Carboxylic Acids (3)

We have seen in a previous blog that carboxylic acids react with alcohols to produce esters and water.

The question I want to deal with in this blog is this: does the alcohol –OH group end up in the water molecule or the ester?

In other words which of these two reactions takes place on esterification:

In this reaction the oxygen of the alcohol –OH group is found in the water molecule.

But in the second reaction the oxygen of the ethanol –OH group is found in the ester.

Which is it and how can we find out which it is?

In a famous experiment in 1938 two American chemists Irving Roberts and Harold Urey found out the destination of the alcoholic oxygen using isotopic labelling of the oxygen atom in the alcohol

In the reaction below, they used isotopically labelled oxygen ¹⁸O in the methanol to see which of the two reactions below actually took place. 

C₆H₅CO | OH     +    CH₃¹⁸OH     ⇋   C₆H₅CO¹⁸OCH₃    +  H₂O ……..A

Or

C₆H₅COO | H    +    CH₃¹⁸OH     ⇋    C₆H₅COOCH₃    +  H₂¹⁸O ……..B

They used a mass spectrometer to determine the masses of the products and found that the labelled oxygen appeared in the ester and not in the water molecule.

This meant that equation A was correct not equation B.

The oxygen of the alcohol appears in the ester.

The mechanism involves the loss of an –OH group from the acid. 

This means that the mechanism begins with the alcohol making a nucleophilic attack on the acid as the mechanism shows below.

Step 1 involves the protonation of the carbonyl group, the strong acid catalyst facilitates this.

Step 2 involves the nucleophilic addition of methanol to the protonated carboxylic acid. 

Methanol is a nucleophile because of the lone pair of electrons on the oxygen atom.

Step 3 involves the elimination of a water molecule due to proximity of the -H and -OH groups.

At this point, the oxygen of the acids -OH group ends up in the water molecule not the ester. 

Step 4 is the final step in which the species on the right above deprotonates to form the ester.

The product in this example is methyl benzoate.

Examination of the reactants and products IR spectra reveal the change in major functional groups.

Here is benzoic acid’s IR spectrum:

Note the large absorption around 1700cm^-1 due to the carbonyl group

Here next is methanol’s IR spectrum

This spectrum is characterised by the large absorption at around 3300cm^-1 due to the H bonded OH group.

Now contrast those two spectra with that of methyl benzoate:

The strong absorption at 3300cm^-1 is no longer present because the ester contains no H bonded OH group. 

The strong carbonyl absorption is still present at about 1700 cm^-1 because this is central to the ester functional group

We have seen then that the mechanism for the esterification of a carboxylic acid involves an addition elimination reaction.

In this reaction substitution of the –OH group of the acid by the CH₃O- group of the methanol occurs.

This reaction mechanism was first confirmed when Irving Roberts and Harold Urey used isotopic labelling to follow the reaction.   

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.  

Carboxylic acids (1)

In this blog we’ll discuss the water solubility of carboxylic acids, their dimerisation, attempt to account for their weakness and look at the structure of the carboxylate anion as another example of the delocalisation of electrons. 

The simplest carboxylic acid is formic or methanoic acid.

It used to be called formic acid and still is by many chemists.

The word formic reminds us of the origins of this acid: it is the acid found in an ant sting–the Latin for ant being Formica. 

The red ants of course also contained the red pigment cochineal and they when crushed produced a beautiful red paste that looked great on lips – the first type of lipstick.

Women in the later Middle Ages and early modern period used this stuff but the effect on the lips of the acidic contents led to infection and probably lip erosion! Ugh! 

You can read more about the early history of lip stick here.

Here is the structural formula of formic acid:  HCOOH  and its displayed formula is this:

Solubility of Carboxylic Acids in Water

The structure of the carboxyl group means it is fully miscible with water with up to four carbon atoms in the carbon chain.

Only as the carbon chain increases considerable in size does the acid become insoluble e.g. when the carbons chain contains 8+ carbon atoms.

So for example benzoic acid with a bulky ring of 6 carbon atoms attached to the carboxyl group is soluble in hot water.  

With a bulky group of carbon atoms attached to the carboxyl group energy is released when the group hydrogen bonds with water.

However, this energy is insufficient to break the stronger van der Waals forces that hold the hydrophobic carbon groups together so the acid is insoluble in water.  

Let’s say that the solubility of these acids in water is due in the main to hydrogen bonding with water, as in the diagram below:

 There are two other considerations for us when looking at solubility in water.

Dimerisation of Carboxylic Acids

First the lower molecular mass acids dimerise in water.

You can see how they do that in this diagram below:

Each acid hydrogen bonds to the other.

Because of dimerisation the molecular mass of a carboxylic acid is often measured to be twice its actual value.  

Reaction with water: acidic properties

Second, the acids actually react with water that’s why they are acids after all and release H+ ions.

So CH₃COOH   +    H₂O  ⇌  CH₃COO^—  +  H₃O⁺

Question is why are all these acids weak acids and not strong acids?

Why is the dissociation of the ethanoic acid into the ethanoate ion and a hydrated proton only partial and incomplete?

To answer this question we need to look at the structure of the ethanoate ion itself.

Here are some representations of this anion:

The first image below shows  the two resonance hybrids of the ethanoate ion in which the negative charge on one oxygen is envisaged as oscillating between the two oxygen atoms in the anion. 

In the next image below, we can see from the reaction that the dissociation happens when water abstracts a proton from the ethanoic acid leading to the formation of the ethanoate anion and an oxonium ion H₃O^+.

It is this reaction that is energetically difficult so that the dissociation is only partial and therefore the acid is weak.   

So why is benzoic acid stronger than ethanoic acid? 

Consider this argument:

If X-COOH and Y-COOH are two carboxylic acids. 

1 If X is more electronegative and electron-pulling than Y then X—COOH will be a stronger acid than Y—COOH. (Note: X or Y can be hydrogen).

2 Carbon shows positive inductive (+I) effect/is less electron pulling than hydrogen. So hydrogen is more electronegative compared to carbon.

3 sp2 hybridised carbon shows weaker +I effect/is more electron–pulling compared to sp3 carbon. Hence the phenyl group (C₆H₅—) is more electronegative than the methyl group (CH₃) but still less electronegative than hydrogen.

If the electronegativity order of the substituents is H—>C₆H₅—>CH₃—, then the order of acid strength is HCOOH > C₆H₅COOH > CH₃COOH.

Consider the case of trichloroethanoic acid Cl₃CCOOH, the three chlorine atoms are very electronegative and electron–pulling and draw the electrons from the –O—H bond.

Thus the –O—H bond is weakened before it even dissociates and this makes its dissociation easier and the acid stronger than ethanoic acid.

This is the key consequence of the structural difference between carboxylic acids.

It is for these structural reasons that the K_a and pK_a values of the carboxylic acids discussed are as follows:

pKa  (at 298K)

Trichloroethanoic acid 0.7

Formic acid 3.8

Benzoic acid 4.2

Ethanoic acid 4.8

Structure of the Carboxylate Anion (—COO^—)

In the next image we can see a molecular orbital picture of the ethanoate anion.

There is a π electron cloud above and below the plane of the ethanoate ion.  

The effect of this distribution of the negative charge on the anion is to delocalise it.

That then stabilises the anion.

The next image brings together the two ideas of the resonance hybrid and the delocalised electron in a molecular orbital: