Curly arrow pushing is one of the essential tools of a mechanistic chemist. Many a published article will speculate about the arrow pushing in a mechanism, although it is becoming increasingly common for these speculations to be backed up by quantitative quantum mechanical and dynamical calculations.

In an earlier post I wrote about the iconic S _N 1 solvolysis reaction, and presented a model for the transition state involving 13 water molecules. Here, I follow this up with an improved molecule containing 16 water molecules, and how the barrier for this model compares with experiment.

We have seen in the series of posts on the topic of hypervalency how the first row main group elements such as Be, B, C and N can sustain apparent hypercoordination and arguably hypervalency. The latter is defined not so much by expanding the total valence shell of electrons surrounding the hypervalent atom beyond eight, but in having more than four well defined bonds to it, as quantified by  AIM and ELF analysis.

Quite a few threads have developed in this series of posts, and following each leads in rather different directions. In this previous post the comment was made that coordinating a carbon dication to the face of a cyclopentadienyl anion resulted in a monocation which had a remarkably high proton affinity.

The previous posts have seen how a molecule containing a hypervalent carbon atom can be designed by making a series of logical chemical connections.

The previous post talked about making links or connections. And part of the purpose for presenting this chemistry as a blog is to expose how these connections are made, or or less as it happens in real time (and not the chronologically sanitized version of discovery that most research papers are). So each post represents an evolution or mutation from the previous one.

Science is about making connections. And these can often be made between the most unlikely concepts. Thus in the posts I have made about pentavalent carbon , one can identify a series of conceptual connections.

In the previous two posts, I noted the recent suggestion of how a stable frozen S _N 2 transition state might be made. This is characterised by a central carbon with five coordinated ligands.

In this follow-up to the previous post, I will try to address the question what is the nature of the bonds in penta-coordinate carbon ? This is a difficult question to answer with any precision, largely because our concept of a bond derives from trying to define what the properties of the electrons located in the region between any two specified atoms are.

The bimolecular nucleophilic substitution reaction at saturated carbon is an icon of organic chemistry, and is better known by its mechanistic label, S _N 2 . It is normally a slow reaction, with half lives often measured in hours.

Chemistry can be very focussed nowadays. This especially applies to target-driven synthesis, where the objective is to make a specified molecule, in perhaps as an original manner as possible. A welcome, but not always essential aspect of such syntheses is the discovery of new chemistry.