Subject aids: organic review
Francine Taylor-Campbell, Contributor

We will continue to prepare for the upcoming examinations by practising questions on the topic.

1. Olive oil contains oleic acid which is unsaturated.

(a) (i) By naming the reagent and the observation describe a simple test to confirm that olive oil contains an acid.

(ii) Explain what is meant by unsaturated.

(iii) How would you confirm that oleic acid is unsaturated? Name reagent and observation.

ANSWERS

1. Acids generally react with carbonates. To prove that the olive oil contains an acid, it can be reacted with sodium carbonate. Observation: Effervescence gas given off is carbon dioxide which turns lime water milky.

Comment: Although oleic acid is a weak acid, it will undergo reactions that are characteristic of acids, such as reaction with metals and carbonates.

(ii) An unsaturated compound is one which contains multiple bonds, e.g., double and triple bonds.

(iii) Unsaturated compounds undergo addition reactions in the presence of bromine in tetrachloromethane and when acidified potassium manganate (VII) reagent is added.

When bromine is added to an unsaturated compound, the bromine becomes decolourised as the bromine is added to the compound, changing double bonds to single bonds (saturated).

Acidified potassium manganate (VII) also changes from a purple to a colourless solution when added to an unsaturated compound.

2. Cracking involves the breakdown of large hydrocarbon molecules into smaller ones.

(i) Give the conditions needed for cracking hydrocarbons

(ii) Explain how cracking can help to match the demand for a fraction with its supply

(iii) A hydrocarbon, C12H26, is cracked to give only two products. One of the products is butane. What is the molecular formula of the other product?

ANSWERS

2. Comment: The lighter fractions of crude oil, e.g., liquefied petroleum gases, petrol and gasoline are the more useful fractions and the ones required in larger amounts than the heavier fractions. To convert the heavier fractions to the more useful and needed fractions, the process of cracking is carried out. This can be done in two ways; thermal cracking (heat) and catalytic cracking using a suitable catalyst.

(i) Conditions needed for cracking are heat and using a suitable catalyst

(ii) Heavier fractions such as heavy oils and bitumen which forms a major component of crude oil and is in large supply can be 'cracked' to give lighter fractions such as gasoline and petrol which are in greater demand.

(iii) C12H26 ====== C4H10 + C8H16 The other product is octene.

ATTEMPT THESE QUESTIONS AS PART OF YOUR REVISION

1. There are three organic compounds, A, B and C. Each contains two carbon atoms.

A. A gaseous compound, readily decolourises a solution of bromine in tetrachloromethane.

B. A colourless pungent liquid reacts with magnesium to give a colourless gas which gives a squeaky 'pop' with a lighted splint. It also reacts with aqueous sodium hydrogen carbonate to give a colourless gas which turns lime water 'milky'.

C. A colourless liquid reacts with sodium to give a neutral gas which is flammable. When C is heated with ethanoic acid, a liquid with a pleasant odour is formed.

Using the information given above, for each compound:

(i) Deduce the structural formula.

(ii) Write a balanced equation for each reaction.

(iii) Explain the reactions described above.

(b) Compound C can be converted to Compound A and also to Compound B. Describe how these two reactions can take place.

2. Carbohydrates, fats and proteins are the three main constituents of food. They can all be hydrolysed, either by aqueous acid or alkali.

(a) What is formed by:

(i) the acid hydrolysis of carbohydrates?

(ii) the alkaline hydrolysis of fats?

(b) What is the name of the class of compound formed when proteins are hydrolysed?

(c) Poly(ethene) and nylon are synthetic polymers. Nylon can be hydrolysed because it contains the same linkage as proteins.

(i) Name this linkage.

(ii) Poly(ethene) cannot be hydrolysed. Give one environmental problem that this lack of hydrolysis may cause.

(d) Wines which contain ethanol, C2H5OH, may have a sour taste of vinegar after long storage.

(i) Give the name and draw the structure of the acid which causes this sour taste.

(ii) Explain how this acid is formed from the ethanol.

Gary Clarke (centre), a physical education practicing teacher, demonstrates a defensive stroke as his students watch during school activities at Jonathan Grant High School in Spanish Town, on Tuesday, February 26. - Anthony Minott/Freelance Photographer

Francine Taylor-Campbell is an independent contributor.