Cahn-Ingold-Prelog Rules

These are rules that allow for the universal description of the geometries (and configurations) about stereocentres. The first thing to do, is to assign priorities: To do this, we simply place the substituents on a stereocentre in order of their atomic numbers (starting with the highest first). To distinguish between different isotopes, rank the heaviest as higher priority. … Read more

Stereoisomers of tartaric acid

Consider the following set of stereoisomers of tartaric acid (Please note the axial bonds are deliberately, and unnaturally, elongated to make the diagrams clearer) : In each case, there are two stereocentres.  1 and 2 are clearly enantiomers, however, what may not be immediately apparent is that 3 and 4 are in fact the same molecule. Molecules that … Read more

Definitions in Stereochemistry

Having discussed some of the basic concepts of stereochemistry, it is necessary to understand some of the language used to describe stereochemistry. We have already met the term enantiomer.  It is defined as: “A species with a non-superimposable mirror image.”  An example of a pair of enantiomers is lactic acid: Optical isomerism is used to describe the … Read more

Introduction to stereochemistry

We have already discussed the structure of chemical compounds, and how that relates to their reactivity.  However, there is one important aspect we ignored; stereochemistry.  Stereochemistry is the study of the orientation in space of the atoms of a molecule.  Drawing the structure of a molecule merely depicts the linkage between atoms, and doesn’t necessarily define … Read more

Huckel’s Rule

Simply put, Huckel’s rule for aromaticity states that a monocyclic system will be aromatic if there are 4n + 2 delocalised electrons, (n an integer) contained within it. e.g.: 2, 6, 10, 14 etc. Huckel arrived at this rule by performing molecular orbital calculations on cyclic systems containing x carbon atoms, and with each carbon atom supplying one pi electron.  He connected high aromatic stability with … Read more

Aromaticity

The most well-known aromatic molecule is benzene, so we shall start with a brief discussion of this molecule.  It must be made clear at this point that “aromatic” here refers to a molecule with certain electronic properties, it does not mean a molecule that smells (although that is in fact where the name first came from)! The … Read more

Resonance

Resonance is a useful concept in chemistry. Earlier, we looked at the following carbocation, and explained it in terms of a spreading of charge.  An alternative (and sometimes more useful) way of descibing this phenomenon of the lowering of the energy of this species is by use of resonance. Let us draw the molecule again, without the p orbitals … Read more

Carbocations

A carbocation is a positively charged species.  The positive charge is localised on an sp2 hybridised carbon atom.  Hence the carbocation itself is planar around this carbon.  The p orbital of the sp2 carbon is empty. Typically they are significantly stabilised if the positive charge can be delocalised across the molecule, and thus they form a part of our discussion … Read more

Oxidation and Reduction: An Introduction

The oxidation and reduction of organic molecules forms a very important part of many organic reactions and syntheses. There is a massive variety of reagents available and a vast array of systems that can be oxidised and/or reduced. However, the simple ‘oxidation is losing’ ‘reduction is gaining’ (‘oilrig‘) way of identifying electron transfer redox processes in inorganic chemistry is difficult to use in terms of organic chemistry. … Read more

Reducing Aromatic Rings (Level 1 Continued)

The Birch Reduction: This extremely useful reaction uses the alkali metal/ammonia solution again to partially reduce aromatic rings. The basic reaction scheme is as follows; However, the reaction becomes more interesting when substituents on the ring are involved – it seems that EWGs and EDGs cause a different arrangement of the final two double bonds: The key to … Read more