Page 36 - Q & A STPM 2022 Chemistry

Chemistry Term 3 STPM Chapter 14 Introduction to Organic Chemistry
(b) Cis-trans (geometrical) isomerism and optical isomerism.
(c) Optical isomers:
CH"CH!COOH COOH!CH"CH
& &
C C
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OH HO CH 2 CH 3
CH 3 CH 2
H H
Cis-trans-isomers:
OH OH
CH 3 CH 2 CH COOH CH 3 CH 2 CH H
C"C C"C
H H H COOH
Cis-isomer Trans-isomer

Section C Essay Questions

Question 5
(a) Using suitable examples explain what is meant by the following species.
(i) Free radical (ii) Nucleophile (iii) Electrophile
(b) Name the three classes of free radicals and explain the factors that affect
their stability.

Answer:
(a) (i) A free radical is a species that has one unpaired electron in its
structure. It is usually formed via the homolytic fission of a covalent
bond. Examples are:
.
Cl 2 !: 2Cl
. .
C 2 H 5 Br !: C 2 H 5 + Br
(ii) A nucleophile is a species that has a lone pair of electrons that can be
donated to other species to form a coordinate bond. A nucleophile is
–
an electron-rich species. Examples are :NH 3 and :OH .
(iii) An electrophile is an electron-deficient species that accepts a lone
pair of electrons from another species to form a coordinate bond.
Examples are NO 2 , Cl and SO 3 .
+
+
(b) The three classes of free radicals are: Primary, secondary and tertiary free
Term
3 radicals.
Free radicals can be considered as electron-deficient species. They are
stabilised by electron-donating groups such as alkyl groups.

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