Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

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

 Index of GCSE level oil and basic organic chemistry notes

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

Sub-index for this page (some overlap with section 6.3)

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

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  , 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.

TOP OF PAGE and sub-index

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).

  +  Cl₂  ===>    +  HCl

The principal product is 3-chlorobenzoic acid, shown above, and 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).

These are electrophilic substitution Friedel-Crafts catalysed reaction mechanisms

See section 7.6 Electrophilic substitution - ring halogenation of benzene & methylbenzene

 

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.

It is not easy to hydrolyse 3-chlorobenzoic acid to 3-hydroxybenzoic acid by simply refluxing it with aqueous sodium hydroxide.

You do not get the same C^δ+-Cl^δ- 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).

TOP OF PAGE and sub-index

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.

  +  2NH₃  ===>    +  NH₄Cl

    +  2NH₃  ===>   +  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.

  +  OH^-  ===>    +  Cl^-

    +  OH^-  ===>   +  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  +  NaC≡N  ===> N≡CCH₂COOH  +  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 ...

N≡CCH₂COOH  +  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