Organic Chemistry Reaction Mechanisms

Essential for both SN1 and SN2 reactions, but their role differs.
SN1: Good leaving group is crucial as the only change in the slow step is C-X bond cleavage.
SN2: A strong nucleophile can compensate for a poor leaving group because the nucleophile binds as the leaving group departs.
Leaving group ability:
- The relative rates of C-X cleavage are: I- > Br- > Cl- > F-.
- HO- and RO- are poor leaving groups but can be improved under acidic conditions via protonation.

SN1: Nucleophile is not involved in the slow step; the carbocation readily reacts with any nucleophile.
SN2: Requires a good nucleophile.
Nucleophilicity: Measure of affinity towards electrophilic $C {+}$ .
- HO- is a good nucleophile, while H2O is weak.
- Bulky bases like $(CH<em>3)</em>3CO^-$ are strong bases but poor nucleophiles due to steric hindrance.

Biological medium is generally H2O (polar solvent, able to form H-bonds).
SN1: Favored by polar solvents, especially those that H-bond, stabilizing the transition state.
SN2: Solvent can influence nucleophilicity through H-bonding.

Common mechanisms: E1 and E2, parallel to SN1 and SN2.
Mechanism is determined by base strength: strong base favors E2, weak base favors E1.

Rate law: rate = k [C-Cl].
Two-step process:
- Step 1: Same as SN1, forming a carbocation (rate-determining step).
- Step 2: A base (e.g., H2O) removes a proton to form a C=C double bond.
Favored by:
- Weak base
- H-bonding solvents
- 3° carbons (same as SN1)
E1 and SN1 often occur together.
The ratio of E1 to SN1 is controlled by the relative rates of the fast steps after the carbocation is formed.

Rate law: rate = k [CH3CH2Br] [HO-].
Single-step, concerted reaction involving halide and base.
Requires a strong base (e.g., HO-).
Possible for 1°, 2°, and 3° compounds.
SN2 vs E2:
- SN2: Requires backside attack, hindered by crowding; rate order is 1° > 2° > 3°.
- E2: Easily accessible H+ is removed; little difference in ease of H+ removal for 1°, 2°, and 3° carbons.