability of a carbocation is influenced by the number of alkyl groups attached to the positively charged carbon; more alkyl groups lead to greater stability due to hyperconjugation and inductive effects. Recognizing the stability of potential carbocations is essential for determining the major product in reactions with HCl.

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