Download Unit 4 Chemical Kinetics and Chemical Equilibrium

SN1 vs. SN2 vs. E1 vs. E2

Factors affecting the type of reaction an alkyl
halide undergoes include:

Type of alkyl halide
o
o
o
 methyl, 1 , 2 , 3 , allylic or benzylic

Strength of nucleophile or base
SN1 vs. SN2 vs. E1 vs. E2

Guidelines for Predicting Products of
Substitution and Elimination Reactions:
 The strength of the nucleophile or base
determines the order of the reaction.
 SN2 and E2 reactions require strong
nucleophiles (or bases)

SN1 and E1 reactions generally use weak
nucleophiles (or bases)
SN1 vs. SN2 vs. E1 vs. E2

Nucleophilicity (nucleophile strength):
 a measure of the rate at which a nucleophile
attacks an electrophilic carbon of a
standard substrate

A species with a negative charge is a stronger
nucleophile than a similar neutral species:
 nucleophilicity of base > conjugate acid
- > CH OH
 CH3O
3
SN1 vs. SN2 vs. E1 vs. E2

Nucleophilicity decreases from left to right in
the periodic table
 look at the element with lone pair of e
 more electronegative elements hold on to
electrons more tightly

OH-
>
F-
SN1 vs. SN2 vs. E1 vs. E2

Nucleophilicity increases from top to bottom
 size increases
 electrons held more loosely
 polarizability increases
 bond can begin forming when
atoms are farther apart
- >
 I
Br-

Polarizable: having electrons that can move
more freely toward a positive charge
SN1 vs. SN2 vs. E1 vs. E2
SN1 vs. SN2 vs. E1 vs. E2
Example: Identify the stronger nucleophile in
each pair. Explain why.
NH3 or NH2H2O or NH3
HS- or Cl-
PH3 or NH3
SN1 vs. SN2 vs. E1 vs. E2

Base Strength:
 Weaker acids produce stronger conjugate
bases.
CH3CH2OH
weak acid

CH3CH2O-
strong base
strong nucleophile
Strong acids produce weak conjugate
bases.
HBr
BrStrong acid
Weak base
good nucleophile
SN1 vs. SN2 vs. E1 vs. E2

The presence of bulky groups on the CH
nucleophile makes it less nucleophilic but3 a
CH3 C O
stronger base.
 hinders approach to the electrophilic
CH3
carbon
CH3
CH3CH2O CH3 C O
CH3
t-butoxide ion
hindered
strong base
weak nucleophile
ethoxide ion
unhindered
weaker base
stronger nucleophile
SN1 vs. SN2 vs. E1 vs. E2
Example: Identify the stronger base in each
pair.
OH- or H2O
CH3OH or CH3O(CH3)3CO- or CH3OWater and alcohols are amphoteric. They can
serve as either an acid or a base. Water and
alcohols are weak acids and weak bases.
SN1 vs. SN2 vs. E1 vs. E2

Guidelines for Predicting Products of
Substitution and Elimination Reactions:
 The type of alkyl halide is important in
determining the type of reaction:
 Methyl halide: SN2 exclusively
o
 1 halide:
 SN2 with a strong nucleophile (favored)

E2 with strong base (sometimes)

SN1 or E1 are sometimes observed at
high temperatures if rearrangement
produces more stable carbonium ion
SN1 vs. SN2 vs. E1 vs. E2

Guidelines for Predicting Products of
Substitution and Elimination Reactions:
 The type of alkyl halide is important in
determining the type of reaction:
o
 3 Halides:
 E2 with a strong base

SN1 and E1 with weak nucleophile or
weak base
SN1 vs. SN2 vs. E1 vs. E2

Guidelines for Predicting Products of
Substitution and Elimination Reactions:
 The type of alkyl halide is important in
determining the type of reaction:
o
 2 Halides:
 Mixtures of products are common

Strong base/nucleophile gives SN2 and
E2

Weak base/nucleophile gives SN1 and
E1
SN1 vs. SN2 vs. E1 vs. E2

Guidelines for Predicting Products of
Substitution and Elimination Reactions:
 Strong bulky bases favor E2 reactions.
 t-butoxide ion

Good nucleophiles with limited basicity
favor SN2 reactions.
 Br
 I
CH
CH2OH
3 CH
CH
OH
3
2
SN1 vs. SN2 vs. E1 vs. E2
Example:
Predict
the product(s) and
CH
CH
CH
CH2OCH
OCH
2
3 3
33
mechanism of the following reaction. If more
CH2 CH
CH2
CH
than one
product
or mechanism is possible,
2
2
explain which is/are most likely.
CH3CH2OCH
CH2OH
CH3CH2Br CH
+ NaOCH
3
CH3CH2Br + NaOCH33
CH3CH2OCH3 + CH2 CH2
CH3OH
major
minor
CH3CH2OCH3
CH3CH2Br + N
NaOCH3 = strong base and strong nucleophile
CH2 CH2
2nd order kinetics CH3CH
S N2
or +E2NaOCH3
Br
2
1o Halide favors SN2 over E2
SN1 vs. SN2 vs. E1 vs. E2
Example: Predict the product(s) and
CH3CH
CH2 If more
mechanism
of the
following
reaction.
NaOCH
CH
2
3
than one product
or mechanism is possible,
CH3CH2OH
explain which is/are most likely.
NaOCH
CH3 3
NaOCH
CH
CH3CHCH
22
3
CH3CHCH3
CH3CH
CH2OH
OH
CH
3 Br
2
Br
CH3CH
CH CH
CH2 2
CH
3
CH3CHCH3
+
NaOCH2CH3
OCH2CH3
CH3CH2OH
CH3CH CH2
NaOCH2CH3CH
=CH
strong
base/nucleophile
CH3CHCH
CHCH3 3
CH
CHCH
CHCH
3
33
33
CH3CHCH3
CH
CHCH
OCH
CH
3
3
OCH
CH
2nd order kinetics
S
2
or
E2
Br
Br
N 22 33
OCH2CH3
Br
Relative amounts of SN2 vs E2 depends on temperature.
SN1 vs. SN2 vs. E1 vs. E2
CH3OH
CH3OH
product(s) Dand
D
Example: Predict the
mechanism of the following reaction.CHIf more
3
CH3OH
CH
3
than one product or mechanism is possible,
CH3OH
Br
D explain
Br
which is/are most likely.
D
CH3
Br
CH3
CH
OH
CH
OH
3
CH
3 3

OCH3

Br
CH3
+
OCH3
major
CH3
CH3
major
+
CH2
CH2
trace
CH3
CH3
CH3OH = weak nucleophile
or weak base
CH3
CH
CH
OCH
CH
Br
3
2
2
3
CH3
CH
1st order
kinetics
SN1 or E1
3
OCH3
Br
Saytzeff’s orientation favored for elimination product.
C
O