Organic chemistry revision
Francine Taylor-Campbell, Contributor

We will attempt a few questions that cover the basic concepts in organic chemistry.

Question 1

(a) Soaps are the sodium or potassium salts of long-chain organic acids. A soap can be represented by the formula

H H

X--C ?= C---? COONa where X represents a long hydrocarbon chain.

(i) Describe in detail how you would prepare a sample of soap.

(ii) Identify one way in which a soap is different from a soapless detergent.

(b) When H H reacts with hydrochloric acid the corresponding organic

X--C ?= C---? COONa acid is formed. Write a balanced equation to show what happens in this reaction.

(c) Describe what you would expect to observe if H H

reacts with

(i) bromine

(ii) acidified potassium manganate VII X-- = C --?- COON?a

(iii) Write an equation to represent the reaction that should take place if H H reacts with (1) bromine

X--x ?= C ---? COONa

(2) hydrogen using a nickel catalyst

(d) Compounds with formulae like

H H

X--C ?= C ---? COOCH3 can bond together under appropriate conditions to form a 'polymer'.

(i) Define the term 'polymer'.

(ii) What type of polymerisation do you expect to occur with compounds in (d) above.

(iii) Use two molecules to show how the units are bonded together.

(iv) Suggest a possible general name for this polymer based on the functional group present in its formula.

ANSWERS

1. A soap sample can be prepared by boiling fats and oils with concentrated sodium hydroxide solution.

(ii) Soaps are formed from vegetable oils and animal fats which are esters. When these are hydrolysed, soaps are produced. Soapless detergents are synthetic detergents which are made from the reaction of hydrocarbons with concentrated sulphuric acid then neutralised with sodium hydroxide to form the sodium salt.

One major difference is that soaps do not lather in hard water, that is, they form scum readily, while soapless detergents do not form scum in hard water.

(b) H H H H

X--C ?= C ---? COONa + HCl ===X-C = C-?COO?H + NaCl

(c) If H H

X--C ?= C --?- COONa reacts with acidified potassium permanganate VII, then one would expect it to be changed from purple to colourless; that is, it is decolourised. When it reacts with bromine it is also decolourised.

(iii) H H H H

X--C ?= C --?- COONa + Br2 == X---C---C--?-COO?Na

Br Br

The bromine adds across the double bond; an addition reaction. A similar situation occurs with hydrogen.

H H H H

X--C? = C --?- COONa + H2 == X--C---C--?-COO?Na

H H

(d) Polymer is a macromolecule formed from the linking of small molecules called monomers. The compound is expected to exhibit addition polymerisation and form polymers called polyalkenes.

H H H H H H H H

C = C C = C == -C --- C --- C --- C-

X Y X Y X Y X Y

Let Y = COOCH3

Team Bickle, a group of high school students based in New York, pose for the camera, after placing third in the high school girls 4x400m event at the the 2009 Gibson Relays held at the National Stadium, St Andrew, on February 28. They are (from left) Charlene Lipsey, Afia Charles, Doris Anyanwu, and Whitney Fountain. - Anthony Minott/ Freelance Photographer

Francine Taylor-Campbell is an independent contributor.