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Ch. 21 - Conjugated Systems I: Stability and Addition Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 21 - Conjugated Systems I: Stability and Addition ReactionsProblem 5
Chapter 20, Problem 5

Predict the product and provide a mechanism for the reaction of 1-methylcyclohexene with HBr.
Chemical structure of 1-methylcyclohexene reacting with HBr, leading to an unknown product indicated by a question mark.\

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1
Identify the type of reaction: The reaction of 1-methylcyclohexene with HBr is an electrophilic addition reaction, where the alkene reacts with the hydrogen halide (HBr) to form a haloalkane.
Determine the first step of the mechanism: The π-electrons of the double bond in 1-methylcyclohexene act as a nucleophile and attack the proton (H⁺) from HBr. This results in the formation of a carbocation intermediate. Use Markovnikov's rule to predict that the proton will add to the carbon of the double bond with the greater number of hydrogens, leading to the more stable carbocation.
Analyze carbocation stability: The carbocation formed will be a secondary carbocation at the carbon adjacent to the methyl group. Check for possible rearrangements, but in this case, the secondary carbocation is already stabilized by the adjacent methyl group through hyperconjugation and inductive effects.
Determine the second step of the mechanism: The bromide ion (Br⁻), which is the nucleophile, attacks the positively charged carbocation, forming a new C-Br bond. This results in the formation of the final product.
Predict the product: The product will be 1-bromo-1-methylcyclohexane, as the bromine adds to the carbon that originally had the double bond and is now part of the carbocation.

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Key Concepts

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

Electrophilic Addition

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, leading to the formation of a more saturated product. In the case of alkenes like 1-methylcyclohexene, the double bond acts as a nucleophile, attacking the electrophilic hydrogen in HBr, resulting in the formation of a carbocation intermediate.
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Carbocation Stability

Carbocation stability is crucial in predicting the outcome of reactions involving alkenes. Carbocations are positively charged species that can vary in stability based on their structure; tertiary carbocations are more stable than secondary or primary ones. In the reaction of 1-methylcyclohexene with HBr, the formation of a more stable carbocation will influence the final product.
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Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX to an alkene, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached, leading to the more stable product. This rule helps predict the regioselectivity of the reaction between 1-methylcyclohexene and HBr, guiding the formation of the major product.
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Related Practice
Textbook Question

There were actually two possible products in the solvolysis reaction from Figure 21.10. Show both products. Which would you expect to be more stable? Why?

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Textbook Question

Rank the following alkenes in order of stability (1 = most stable; 5 = least stable). Explain your order.                                                                                                                                                                                   

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Textbook Question

Draw all possible resonance structures for the reactive intermediates shown.

(c) 

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Textbook Question

Rank the following carbocations in order of stability (1 = most stable; 5 = least stable ). Explain your order.

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Textbook Question

To this point, hydrogenation has always been an exothermic process. Using the numbers from Fig­ure 21.6, calculate ∆Hohydr for each step of the reduction of benzene.

∆H1 + ∆H2 + ∆H3 = ∆Htot = ―49.5 kcal /mol (―208 kJ/mol)

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Textbook Question

Hydroboration, an electrophilic addition reaction like those studied in Section 21.4, only gives 1,2-addition to buta-1,3-diene, regardless of temperature. Why?

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