page 15 Flashcards
Substitution Reactions:
Mechanism: A nucleophile (
Nu−
) attacks the carbon atom bonded to the halogen (X), replacing it.
General Reaction:
R−X+Nu−→R−Nu+X−
Examples:
Nucleophiles:
OH−, CN−, NH3.
Products: Alcohols, nitriles, or amines.
Reactivity:
Favors primary and secondary alkyl halides in
SN2 reactions.
Tertiary alkyl halides favor
SN1 due to carbocation stability.
Elimination Reactions:
Mechanism: A Bronsted-Lowry base (:B) removes a proton (H+) from a carbon adjacent to the one bonded to the halogen, forming a double bond (alkene).
General Reaction:
R−CH2−CHX−R′+:B→R−CH-CH−R′+H−B+ +X−
Examples:
Bases:
OH− , RO−.
Product: Alkene (via elimination of HX).
Reactivity:
Favors secondary and tertiary alkyl halides due to the stability of the transition state.
Comparison Between Substitution and Elimination
Reagents:
Substitution uses nucleophiles.
Elimination uses bases.
Products:
Substitution: Forms a new compound where
X
is replaced.
Elimination: Forms an alkene by removing
HX.
Reaction Conditions:
Substitution: Low temperatures.
Elimination: High temperatures favor elimination.
What happens to the halogen in a substitution reaction?
Answer: It is replaced by a nucleophile.
What type of product is formed in a substitution reaction?
Answer: A new compound where the halogen is replaced by a nucleophile (e.g., alcohols, nitriles).
What type of bond is formed during elimination reactions?
Answer: A double bond (alkene).
What are the byproducts of elimination reactions?
Answer: A halide ion (
X− ) and a proton (
H+).
What determines whether a reaction undergoes substitution or elimination?
Answer: Reaction conditions, the nature of the alkyl halide, and the strength of the base/nucleophile.
Which reaction is favored at high temperatures?
Answer: Elimination reactions.
