6.2.4 Carbon–carbon bond formation Nucleophilic …

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6.2.4 Carbon–carbon bond formation

Various reactions are used the involve formation of a C–C bond in synthesis to increase the
length of a carbon chain

Nucleophilic substitution of haloalkanes with cyanide ions

Change in functional group: haloalkane  nitrile
Reagent: KCN dissolved in ethanol/water mixture
Conditions: Heating under reflux
Mechanism: Nucleophilic substitution
Type of reagent: Nucleophile, :CN-

H

C

H

H

C

H

H

C

H

H

Br

+ :CN- 

H

CN

+ Br-

1-bromopropane

butanenitrile

H

C

H

H

C

H

H

C

H

δ+

CH3

δ-

Br

-NC:

H

C

H

CH3

CN

+ :Br –

H

C

H

This reaction increases the length
of the carbon chain (which is
reflected in the name) In the
above example butanenitrile
includes the C in the nitrile group.

Naming Nitriles

Nitrile groups have to be at the end of a chain. Start
numbering the chain from the C in the CN

CH3CH2CN : propanenitrile

CH3

CH CH2 C N

3-methylbutanenitrile

CH3

Note the naming: butanenitrile and not
butannitrile.

Addition of hydrogen cyanide to carbonyls to form hydroxynitriles

Reaction: carbonyl  hydroxynitrile
Reagent: sodium cyanide (NaCN) and dilute sulfuric
acid.
Conditions: Room temperature and pressure
Mechanism: nucleophilic addition

The NaCN supplies the
nucleophilic CN-
ions. The
H2SO4 acid supplies H+ ions
needed in second step of the
mechanism

R

C

NC

OH

H
hydroxynitrile

CH3COCH3+ HCN  CH3C(OH)(CN)CH3

2-hydroxy-2-methylpropanenitrile

NC C

OH

CH3

CH3

When naming hydroxy
nitriles the CN becomes
part of the main chain

CH3CHO + HCN  CH3CH(OH)CN

2-hydroxypropanenitrile

NC C

OH

We could use HCN for this reaction but it
is a toxic gas that is difficult to contain.
The KCN/NaCN are still, however, toxic,
because of the cyanide ion.

CH3

H

H+

Nucleophilic Addition Mechanism

H+ from sulfuric acid

δ-

O

δ+

C

CH3

CH3

:CN-

–
:

O

C

CN

CH3

CH3

CH3

C

CH3

O H

CN

N Goalby chemrevise.org

1

The nitriles made in the previous two reactions can then be converted into other functional groups by the
following reactions

Preparing amines from nitriles

Reduce nitrile to amine by using LiAlH4 in ether or by reducing with H2 using a Ni catalyst

CH3CH2CN + 4[H] CH3CH2CH2NH2

This is a reduction reaction

Preparing carboxylic acids from nitriles

Hydrolysing nitriles by reacting them with strong acids will produce a carboxylic acid

CH3CH2CN + H+ + 2H2O  CH3CH2CO2H +NH4
+

This is a hydrolysis reaction

C-C bonds can be added to aromatic compounds through the Friedal Craft’s Reactions met in 6.1.1
aromatic compounds

Any chloroalkane can be used RCl where
R is any alkyl group Eg –CH3 , -C2H5.
The electrophile is the R+.

Friedel Crafts Alkylation

Change in functional group: benzene  alkylbenzene
Reagents: chloroalkane in the presence of anhydrous
aluminium chloride catalyst
Conditions: heat under reflux
Mechanism: Electrophilic substitution

Formation of the electrophile.
AlCl3 + CH3CH2Cl  CH3CH2

+ AlCl4

–

Overall Equation for reaction

CH3CH2

–
+ AlCl4

+

Friedel Crafts Acylation

Change in functional group: benzene  phenyl ketone
Reagents: acyl chloride in the presence of anhydrous
aluminium chloride catalyst
Conditions: heat under reflux (50OC)
Mechanism: Electrophilic substitution

Equation for Formation of the electrophile.
AlCl3 + CH3COCl  CH3CO+ AlCl4
–

Overall Equation for reaction

–
CH3CO+ AlCl4

+

CH2CH3

+ AlCl3 + HCl

ethylbenzene

Any acyl chloride can be used RCOCl where
R is any alkyl group e.g. –CH3 , -C2H5. The
electrophile is the RCO+.

O

C

CH3

+ AlCl3 + HCl

phenylethanone

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These are important
reactions in organic
synthesis because they
introduce a reactive
functional group on to the
benzene ring

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