Organic 2 Review Problem 193

Organic 2 Review Problem 193 - very low concentration of...

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PREPARATIONS 177 Ethyl y-Bromocrotonate. BrCH 2 CHiCH-COOC 2 H 5 . (Using N-Bromosuccinimide.) A T -Bromosuccinimide (prepared by the action of bromine on succinimide at o° in the presence of sodium hydroxide) is a valuable specific reagent for brominating defines in the a-methylene position to the double bond without simultaneously adding bromine to this bond. For example, if A T -bromosuccini- mide is represented by (C 4 H 4 O 1 )NBr:— (C 4 H 4 O 2 )NBr-fCH s -CH:CH-COOEt->(C 4 H 4 O8)NH+BrCH 2 -CH:CH-COOEt The reaction was formerly considered to involve a radical mechanism initiated by the non-ionic fission of the very weak N-Br bond. H 2 C-CO H 2 C-CO I NNBF -* j ^>N + Br H 2 C-CO H 2 C-CO H 8 C-CO H 2 C-CO j \N --!-CH 3 -CHiCH-COOEt -> j ^)NH + CH 2 CHiCH-COOEt H 2 C-CO H 2 C-CO H 1 C-CO H 2 C-CO - CH 2 CH:CH-COOEt + ^)NBr -> BrCH 2 -CHrCH-COOEt + ^)N H 2 CCO H 2 C-CO Such a mechanism is supported by the fact that the reaction is accelerated by benzoyl peroxide and other radical-producing agents. It is now however considered that the function of the A T -bromosuccinimide is to provide a constant,
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Unformatted text preview: very low concentration of molecular bromine (Tedder et al.). The substitution is best carried out by boiling zV-bromosuccinimide with the olefine in carbon tetrachloride. Succinimide crystallises out from the carbon tetrachloride on cooling whereas the brominated product remains dissolved in the carbon tetrachloride. Required: Ethyl crotonate, 22-8 g.; A-bromosuccinimide, 35-6 g.; carbon tetrachloride, 40 ml. Dissolve 22-8 g. of ethyl crotonate in 40 ml. of dry carbon tetrachloride and add 35 -6 g. of A r-bromosuccinimide.* Heat the mixture under reflux for three hours. Cool to o° and filter off the succinimide which is insoluble in cold carbon tetrachloride. Now shake the filtrate with water in a separating funnel, separate and dry the carbon tetrachloride layer with sodium sulphate. Filter through a fluted filter-paper into a Claisen flask and distil *Add o-2 g. of benzoyl peroxide to accelerate reaction....
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This note was uploaded on 09/19/2011 for the course CHM 2211 taught by Professor Castalleano during the Fall '06 term at University of Florida.

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