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Ch. 17 - Reactions of Aromatic Compounds
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 17 - Reactions of Aromatic CompoundsProblem 38a,b
Chapter 17, Problem 38a,b

(a) Based on what you know about the relative stabilities of alkyl cations and benzylic cations, predict the product of addition of HBr to 1-phenylpropene.
(b) Propose a mechanism for this reaction.

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Step 1: Analyze the structure of 1-phenylpropene. It consists of a phenyl group attached to a propene chain. The double bond in the propene chain is the reactive site for electrophilic addition reactions.
Step 2: Predict the product of HBr addition. The reaction involves the electrophilic addition of HBr to the double bond. The proton (H⁺) from HBr will add to one of the carbons in the double bond, forming a carbocation intermediate. Consider the relative stabilities of alkyl cations and benzylic cations to determine which carbon will form the carbocation.
Step 3: Explain carbocation stability. Benzylic carbocations are stabilized by resonance with the aromatic ring, making them more stable than alkyl carbocations. Therefore, the proton will add to the terminal carbon of the double bond, leaving the benzylic carbon as the carbocation intermediate.
Step 4: Propose the mechanism for the reaction. The mechanism begins with the electrophilic attack of H⁺ on the double bond, forming the benzylic carbocation. Next, the bromide ion (Br⁻) attacks the carbocation, resulting in the formation of the final product.
Step 5: Conclude with the predicted product. The addition of HBr to 1-phenylpropene will yield 1-bromo-1-phenylpropane as the major product, due to the formation of the more stable benzylic carbocation intermediate during the reaction.

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Key Concepts

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

Alkyl Cation Stability

Alkyl cations are positively charged carbon species whose stability varies based on their structure. Tertiary cations are more stable than secondary, which are more stable than primary due to hyperconjugation and inductive effects. Understanding this stability is crucial for predicting reaction pathways and products in electrophilic addition reactions.
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Benzylic Cation Stability

Benzylic cations, which are cations adjacent to a benzene ring, are particularly stable due to resonance stabilization. The positive charge can be delocalized into the aromatic system, making these cations more stable than their non-aromatic counterparts. This concept is essential for predicting the major product in reactions involving compounds like 1-phenylpropene.
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Electrophilic Addition Mechanism

The electrophilic addition mechanism involves the attack of an electrophile (like HBr) on a nucleophilic site in an alkene. In the case of 1-phenylpropene, the double bond reacts with HBr, leading to the formation of a carbocation intermediate. The stability of the resulting carbocation will dictate the final product, highlighting the importance of understanding reaction mechanisms in organic chemistry.
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