Carboxylic Acids (2)

In this blog we are going to look at some of the reactions of the carboxylic acids, both those reactions that are typical of acids generally and those specific to this class of organic acids.

1. with reactive metals: 

This is a redox reaction.

Note the increase in oxidation number of the magnesium from 0 to +2.

It is not always easy to identify the organic atom or species that is reduced however.

The redox reactions are typical of minerals acids generally and because organic acids have replaceable hydrogens in their carboxyl groups then they too can take part in these reactions.

2. with bases 

This is not a redox reaction but a neutralisation reaction.

There is no change to the oxidation number of the magnesium ion.

Water forms as a result of the oxide ion abstracting two protons from two ethanoic acid molecules.

The resultant magnesium ethanoate is water soluble

3. with alkalis 

As we can see here with alkalis or soluble bases the hydroxide ion abstracts a proton and forms water.

Again the reaction is a neutralisation not a redox reaction.

The reaction is the typical weak acid–strong base titration example often given to illustrate the formation of an acid buffer. (which see here )

4. with PCl5

First, please note for simplicity I have drawn PCl₅ as a molecule whereas we are well aware that the white solid is an ionic structure formed from two ions PCl₄⁺ and PCl₆^-.

The cation is tetrahedral in shape and the anion is octahedral in shape.  

The product is ethanoyl chloride, one of a number of carboxylic acid derivatives which we shall consider in another blog.  

5. with alcohols: esterification

  

Much is made of the esterification reaction in college and Advanced level chemistry partly because it is known to be challenging on several levels not least in the naming of esters.  

I have colour coded the names in an attempt to bring home the fact that the ester derives its name from both the acid and the alcohol.

The reaction does not go to completion but enters an equilibrium state.

Water is the only other product

Esters are insoluble in water.

They are another type of carboxylic acid derivative.

The reaction requires a catalytic amount of a strong acid such as H₂SO₄

6. with SOCl₂ thionyl chloride

Thionyl chloride acts as a powerful nucleophile and attacks the carbon of the carbonyl group.

The product is the highly volatile and easily hydrolysed ethanol chloride.  

This reaction occurs at room temperature and is as equally dangerous as the PCl₅ reaction given the toxicity of the products SO₂ and HCl.  

7. with LiAlH4

LiAlH4 is Lithium tetrahydridoaluminate (III) and it is a very powerful reducing agent.  

In dry ether and in the absence of moisture is acts as a nucleophile attacking the carboxylic acid with H-ions.  

The acid is reduced down in two stages to its original alcohol.  

  

The reactions described above are typical of the lower molecular mass carboxylic acids that are soluble in water.  

Those acids of higher molecular mass that are waxy solids do not enter into these reactions since they do not dissolve in the ionic media necessary. 

There is more discussion of the carboxylic acid derivatives such as  esters, acid chloride etc. in a later blog. 

Chemical Test

Finally a chemical test exists for the carboxyl group –COOH in water.

Simply add a spatula measure of sodium carbonate powder (Na₂CO₃)and watch the aqueous solution of carboxylic acid fizz as CO₂ is evolved.   

This is your opportunity to construct the balanced symbol equation for this reaction taking place in this chemical test.

As the pKa of carboxylic acids are greater than those of phenols this test easily distinguishes the acids from phenolic compounds.  

Phenolic compounds do not fizz with sodium carbonate powder or solution.