There are three main ‘reactive species’ in organic chemistry; 1. The Nucleophile: In general, a reactive species with an available pair of electrons. In practice, this splits neatly into two categories – neutral species with a lone pair (e.g. H2O where the Oxygen has two lone pairs) and species with a negative charge (anionic) (e.g. OH–, the hydroxide ion). 2. The Electrophile: … Read more
We have established that in order for a reaction to occur at all, ΔG < 0, however, we still have not concluded anything about the rate of reaction. This is the realm of kinetics. For example, petrol at room temperature is stable, although volatile. Cars do not spontaneously explode; the petrol sits in the tank essentially … Read more
Now we have discussed, and looked at, various factors affecting the reactivity of a species, and which types of reagents attack where, we can look at the variation of these factors from one structure to another, and hope these factors also affect the course, and rate, of a reaction. Firstly, a basic tenet of chemistry is that … Read more
Probably one of the most useful tools for discussing conformation (a subject we have touched on) is the Newman Projection. This is representation of a view looking along the bond of a molecule of interest. For example, the following diagram is a Newman Projection of the staggered form of ethane: The circle represents the nearest carbon atom, and … Read more
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
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
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
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
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
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