1-bromo-2-butene

Definition

1-bromo-2-butene is an organic compound with the formula CH3CH=CHCH2Br. It is a halogenated alkene that is important in the context of understanding the stability of the allyl radical and the electrophilic additions to conjugated dienes, specifically the formation of allylic carbocations.

5 Must Know Facts For Your Next Test

1. The presence of the bromine atom at the 1-position of the 2-butene molecule makes the carbon at that position more susceptible to radical or carbocation formation.
2. The allyl radical formed from 1-bromo-2-butene is stabilized through resonance, with the unpaired electron being delocalized across the three-carbon system.
3. During electrophilic additions to the conjugated diene system of 1-bromo-2-butene, an allylic carbocation intermediate can form, which is also stabilized through resonance.
4. The stability of the allyl radical and the allylic carbocation intermediates influences the reactivity and product distribution in reactions involving 1-bromo-2-butene.
5. The bromine atom in 1-bromo-2-butene can undergo substitution or elimination reactions, depending on the reaction conditions and the presence of other nucleophiles or bases.

Review Questions

• Explain how the presence of the bromine atom in 1-bromo-2-butene affects the stability of the allyl radical formed during radical reactions.
  • The bromine atom in the 1-position of 1-bromo-2-butene makes the adjacent carbon more susceptible to radical formation. The resulting allyl radical is stabilized through resonance delocalization of the unpaired electron across the three-carbon system. This increased stability of the allyl radical, compared to a non-halogenated alkene, influences the reactivity and product distribution in radical reactions involving 1-bromo-2-butene.
• Describe the role of the conjugated diene system in 1-bromo-2-butene during electrophilic addition reactions, and how the formation of an allylic carbocation intermediate affects the reaction outcome.
  • The conjugated diene system in 1-bromo-2-butene allows for the formation of a stabilized allylic carbocation intermediate during electrophilic addition reactions. The positive charge in the allylic carbocation is delocalized through resonance, making it a more stable and reactive species compared to a non-conjugated alkene. The stability of this allylic carbocation intermediate influences the regio- and stereochemistry of the final addition products, as the reaction will tend to proceed through the most stable carbocation intermediate.
• Analyze how the reactivity of 1-bromo-2-butene can be influenced by both the presence of the bromine atom and the conjugated diene system, and discuss the implications for its use in organic synthesis.
  • The dual reactivity of 1-bromo-2-butene, stemming from both the bromine atom and the conjugated diene system, makes it a versatile and valuable building block in organic synthesis. The bromine atom can undergo substitution or elimination reactions, while the conjugated diene system allows for the formation of stabilized allylic carbocation intermediates during electrophilic additions. The ability to selectively control and exploit these different reactivity patterns makes 1-bromo-2-butene a useful reagent for the synthesis of a wide range of organic compounds, with the potential to access diverse product structures and stereochemistries.

Related terms

Allyl Radical:

The allyl radical is a resonance-stabilized radical species that features a carbon atom with an unpaired electron flanked by two carbon-carbon double bonds.

Conjugated Diene:

A conjugated diene is an organic compound with two carbon-carbon double bonds separated by a single carbon-carbon bond, resulting in a system of alternating double and single bonds.

Allylic Carbocation:

An allylic carbocation is a positively charged intermediate that forms during electrophilic additions to conjugated dienes, where the positive charge is stabilized by resonance with the adjacent double bond.