A. Draw the major product(s) of the reaction of 1...

a. Draw the major product(s) of the reaction of 1 -methylcyclohexene with the following reagents, disregarding stereoisomers: 1. NBS/ $\Delta /$ peroxide 2. $\mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$ 3. HBr 4. HBr/peroxide b. For each reaction, show which stereoisomers are obtained.

a. Draw the major product(s) of the reaction of 1 -methylcyclohexene with the following reagents, disregarding stereoisomers:
1. NBS/ $\Delta /$ peroxide
2. $\mathrm{Br}_{2} / \mathrm{CH}_{2} \mathrm{Cl}_{2}$
3. HBr
4. HBr/peroxide
b. For each reaction, show which stereoisomers are obtained.

Key Concepts

Free Radical Mechanism

The free radical mechanism is a pathway that involves reactive radical intermediates generated by homolytic bond cleavage under the influence of light or peroxides. This mechanism is key in processes like allylic bromination and anti?Markovnikov addition of HBr, where the stability of radicals and the selectivity of hydrogen abstraction determine the overall outcome of the reaction. It contrasts with ionic mechanisms, providing different regio- and stereochemical products.

Markovnikov and Anti?Markovnikov Addition

The addition of HX to alkenes can proceed with Markovnikov or anti?Markovnikov regioselectivity depending on the reaction conditions. Under standard electrophilic addition conditions (e.g. HBr without peroxides), the hydrogen adds to the less substituted carbon, following Markovnikov's rule due to the stability of the ensuing carbocation intermediate. In contrast, in the presence of peroxides, the reaction follows a free radical mechanism leading to anti?Markovnikov addition, where the bromine atom adds to the less substituted carbon.

Stereochemical Considerations in Addition Reactions

Stereochemistry plays a crucial role in addition reactions, particularly those that involve cyclic systems or formation of new chiral centers. For example, in the halogenation of alkenes, the anti addition mechanism leads to the creation of stereoisomers (enantiomers or diastereomers depending on the substrate) by adding substituents on opposite faces of the double bond. Understanding the stereochemistry is essential for predicting the configuration of the resulting products.

Allylic Bromination

This concept involves the selective substitution of a hydrogen atom at an allylic position (a carbon adjacent to a double bond) by a bromine atom using reagents such as NBS in the presence of heat and peroxides. The reaction proceeds via a free radical mechanism, where the stability of the radical intermediate governs the product distribution. It is a useful transformation for functionalizing alkenes at positions that are less reactive toward direct electrophilic addition.

Electrophilic Addition to Alkenes

In electrophilic addition reactions, an electrophile reacts with the electron?rich double bond of an alkene, leading to the formation of a carbocation intermediate, followed by nucleophilic attack. When Br2 is added to an alkene in an inert solvent like CH2Cl2, the reaction typically proceeds via an anti addition mechanism, where the two bromine atoms add to opposite faces of the former double bond, resulting in a vicinal dibromide with specific stereochemical configurations.

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