Carboxylic acids, acyl chlorides and esters

In this lesson, we will learn:

• To identify carboxylic acids, acyl chlorides and esters by their functional groups.
• The properties of these functional groups as influenced by their bonding.
• How to produce these functional groups and their key reactions.

• This lesson will look at three more common organic functional groups, their reactions and how they are prepared. They all contain the C=O carbonyl group that we saw last lesson in Aldehydes and ketones.


• Carboxylic acids are a very common type of acid, identifiable by the -COOH attachment at the end of a carbon chain. Compared to an aldehyde, the C-H chain end is an -OH group instead.
• Their naming suffix is -oic acid, and as a high priority group always found on the end of a carbon chain, numbering is not necessary.
  For example, a two-carbon chain containing a carboxylic acid group on one of the carbons is called ethanoic acid. This is the compound that gives vinegar its taste and intense smell – carboxylic acids are generally smelly compounds.
As an acid, carboxylic acids can be deprotonated and become carboxylates (COO^-). A few examples of both are below:


• The physical properties of carboxylic acids, just like with other compounds, are dictated by their bonding.
  Unlike aldehydes and ketones, carboxylic acids are capable of hydrogen bonding between individual molecules in a pure sample due to the presence of both $\delta +$ H atoms and $\delta -$ O atoms. This is similar to alcohols, but compared to them, carboxylic acid analogues have even higher melting and boiling points than analogous alcohols.


A pure sample of carboxylic acids will show intermolecular hydrogen bonds and form dimers – ‘units’ of two molecules joining, where the carboxylic acid ends face one another.

Carboxylic acids are soluble in water. In water, carboxylic acids form hydrogen bonds with the water solvent molecules, just like they would do with if in a pure sample. Like with aldehydes and ketones, solubility decreases with increasing carbon chain length, as the solubility is based on the -COOH / H₂O interactions, not the alkyl chain.


• There are two main ways to make carboxylic acids:
• We’ve already seen in Aldehydes and ketones that oxidising aldehydes produces carboxylic acids. Aldehydes can be made from alcohols, which are very common reagents, so producing carboxylic acids usually starts with alcohols, which oxidise to aldehydes, which oxidise to acids.
  Acidified potassium dichromate will oxidise primary alcohols to aldehydes and then on to carboxylic acids. Remember, as we also saw earlier, you can’t oxidise ketones, only aldehydes!
  You can represent the oxidising agent (which is Cr₂O₇^2-) as [O] in simple oxidation reactions, such as:

RCH₂OH + [O] $\,$→$\,$ RCHO + CH₂O
Even further, to the carboxylic acid:

RCHO + [O] $\,$→$\,$ RCOOH + H₂O
If a secondary alcohol was used, the reaction would stop at the first step when a ketone is made. As the [O] is dichromate ions, this reaction vessel would show a colour change from orange to green, as Cr₂O₇^2- becomes Cr³⁺ ions.


• Nitriles (R-C$\equiv$N) can be hydrolysed to make carboxylic acids. We also saw these in Aldehydes and ketones, where aldehydes and ketones react with the CN^- ion to make cyanohydrins. A nitrile with a hydroxyl group on the same carbon is a cyanohydrin.
  The -C$\equiv$N nitrile group can be hydrolysed to -COOH in acidic or basic conditions if heated under reflux:

Acidic conditions:
CH₃CH₂CN + 2H₂O + H⁺ $\,$→$\,$ CH₃CH₂COOH + NH₄⁺
Basic conditions:
CH₃CH₂CN + H₂O + OH^- $\,$→$\,$ CH₃CH₂COO^- + NH₃
The only difference is that in acidic conditions, H⁺ is abundant so you make the acid and NH₄⁺ (protonated ammonia).
In basic conditions, H⁺ is scarce so the acid is deprotonated and a carboxylate is obtained instead. Just add concentrated acid to convert the carboxylate to an acid.


