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The programme, dealing with the mechanisms of a reaction, examines nucleophilic substitution of aromatic substances.
Metadata describing this Open University video programme
Item code: S24-; 10
First transmission date: 27-05-1972
Published: 1972
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Duration: 00:23:10
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Producer: David Jackson
Contributors: Len Haynes; David Roberts
Publisher: BBC Open University
Keyword(s): `Benzyne mechanism'; Alternative mechanisms; Nucleophilic aromatic substitution; O-bromotoluene and sodamide in liquid ammonia; Saturated/unsaturated carbon; SN1 and SN2 mechanisms
Footage description: Len Haynes introduces the programme. Haynes explains that the study of reaction mechanisms is essentially an inventive process. Slot of the benzyne ring shown on the screen. Haynes briefly discusses the benzyne mechanism for nucleophilic aromatic substitution first proposed in the 1950s. (The programme will test this hypothetical mechanism) Haynes examines the background to nucleophilic aromatic substitution and the mechanism which was suggested for this kind of reaction before the 1950s. Haynes performs a simple experiment to show that ideas of nucleophilic substitution must change when dealing with aromatic rather than aliphatic compounds. He adds benzyl chloride to a solution of silver nitrate in nitric acid. A precipitate forms. Haynes with a sample of 3 hour refluxed chlorobenzene in 10% NaOH solution. He neutralises the hydroxide with nitric acid and then adds silver nitrate. There is no visible reaction. Shots of the structures of the two organic halides are shown on the screen. Haynes explains why one reacted and the other not. Roberts looks at the differences in reactivity of the aromatic and aliphatic halides. He examines some possible intermediaries involved in the hydrolysis of chlorobenzene and comes up with a modified SN2 mechanism for nucleophilic aromatic substitution. The reaction formula is shown on the screen. Film shots of a laboratory demonsteation of the reaction of O-bromotoluene with sodamide in liquid ammonia. Production of sodamide shown first, then the rest of the experiment) Commentary by Roberts. The products of this reaction, two isomers, are O- and m-aminotoluene in equal yields. Shots of the product purity being tested by Gas-liquid chromatography. Roberts now introduces the "benzyne mechanism" to explain the formation of the two isomers in the above reaction. He uses a mag-board to show the reaction formula. Roberts explains the format for the rest of the programme. It will consist of a series of objections raised by Professor Haynes, against the benzyne mechanism and answers to these by Roberts. Haynes uses a mag-board to put forward his first objection - that benzyne is an acetylene and that the acetylene triple bond is linear not ring. Roberts refutes this argument. Haynes uses the mag-board to suggest an alternative mechanism - that two reactions actually take place, substitution followed by isomerization. Roberts refutes this argument. Haynes next suggests that a benzyne mechanism ought not to give equal yields of products because substituents other than the methyl group do not always give a 50/50 ratio. He again uses a mag-board to aid. Roberts refutes this argument. Haynes suggests another alternative which involves two completing mechanisms - substitution and an attack by amide ion at the positive one along, followed by a hydride ion shift to eliminate bromide. Roberts refutes this argument. Haynes tries to explain the equal yields by postulating another symmetrical intermediate. This mechanism becomes an addition - substitution - elimination process. Roberts answers this argument. Haynes challenges anyone to prove by some independent experiment that benzyne exists. Roberts suggests such an experiment. Shots of a molecular model of triptycene, the product of such an experiment. Haynes sums up.
Master spool number: 6HT/70504
Production number: 00521_2197
Videofinder number: 1727
Available to public: no