Predict the major product(s) of the following reactions: (a) phenylacetylene+2 HBr. (b) hex-1-yne+2 HCI. (c) cyclooctyne+2 HBr. (d) hex-2-yne+2 HCI

To predict the major products of the given reactions, we need to consider the addition of hydrogen halides (HX) to alkynes. This process typically follows Markovnikov's rule, where the hydrogen atom from the hydrogen halide adds to the carbon with more hydrogen atoms, and the halide adds to the carbon with fewer hydrogen atoms. Let's analyze each reaction step by step.

Step 1: Define the Problem

Identify the reactions and the reagents involved:

• (a) Phenylacetylene + 2 HBr
• (b) Hex-1-yne + 2 HCl
• (c) Cyclooctyne + 2 HBr
• (d) Hex-2-yne + 2 HCl

Step 2: Break Down the Problem

For each reaction, determine the structure of the alkyne and predict the addition of hydrogen halides.

Step 3: Apply Relevant Concepts

Use Markovnikov's rule to predict the major product for each reaction.

Step 4: Analysis, Verify and Summarize

(a) Phenylacetylene + 2 HBr

• Structure: Phenylacetylene is an alkyne with a phenyl group attached to one end.
• Reaction: Addition of HBr will occur twice.
• Product: The major product is 1,1-dibromo-1-phenylethane, where both Br atoms add to the same carbon (Markovnikov addition).

(b) Hex-1-yne + 2 HCl

• Structure: Hex-1-yne is a terminal alkyne.
• Reaction: Addition of HCl will occur twice.
• Product: The major product is 1,1-dichlorohexane, where both Cl atoms add to the terminal carbon.

(c) Cyclooctyne + 2 HBr

• Structure: Cyclooctyne is a cyclic alkyne.
• Reaction: Addition of HBr will occur twice.
• Product: The major product is 1,1-dibromocyclooctane, where both Br atoms add to the same carbon.

(d) Hex-2-yne + 2 HCl

• Structure: Hex-2-yne is an internal alkyne.
• Reaction: Addition of HCl will occur twice.
• Product: The major product is 2,2-dichlorohexane, where both Cl atoms add to the same carbon.

Final Answer

• (a) Phenylacetylene + 2 HBr → 1,1-dibromo-1-phenylethane
• (b) Hex-1-yne + 2 HCl → 1,1-dichlorohexane
• (c) Cyclooctyne + 2 HBr → 1,1-dibromocyclooctane
• (d) Hex-2-yne + 2 HCl → 2,2-dichlorohexane