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Advanced Level Organic Chemistry: The chlorination/bromination of carboxylic acids

Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 6.6 The chlorination and bromination of carboxylic acids and use of the products in organic synthesis

Sub-index for this page

6.6.1 The chlorination/bromination of aliphatic carboxylic acids

6.6.2 The chlorination of aromatic carboxylic acids - substitution in the benzene ring

6.6.3 The use of chlorinated aliphatic carboxylic acids in organic synthesis reactions

INDEX of all carboxylic acids and derivatives notes

All Advanced A Level Organic Chemistry Notes

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6.6.1 The chlorination and bromination of aliphatic carboxylic acids

The alkyl hydrocarbon sections of aliphatic carboxylic acid can be chlorinated using chlorine and uv light.

You can prepare chlorinated aliphatic monocarboxylic acids by bubbling chlorine gas into the liquid acid in the presence of uv light or bright sunlight.

You can carry out the same reaction using liquid bromine to make bromocarboxylic acids.

For ethanoic acid, you can replace all three hydrogen atoms of the alkyl methyl group.

(i) CH3COOH  +  Cl2  ===>  ClCH2COOH  +  HCl   (chloroethanoic acid)

(ii) ClCH2COOH  +  Cl2  ===>  Cl2CHCOOH  +  HCl   (dichloroethanoic acid)

(iii) Cl2CHCOOH  +  Cl2  ===>  Cl3CCOOH  +  HCl  (trichloroethanoic acid)

 

The chlorination of an aliphatic carboxylic acid is a free radical chain reaction mechanism

(just like the chlorination of methane: Free radical chlorination/bromination to give halogenoalkanes

Initiation

Cl2  ===> 2Cl•

(note the denotes the unpaired electron on the free radical)

Propagation steps

 (i)  for the 1st substitution product

Cl•  +  CH3COOH  ===>  •CH2COOH   +  HCl

•CH2COOH   +  Cl2  ===> ClCH2COOH  +  Cl•

(ii) for the 2nd substitution product

ClCH2COOH  +  Cl•  ===>  •CHClCOOH  +  HCl

•CHClCOOH  +  Cl2  ===>  Cl2CHCOOH  +  Cl•

(iii) for the 3rd substitution product

Cl2CHCOOH  + Cl•  ===>  •CCl2COOH  +  HCl

•CCl2COOH  +  Cl2  ===>  Cl3CCOOH  +  Cl•

Termination

Any two of all the radicals mentioned above e.g.

Cl•  + Cl•  ===> Cl2

R•  +  Cl•  ===>  RCl

R•  + R'•  ===> R-R'

 

Chlorinating/brominating higher members of the aliphatic carboxylic acids e.g.

(i) Chlorinating propanoic acid with Cl2/uv gives mainly 2-chloropropanoic acid.

CH3CH2COOH  +  Cl2  ===>  H3CCHClCOOH  +  HCl 

The product will be a racemic mixture of the two R/S stereoisomers of  (c) doc b, (c) doc b because the middle carbon (known as the alpha carbon) is chiral - 4 different groups attached to it.

(ii) Bromination of propanoic acid ('propionic acid') with bromine and phosphorus(III) bromide gives 2-bromopropanoic acid.

CH3CH2COOH  +  Br2  ===>  H3CHBrCOOH  +  HBr

Structures as above, but Br instead of Cl and will also exhibit R/S stereoisomerism.


6.6.2 The chlorination of aromatic carboxylic acids - substitution in the benzene ring

The benzene ring is very stable and you cannot get controlled chlorination of it using chlorine and uv light.

However, you can replace the hydrogen atoms in the benzene ring using chlorine and a catalyst such as aluminium chloride (AlCl3) or iron(III) chloride (FeCl3).

(c) doc b  +  Cl2  ===>  2-chlorobenzoic acid structural formula  +  HCl

The principal product is 3-chlorobenzoic acid (with smaller amounts of 2-chlorobenzoic acid and 4-chlorobenzoic acid.

 

You can brominate i.e. substitute bromine into the benzene ring using bromine and a catalyst of aluminium bromide (AlBr3)  or iron(III) bromide (FeBr3).

 

Aromatic halogen compounds, where the halogen atom is directly attached to the ring, do not hydrolyse easily to a phenol group with alkali like ethanolic or aqueous potassium/sodium hydroxide.

You do not get the same Cδ+-Clδ- polarising effect because of the benzene ring,

The carbon - chlorine bond is less polar because of the interaction of chlorine's orbitals with the pi electron cloud of the benzene ring and this same 'resonance' interaction increases the strength of the C-Cl bond (increases the C-Cl bond order to > 1.0).


6.6.3 The use of chlorinated aliphatic carboxylic acids in organic synthesis

A monochlorinated aliphatic carboxylic acid has two functional groups.

The -COOH is dealt with on other pages, so we can concentrate on the other functional group, the chlorine atom -Cl.

This gives the molecule the chemistry of halogenoalkanes.

We'll illustrate the possibilities starting with ethanoic acid.

(a) Amine preparation - amino acid

Reacting chloroethanoic acid with ammonia

ClCH2COOH  +  2NH3  ===> H2NCH2COOH  +  NH4Cl

The product is aminoethanoic acid, the simplest alpha amino acid.

This molecule has now also got the functional group chemistry of an amine.

 

2-chloropropanoic acid gives 2-aminopropanoic acid.

(c) doc b  +  2NH3  ===>  (c) doc b  +  NH4Cl

 (c) doc b   +  2NH3  ===> (c) doc b  +  NH4Cl

 

For more mechanistic details of this reaction see

Part 3.6 Nucleophilic substitution reaction between halogenoalkanes (haloalkanes) and ammonia

 

(b) Hydroxy acid preparation

Refluxing chloroethanoic acid with ethanolic sodium hydroxide

ClCH2COOH  +  NaOH  ===> HOCH2COOH  +  NaCl

The product is hydroxyethanoic acid, the simplest alpha amino acid.

This molecule has now also got the functional group chemistry of a primary alcohol.

This is a nucleophilic substitution reaction.

 

2-chloropropanoic acid gives 2-hydroxypropanoic acid.

(c) doc b  +  OH-  ===>  2-hydroxypropanoic acid structural formula doc brown's advanced A level organic chemistry revision notes  +  Cl-

 (c) doc b   +  OH-  ===> skeletal formula of 2-hydroxypropanoic acid 2-hydroxypropionic acid  +  Cl-

 

For more mechanistic details for this reaction see

Part 3.4 Nucleophilic substitution of halogenoalkanes (haloalkanes) with sodium hydroxide

 

(c) Nitrile preparation and hydrolysis to a dicarboxylic acid

Refluxing chloroethanoic acid with ethanolic potassium cyanide

ClCH2COOH  +  NaCN  ===> NCCH2COOH  +  NaCl

This molecule has now also got the functional group chemistry of a nitrile.

This is a nucleophilic substitution reaction, Cl- exchanged (displaced) with a CN- ion.

Therefore it can be hydrolysed to a dicarboxylic acid by refluxing with dilute aqueous alkali (NaOH) or dilute aqueous acid (HCl, H2SO4). The equation for acid catalysed hydrolysis is ...

NCCH2COOH  +  2H2O  +  H+  ===>  HOOCCH2COOH  +  NH4+

The hydrolysis product is propanedioic acid.

 

For more details see

Part 3.5 The nucleophilic substitution reaction between halogenoalkanes (haloalkanes) and potassium cyanide and hydrolysis of nitriles to carboxylic acids

 

(d)


Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of all carboxylic acids and derivatives notes

 All Advanced Organic Chemistry Notes

 

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