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Ch.6 - Alkyl Halides; Nucleophilic Substitution
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.6 - Alkyl Halides; Nucleophilic SubstitutionProblem 57a
Chapter 6, Problem 57a

Propose mechanisms to account for the observed products in the following reactions.
(a)

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1
Step 1: Recognize that the reaction involves allylic bromination using NBS (N-bromosuccinimide) under photochemical conditions (hv). NBS selectively brominates allylic positions in the presence of light or heat.
Step 2: Identify the allylic positions in the starting compound (bicyclic alkene). Allylic positions are the carbons adjacent to the double bond, which are stabilized by resonance.
Step 3: Initiate the mechanism with the generation of a bromine radical (Br•) from NBS under photochemical conditions. This radical is responsible for abstracting a hydrogen atom from the allylic position of the substrate.
Step 4: After the hydrogen abstraction, a resonance-stabilized allylic radical is formed. Draw the resonance structures to show the delocalization of the unpaired electron across the π-system of the double bond.
Step 5: The bromine radical then reacts with the allylic radical to form the brominated products. The two observed products arise from the different resonance structures of the allylic radical, leading to bromination at two distinct allylic positions.

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Key Concepts

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

Allylic Bromination

Allylic bromination is a reaction where bromine is added to the allylic position of an alkene, typically using reagents like N-bromosuccinimide (NBS) in the presence of light. This process involves the formation of a radical intermediate, allowing for the selective substitution of hydrogen atoms at the allylic position, leading to the formation of multiple products due to the possibility of bromination at different sites.
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Mechanism of Allylic Bromination.

Radical Mechanism

The radical mechanism is a reaction pathway that involves the formation and reaction of free radicals, which are highly reactive species with unpaired electrons. In the context of allylic bromination, the mechanism typically proceeds through three steps: initiation (formation of radicals), propagation (reaction of radicals with the substrate), and termination (recombination of radicals). Understanding this mechanism is crucial for predicting the products formed in the reaction.
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Regioselectivity

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In allylic bromination, regioselectivity is influenced by the stability of the radical intermediates formed during the reaction. More stable radicals, such as those that are tertiary or resonance-stabilized, are favored, leading to specific products that can be predicted based on the structure of the starting material.
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Related Practice
Textbook Question

Triethyloxonium tetrafluoroborate, (CH3CH2)3O+ BF4–, is a solid with melting point 91–92°C. Show how this reagent can transfer an ethyl group to a nucleophile (Nuc:−) in an SN2 reaction. What is the leaving group? Why might this reagent be preferred to using an ethyl halide? (Consult Table 6-2)

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Textbook Question

Propose mechanisms to account for the observed products in the following reactions.

(b)

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Textbook Question

Show the products you expect when each compound reacts with NBS with light shining on the reaction.

(c)

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Textbook Question

Show the products you expect when each compound reacts with NBS with light shining on the reaction.

(a)

(b)

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Textbook Question

The following reaction takes place under second-order conditions (strong nucleophile), yet the structure of the product shows rearrangement. Also, the rate of this reaction is several thousand times faster than the rate of substitution of ­hydroxide ion on 2-chlorobutane under similar conditions. Propose a mechanism to explain the enhanced rate and ­rearrangement observed in this unusual reaction. (“Et” is the abbreviation for ethyl.)

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Textbook Question

Furfuryl chloride can undergo substitution by both SN2 and SN1 mechanisms. Because it is a 1° alkyl halide, we expect SN2 but not SN1 reactions. Draw a mechanism for the SN1 reaction shown below, paying careful attention to the structure of the intermediate. How can this primary halide undergo SN1 reactions?

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