1

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