Chemistry 5

Organic molecules form the basis of living things and all contain carbon atoms. Carbon atoms covalently bond to each other to form the backbone of many series of organic molecules. Series of molecules that have a general formula are called homologous series. 

Alcohols contain the functional group -O-H, if one hydrogen atom from each alkane molecule is replaced with an -O-H group we get a homologous series of alcohols. 

A structural formula shows which atoms are bonded to each carbon atom and the functional group. 

Carboxylic acids have the functional group -COOH. 

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Esters have the functional group -COO-. If the H atom in the -COOH group of carboxylic acid is replaced by a hydrocarbon group, the compound is an ester.

Alcohols like methanol, ethanol and propanol mix well with water and produce neutral solutions. Many organic substances dissolve in alcohols which makes them useful solvents. Alcohols burn in air, when burned completely they produce carbon dioxide and water so are used as fuels. 

Alcohols can be oxidised by chemical oxidising agents such as potassium dichromate to poduce carboxylic acids. Some microbes in the air can also oxidise solutions of ethanol to produce ethanoic acid. Sodium reacts with alcohol to produce hydrogen gas and a solution. 

Carboxyilc acids dissolve in water to produce solutions with pH value less than 7. They have the properties typical of all acids. However they are also different from other acids because they reacts with alcohol in the presence of an acid catalyst to produce esters. Esters are volatile compounds and have distinctive smells. 

In aqueous solution, hydrochloric acid ionises completely to hydrogen and chloride ions. Acids that ionise completely in aqueous solutions are known as strong acids. When ethanoic acid dissolves in water, it does not ionise completely and some of the ethanoic molecules remain. Acids that do not ionise in aqueous solutions are known as weak acids. 

In aqueous solutions of equal concentration, weak acids have a higher pH and react slower than strong acids. 

Alcohols, carboxylic acids and esters are important organic chemicals that can be used in foods, drinks, solvents and fuels for the benefit of society. However they also cause many problems:

- alcoholic drinks and solvents can be abused and lead to health problems

- biofuels offer and alternative to fossil fuels however some crops used for the production of biofuels require land that could be used to grow food.