Chemistry - Organic Chemistry Mechanisms

ORGANIC CHEMISTRY MECHANISM SUMMARY FREE RADICAL SUBSTITUTION (FRS) Exemplified by the substitution of methane by chlorine

Step 1: Initiation Cl—Cl  uv   Cl• + Cl•

Step 2: Propagation a) Cl• + CH4  CH3• + HCl b) CH3• + Cl2  CH3Cl + Cl• Then (a), (b), (a), (b)…

Step 3: Termination a) Cl• + Cl•  Cl2 b) CH3• + Cl•  CH3Cl c) CH3• + CH3•  CH3CH3

ELECTROPHILIC ADDITION Exemplified by the addition of HBr to an alkene

H

H C Step 1

+

C

H

H

 H

Br 

H

H

H

H

C H

+

H

-

carbocation

H H

+

C

Br

H

H

C

+

C +

intermediate

H Step 2

s lo w

:Br

-

fa s t

H

C C H H Br

H

ELECTROPHILIC AROMATIC SUBSTITUTION Exemplified by the addition of NO2 to benzene

Step 1 Generation of electrophile

HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4-

H Step 2 Electrophilic attack

NO2

s lo w

+

+

+ benzoniu

m ion

H

Step 3 Loss of proton

HSO4

NO2

-

+

NO2

NO2 fa s t

+

+

H2SO 4

NUCLEOPHILIC SUBSTITUTION – S 2 – ONE STEP N

R3

-

 C X 

Nu : R1

‡

R3  Nu R1

R2

C

 X R2

pentavalent transition state

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R3 Nu

+

C R2

R1

-

X

NUCLEOPHILIC SUBSTITUTION – S 1 – TWO STEP N

-

Nu :

R3 X 

s lo w

C 

R3

R3 fa s t

+

C

R1 R2

R1 R2

Nu

-

C

R3

+

C

R 1 R2

: Nu

Nu

R2 R1

carbocation intermediate (planar)

two products racemic mixture

NUCLEOPHILIC ADDITION Exemplified by the addition of CN- to ethanal

Step 1 Generatio n of nucleophi le

Step 2 Nucleophi lic attack

KCN → K+ + CN-

CN

H3C

 O

 C

+

:CN

s lo w

-

H3C

H

C

Protonati on

H3C

-

H CN

Step 3

O

C

CN O

-

+

H

H

C

N

fa s t

H3C

C

OH

+

CN

H Cyanohydrin

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