The main difference between allylic and benzylic halides is that allylic halide is a halogenated compound where the halogen atom is attached to a carbon atom that is adjacent to a carbon-carbon double bond, whereas benzylic halide is a halogenated compound where the halogen atom is attached to a carbon atom that is directly bonded to a benzene ring.
Both allylic halides and benzylic halides are organic compounds. They contain both carbon and hydrogen atoms in them.
Key Areas Covered
1. What are Allylic Halides
– Definition, Structure, Features
2. What are Benzylic Halides
– Definition, Structure, Features
3. Similarities Between Allylic and Benzylic Halides
– Outline of Common Features
4. Difference Between Allylic and Benzylic Halides
– Comparison of Key Differences
Key Terms
Allylic Halides, Benzylic Halides
What are Allylic Halides
Allylic halides are a group of organic compounds that possess a halogen atom attached to a carbon atom adjacent to a carbon-carbon double bond. Allylic halides have resonance stability. The presence of a conjugated double bond system adjacent to the halogen atom allows for resonance delocalization of electrons. This resonance stabilization contributes to the enhanced stability of the allylic halides compared to simple alkyl halides. The resonance stabilization of allylic halides is also responsible for their distinct reactivity. The carbon-carbon double bond adjacent to the halogen atom can undergo various transformations making allylic halides valuable intermediates in organic synthesis.
One common reaction involving allylic halides is the allylic substitution. Here, a nucleophile replaces the halogen atom at the allylic position. The nucleophile can be anionic or neutral, and the reaction can proceed through either an SN1 or SN2 mechanism, depending on the specific conditions and substrate. Allylic substitution reactions offer a versatile strategy for the synthesis of functionalized molecules with diverse applications in many industries.
Allylic rearrangements involve the migration of a group from one carbon atom to the adjacent allylic carbon. This rearrangement can occur via various mechanisms such as 1,2 – alkyl shift, 1,2- hydride shift, or 1,2-carbon shift. Alkyl rearrangements are valuable synthetic tools for generating complex molecular structures and have been widely employed in natural product synthesis and total synthesis of complex organic compounds.
Allylic halides also can undergo oxidation reactions, which involve the conversion of the allylic carbon to an allylic alcohol or carbonyl group. This transformation is typically achieved using reagents such as selenium dioxide, potassium permanganate, or palladium catalysts. Allylic oxidation provides a powerful method for introducing oxygen functionality into organic molecules and has significant implications in the synthesis of natural products and bioactive compounds.
What are Benzylic Halides
Benzylic halides are organic compounds in which a halogen atom is attached to a carbon atom adjacent to a benzene ring. Benzylic halides have unique reactivity due to the stability and resonance effects of the benzene ring. One feature of benzylic halides is their enhanced reactivity towards nucleophilic substitution reactions. The benzene ring, through its resonance effects, stabilizes the positive charge that develops during the transition state of the substitution reaction. This stabilization lowers the energy barrier for the reaction, making it more favorable and faster compared to similar reactions with non-benzylic substrates. As a result, benzylic halides undergo nucleophilic substitution reactions more readily.
SN1 and SN2 are the two main types of nucleophilic substitution reactions benzylic halides can undergo. In SN1 reactions, the benzylic halide undergoes a stepwise mechanism involving the formation of a carbocation intermediate, which is stabilized by the resonance effects of the benzene ring. The intermediate can then react with a nucleophile to form the substitution product. SN2 reactions, on the other hand, proceed through a concentrated mechanism where the nucleophile attacks the benzylic carbon while the leaving group departs. The benzene group stabilizes the transition state, facilitating the reaction. These also undergo oxidation reactions.
Similarities Between Allylic and Benzylic Halides
- Allylic and benzylic halides have reactive carbon atoms adjacent to the halogen.
- Both are organic compounds.
Difference Between Allylic and Benzylic Halides
Definition
Allylic halide is a halogenated compound where the halogen atom is attached to a carbon atom that is adjacent to a carbon-carbon double bond, whereas benzylic halide is a halogenated compound where the halogen atom is attached to a carbon atom that is directly bonded to a benzene ring.
Substitution Reactions
Due to the resonance stabilization, allylic halides tend to undergo nucleophilic substitution reactions more readily than benzylic halides.
Synthetic Applications
Allylic halides are commonly employed in the allylic substitution reactions and allylation processes. Benzylic halides are often used for benzylic substitution reactions as well as in the synthesis of various aromatic compounds and natural products.
Radical Reactions
The radical reactivity is more pronounced in allylic halides than in benzylic halides.
Conclusion
Both allylic halides and benzylic halides are organic compounds. They contain both carbon and hydrogen atoms in them. The main difference between allylic and benzylic halides is that allylic halide is a halogenated compound where the halogen atom is attached to a carbon atom that is adjacent to a carbon-carbon double bond, whereas benzylic halide is a halogenated compound where the halogen atom is attached to a carbon atom that is directly bonded to a benzene ring.
Reference:
1. “Benzyl Halide – Overview.” Science Direct.
Image Courtesy:
1. “Allyl-chloride-3D-balls” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
2. “Benzal-chloride-3D-balls” By Jynto (talk) – created with Discovery Studio Visualizer. (CC0) via Commons Wikimedia
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