The names of four esters are listed.
methyl propanoate
ethyl propanoate
propyl propanoate
butyl propanoate
(a) Esters are a family of organic compounds with similar chemical properties. They can be represented by the formula CnH2nO2.
(i) State the name given to a family of organic compounds with similar chemical properties.
(ii) Explain why members of a family of organic compounds have similar chemical properties.
(iii) State the name given to a formula such as CnH2nO2.
(iv) Determine the value of ‘n’ in butyl propanoate.
(b) All four of the esters in the list are liquids at room temperature.
Name the technique used to separate ethyl propanoate from a mixture of the four esters.
(c) All four esters can be made by reacting different alcohols with the same substance.
(i) Name this substance and draw its structure. Show all of the atoms and all of the bonds.
(ii) Name the alcohol used to make methyl propanoate.
(d) Other esters, not in the list, have the same molecular formula as propyl propanoate, but different structures.
(i) State the term used to describe substances with the same molecular formula but different structures.
(ii) Name two esters with the same molecular formula as propyl propanoate.
(e) Polyesters can be made from the two different molecules shown.
(i) Complete the diagram to show a section of the polyester made from these two molecules. Include all of the atoms and all of the bonds in the linkages.
(ii) Name the type of polymerization that takes place when this polymer forms.
(iii) Name a polyester.
▶️ Answer/Explanation
(a) (i) Homologous series – A family of organic compounds with the same functional group and similar chemical properties.
(a) (ii) They have the same functional group (ester group, -COO-), which determines their chemical behavior.
(a) (iii) General formula – Represents all members of a homologous series.
(a) (iv) For butyl propanoate (C4H9COOC3H7), n = 7 (total carbons: 4 + 3 = 7).
(b) Fractional distillation – Separates liquids based on differences in boiling points.
(c) (i) The common substance is propanoic acid (C2H5COOH). Its structure:
(c) (ii) Methanol (CH3OH) reacts with propanoic acid to form methyl propanoate.
(d) (i) Structural isomers – Compounds with the same molecular formula but different arrangements of atoms.
(d) (ii) Two esters with the same formula as propyl propanoate (C6H12O2):
1. Ethyl butanoate (C2H5COOC4H9)
2. Butyl ethanoate (CH3COOC4H9)
(e) (i) The polyester forms through ester linkages (-COO-). The diagram should show:
– Repeating units with alternating diol and diacid segments.
– Continuation bonds (extending lines) at both ends.
(e) (ii) Condensation polymerization – Water is eliminated as the polymer forms.
(e) (iii) Example: Terylene (a common polyester).
Alkenes and carboxylic acids are both families of similar compounds with similar chemical properties.
Alkenes and carboxylic acids have different reactions.
(a) State the term used for a ‘family’ of similar compounds.
(b) State the general formula of alkenes.
(c) The structure of but-2-ene is shown.
(i) But-2-ene reacts with aqueous bromine in an addition reaction.
Describe the colour change seen when but-2-ene is added to aqueous bromine.
(ii) State what is meant by the term addition reaction.
(iii) Write the chemical equation for the reaction between but-2-ene and bromine.
(iv) But-2-ene forms a polymer.
Suggest the name of the polymer formed from but-2-ene.
(v) Name and draw a structural isomer of but-2-ene.
Show all of the atoms and all of the bonds.
(d) Butanoic acid, \(\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}\), is a carboxylic acid.
(i) Deduce the empirical formula of butanoic acid.
(ii) Complete the chemical equation for the reaction of butanoic acid and sodium carbonate, \(\text{Na}_2\text{CO}_3\).
\(2\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{Na}_2\text{CO}_3\) → ……………………………… + …………… + ……………
(iii) Butanoic acid reacts with methanol to form an organic compound and water.
● Name the organic compound formed.
● Draw the structure of the organic compound formed.
Show all of the atoms and all of the bonds.
▶️ Answer/Explanation
(a) The term is homologous series (compounds with same functional group and general formula).
(b) The general formula of alkenes is \(\text{C}_n\text{H}_{2n}\) (one double bond per molecule).
(c) (i) Colour change: orange to colourless (bromine adds across the double bond).
(ii) Addition reaction: Two molecules combine to form one product (no byproducts).
(iii) Equation: \(\text{C}_4\text{H}_8 + \text{Br}_2 → \text{C}_4\text{H}_8\text{Br}_2\) (1,2-dibromobutane).
(iv) Polymer name: polybut-2-ene (addition polymer of but-2-ene).
(v) Isomer: but-1-ene (\(\text{CH}_2=\text{CH}-\text{CH}_2-\text{CH}_3\)).
(d) (i) Empirical formula: \(\text{C}_2\text{H}_4\text{O}\) (simplest ratio of atoms).
(ii) Equation: \(2\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} + \text{Na}_2\text{CO}_3 → 2\text{CH}_3\text{CH}_2\text{CH}_2\text{COONa} + \text{H}_2\text{O} + \text{CO}_2\).
(iii) Organic compound: methyl butanoate (ester).
Structure: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{COOCH}_3\) (ester linkage shown).