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

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Part 6.5 Selective reduction of carboxylic acids and use of the products in organic synthesis routes and a unique carboxylic acid oxidation!

6.5.1 Reduction of a carboxylic acid to a primary aliphatic alcohol

Sodium tetrahydridoborate(III) NaBH₄, (sodium borohydride), is not a powerful enough reducing agent to reduce carboxylic acids.

Lithium tetrahydridoaluminate(III), LiAlH₄, (lithium aluminium tetrahydride), is a much more powerful reducing agent than NaBH₄, and in ether solvent, readily reduces carboxylic acids to primary alcohols.

The LiAlH₄ effectively releases a hydride ion, :H^-, a powerful nucleophile - electron pair donor, which can attacks the δ+ carbon of polarised carbonyl bond ^δ+C=O^δ-. (electronegativity of O > C).

The reaction must be carried with a dry solvent such as ethoxyethane ('ether') because LiAlH₄ reacts rapidly with water (and ethanol too).

The reaction is complex and goes through various stages and can be summarised as:

RCOOH  +  4[H]  ===>  RCH₂OH  +  H₂O  (R = H, alkyl or aryl)

The initial product must then be hydrolysed with water to release the primary alcohol.

e.g. propanoic acid to propan–1–ol: CH₃CH₂COOH + 4[H] ==> CH₃CH₂CH₂OH + H₂O

or benzoic acid to phenylmethanol: C₆H₅COOH + 4[H] ==> C₆H₅CH₂OH + H₂O

  +  4[H]  ===>    +  H₂O

 

So, LiAlH₄ (not NaBH₄) readily reduces the carbonyl group (>C=O) in carboxylic acids and derivatives to the primary alcohol functional group.

 

As far as I know, metal/acid reducing agents like Zn(s)/HCl(aq) is not powerful enough to reduce carboxylic acids.

H₂(g)/Ni(s) will NOT reduce carboxylic acids, but there are other specialised catalysts that can effect this reduction using hydrogen gas in the chemical industry.

RCOOH  +  2H₂  ===>  RCH₂OH  +  H₂O   (R = H, alkyl or aryl)

 

A simple example to illustrate what is, and is not, reduced in terms of the functional groups.

e.g. propenoic acid H₂C=CH-COOH ('acrylic acid'),

which has two functional groups - alkene and carboxylic acid

(i) H₂C=CH-COOH  +  [H]  == NaBH₄ ==>  No reaction, no reduction

 

(ii) H₂C=CH-COOH  +  4[H]  == LiAlH₄ ==>  H₂C=CH-CH₂OH  +  H₂O

The alkene is NOT reduced, but the carbonyl group is, giving prop-2-en-1-ol. via the negative hydride ion nucleophile attacking the polarised carbonyl bond.

Nucleophilic attack:  ^-H: ==> ^δ+C=O^δ-

This product is an unsaturated primary alcohol, still with two functional groups.

The >C=C< alkene group is NOT reduced by LiAlH₄ because the attacking nucleophile is essentially a negative hydride ion (:H^-) which would be repelled by the high electron density of the pi electron cloud of the non-polar C=C double bond.

 

(iii) H₂C=CH-COOH  +  H₂  == Ni ==>  H₃C-CH₂-COOH

Only the alkene group is reduced with a nickel catalyst, giving propanoic acid with only one functional group.

 

This means you can selectively reduce either functional groups or you need two reductions to form propan-1-ol.

 

The molecule now has the chemistry of primary alcohols

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6.5.2 A unique oxidation

Carboxylic acids are usually quite stable against oxidation.

Think of the end product of oxidising alcohols and aldehydes with acidified potassium dichromate(VI) - carboxylic acids.

However, in terms of lower members of the simple aliphatic carboxylic acids, there is one clear exception.

Apart from a carboxylic acid with an aldehyde group as or in a side chain, the first in this homologous series of aliphatic carboxylic acids, methanoic acid (HCOOH) is the only carboxylic acid that is the only one easily oxidised, acting as a reducing agent, it gives the following results with the following reagents:

(i) It gives a silver mirror with ammoniacal silver nitrate (Tollen's reagent).

(ii) It gives a red-brown copper(I) oxide (Cu₂O) with blue Fehling's solution.

(iii) However, it does not give a yellow-orange precipitate with 2,4-dinitrophenylhydrazine - carboxylic acids themselves do not usually undergo nucleophilic addition - elimination reactions like aldehydes do.

Explanation

In tests (i) and (ii) methanoic acid is oxidised to carbon dioxide and water.

HCOOH  +  [O]  ===>  CO₂  +  H₂O

So methanoic acid acts as a reducing agent and readily oxidised to carbon dioxide and water by these two reagents.

When you write the formula of methanoic acid, HCOOH, the 'usual' abbreviated structural formula, the explanation isn't as obvious until you write it another way ...

i.e.  HOCHO is now a formula of a hydroxy-aldehyde, hence its ease of oxidation, giving positive results for two the simple tests for aldehydes !!!

So methanoic acid behaves like an aldehyde, the left of this particular carboxylic acid molecule has a H-C=O grouping, identical to an aldehyde group.

Methanoic acid esters have the same 'end' grouping e.g. ethyl methanoate, so I assume they can also give a positive 'aldehyde' result?

Ethanoic acid CH₃COOH, cannot behave in this way, the left H is replaced by an alkyl group, so there is no equivalent of an aldehyde group present in the molecule.

You can't 'rearrange' any of the other carboxylic acids to be both aldehyde and carboxylic acid.

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