4. The Unimolecular (SN1) Reaction: 2 steps
(1) R X R + X
R-X dissociates, forming a carbocation intermediate, R+, and the
leaving group, X-.
Slow step = rate determining step (r.d.s.).
Entails only one species, R-X.
(2) R + Nu R-Nu
Carbocation and nucleophile combine.
Fast step.
Overall reaction: R-X + Nu- R-Nu + X-.
Rate of the reaction = k[R-X]
5. SN1 Reaction
R R
rate determining s tep Nu
R C X C+
R R R
R R
R C Nu + Nu C R
R R
x
Carbocation stability is important factor: 3º > 2º> 1º.
Polar, protic solvents such as ethanol support carbocation formation
(stabilize intermediate).
Aprotic solvents such as acetone do NOT support carbocation
formation.
Weak nucleophile is OK.
6. The Bimolecular (SN2) Reaction: 1 step
δ+ δ-
(1) Nu + R-X Nu R X
Nucleophile combines with alkyl halide to form pentacoordinate
transition state.
Slow step = rate determining step (r.d.s.).
Entails two species, R-X and Nu-.
δ+ δ-
(2) Nu R X Nu-R + X
Transition state dissociates to form product, Nu-R, and halide ion, X-.
Rate of the reaction = k[R-X][Nu-]
7. SN2 reaction
H H H
Nu : C X Nu C X Nu C
H H
H H H H
Steric factors are important: 1º > 2º > 3º.x
Polar, aprotic solvents such as acetone increase rate of SN2.
Aprotic solvent destabilizes nucleophile (increases its reactivity).
Strong nucleophile is necessary.
The nature of the leaving group is important in both
SN1 and SN2 reactions: I > Br > Cl.
8. Experiment: Structural Effects (Part A only)
Possible Products 0.2 mL (4 drops) Possible Products
OEt Cl I
OEt Br I
2 mL 2 mL
95% EtOH NaI
+ AgCl AgNO3 acetone + NaCl
OEt Cl I
SN1 SN2
OEt Cl I
OEt Br I
9. Experiment
- You will have 2 series of 5 test tubes with 0.2 mL (4 drops) of each reactant.
- You will monitor time for cloudiness.
- If NaI/acetones do not react after 30 min, heat to 50 °C in water bath (loose
corks!!!)
- If 95% EtOH/AgNO3 tubes do not react after 30 min, also heat to 50 °C.
Students must include statements in their lab reports about the following:
1. Alkyl halide structure (1º, 2º, 3º) and SN2
2. Alkyl halide structure (1º, 2º, 3º) and SN1
3. The leaving group (Br vs. Cl)
4. Aryl halide reactivity
5. Temperature and rate
All statements must be based on observations.
Grading: 50% on the written report and
50% on observations, results, discussion and conclusions.
