A laboratory student was converting cyclohexanol to cyclohexyl bromide by using one equivalent of sodium bromide in a large excess of concentrated sulfuric acid. The major product she recovered was not cyclohexyl bromide, but a compound of formula C6H10 that gave the following 13C NMR spectrum:(a) Propose a structure for this product.(b) Assign the peaks in the 13C NMR spectrum to the carbon atoms in the structure.(c) Suggest modifications in the reaction to obtain a better yield of cyclohexyl bromide.

Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

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Wade 9th Edition

Ch. 13 - Nuclear Magnetic Resonance Spectroscopy

문제 31

13장, 문제 31

A laboratory student was converting cyclohexanol to cyclohexyl bromide by using one equivalent of sodium bromide in a large excess of concentrated sulfuric acid. The major product she recovered was not cyclohexyl bromide, but a compound of formula C₆H₁₀ that gave the following ¹³C NMR spectrum:
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(a) Propose a structure for this product.
(b) Assign the peaks in the ¹³C NMR spectrum to the carbon atoms in the structure.
(c) Suggest modifications in the reaction to obtain a better yield of cyclohexyl bromide.

검증된 단계별 안내

Step 1: Analyze the chemical formula C6H10 provided in the problem. This indicates the compound is unsaturated, as it follows the general formula for alkenes (CnH2n). The presence of double bonds or rings is likely.

Step 2: Examine the 13C NMR spectrum. The peaks at approximately 130 ppm suggest the presence of sp2-hybridized carbons, which are characteristic of double bonds in alkenes. Peaks at lower ppm values (around 20-40 ppm) correspond to sp3-hybridized carbons, likely from alkyl groups.

Step 3: Propose a structure for the compound based on the NMR data. The sp2-hybridized carbons suggest a double bond, and the sp3-hybridized carbons indicate alkyl groups attached to the double bond. A plausible structure is cyclohexene, which matches the formula C6H10 and the NMR data.

Step 4: Assign the peaks in the 13C NMR spectrum to the carbon atoms in cyclohexene. The peak at ~130 ppm corresponds to the two sp2-hybridized carbons in the double bond. The peaks at ~20-40 ppm correspond to the sp3-hybridized carbons in the ring.

Step 5: Suggest modifications to improve the yield of cyclohexyl bromide. To avoid elimination reactions leading to cyclohexene, use milder conditions such as phosphorus tribromide (PBr3) or thionyl bromide (SOBr2) instead of concentrated sulfuric acid, which promotes elimination.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. In the context of converting cyclohexanol to cyclohexyl bromide, the nucleophile (bromide ion) attacks the carbon atom bonded to the hydroxyl group, leading to the formation of the desired product. Understanding this mechanism is crucial for predicting the outcome of the reaction and the potential side products formed.

NMR Spectroscopy

Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. The <sup>13</sup>C NMR spectrum provides information about the number and environment of carbon atoms in a molecule. By analyzing the chemical shifts and splitting patterns in the spectrum, one can deduce the connectivity and arrangement of carbon atoms in the product, which is essential for answering part (b) of the question.

Reaction Conditions and Mechanisms

The conditions under which a reaction occurs can significantly influence the products formed. In this case, the use of concentrated sulfuric acid as a catalyst can lead to dehydration reactions, resulting in the formation of alkenes instead of the desired alkyl bromide. Understanding how to modify reaction conditions, such as using a different solvent or adjusting the concentration of reagents, is key to optimizing the yield of cyclohexyl bromide, as discussed in part (c) of the question.