Previously, I explored the Graham reaction to form a diazirine. The second phase of the reaction involved an Sn2′ displacement of N-Cl forming C-Cl. Here I ask how facile the simpler displacement of C-Cl by another chlorine might be and whether the mechanism is Sn2 or the alternative Sn1.
The reason for posing this question is that as an Sn1 reaction, simply ionizing off the chlorine to form a diazacyclopropenium cation might be a very easy process. Why? Because the resulting cation is analogous to the cyclopropenium cation, famously proposed by Breslow as the first example of a 4n+2 aromatic ring for which the value of n is zero and not 1 as for benzene.[cite]10.1021/ja01576a067[/cite] Another example of a famous “Sn1” reaction is the solvolysis of t-butyl chloride to form the very stable tertiary carbocation and chloride anion (except in fact that it is not an Sn1 reaction but an Sn2 one!)
Posts Tagged ‘Substitution reactions’
Smoke and mirrors. All is not what it seems with this Sn2 reaction!
Thursday, April 4th, 2019An alternative mechanism for nucleophilic substitution at silicon using a tetra-alkyl ammonium fluoride.
Friday, May 27th, 2016In the previous post, I explored the mechanism for nucleophilic substitution at a silicon centre proceeding via retention of configuration involving a Berry-like pseudorotation. Here I probe an alternative route involving inversion of configuration at the Si centre. Both stereochemical modes are known to occur, depending on the leaving group, solvent and other factors.[cite]10.1016/S0040-4020(01)89077-3[/cite],[cite]10.1021/ja01006a024[/cite],[cite]10.1021/ja00784a081[/cite]
Mesomeric resonance in substituted benzenes: a crystallographic reality check.
Wednesday, August 26th, 2015Previously, I showed how conjugation in dienes and diaryls can be visualised by inspecting bond lengths as a function of torsions. Here is another illustration, this time of the mesomeric resonance on a benzene ring induced by an electron donating substituent (an amino group) or an electron withdrawing substituent (cyano).
A new way of exploring the directing influence of (electron donating) substituents on benzene.
Friday, April 17th, 2015The knowledge that substituents on a benzene ring direct an electrophile engaged in a ring substitution reaction according to whether they withdraw or donate electrons is very old.[cite]10.1039/CT8875100258[/cite] Introductory organic chemistry tells us that electron donating substituents promote the ortho and para positions over the meta. Here I try to recover some of this information by searching crystal structures.