OpenOChem Learn First Semester Topics Alkyl Halides and Alcohol Strategy for Predicting Products of Allylic Brominations

Strategy for Predicting Products of Allylic Brominations

Allylic bromination, typically achieved using N-bromosuccinimide (NBS), offers a selective method for introducing a bromine atom at the allylic position, the carbon adjacent to a double bond. Predicting the products of these reactions, especially with unsymmetrical alkenes, requires a careful and systematic approach. The key lies in understanding the formation and stability of allylic radicals. By following these steps, you can systematically analyze allylic bromination reactions and predict the major and minor products formed, even with complex, unsymmetrical alkenes. Remember to always consider all possible allylic positions and the stability of the resulting radicals.

Example

Identify All Possible Allylic Hydrogen Atoms: Begin by carefully examining the structure of the alkene. Identify all hydrogen atoms attached to carbons that are allylic, meaning they are adjacent to the double bond. In 3-methylcyclohexene, you'll find allylic hydrogens on both sides of the double bond. I've colored the allylic protons GREEN.
Generate All Possible Allylic Radicals: For each unique allylic hydrogen you identified in step 1, imagine abstracting that hydrogen atom. This generates an allylic radical. Draw the structure of each possible allylic radical. With 3-methylcyclohexene, there are three distinct allylic positions where hydrogen abstraction can occur, leading to three different allylic radicals.
Draw Resonance Structures for Each Allylic Radical: Allylic radicals are stabilized by resonance. For each of the allylic radicals you drew in step 2, draw all possible resonance structures. Remember that the unpaired electron can be delocalized across the allylic system. Use curved arrows to show the movement of electrons. In 3-methylcyclohexene, the allylic radical formed by abstracting a hydrogen from the methyl group will have two resonance structures. The allylic radicals formed by abstracting a hydrogen from the other two allylic positions on the ring will also have two resonance structures.
Assess the Stability of Each Allylic Radical: Compare the stabilities of all the allylic radicals (and their resonance structures). Factors that influence radical stability include:
- Number of alkyl substituents: More substituted radicals are generally more stable (tertiary > secondary > primary).
- Resonance: More resonance structures generally indicate greater stability.
Other factors: While less common, other factors like allylic strain can also influence the stability of the radical.
In the case of 3-methylcyclohexene, one of the allylic radicals is tertiary and thus more stable than the two secondary allylic radicals.
Brominate Each Resonance Structure: For each resonance structure of each allylic radical, attach a bromine atom at the position where the unpaired electron is located. This gives you all the possible allylic bromide products. With 3-methylcyclohexene, the more stable, tertiary radical will lead to the major product, while the less stable, secondary radicals will lead to minor products.
Determine Major and Minor Products: The major product(s) will be derived from the most stable allylic radical(s). If two resonance structures of the same radical lead to different products, both may be formed, possibly in unequal amounts. The less stable allylic radicals will lead to minor products. In 3-methylcyclohexene, the major product will be the tertiary allylic bromide, and the minor products will be the secondary allylic bromides.