Ch.9 - Alkynes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.9 - Alkynes

Problem 16c,d

Chapter 9, Problem 16c,d

Predict the major product(s) of the following reactions:
c. cyclooctyne + 2 HBr
d. *hex-2-yne + 2 HCl

Verified step by step guidance

Step 1: Analyze the structure of the alkyne in each reaction. Cyclooctyne is a cyclic alkyne, while hex-2-yne is a linear alkyne with the triple bond located between carbons 2 and 3.

Step 2: Recall the mechanism of electrophilic addition of hydrogen halides (HX) to alkynes. The reaction proceeds in two steps: (1) addition of one molecule of HX to form a vinyl halide, and (2) addition of a second molecule of HX to form a geminal dihalide.

Step 3: For cyclooctyne + 2HBr, the first molecule of HBr adds across the triple bond, forming a vinyl bromide intermediate. The second molecule of HBr then adds to the same carbon, resulting in a geminal dibromide. The cyclic structure remains intact.

Step 4: For hex-2-yne + 2HCl, the first molecule of HCl adds across the triple bond, forming a vinyl chloride intermediate. The second molecule of HCl adds to the same carbon, resulting in a geminal dichloride. The major product will have the halogens attached to the same carbon (geminal position).

Step 5: Consider regioselectivity and Markovnikov's rule. In both reactions, the halogen will preferentially add to the more substituted carbon of the triple bond during each step, ensuring the major product follows Markovnikov's rule.

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Key Concepts

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

Electrophilic Addition Reactions

Electrophilic addition reactions involve the addition of electrophiles to nucleophiles, typically occurring in alkenes and alkynes. In the case of cyclooctyne and hex-2-yne, the triple bond acts as a nucleophile, reacting with the electrophilic hydrogen atoms from HBr or HCl. This process leads to the formation of more saturated products, often resulting in the addition of halogens across the multiple bonds.

Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX 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 products, guiding the formation of major products when reacting compounds like hex-2-yne with HCl, where the more substituted carbon will preferentially bond with the halide.

Rearrangement and Stability of Carbocations

During electrophilic addition, carbocations may form as intermediates, and their stability is crucial for determining the reaction pathway. More stable carbocations, such as tertiary or secondary, are favored over less stable primary ones. Understanding the stability of these intermediates helps predict the major products of the reactions, as the pathway leading to the most stable carbocation will generally dominate.

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Related Practice

Textbook Question

Show how hex-1-yne might be converted to

e. 2-bromohexane.

1183

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Textbook Question

Propose a mechanism for the entire reaction of pent-1-yne with 2 moles of HBr. Show why Markovnikov's rule should be observed in both the first and second additions of HBr.

1345

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Textbook Question

In the addition of just 1 mole of bromine to 1 mole of hex-1-yne, should the hex-1-yne be added to a bromine solution or should the bromine be added to the hex-1-yne? Explain your answer.

1236

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Textbook Question

Show how hex-1-yne might be converted to

a. 1,2-dichlorohex-1-ene.

b. 1-bromohex-1-ene.

c. 2-bromohex-1-ene.

1056

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Textbook Question

Predict the major product(s) of the following reactions:

a. phenylacetylene + 2 HBr