There are four major categories into which organic reactions fall. These are: Substitution Addition Elimination Rearrangement Substitution To deal with 1. first, substitution usually occurs on carbon. There are two sub-categories of this type: nucleophilic, and electrophilic. This is an example of electrophilic substitution (nitration of benzene): Below is a characteristic example of nucleophilic substitution: NC– is the … Read more
Whereas electrophiles have areas of electron deficiency, nucleophiles have areas that are electron rich. A nucleophilic “nucleus-loving” species will tend to attack electron-deficient areas.e.g.: Chloromethane (left) has an electron deficient carbon atom due to the presence of the inductively withdrawing chlorine atom. This is therefore most readily attacked by negatively charged species (OH–, CN– etc.), or by species … Read more
In our discussion of inductive and mesomeric effects, we have covered the concept that certain groups donate or withdraw electrons. This has the effect of making areas of molecules electron rich or poor, and this in turn will clearly influence their reactivity with other molecules. An example of an electron rich molecule is the phenoxide ion shown above. Clearly, this is … Read more
The hybridisation and therefore shape of a radical can be anywhere between sp3 and sp2, although the latter is the more common (with the unpaired electron in the p orbital) – there is some preference for it, shown by the difficulty of forming radicals in bridged species that cannot reach sp2 hybridisation. Radicals are not particularly stable in general, as … Read more
A radical is a highly reactive species which has one unpaired electron. They tend to be formed by homolytic fission of single bonds – this is in contrast to the heterolytic fission which dominates the chemistry of nucleophiles and electrophile – as shown below; In the gas phase, this bond-breaking is always more energetically favourable than the alternative heterolytic fission … Read more
It may be useful to refer to the physical chemistry sections on orbital theory and “electron in a box” diagrams before continuing with this section if they are not familiar concepts. The analogy of the harmonics (standing waves) of a piece of string fastened at both ends to an electron in an energy level is useful in this instance. … Read more
After mechanisms had been developed which brought order and reason to many organic reactions, a few were still elusive. They did not appear to proceed with a polar or radical pathway. A famous example of one such reaction is the Diels-Alder reaction: This involves the addition of an alkene to a conjugated diene. Reactions such as the Diels-Alder have one very … Read more
A substitution reaction is one with the general form; For example; For a nucleophilic substitution, Z is the nucleophile. This nucleophile will attack an area of a molecule which has partial positive charge – or in other words is electrophilic. In the example above, the carbon-iodine bond is polar (because iodine is more electronegative than carbon), so there is partial positive charge … Read more
Carbonyls The other unsaturated system that will commonly undergo nucleophilic addition. However, a carbonyl system is most likely to undergo nucleophilic substitution with a different mechanism to SN1, SN2 etc. and this is dealt with in a separate section. The reason for the attractiveness of carbonyl systems to nucleophiles is that the Carbon-Oxygen double bond is quite polar, as shown by the resonance structures which leave a δ+ on the carbon. Compared … Read more
Nucleophilic addition is essentially the opposite of elimination – shown in general form below; The two main types of unsaturated systems that will undergo this are alkenes and carbonyls. These two systems are very different, so their examples will be kept separate, but a general pattern should become evident. The other type of addition that occurs (also onto unsaturated systems) is electrophilicaddition, … Read more