• As with other acids, carboxylic acids readily react with bases to produce a salt and water. The salt produced will generally be a carboxylate salt . For example:

CH₃COOH + NaOH $\,$→$\,$ CH₃COONa + H₂O
If ethanoic acid reacts with sodium hydroxide, the salt produced has a sodium (Na⁺) positive ion and the carboxylate of ethanoic acid – ethanoate, CH₃COO^-.
The products are therefore sodium ethanoate and water. Like all salts, carboxylate salts are ionic, so do not draw a covalent bond between COO^- and Na⁺!


• As carboxylic acids are made by oxidising alcohols and aldehydes, carboxylic acids can be reduced back to aldehydes and alcohols. This can be done by lithium aluminium hydride in an ether solvent.
  Again, just like in the oxidation, this works in two steps: acid to aldehyde first, then the aldehyde to alcohol. However, LiAlH₄ is an extremely good reducing agent, so it will react straight through to an alcohol. As this level we can represent LiAlH₄, the reducing agent, as [H].

RCOOH + 4 [H] $\,$→$\,$ RCH₂OH + H₂O


• Carboxylic acids can react with PCl₃ or PCl₅ to create acyl chlorides . These reactions have slightly different side products, but the main acyl chloride product is effectively replacing the -OH on the acid with a -Cl instead:
• The reaction with PCl₃ produces phosphoric acid and the acyl chloride, for example here with ethanoic acid:

3 CH₃COOH + PCl₃ $\,$→$\,$ 3 CH₃COCl + H₃PO₄

• The reaction with PCl₅ (phosphorus pentachloride) produces hydrochloric acid and phosphorus oxychloride as side products, as well as the acyl chloride. Again, with ethanoic acid:

CH₃COOH + PCl₅ $\,$→$\,$ CH₃COCl + POCl₃ + HCl
This reaction has more side products but is very useful because all three products are valuable industrial chemicals. Acyl chlorides are just one of them.

Below is a summary table for carboxylic acids:

• Acyl chlorides are compounds with the -COCl functional group.
  The naming suffix for an acyl chloride is -oyl chloride. For example, a three-carbon chain with an acyl chloride group would be propanoyl chloride.
  They are a reactive group of chemicals but with this they can perform many reactions and make a variety of other important substances.
• Acyl chlorides readily react with water and alcohols to produce carboxylic acids and esters, respectively. In both cases, HCl is produced. The other product is the leftover of the acyl group and the alcohol group or water molecule. Both examples are below, with the example of ethanoyl chloride (the most common acyl chloride):
• Reaction of ethanoyl chloride with water:

CH₃COCl + H₂O $\,$→$\,$ CH₃COOH + HCl
Here a carboxylic acid is produced.


• Reaction of ethanoyl chloride with ethanol:

CH₃COCl + CH₃CH₂OH $\,$→$\,$ CH₃COOCH₃CH₃ + HCl
Here an ester is produced – another type of functional group which we’ll look as later this lesson.


• Acyl chlorides react with ammonia and amines to produce amides . This reaction also produces HCl as a product like the reactions with water and alcohols.
  Amides are identified by their functional group -CONR₂, where R can be H or a hydrocarbon group. The only difference between ammonia and amines reacting with acyl chlorides is the number of hydrogens attached to nitrogen in the product:
• Reaction of ethanoyl chloride with ammonia :

CH₃COCl + NH₃ $\,$→$\,$ CH₃CONH₂ + HCl
This product is called a primary amide, because the nitrogen has only one bond to a non-hydrogen atom. The amide here would be called ethanamide.

Ammonia, being a weak base, and HCl can also react once this reaction begins, which leads to ammonium chloride being made.

NH₃ + HCl $\,$→$\,$ NH₄Cl

• Reaction of ethanoyl chloride with ethylamine:

CH₃COCl + NH₂CH₂CH₃ $\,$→$\,$ CH₃CON(H)CH₂CH₃ + HCl
This product is a secondary amide for the same reason above – the nitrogen this time is bonded to two non-hydrogen atoms. This is the same as how amines are grouped.

