Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

6.14 Examples of the uses of carboxylic acids and derivatives and their occurrence in nature

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 uses of domestic and industrial uses of carboxylic acids

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6.14 Examples of the uses of carboxylic acids and derivatives and their occurrence in nature

See also Part 6.8 Esters - preparation, reactions including hydrolysis and transesterification, uses

The uses of carboxylic acids and their derivatives

A few reminders

The aliphatic homologous series of carboxylic acids are colourless liquids with very strong odours (pungent smell) and tastes.

The obnoxious smell of rancid butter and sweaty socks is due to the formation of carboxylic acids.

It should be mentioned here that carboxylic acids with long carbon chains are called fatty acids, because they are combined with an alcohol called glycerol to form large ester molecules (glycerides/triesters) that make up most natural oils from plants and fat in animals.

Long chain fatty acids may be saturated (no C=C double bonds) or unsaturated (with at least one C=C double bond) in the carbon chain and are associated with triglyceride esters found in animal fats and vegetable oils.

This page includes links to previously written material on the uses of carboxylic acids and their derivatives - some of it is repeated here, but other new material has been added.

Domestic uses of carboxylic acids, connection with pharmaceuticals, nutrition and the natural world

Vinegar contains ethanoic acid (old name 'acetic acid')

In the chemical industry ethanoic acid can be manufactured on a large scale by oxidising the alcohol ethanol.

See in section 9 Oxidation of the alcohol ethanol

It is used as a food preservative and in food flavourings.

It is the oxidation of ethanol to ethanoic acid that results in alcoholic drinks turning sour (e.g. cider, wine) when exposed to air.

The fruit material already contains the enzymes that catalyse the oxidation of ethanol ('alcohol') in the presence of air.

ethanol + oxygen ====> ethanoic acid + water

CH₃CH₂OH + O₂ ====> CH₃COOH + H₂O 

+ O₂ ====>  +  H₂O

This reaction occurs if wine, beer or cider is left out open to the air, it eventually becomes vinegar and not very nice to drink!, but vinegar is very nice in salad dressings and on your fish and chips.

Methanoic acid (formic acid), HCOOH, is found in the sting of ants, its not quite as 'weak' as carboxylic acids are supposed to be.

Butanoic acid, CH₃CH₂CH₂COOH, a very smelly acid formed when butter goes rancid.

Carboxylic acids found in nature include citric acid and ascorbic acid (Vitamin C) both found in citrus fruits.

The ascorbic acid (vitamin C) present in fresh fruit and vegetables and is vital for good health AND the body cannot synthesise it, so you must eat fruit and vegetables regularly!

A lack of vitamin C can cause the disease scurvy. The symptoms of scurvy are skin sores, spongy gums and bleeding from mucous membranes. This is one example of malnutrition diseases caused by a vitamin deficiency in a diet.

Citrus fruits like oranges and lemons and many soft drinks contain the tri–carboxylic acid citric acid. and contribute to the 'tarter' or 'sour' taste of fruit. The molecule contains three acidic carboxylic acid groups –COOH (molecular structure on the right).

Citric acid is a natural preservative (E330 on food labels) and is found in the largest quantities in oranges, lemons, limes and grapefruit.

It is an anti–oxidant. Metal salts from citric acid, i.e. citrates, are used in dietary supplements to deliver trace metal minerals in a biologically available/absorbable chemical form.

(It's a nice little exercise in working out a skeletal formula!)

Citric acid can be used in baking powder to react with sodium bicarbonate giving the raising action from carbon dioxide gas formation.

The same combination can be used to give the fizzy drink effect in medicines like ant–acid stomach powders.

Citric acid is made in large quantities for the food industry, including fizzy drinks – which can be quite acid – dental concerns about the health of teeth here!

Citric acid is strong enough to be used in some limescale removers – so think about your teeth when consuming all those fizzy drinks we might like!

Citric acid is an intermediate compound in the Krebs cycle in the metabolic chemistry of carbohydrates, fats and proteins.

2-hydroxybutanedioic acid (Malic acid) is found in apples and gives the fruit its sharp taste. This molecule will exhibit R/S stereoisomerism (optical isomers, two non-superimposable mirror image forms), the 2nd carbon is chiral (asymmetric) with four different groups attached to it.

Ethanedioic acid ('oxalic acid') is found in the leaves and stems of rhubarb. It is a harmful acid and a high concentration in the leaves, which should not be eaten.

