Topic 5.3 Organic Chemistry III

Return to AS and A2 chemistry
Questions set on this topic may require the application of knowledge covered in topic 2.2 and topic 4.5; specifically the rules for nomenclature and the ideas of isomerism, bond polarity and bond enthalpy, including reagents and the reaction conditions in these topics.  The application of reaction mechanisms to organic chemical changes is considered a synoptic skill within this unit and Unit 6B.  The inclusion of a compound in this topic does not imply that practical work should be carried out with the compound. Benzene must not be used in laboratories at this level.

5.3a Structure of benzene and reactions of aromatic compounds
Students should be able to use the concepts of the different types of covalent bonds, and bond enthalpy to explain the structure and stability of the benzene ring
· students may represent the structure of benzene as   in equations and mechanisms
· benzene is chosen as the exemplar for electrophilic substitution in aromatic systems to remove the need for students to become involved in discussion about the orientation of substitution
Students should be able to recall, in terms of reagents and general reaction conditions, the reaction of:
(i) benzene with a nitrating mixture, bromine, chloroalkanes or acid chlorides in the presence of anhydrous aluminium chloride
(ii) aromatic compounds with carbon-containing side chains with alkaline potassium manganate(VII) solution, resulting in the oxidation of the side-chains
(iii) phenol with sodium hydroxide, bromine and ethanoyl chloride
(iv) aromatic nitro-compounds with tin and concentrated hydrochloric acid reducing them to amines
(v) phenylamine with nitrous acid; and the subsequent coupling reaction of benzenediazonium ions with phenol.
5.3b Reaction mechanisms
The convention of  to represent movement of an electron pair will be expected.  Students should be able to recall the following reaction mechanisms together with reagents and general conditions for the reactions shown and apply them to simple allied reactions.
· reaction mechanism: students should be able to illustrate each of the reaction types with specific examples and be able to write an overall equation for the reaction chosen
(i) homolytic, free radical substitution (alkanes with chlorine)
· students should understand that the reaction of a molecule with a free radical will generate another free radical and that reaction between free radicals provides a termination reaction
· students should be encouraged to use the convention to represent the movement of a single electron from a pair of electrons in radical reactions
· students are not expected to carry out these reactions in the laboratory
(ii) homolytic, free radical addition (polymerisation of ethene)
(iii) heterolytic, electrophilic addition (symmetrical and unsymmetrical alkenes with halogens and hydrogen halides)
· a statement of Markovnikov’s Rule will not be examined
· explanations of the orientation of addition should be in the context of the relative stability of the intermediate carbocation.
(iv) heterolytic, electrophilic substitution (benzene with a nitrating mixture, with bromine and with chloroalkane and acid chlorides)
· the generation of the electrophile, eg NO₂⁺ must be shown as part of the mechanism
· the orientation of substitution in benzene derivatives will not be examined
(v) heterolytic, nucleophilic substitution (halogenoalkanes with hydroxide ions and cyanide ions) SN1 and SN2.
· students are not expected to carry out reactions involving cyanide
(vi) heterolytic, nucleophilic addition (carbonyl compounds with hydrogen cyanide)