page 3 Flashcards
Vinyl Halides:
The halogen atom (X) is bonded directly to a C=C double bond.
Example:
CH 2=CH−Cl (vinyl chloride).
Key Feature: The sp² hybridization of the carbon makes these compounds less reactive in nucleophilic substitution reactions.
Aryl Halides:
The halogen atom (
X) is bonded directly to a benzene ring.
Example:
C6H5−Cl(chlorobenzene).
Key Feature: Aryl halides are less reactive due to the delocalization of electrons in the benzene ring, which stabilizes the C-X bond.
Allylic Halides:
The halogen atom (
X) is bonded to a carbon atom that is adjacent to a C=C double bond.
Example:
CH2 =CH−CH2 −Br (allyl bromide).
Key Feature: The allylic position is stabilized by resonance, making these halides more reactive in substitution reactions.
Benzylic Halides:
The halogen atom (
X) is bonded to a carbon atom that is adjacent to a benzene ring.
Example:
C6H5−CH2 −Cl (benzyl chloride).
Key Feature: The benzylic position is stabilized by resonance with the benzene ring, enhancing reactivity in substitution and elimination reactions.
What distinguishes organic halides like vinyl halides, aryl halides, allylic halides, and benzylic halides from regular alkyl halides?
Answer: Their halogen atom (
X) is bonded to specific carbon environments (double bonds, benzene rings, or adjacent carbons).
Which type of organic halide has
X bonded directly to a C=C double bond?
Answer: Vinyl halides.
Why are vinyl halides less reactive in nucleophilic substitution reactions?
Answer: The carbon bonded to the halogen is sp² hybridized, and the double bond restricts attack by nucleophiles.
Give an example of a vinyl halide.
Answer:
CH 2=CH−Cl (vinyl chloride).
What makes aryl halides less reactive than alkyl halides?
Answer: The delocalized electron system of the benzene ring stabilizes the C-X bond.
Give an example of an aryl halide.
Answer:
C6H5−Cl
(chlorobenzene).
Where is the halogen bonded in an allylic halide?
Answer: To a carbon atom adjacent to a C=C double bond.
Why are allylic halides more reactive in substitution reactions?
Answer: The allylic position is stabilized by resonance.
Give an example of an allylic halide.
Answer:
CH2=CH−CH2−Br (allyl bromide).
What makes benzylic halides highly reactive in substitution and elimination reactions?
Answer: The benzylic position is stabilized by resonance with the benzene ring.
Where is the halogen bonded in a benzylic halide?
Answer: To a carbon atom adjacent to a benzene ring.
Give an example of a benzylic halide
.
Answer:
C6H5−CH2 −Cl (benzyl chloride).
summary table
Reactivity Based on Halogen (Leaving Group Ability):
The halogen’s ability to act as a leaving group significantly affects reactivity. Better leaving groups are weaker bases that can stabilize the negative charge.
Order of Reactivity (Best to Worst):
R−I>R−Br>R−Cl>R−F
Reason:
Iodide (I−) is a large, weakly basic ion and highly stable after leaving, making it the best leaving group.
Fluoride (F−) is a strong base and poorly stabilized, making it the least reactive.
Reactivity Based on Carbon Type (Alkyl Halides):
SN1 (Carbocation Formation):
Tertiary(3°)
>
Secondary(2°)
>
Primary(1°)
>
MethylHalide
Tertiary(3°)>Secondary(2°)>Primary(1°)>MethylHalide
Reason:
The reaction depends on the formation of a stable carbocation intermediate.
Tertiary carbocations are most stable due to hyperconjugation and inductive effects from surrounding alkyl groups.
SN2 (Backside Attack):
MethylHalide
>
Primary(1°)
>
Secondary(2°)
>
Tertiary(3°)
MethylHalide>Primary(1°)>Secondary(2°)>Tertiary(3°)
Reason:
SN2 reactions involve a one-step concerted mechanism where the nucleophile attacks the carbon directly.
Methyl and primary carbons experience minimal steric hindrance, while tertiary carbons are too crowded to allow backside attack.
Reactivity Based on Special Types of Organic Halides:
Benzylic Halides:
Highly reactive in both SN1and SN2 due to resonance stabilization of the carbocation or transition state.
Allylic Halides:
Highly reactive due to resonance stabilization of the carbocation or transition state.
Aryl Halides:
Very unreactive in nucleophilic substitution because the C-X bond is part of the benzene’s delocalized electron system, making it very strong.
Vinyl Halides:
Very unreactive because the C-X bond is attached to an sp²-hybridized carbon, and the double bond restricts nucleophilic attack.
Key Takeaways:
SN1 favors:
Tertiary, benzylic, and allylic halides due to carbocation stability.
Polar protic solvents that stabilize intermediates.
SN2 favors:
Methyl, primary, and secondary halides with minimal steric hindrance.
Strong nucleophiles and polar aprotic solvents that don’t hinder the nucleophile.