However, there is much less in the stem which is safe and delicious to eat after cooking (with sugar!).

Aspirin is a carboxylic acid. Aspirin is a drug used for pain relief and is taken regularly by those at risk from heart attacks (see also Drugs).

The molecule is based on an aromatic carboxylic acid, often used as the sodium salt to make it more soluble..

from  +  NaOH  ===>   +  H₂O 

 

Note that the benzene rings shows up in other medications used for pain and inflammation relief are:

Paracetamol is a secondary amide made from ethanoylating 4-aminophenol with ethanoic anhydride.

Ibuprofen is an aliphatic carboxylic acid with an alkyl substituted benzene ring tacked onto the middle carbon of propanoic acid.

An 'aspirin' related carboxylic acid derived molecule is C₈H₈O₃, or , methyl 2-hydroxybenzoate.

Commonly known as methyl salicylate or 'oil of wintergreen', a molecule that has mild pain killing properties. This methyl ester of salicylic acid is a colourless, viscous liquid with a sweet, fruity minty odour and used in mint candies. It  is produced by many species of plants, particularly wintergreens, from which it can be obtained by distillation. These days it is mostly manufactured synthetically, by esterifying 2-hydroxybenzoic acid with methanol, and used as a fragrance and flavouring agent.

Many plants produce methyl salicylate in small quantities, often in response to biotic stress e.g. infection by pathogens, where it plays a role in the induction of resistance. Methyl 2-hydroxybenzoate is also released in some plants when they are damaged by herbivorous insects, where they may function as a cue aiding in the recruitment of predators. Some plants produce methyl salicylate in larger quantities to aid direct defence against predators or pathogens

Many traditional soaps are made from natural oils and fats.

Long chain carboxylic acids, known as 'fatty acids', are used to make soaps and detergents, originally derived from plant oils.

Below are some diagrams of the organic molecules or ions involved, typically with 16 to 20 carbon atoms in the chain.

Diagram S1: The stearic acid molecule C₁₇H₃₅COOH or CH₃(CH₂)₁₆COOH is a typical long chain fatty acid obtained from naturally occurring plant oils and used to make traditional soaps.

Diagram S2: The salt sodium stearate C₁₇H₃₅COO^–Na⁺, formed when stearic acid is neutralised with sodium hydroxide is a typical soap molecule.

Vegetable oils and animal fats, both saturated and unsaturated, are made of esters synthesised from long chain fatty acids and propane-1,2,3-triol (glycerol), so found in most living organisms - plants and animals.

Vegetable and fish oils contain esters from unsaturated long chain fatty acids which are supposed to be more healthy to eat that saturated fatty acids (actually we need both, but not too much fat !!!).

For more details on glycerides and soaps see Oils, fats, margarine and soaps 

and Natural esters - triglyceride fats and oils, manufacture of margarine and biodiesel

Esters occur in the milk of animals e.g.

Esters of 2-hydroxypropanoic acid (lactic acid) and butanoic acid are found in the milk of cows.

Ethyl 2-hydroxypropanoate

and ethyl butanoate 

or skeletal formula 

An ester of octanoic acid are found in the milk of goats.

Ethyl butanoate is also found in fruit and is used as a flavour enhancer in the food and drinks industry.

I've already described many uses of esters on Esters, chemistry and uses including perfumes, solvents basic notes for IGCSE/GCSE chemistry students.

For more advanced notes on esters see Esters - preparation, reactions including hydrolysis and transesterification, uses

 

Amino acids are the main molecular constituent of protein - the amino acids are polymerised by forming peptide bonds (a secondary amide connection) the natural condensation polymer. For more advanced level notes see

Part 6.13 Amino acids - molecular structure, preparation and reactions - two functional group chemistries

and Part 8.8 LINKS to all my notes on amino acids, peptides, polypeptides and types of proteins

Industrial uses of carboxylic acids

Ethanoic acid is used in the manufacture of the fibre, acetate rayon.

Esters and amides are used in the manufacture of other fibres and plastics e.g.

The manufacture, molecular structure, properties and uses of polyesters

Amides - a brief mention of polyamides like Kevlar and Nylon

Esters are used in the perfume and food industries for pleasant aromas and enhancing taste.

For basic notes on this see Esters, chemistry and uses including perfumes, solvents

For more advanced notes on esters see Esters - preparation, reactions including hydrolysis and transesterification, uses

See also manufacture of plasticisers in section 6.8.5

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