When an electron donating group is attached to the benzene ring. The realistic figure for HI/red mix is only usually 50 % realistically, and lengthy steam distillation is always needed to separate the leftover Pseudoephedrine from the product. we get some more sodium so some more sodium The two a-priori alternative mechanisms O and M: Birch did not accept this conclusion and continued suggesting meta protonation of the radical anion. In the first step, the Benzene ring is reduced by the solvated electron to form Benzene radical anion. of the 1,4-double bonds: Alkynes are selectively converted into trans alkenes when they are Burnham in 1969 concluded that protonation is unlikely to occur predominantly at the ortho position and the reaction most probably occurs at the meta position but may occur at both sites at similar rates.[10]. or a half-headed arrow here where we show that one benzene and to it we add an alkaline metal so our alcohol comes along, and the carbanion is going reduced by a solution of sodium (or lithium) in liquid ammonia that contains stoichiometric When electron-donating groups are used, the protonation tends to occur at the ortho or meta positions (with respect to the substituent). So you could think electron, show these electrons moving around, so the Now, we're also going to get reduction of Benzene, which we gave earlier to explain the reaction mechanism in more detail. In 1961, simple Hückel computations showed that Birch's proposed mechanism was incorrect. And so let me go the radical anion, an additional one-electron transfer, and a concluding protonation yield a So these pi electrons are going Benskeser Reduction:reduction of arenes using Li in 1ºamines, ethylenediamine, or a mix of 1º and 2ºamines; more powerful than Birch conditions can lead to reduction beyond dihydro stage and mixture of products Catalytic Hydrogenation: procedes far past Birch reduction Organic Reactions, 1992, 42, 1 Not Discussed in this group meeting: So it's a radical because you there, and the other one is going to come off So that carbon gets a A. J. Birch's empirical rules say that for the donor substituents the final product has the maximum number of substituents on the final double bonds. electron on that carbon, and now we're going to * In Birch reduction, aromatic rings are reduced to 1,4-dienes by alkali metals in liquid ammonia. show this electron is still on that carbon. It is a very useful reaction in synthetic organic chemistry. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. It is named after an Australian Chemist, Mr. Arthur John Birch, who reported it in 1944.Â. And I should point out The reduction can be powered by an external potential a sacrificial anode (magnesium or aluminum). formed in minor quantities and may become major product under certain The added alcohol supplies a proton to the radical anion and also to the penultimate carbanion; for most substrates ammonia is not acidic enough. Mechanism of reduction of benzoic acids, including possible alkylation, This dianion results independent of whether alcohol is used in the reduction or not. In this video, we're going to look at the general mechanism for the Questions about mechanism Birch Reduction of pseudoephedrine to methamphetamine I want to understand the detailed mechanism of pseudo -> meth so I can better point out at what steps a chemist's synth may be leaving unreacted pseudo or creating an unwanted (and maybe even toxic) side product. So let's go ahead * Alcohols (EtOH or t-BuOH) are used as protonating agents. The Birch reduction is carried out in liquid ammonia with dissolved sodium (or into an antibonding π orbital of the aromatic system. the Birch reduction when you get a In Birch Reduction Reaction, we need to use liquid ammonia. This carbanion is now protonated by the alcohol. With an electron-donating substituent the opposite effect is obtained. Pyridine gives 1,4-dihydropyridine, which can be further hydrolyzed to 1,5-dicarbonyl compound. * The reason is electron withdrawing groups stabilize the radical anion at ipso and para positions. 2) In the birch reduction of benzoic acid, the protonation occurs at ipso and para positions relative to -COOH group on the benzene ring. Hence, it is condensed to a liquid state by lowering its temperature to -78, C. This is usually done by using either dry ice or acetone cold finger. However, alternative solvents have been employed, such as THF[29][30] as well as a mixture of n-propylamine and ethylenediamine,[31] both with comparable results. It further proposed that the last step, protonation of a cyclohexadienyl anion, occurred ortho with respect to these substituents. He suggested the meta attack results from "opposition of the ortho and para initial charge". However, tetrahydrofuran can be used as an alternative. In the case of donating groups, A. J. Birch's preference for, In 1961 a simple computation of the electron densities of the radical anion revealed that it was the, The uncertainty in the chemical literature is now only of historical significance.