Choose the best reagent or sequence of reagents from the list provided below for carrying out the following transformations. Place the letter of your response to the left of the reaction. a. PBr3 b. HCI (gas), ether C. 1. Mg, ether 2. D2O CH3 H3C-C-OH CH3 CH3 Br OH d. SOCI2, pyridine e. HBr (gas), ether f. 1. Mg, ether 2. NBS CH3 H2C-C-Br CH3 CH3 D ClXII. Choose the best reagent or sequence of reagents from the list provided below for carrying out the following transformations. Place the letter of your response to the left of the reaction. a. PBr3 b. HCI (gas), ether C. 1. Mg, ether 2. D2O CH3 H3C-C-OH CH3 CH3 Br OH d. SOCI2, pyridine e. HBr (gas), ether f. 1. Mg, ether 2. NBS CH3 H2C-C-Br CH3 CH3 D Cl
The Correct Answer and Explanation is :
To convert the alcohol CH₃CH₂CH₂OH into the alkyl halide CH₃CH₂CH₂Br, the most suitable reagent from the provided list is a. PBr₃.
Explanation:
Phosphorus tribromide (PBr₃) is a reagent commonly used to convert alcohols into alkyl bromides through an SN₂ mechanism. This reaction proceeds with inversion of configuration at the carbon center. The mechanism involves the alcohol attacking the phosphorus atom, leading to the displacement of a bromide ion, which then attacks the carbon center, resulting in the formation of the alkyl bromide.
Other reagents listed have different reactivity profiles:
- b. HCl (gas), ether: While HCl can convert alcohols to alkyl chlorides, it may lead to carbocation formation, especially with secondary or tertiary alcohols, potentially causing rearrangements. This is less ideal for primary alcohols like CH₃CH₂CH₂OH.
- c. 1. Mg, ether 2. D₂O: This sequence is used to prepare Grignard reagents (RMgX) from alkyl halides, not for converting alcohols to alkyl halides.
- d. SOCl₂, pyridine: Thionyl chloride (SOCl₂) is effective for converting alcohols to alkyl chlorides, proceeding via an SN₂ mechanism with inversion of configuration. However, it is specifically suited for chlorination, not bromination.
- e. HBr (gas), ether: Hydrobromic acid (HBr) can convert alcohols to alkyl bromides, but like HCl, it may lead to carbocation formation and rearrangements, making it less ideal for primary alcohols.
- f. 1. Mg, ether 2. NBS: This sequence is used to prepare Grignard reagents and then brominate compounds, but it is not suitable for converting alcohols directly to alkyl bromides.
Therefore, a. PBr₃ is the most appropriate choice for converting CH₃CH₂CH₂OH to CH₃CH₂CH₂Br.
For a visual explanation of alcohol reactions with PBr₃, SOCl₂, and HBr, you might find the following video helpful: