Organic Chemistry: Part 2 - Carboxylic Acids, esters, fats, oils and soaps


Carboxylic Acids

This is the term for organic acids. The simplest one is methanoic acid.:
Formic-acid.png
The general structure of carboxylic acids is often written as RCOOH, where "R" is the rest of the molecule. Methanoic acid would be written as HCOOH in this format.
Larger carboxylic acids are formed by the addition of alkyl chains onto the COOH functional group. Below is ethanoic acid, which has one extra methyl group:


Aceticsmall.png
The hydrogen on the right in the model is only loosely held. When the acid is mixed with water, a small percentage of these hydrogens dissociate (separate) creating hydrogen ions and carboxylate anions. Typically, the percentage is 0.001 – 1%. It is the presence of the hydrogen ions that makes aqueous solutions of short chain carboxylic acids acidic (a detailed understanding of acid-base theory is not required in this course).
Carboxlylic acids can be distinguished from other water-soluble organic compounds in the laboratory by using blue litmus, which they will turn red (alcohols will not change the colour). They also react as other acids e.g. causing sodium carbonate to fizz. They are weak acids i.e. their pH is typically 3-4 at the most acidic concentration.

Some short chain carboxylic acids are known by 'common' non-systematic names:
Systematic name
Common name
Methanoic acid
Formic acid
Ethanoic acid
Acetic acid, vinegar
Propanoic acid
Propionic acid
Butanoic acid
Butyric acid
Like alcohols, miscibility with water decreases as the length of the chain increases. Methanoic to butanoic acids are completely miscible with water, but from pentanoic (valeric) acid upwards there is some insolubility. Acids with 4 or more carbons are known as fatty acids; 4-8 carbons are termed 'short chain' fatty acids. Fatty acids with 8 or more carbons are a component of natural fats and oils.
Naturally ocurring carboxylic acids
Many natural carboxylic acids are known only or mostly by their common (non-systematic) name. Examples are salicylic acid (willow bark), malic acid (apples) tartaric acid (grapes), citric acid (citrus) and oxalic acid (oxalis plants). Some of these have more than one carboxyl group; for instance, oxalic acid is known as ethandioic acid and has two carbon atoms, both with the -OOH group attached. Citric acid has three carboxyl groups and is known as a tribasic acid. You will not be required to discuss the chemical reactions or give structures for these acids. (follow the links for more on: Carboxylic acids and carboxylates).


Reactions of carboxylic acids:

1. Reaction with bases:
Carboxylic acids react with alkalis to produce carboxylate salts e.g.:

sodium bicarbonate + ethanoic acid --> sodium ethanoate + carbon dioxide + water
Sodium ethanoate is very commonly known as sodium acetate.
Sodium or potassium salts of fatty acids are known as soaps.

2. Reaction with alcohols:
Carboxylic acids react with alcohols to form esters. See the next section for more details on this.

Production of carboxylic acids

Carboxylic acids can be produced by partial oxidation of the corresponding alcohol with a strong oxidizing agent such as potassium dicthromate:

ethanol + potassium dichromate + hydrochoric acid -------> ethanoic acid + chromic chloride + water

As potassium dichromate is bright orange and chromic chloride dark green, the reaction is readily observable. It is the reaction used for the old chemical 'breathalizer' bags. Any alcohol will be oxidized this way.

Rice vinegar
Rice vinegar
Commercial manufacture of vinegar:
Ethanol (alcohol) produced by fermentation can be partially oxidized by bacteria to produce ethanoic acid. This is how vinegar is made. A sugar solution is first fermented to produce an alcoholic mixture (e.g. apple juice to cider).
glucose ----(yeast fermentation)----> ethanol + carbon dioxide + water


Natural or cultured bacteria belonging to the genus Acetobacter are added, and in time all the alcohol is converted to acetic acid.
ethanol + oxygen----(bacterial aerobic fermentation)----> ethanoic acid

Some chemicals from the original fermented mixture remain, providing extra flavours, so apple juice produces cider vinegar. The source of the sugar can be a carbohydrate such as rice, producing rice vinegar (picture on right)


Esters

Introduction

Esters are compounds formed by the reaction of carboxylic acids with alcohols. The word equation for this is

alcohol + carboxylic acid <----> alkyl carboxylate + water

for example

ethanol + propanoic acid <----> ethyl propanoate + water

Click here for a YouTube animation showing ethyl ethanoate being produced (the names are in German but the formula is the same)

ester_link.pngThe diagram to the left shows the ester ethyl propanoate, with the ester link shaded yellow.
Naming esters: remember that the part of the ester with the C=O double bond will be the 'ate' bit (carboxylate group); so count how many carbons are on this bit (don't forget the one with the C=O itself). Here there are 3, so it is a propanoate ('prop' is 3). Then count the carbons on the other side of the link - here there are 2. This is the '-yl' part (alkyl group), so this one is ethyl. The alkyl part goes first in the name, so this one becomes ethyl propanoate. Don't be fooled if the molecule is drawn back to front as it is here.

Reaction conditions

The reaction is reversible, as indicated by the double headed arrow. It is sped up by heat, and further by acid conditions, which catalyze it. Concentrated sulfuric acid is used because it both catalyzes the reaction (speeds it up without being used up) and removes the water from the right hand side of the reaction (moving the equilibrium to the right). This increases the yield (the amount of product).
For example, to prepare methyl benzoate (smells of feijoas) in the lab, you would place about half a teaspoon of benzoic acid (white powder) in a test tube, add 2 or 3 mL of methanol, and 1 mL concentrated sulfuric acid. You would then heat it in a water bath (a beaker of water to prevent it overheating) until a smell of feijoas is detected. The temperature needs to be kept below the boiling point of methanol (68 degrees celsius).
If a longer reaction time is needed, the heating can be done in a reflux apparatus:
Fractional distillation and reflux apparatus
Fractional distillation and reflux apparatus

De-esterification

Because the reaction to form esters is reversible, esters can revert to their component carboxylic acid and alcohol under suitable conditions, particularly the presence of water. Some of the unpleasant smells resulting from fat going rancid can be due to this process.

Properties and uses

Esters are liquids with a fruity, pleasant smell. Certain esters smell distinctly of certain fruit, e.g. methyl benzoate of feijoa. For a list of which esters smell of what, visit the Wikipedia entry on esters.
Esters are used as solvents, e.g. nail polish remover. They are relatively non-toxic compared to some alternatives. They are also used as flavouring agents and in perfumery; however, the tendency to de-esterify into unpleasant smelling carboxylic acids and alcohols can limit this use.
Methyl esters of fatty acids produced from waste food oils by de-esterification can be suitable for use as biodiesel.
Fats and oils from animals and plants are natural tri-esters of the alcohol 1,2,3 propantriol, more commonly known as glycerol. See the next section for more information about this.

Fats and oils

Triglycerides
Representative triglyceride; source: Wikipedia (click on picture for Wiki entry)
Representative triglyceride; source: Wikipedia (click on picture for Wiki entry)

This is another name for animal and plant fats (they are a subset of a broader group of compounds called lipids). Three fatty acids are joined onto a glycerol molecule via an ester link. The fatty acids can be saturated (the top one in the diagram), mono-unsaturated (the middle one) or polyunsaturated. The double bond has the effect of ‘straightening’ the ‘zig-zag’ shape of the molecule, which in turn has the effect of lowering the melting point.
The bottom fatty acid in this oil is termed an ‘omega-3’ fatty acid, because it contains three double bonds at the omega end of the molecule.


cis configuration of 2-butene (Wikipedia)
cis configuration of 2-butene (Wikipedia)
There is a variation in how the double bond can be arranged: the methyl groups on either side can be on the same side of the double bond, termed 'cis', shown on the left
trans configuration of 2-butene
trans configuration of 2-butene

or they can be diagonally across the double bond, termed 'trans': as shown on the right.