A general summary of acyl chlorides is below:


Formula	Made by:	Rxn with water	Rxn with alcohols:	Rxn with ammonia, amines
R-COCl	Carboxylic acids with PCl5	Makes carboxylic acid and HCl	Makes esters and HCl	Makes 1° amides + HCl (ammonia) Or 2° / 3° amides + HCl (amines)



• Carboxylic acids can also react with alcohols and an acid catalyst to produce esters . Esters are functional groups where the H on a carboxylic acid has been replaced by a hydrocarbon group; there are many varieties. Esters are defined by the R-COOR’ group, where R’ can be alkyl or aryl group – like an alkyl has bonded to a carboxylate.
  The naming of esters is based on this – esters have two parts to their name:
• The alkyl/aryl (e.g. ethyl) group named first.
• The carboxylate (e.g. ethanoate) group which is named second.
See some examples below:

These esters could all be made from a carboxylic acid and an alcohol by a condensation reaction (H₂O is lost). See the example below, where ethanol and butanoic acid make ethyl butanoate:


• Esters are related to carboxylic acids but have some notable differences in physical properties and features:
• Esters are relatively sweet, pleasant smelling compared to the unpleasant intense odours of carboxylic acids.
• Esters do not create a $\delta +$ H atom in the molecule, so there is no hydrogen bonding between ester molecules. Just like in aldehydes and ketones then, similarly-sized carboxylic acids have a higher melting and boiling point than their ester analogues. For example, pentanoic acid (a five-carbon acid) has a higher boiling than ethyl propanoate (a five-carbon ester).
• Hydrogen bonding can occur with water molecules but ester solubility in water rapidly drops with chain length.
• Many esters of medium to large chain length are common oils and fats – the acids that make these esters are known as ‘fatty acids’, as in the ones found in animal products and other foods.


• Just as esters are created by removing water (a condensation reaction), esters can be broken up by hydrolysis in acidic and basic conditions.
• In acidic conditions: The ester reacts with water and H⁺ as a catalyst to return to the acid and alcohol it was made from. For example with ethyl ethanoate:

CH₃COOOCH₂CH₃ + H₂O + (H⁺ cat.) $\, \rightleftharpoons \,$ CH₃COOH + CH₃CH₂OH
The reaction is reversible, of course – we have already seen acids and alcohols reacting to make the ester. Use your knowledge of Le Chatelier’s principle – how would you push the process to the right?


• In basic conditions: The ester reacts with a strong base like NaOH or KOH to produce a carboxylate salt and alcohol. With ethyl ethanoate:

CH₃COOCH₂CH₃ + NaOH $\,$→$\,$ CH₃COONa + CH₃CH₂OH
As you can see, this reaction isn’t a reversible/equilibrium process. An alcohol and the aqueous salt can be easily separated by distillation too. The acid will be obtained if dilute acid is added to the sodium carboxylate salt.


• We now know that esters will form when a carboxylic acid and an alcohol react together.
  The carboxylic acid must be on the end of a carbon chain. So what if you react a diol with a dioic acid ? This can create a polymer called a polyester.
  The process is just like forming one ester, but as a dioic acid has a -COOH group at both ends of the carbon chain, it happens twice per molecule. The two diol molecules it joins with each have two terminal -OH groups, will each combine with another carboxylic acid molecule to continue the polymer chain.
  The product is a polymer chain with the repeating unit containing one dioic acid and one diol combined together. A common example:


Polyesters like polyethylene terephthalate or PET (the one in the diagram above) are very useful materials for plastic bottles and clothing – this is what ‘polyester’ in clothing is. We will see more of these in the next lesson.

Below is a summary of esters:


Formula	Naming:	Made by:	Properties:	Hydrolysis of esters:
R-COOR’	1. Alcohol section (e.g. ethyl-) 2. Carboxylate section (e.g. propanoate)	1. Alcohols with carboxylic acids. 2. Haloalkanes with acyl chloride, 3. Haloalkanes with acid anhydride, less corrosive. 4. Polyesters by dioic acids and diols (polymer)	1. Lower BP than alcohols (no pure H-bonds) 2. Water soluble, drops with size	1. Catalysed by OH- (carboxylate product, distil) 2. H+ (reversible, use excess)