Abstract
The allylic bromination of alkenes using N-bromosuccinimide (NBS) is a useful and selective substitution reaction, in which bromine radicals abstract hydrogen from the alkene to form a resonance-stabilized radical intermediate. However, allylic rearrangement can be a consequence of the intermediate's two resonance structures. Some literature sources imply that NBS bromination gives only one major product from asymmetrical alkenes, e.g., 2-hexene and 2-heptene, even when they should form equally stable but non-equivalent resonance contributors. In this research, bromination of trans-2-hexene was performed using cyclohexane as the solvent, and the products were analyzed using GC-MS to determine if only one product, 4-bromo-2-hexene, is formed or if allylic rearrangement also occurs to give 2-bromo-3-hexene. Additionally, 1-hexene and 3-hexene were reacted with NBS to provide support for our peak identifications in the case of trans-2-hexene. The experimental data aligns with resonance theory with two major products, 4-bromo-2-hexene (50%) and 2-bromo-3-hexene (32%) relative to smaller amounts of six other monobrominated products. As expected, 1-hexene yielded 1-bromo-2-hexene (56%) and 3-bromo-1-hexene (10%), and 3-hexene yielded both 4-bromo-2-hexene (58%) and 2-bromo-3-hexene (41%). Both reactions also demonstrate the occurrence of allylic rearrangement. All of the reactions also yielded a small amount of bromocyclohexane. The data provides strong evidence that NBS bromination of non-symmetrical alkenes always yields two major products, consistent with resonance theory, and contrary to some literature references.
College
College of Science & Engineering
Department
Chemistry
Campus
Winona
First Advisor/Mentor
Thomas Nalli
Location
Ballroom - Kryzsko Commons
Start Date
4-18-2024 2:00 PM
End Date
4-18-2024 3:00 PM
Presentation Type
Poster Session
Format of Presentation or Performance
In-Person
Session
2b=2pm-3pm
Poster Number
16
Included in
Allylic Rearrangement in NBS Bromination Reactions
Ballroom - Kryzsko Commons
The allylic bromination of alkenes using N-bromosuccinimide (NBS) is a useful and selective substitution reaction, in which bromine radicals abstract hydrogen from the alkene to form a resonance-stabilized radical intermediate. However, allylic rearrangement can be a consequence of the intermediate's two resonance structures. Some literature sources imply that NBS bromination gives only one major product from asymmetrical alkenes, e.g., 2-hexene and 2-heptene, even when they should form equally stable but non-equivalent resonance contributors. In this research, bromination of trans-2-hexene was performed using cyclohexane as the solvent, and the products were analyzed using GC-MS to determine if only one product, 4-bromo-2-hexene, is formed or if allylic rearrangement also occurs to give 2-bromo-3-hexene. Additionally, 1-hexene and 3-hexene were reacted with NBS to provide support for our peak identifications in the case of trans-2-hexene. The experimental data aligns with resonance theory with two major products, 4-bromo-2-hexene (50%) and 2-bromo-3-hexene (32%) relative to smaller amounts of six other monobrominated products. As expected, 1-hexene yielded 1-bromo-2-hexene (56%) and 3-bromo-1-hexene (10%), and 3-hexene yielded both 4-bromo-2-hexene (58%) and 2-bromo-3-hexene (41%). Both reactions also demonstrate the occurrence of allylic rearrangement. All of the reactions also yielded a small amount of bromocyclohexane. The data provides strong evidence that NBS bromination of non-symmetrical alkenes always yields two major products, consistent with resonance theory, and contrary to some literature references.
