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Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 33b
Chapter 9, Problem 33b

Predict the major products resulting from the addition of one equivalent of HX to the following alkynes.
(b) Structural representation of an alkyne reacting with HCl, indicating the addition reaction process.

Verified step by step guidance
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Step 1: Identify the alkyne functional group in the given molecule. The molecule contains a triple bond between two carbon atoms, which is the site of reactivity for the addition of HCl.
Step 2: Recall the mechanism of electrophilic addition of HX (hydrogen halide) to alkynes. The reaction proceeds via Markovnikov's rule, where the hydrogen atom from HX adds to the carbon of the triple bond with more hydrogens, and the halide (Cl⁻) adds to the carbon with fewer hydrogens.
Step 3: Analyze the structure of the alkyne. The triple bond is located between two carbons, one of which is more substituted (attached to more carbon groups) than the other. According to Markovnikov's rule, the chlorine atom will add to the more substituted carbon.
Step 4: Consider the stereochemistry and regioselectivity of the product. Since only one equivalent of HCl is added, the reaction stops at the alkene stage, forming a double bond. The major product will be a vinyl halide (an alkene with a halogen attached).
Step 5: Draw the structure of the major product based on the addition pattern. The double bond will remain between the two carbons, and the chlorine atom will be attached to the more substituted carbon. Ensure the product reflects Markovnikov's addition and retains the original substituents on the molecule.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alkyne Reactivity

Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. They are more reactive than alkenes due to the high electron density of the triple bond, which can undergo addition reactions. When treated with HX (where X is a halogen), alkynes can react to form haloalkenes, with the addition occurring across the triple bond.
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Markovnikov's Rule

Markovnikov's Rule states that when HX is added to an unsymmetrical alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This results in the more stable carbocation intermediate and leads to the formation of the major product. Understanding this rule is crucial for predicting the outcome of the addition reaction.
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Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple products are possible. In the context of alkyne addition reactions, regioselectivity is influenced by the stability of the intermediates formed during the reaction, which is guided by factors such as sterics and electronic effects. This concept is essential for predicting the major product in the reaction with HX.
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Related Practice
Textbook Question

Long, polarized bonds are also reducible in the same way that the C―C π bond of an alkyne is. Show a mechanism by which a C―Br bond might be reduced by sodium metal.

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Textbook Question

The alkyl carbocation is estimated to be 15 kcal/mol more stable than the alkenyl carbocation. If this is also the difference in the energies of the transition state leading to each, what is the expected rate difference? 

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Textbook Question

We have studied the following reactions in previous chapters. For each, (i) indicate which reaction sheets they should appear on, (ii) show the best structure to use to represent them, and (iii) write the notes you could put in the margin so that the mechanism is implied.

(b) Radical halogenation of an alkane

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Textbook Question

Draw the ketone(s) you would expect to form by reacting the following alkynes under the conditions of oxymercuration: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne.

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Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(a)

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Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(b)

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