Trans fatty acids tend to be produced during the partial hydrogenation of plant oils, a process carried out in the food technology industry to increase the shelf life and raise the melting point of these oils (e.g. to make vegetable shortening and margarine). There is some evidence that these trans fatty acids have similar detrimental effects in terms of coronary heart disease and other circulatory problems as saturated fats (such as those found in animal fats) (read more...). Cis fatty acids, such as this:
ALAnumbering.png
Alpha-linolenic acid

are thought to be better for your health. These are found in fish oils and oils of certain plants such as linseed (European flax) and evening primrose. Click on the picture for more information.
An article on the health effects of saturated and trans fats can be found here.

Double bonds and melting point:
The presence of cis double bonds decreases the melting point of the fats formed by the fatty acids. This is due to the 'straightening' effect of these cis double bonds on the molecule; this results in less surface area for inter-particle forces. Cold water fish have evolved to be rich in these, because otherwise their oils would solidify. This would interfere with their metabolic function and make the fish flesh too stiff for swimming. Plants also tend to have triglycerides with lower melting points because the oils need to be liquid to be metabolized. Mammals, by contrast, being warm blooded, do not need such low melting points. They have not needed to evolve the ability to produce these substances.
Testing for saturation
Saturation in fats can be tested for using bromine or iodine. The halogen (Br2 or I2) reacts with the double bond, breaking it and joining on one atom on either side. This is called an addition reaction. As the halogen is coloured and the halogenated fat is not, this results in a rapid decolorization of the halogen solution (a slow decolorization can occur via another reaction called substitution).
A semi-quantitative indication of the degree of saturation of the fat is given by the iodine number. Roughly speaking, this is the number of grams of iodine that can be decolorized by 100 grams of the lipid in question, when the reaction is done following certain procedures (click on the link for more details).

Soaps
Soaps are salts of fatty acids. They are made by reacting a fat or oil with hot sodium hydroxide. This causes it to de-esterify into the fatty acid and glycerol. The fatty acid immediately reacts with the sodium ions to form the sodium fatty salt and water e.g.
Sodium hydroxide + stearic acid --> sodium stearate + water
SOAP.png
Above is a fat molecule (the fatty acid here has 13 carbons; stearic acid has 18)
When you add hot sodium hydroxide you get:
soapmake.png
Formation of soap 'scum':
Soaps become useless in 'hard' water (water containing calcium ions, such as seawater, or water with calcium bicarbonate in it from lime-rich soils), because they react with the calcium ions to form a calcium soap. This is commonly called 'soap scum'.

Sodium stearate + calcium bicarbonate --> calcium stearate + sodium bicarbonate

Calcium stearate is insoluble. It stops the soap cleaning, and builds up in washing machines. Adding chemicals that remove calcium ions can prevent this. Chemicals used for this include sodium carbonate, sodium phosphate or sodium EDTA.
Some insoluble soaps e.g. zinc stearate, can be used as waterproofing agents for canvas and other fabrics (one end binds to the fabric, the other repels water).

Detergents have a similar structure to soaps, but have a sulphate group where the carboxylate group is normally found:

Sodium lauryl sulfate, a common detergent (click for more info)
Sodium lauryl sulfate, a common detergent (click for more info)


Detergents like this do not have the same degree of reaction with calcium ions, so can be used in hard water.
Detergents come in several varieties: cationic, anionic and non-ionic. The updated achievement standard no longer mentions these, but if you want to get more information, click here.

How soaps and detergents work:
The long carbon chain on the soap or detergent molecule is non polar and therefore is repelled by water (hydrophobic) but is soluble in other non-polar substances such as fat and grease.
detergency.pngThe ionic end of the molecule is polar and attracted to water (hydrophilic).
When soap molecules encounter a grease particle, the hydrophobic end 'sticks' into the grease particle, leaving the hydrophilic end of it sticking into the water. Eventually, the grease particle is entirely surrounded by the soap molecules:
In the diagram to the left, the red dot marks the polar end of the soap molecule. In practice there would be many more of them than is shown here.