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Chemistry in real life: epoxy resins
prepolymer hardener
7. Substitution/elemination
2 2
Factors affecting the SN2 reaction
1. the nucleophile 2. the structure of the substrate 3. the nature of the leaving group (X) 4. the solvent
Nu X solvent Nu XR R
7. Substitution/elemination
3
2. The structure of the substrate -steric hindrance in practice
H3C Br Br Br BrBr
methyl primary secondary tertiary "neopentyl"
6 x 103 30 1 5 x 10-5 about 10-5relative rates:
Nu R Br acetone R Nu Br
The greater the steric hindrance, the slower the SN2 reaction. Remember that the Nu- must be able to approach the σ* orbital of the electrophile!
7. Substitution/elemination
4
3. The leaving group (X) -the more stable, the better
The weaker the base, the better leaving group! I– > Br– > Cl– > F–
• converting a poor leaving group into a good:
instead:
Nu OH Nu HOSN2
poor leaving group;strong base!
R R
OH +
SCl
OO
Et3N
TsCl
R
SO
OO
RCH2OTs
R NuNu
R
SO
OO
TsO–
[pKa(TsOH) -2.8 (H2O)]
Nu X Nu XSN2
R R
7. Substitution/elemination
5
4. The Solvent -not an innocent bystander!
(i) the solvent will affect the activity of the nucleophile: already discussed
(ii) for a given reaction, the solvent can influence the reaction rate:
7. Substitution/elemination
6
Chemistry in action: epoxy resins (SN2)
prepolymer hardener
7. Substitution/elemination
7 7
Chemistry in action: epoxy resins (SN2)
(i) preparation of the prepolymer
HO O
bisphenol A
HOH+
acid-basereaction
-H2O
HO O
ClO
+
SN2
HO OO
Cl
SN2 HO OO
O O OOH
OO O
n
repeatprevious steps
prepolymer
7. Substitution/elemination
8
O O OOH
OO O
nprepolymer
H2NHN
NH2
hardener
O O OOH
O
nan epoxy resin
RHNOH
NHROH
RO
H2N R'+
SN2
RN
O
R'
H H
proton transfer
RNH
OH
R'
Chemistry in action: epoxy resins (SN2)
(ii) mixing of the prepolymer and the hardener
7. Substitution/elemination
9
E2 eliminations
How to explain:
• the faster reaction of diastereomer A?
• the formation of two regioisomers in the first reaction but only one in the second?
Different diastereomeric starting materials can behave differently:
7. Substitution/elemination
10
SN2 versus E2
7. Substitution/elemination
NuH
XR'
RSN2
E2
SN2
E2
HNu
R'R
RR'
X
+
+
X Nu H+ +
Some rules of thumb:
• methyl halides cannot eliminate → only SN2
• increasing branching favours E2
• strongly basic and hindered nucleophiles favours E2
• good nucleophiles prefers SN2, unless for tertiary substrates
• high temperatures favour E2
11
Chemistry in action: Jaguar cars and SN1 reactions
7. Substitution/elemination
12 12 12
Factors affecting the SN1 reaction
1. the nucleophile 2. the structure of the substrate 3. the nature of the leaving group (X) 4. the solvent
Nu–H X solventR
RR
RRR
Nu–H X NuR
RR
HX Nu
R
RR
H–X
7. Substitution/elemination
13 13
2. The structure of the substrate
In practice, only tertiary, and sometimes secondary, electrophiles participate!
7. Substitution/elemination
14
Chemistry in action: Jaguar cars and SN1 reactions
isobutylene,2-methy-1-propene
H
MeOHMeOH
OMeH
-H OMe
methyl tert-butyl ether(MTBE)
CO2Memethyl methacrylate
polymerizationMeO2C CO2Me
n
poly(methyl methacrylate)[plexiglas™]
7. Substitution/elemination