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Ch.11 - Reactions of Alcohols
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.11 - Reactions of AlcoholsProblem 17
Chapter 11, Problem 17

When cis-2-methylcyclohexanol reacts with the Lucas reagent, the major product is 1-chloro-1-methylcyclohexane. Propose a mechanism to explain the formation of this product.

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Step 1: Understand the Lucas reagent reaction. The Lucas reagent is a mixture of concentrated hydrochloric acid (HCl) and zinc chloride (ZnCl₂). It is used to convert alcohols into alkyl chlorides via an SN1 or SN2 mechanism, depending on the structure of the alcohol. Tertiary alcohols typically undergo an SN1 mechanism due to the stability of the carbocation intermediate.
Step 2: Analyze the structure of cis-2-methylcyclohexanol. The hydroxyl (-OH) group is attached to a secondary carbon, and the molecule has a methyl group on the adjacent carbon. This secondary alcohol is likely to undergo an SN1 mechanism because the resulting carbocation can be stabilized by hyperconjugation and inductive effects from the methyl group.
Step 3: Protonation of the hydroxyl group. In the presence of the Lucas reagent, the hydroxyl group is protonated by HCl, forming water as a leaving group. This step increases the electrophilicity of the carbon attached to the hydroxyl group, making it more susceptible to nucleophilic attack.
Step 4: Formation of the carbocation intermediate. After the water molecule leaves, a carbocation is formed at the secondary carbon. This carbocation is stabilized by the adjacent methyl group through hyperconjugation and inductive effects. The stability of the carbocation is crucial for the SN1 mechanism.
Step 5: Nucleophilic attack by chloride ion. The chloride ion (Cl⁻) from HCl acts as a nucleophile and attacks the carbocation, forming 1-chloro-1-methylcyclohexane as the major product. The stereochemistry of the product is determined by the planar nature of the carbocation, allowing the chloride ion to attack from either side.

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Key Concepts

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

Cis-Trans Isomerism

Cis-trans isomerism refers to the different spatial arrangements of atoms in a molecule, particularly in cyclic compounds. In the case of cis-2-methylcyclohexanol, the methyl group and the hydroxyl group are on the same side of the cyclohexane ring, influencing the reactivity and sterics during reactions. Understanding this concept is crucial for predicting how the molecule will interact with reagents like the Lucas reagent.
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Lucas Reagent

The Lucas reagent, typically a mixture of zinc chloride and hydrochloric acid, is used to convert alcohols into alkyl halides. It works through the formation of a carbocation intermediate, which is more favorable for secondary and tertiary alcohols. Recognizing how the Lucas reagent functions helps in understanding the mechanism of the reaction and the stability of the resulting carbocation.
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Carbocation Stability

Carbocation stability is a key concept in organic chemistry, as the stability of the carbocation intermediate significantly influences the reaction pathway. Tertiary carbocations are more stable than secondary or primary ones due to hyperconjugation and inductive effects. In the reaction of cis-2-methylcyclohexanol with the Lucas reagent, the formation of a stable carbocation leads to the major product, 1-chloro-1-methylcyclohexane.
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Related Practice
Textbook Question

Show how you would use a simple chemical test to distinguish between the following pairs of compounds. Tell what you would observe with each compound.

(d) allyl alcohol and propan-1-ol

(e) butan-2-one and tert-butyl alcohol

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Textbook Question

Write balanced equations for the three preceding reactions.

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Textbook Question

Two products are observed in the following reaction.

a. Suggest a mechanism to explain how these two products are formed.

b. Your mechanism for part (a) should be different from the usual mechanism of the reaction of SOCl2 with alcohols. Explain why the reaction follows a different mechanism in this case.

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Textbook Question

Explain the products observed in the following reaction of an alcohol with the Lucas reagent.

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Textbook Question

Neopentyl alcohol, (CH3)3CCH2OH, reacts with concentrated HBr to give 2-bromo-2-methylbutane, a rearranged product. Propose a mechanism for the formation of this product.

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Textbook Question

Suggest how you would convert trans-4-methylcyclohexanol to

a. trans-1-chloro-4-methylcyclohexane.

b. cis-1-chloro-4-methylcyclohexane.

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