Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?
Posts Tagged ‘Tutorial material’
The mechanism of the Birch reduction. Part 3: reduction of benzene
Tuesday, December 4th, 2012The mechanism of the Birch reduction. Part 1: reduction of anisole.
Saturday, December 1st, 2012The Birch reduction is a classic method for partially reducing e.g. aryl ethers using electrons (from sodium dissolved in ammonia) as the reductant rather than e.g. dihydrogen. As happens occasionally in chemistry, a long debate broke out over the two alternative mechanisms labelled O (for ortho protonation of the initial radical anion intermediate) or M (for meta protonation). Text books seem to have settled down of late in favour of O. Here I take a look at the issue myself.
The regiospecificity of di-imide reduction of an alkene.
Sunday, November 25th, 2012Not a few posts on this blog dissect the mechanisms of well known text-book reactions. But one reaction type where there are few examples on these pages are reductions. These come in three types; using electrons, using a hydride anion and using di-hydrogen. Here I first take a closer look at the third type, and in particular di-hydrogen as delivered from di-imide.
Secrets of a university tutor. An exercise in mechanistic logic: second dénouement.
Monday, October 29th, 2012Following on from our first mechanistic reality check, we now need to verify how product A might arise in the mechanism shown below, starting from B.
Secrets of a university tutor. An exercise in mechanistic logic: first dénouement.
Sunday, October 28th, 2012The reaction described in the previous post (below) is an unusual example of nucleophilic attack at an sp2-carbon centre, reportedly resulting in inversion of configuration[1]. One can break it down to a sequence of up to eight individual steps, which makes teaching it far easier. But how real is that sequence?
References
- T.C. Clarke, and R.G. Bergman, "Olefinic cyclization at a vinyl cation center. Inversion preference for intramolecular nucleophilic substitution by a double bond", Journal of the American Chemical Society, vol. 94, pp. 3627-3629, 1972. http://dx.doi.org/10.1021/ja00765a062
Secrets of a university tutor. An exercise in mechanistic logic, prequel.
Saturday, October 27th, 2012The reaction below plays a special role in my career. As a newly appointed researcher (way back now), I was asked to take tutorial groups for organic chemistry as part of my duties. I sat down to devise a suitable challenge for the group, and came upon the following reaction[1]. I wrote it down on page 2 of my tutorial book, which I still have. I continue to use this example in tutorials to this day, some 35 years later.
References
- T.C. Clarke, and R.G. Bergman, "Olefinic cyclization at a vinyl cation center. Inversion preference for intramolecular nucleophilic substitution by a double bond", Journal of the American Chemical Society, vol. 94, pp. 3627-3629, 1972. http://dx.doi.org/10.1021/ja00765a062
How is the bromination of alkenes best represented?
Sunday, October 14th, 2012I occasionally delve into the past I try to understand how we got to our present understanding of chemistry. Thus curly arrow mechanistic notation can be traced back to around 1924, with style that bifurcated into two common types used nowadays (on which I have commented and about which further historical light at the end of this post). Here I try to combine these themes with some analysis of wavefunctions for a particularly troublesome reaction to represent, the dibromination of an alkene, which I represented in the previous post as shown below.