Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

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

Wade 9th Edition

Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Problem 38c

Chapter 15, Problem 38c

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.
(c) Propose a mechanism for the dehydration of alcohol A to compound B.

Verified step by step guidance

Step 1: Protonation of the alcohol group in alcohol A occurs when sulfuric acid (H₂SO₄) is added. The hydroxyl group (-OH) is converted into a better leaving group, forming a protonated alcohol intermediate.

Step 2: The protonated alcohol undergoes elimination to form a carbocation. The leaving group (water, H₂O) departs, generating a tertiary carbocation at the carbon where the hydroxyl group was originally attached.

Step 3: The carbocation undergoes rearrangement or stabilization if necessary. In this case, the tertiary carbocation is already stable due to hyperconjugation and inductive effects from the surrounding alkyl groups.

Step 4: A β-hydrogen elimination occurs from an adjacent carbon atom, leading to the formation of a double bond. This step is facilitated by the acidic environment, resulting in the formation of an alkene.

Step 5: The final product, compound B, is formed as the major product due to its conjugated structure, which is stabilized by resonance. The λ_max = 269 nm indicates the presence of conjugation in the molecule, consistent with the desired product structure.

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Key Concepts

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

Dehydration Reaction

A dehydration reaction involves the removal of a water molecule from an alcohol to form an alkene. In this case, alcohol A undergoes dehydration when heated with sulfuric acid, leading to the formation of compound B. This reaction typically follows an E1 or E2 mechanism, depending on the structure of the alcohol and the reaction conditions.

E1 and E2 Mechanisms

E1 and E2 are two types of elimination mechanisms in organic chemistry. E1 is a two-step process where the leaving group departs first, forming a carbocation, followed by deprotonation to form the alkene. E2 is a one-step process where the base abstracts a proton while the leaving group departs simultaneously. The choice between these mechanisms depends on the substrate structure and the strength of the base used.

Terpene Synthesis

Terpene synthesis refers to the chemical processes used to create terpenes, which are a large class of organic compounds produced by various plants. Understanding the structure and reactivity of terpenes is crucial for predicting the outcomes of reactions like the one described. The desired product's structure suggests a specific rearrangement or elimination pathway that is common in terpene chemistry.

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Related Practice

Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.

h. Add an electron to the pentadienyl radical to give the pentadienyl anion. Which carbon atoms share the negative charge? Does this picture agree with the resonance picture?

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Textbook Question

A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted 3-bromo-6-methylcyclohexene to alcohol A. She heated alcohol A with sulfuric acid and purified one of the components (compound B) from the resulting mixture. Compound B has the correct molecular formula for the desired product.

a. Suggest how 3-bromo-6-methylcyclohexene might be converted to alcohol A.

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Textbook Question

The pentadienyl radical, H₂C=CH–CH=CH–CH₂•, has its unpaired electron delocalized over three carbon atoms.

g. Remove the highest-energy electron from the pentadienyl radical to give the pentadienyl cation. Which carbon atoms share the positive charge? Does this picture agree with the resonance picture?

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Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(a)

(b)

(c)

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Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(g)

598

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Textbook Question

Determine whether each structure is likely to be colored or not. For those that you predict to be colored, indicate the extended conjugation by marking the series of continuous sp² hybridized atoms.

(d)

(e)

(f)

1283

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