(a)(i) Give three characteristics of homologous series.
(ii) Name two groups of compounds which form such a series.
(b) A saturated organic compound A containing two carbon atoms reacted with ethanoic acid in the presence of a mineral acid to form a compound B with a sweet smell. (i) Name the functional group present in A.
(ii) Draw the structure of A.
(iii) Write a chemical equation to show the formation of B.
(iv) Name the compound B.
(c)(i) Write a balanced equation for the reaction between ethyne and excess bromine.
(ii) Give IUPAC name of the product of the reaction in (c)(i)
(iii) State two conditions under which cracking takes place
(d)(i) Outline the preparation of ethanol from starch.
(ii) Give two properties of starch.
(iii) Give a reason why starch does not reduce Fehlings solution
(iv) Describe briefly a chemical test to confirm the presence of starch.
Explanation
(a)(i) Characteristics of homologous series: –All members can be represented by a general molecular formular. –Each member of the series differs from the next by a constant/Fixed number of atom-CH\(_2\)/Successive member differs by CH–All members of the group have similar chemical proterties. –All members have same functional group. –All members of the; roup are prepared using similar/same general methods. All members of the series show a gradual change in physical properties (e.g. density, solubility, etc).
(ii) Groups of compounds which form such a series: –Alkanes –Alkenes –Alkynes –Alkanols –Alkanoic acid –Alkanoates.
(b) A saturated organic compound A containing two carbon atoms reacted with ethanoic acid in the presence of a mineral acid to form a compound B with a sweet smell: The functional group present in A. Hydroxy/Hydroxyl.
(ii) Structure of A.
(iii) Chemical equation to show the formation of B:
CH\(_3\)COOH + CH\(_3\)CH\(_2\)OH \(\rightleftharpoons\) CH\(_3\)COOH\(_2\)CH\(_3\) + H\(_2\)O.
(iv) Name of compound B: Ethyl ethanoate
(c)(i) Balance equation for reaction between Ethyne and Excess bromine:
C\(_2\)H\(_2\) + 2Br\(_2\) --> C\(_2\)H\(_2\) Br\(_4\)
OR
H— C \(\equiv\) C— H H 2Br\(_2\) —> H-CBr\(_2\)-CBr\(_2\)-H
(ii) IUPAC name of the product of the reaction in (c)(i) 1,1, 2, 2–tetrabromoethane.
(iii) Conditions under which cracking takes place: –High pressure –High temperature –Presence of a suitable catalyst.
(d)(i) Preparation of ethanol from starch: Treat starch with malt, diatase, convet starch to maltose.
2(C\(_6\) H\(_{(10)}\)O\(_5\) )n + nH\(_2\) O --> nC\(_{(12)}\) H\(_{(22)}\)O\(_{(11)}\). Add yeast. Ezyme maltase convert maltose to glucose.
C\(_{(12)}\)H\(_{(22)}\) + H\(_2\)O —> 2C\(_6\)H\(_{(12)}\)O\(_6\). Ezyme zymase convert glucose to maltase to ethanol and CO\(_2\)
C\(_6\)H\(_{(12)}\)O\(_6\) —> 2C\(_2\)H\(_5\)OH + 2CO\(_2\).
(ii) Properties of starch: –Can be hydrolysed. –Has no taste. –When suspension of starch is warmed, it forms colliodal solution. –It is insoluble in water. –It has no smell.
(iii) Reason why starch does not reduce Fehlings solution: It does not contain any free carbonyl group/No alkanol group/No hydrogen attached to the carbonyl group.
(iv) Chemical test to confirm the presence of starch: Add a few drops of iodine to a boiled starch. A dark-blue colouration appears.