Reactions of alcohols and carboxylic acids
QUICK POINTS
Alcohols are organic compounds containing the –OH functional group with a general formula of CnH2n+1 OH.
The presence of the –OH group makes alcohols soluble in water and increases their boiling points.
Ethanol can be prepared by fermentation of sugars using the enzymes in yeast.
Alcohols can also be prepared by the hydration of alkenes in the presence of a phosphoric acid catalyst at high pressures.
Alcohols are oxidised to organic acids using oxidising agents such as acidified potassium dichromate.
Carboxylic acids contain the – COOH functional group and have the general formula CnH2n+1 COOH.
Carboxylic acids are weak acids which show the typical reactions of acids.
Carboxylic acids react with alcohols in the presence of concentrated sulphuric acid to form esters. This is a reversible reaction.
Ester formation is a type of condensation reaction in which a large molecule is formed from the joining of two reactive groups, with the elimination of a small molecule such as H2O. Esters can be hydrolysed (broken down) using acid or alkali to yield the acid (or salt of the acid) and alcohol that formed the ester.
Esters are responsible for the sweet smells of fruits and perfumes.
QUESTION 1
Ethanol can be made by two different methods.
(a) Show what these methods are, using equations.
(b) Give an advantage and a disadvantage of each method.
(c) Describe two uses of ethanol.
ANSWERS
(a) Ethanol can be prepared by fermentation and by catalytic hydration of an alkene.
FERMENTATION
In this process, the enzyme acts as a catalyst to convert the glucose to alcohol and carbon dioxide. If starch (polysaccharide) is used, the enzyme invertase first breaks it down into glucose (monosaccharides), then the enzyme zymase converts the sugar into alcohol. This process is used to make beers and wines.
Hydration of ethene (or addition of water) is the second method used to produce ethanol. Conditions: catalyst phosphoric acid on silica 3000C, 60 atm.
C 2H4 + H2O ––– C2H5OH (ethanol)
(b) Ethanol made from hydration of ethane uses up valuable crude oil reserves, which are already being depleted. This method can be run continuously, increasing the efficiency of the process, since the reaction is reversible. The process of fermentation is ideal to conserve crude oil supplies; however, it is quite slow. The use of sugar cane (and other plants) as raw materials also aid in removing carbon dioxide levels in the atmosphere.
(c) Ethanol is a useful solvent often found in methylated spirits. Ethanol can also be combined with petrol to form gasohol, which is a more environmentally friendly fuel.
QUESTION 2
(a) How are esters formed?
(b) Write a balanced equation for the reaction to show the formation of the ester methyl ethanoate.
(c) Describe how the smell of perfume spreads.
(d) Describe two uses of esters.
ANSWERS
(a) Esters are formed from the condensation reaction of an alcohol and a carboxylic acid. A small molecule of water is eliminated during the process.
(b) Methyl ethanoate would be formed from the reaction of methanol (alcohol) and ethanoic acid (carboxylic acid). This is done in the presence of concentrated sulphuric acid as the catalyst.
(c) Esters produce a sweet or fruity smell. These substances are volatile (have a low boiling point) and quickly vaporise to gases at room temperature. Like all gases, they would quickly spread to occupy any space they are placed in by the process of diffusion.
(d) The sweet smell of esters are useful in food flavourings such as vanilla essence. These provide artificial flavourings in food. Esters are also used as solvents; for example, in nail varnish removers.
Francine Taylor-Campbell is an independent contributor. Send comments to kerry-ann.hepburn@ gleanerjm.com.
CH3OH (l) + CH3COOH (l) ––– CH3COOCH3
Methanol + Ethanoic acid –––– Methyl ethanoate (ester)