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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Tutti i libri di testoWade 9th EditionCh.6 - Alkyl Halides; Nucleophilic SubstitutionProblema 24a
Capitolo 6, Problema 24a

3-Bromocyclohexene is a secondary halide. It undergoes SN1 substitution about as fast as most tertiary halides. Use resonance structures to explain this enhanced reactivity.

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1
Identify the structure of 3-bromocyclohexene, which consists of a cyclohexene ring with a bromine atom attached to the third carbon.
Recognize that the presence of the double bond in the cyclohexene ring allows for resonance stabilization of the carbocation formed during the SN1 reaction.
Draw the resonance structures: When the bromine leaves, a carbocation is formed at the third carbon. The positive charge can be delocalized to the adjacent carbon atoms through resonance with the double bond.
Illustrate the movement of electrons: The pi electrons from the double bond can move to form a new double bond with the carbocation, shifting the positive charge to the other carbon atom in the ring.
Explain that this resonance stabilization of the carbocation intermediate makes the SN1 reaction more favorable, enhancing the reactivity of 3-bromocyclohexene compared to other secondary halides without such stabilization.

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SN1 Reaction Mechanism

The SN1 reaction mechanism involves a two-step process where the leaving group departs first, forming a carbocation intermediate, followed by nucleophilic attack. The rate of SN1 reactions depends on the stability of the carbocation; more stable carbocations lead to faster reactions. This mechanism is common in tertiary halides due to their ability to stabilize positive charges.
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Drawing the SN1 Mechanism

Carbocation Stability

Carbocation stability is crucial in determining the rate of SN1 reactions. Stability is enhanced by factors such as hyperconjugation and resonance. Tertiary carbocations are generally more stable than secondary or primary ones due to greater hyperconjugation and inductive effects. In 3-bromocyclohexene, resonance stabilization plays a key role in enhancing carbocation stability.
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Determining Carbocation Stability

Resonance Structures

Resonance structures are different Lewis structures for a molecule that depict the delocalization of electrons. In 3-bromocyclohexene, the positive charge on the carbocation can be delocalized over the double bond, creating resonance structures that stabilize the intermediate. This delocalization increases the carbocation's stability, making the SN1 reaction proceed more rapidly, similar to tertiary halides.
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Drawing Resonance Structures
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In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

d. Explain why cyclohexene reacts with bromine much faster than cyclohexane, which must be heated to react.

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Benzyl bromide is a primary halide. It undergoes SN1 substitution about as fast as most tertiary halides. Use resonance structures to explain this enhanced reactivity.

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Choose the member of each pair that will react faster by the SN1 mechanism.

c. n-propyl bromide or allyl bromide

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Give the SN1 mechanism for the formation of 2-ethoxy-3-methylbutane, the unrearranged product in this reaction.

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Choose the member of each pair that will react faster by the SN1 mechanism.

e. 2-iodo-2-methylbutane or tert-butyl chloride

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Choose the member of each pair that will react faster by the SN1 mechanism.

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