Substitution is the replacement of one group by another. You met such reactions in Chapter 10, and an example is shown in the margin. This reaction is a substitution because the Cl group is replaced by the NH₂ group. You learnt to call the molecule of ammonia (NH₃) the nucleophile and the chloride you called the leaving group. In Chapter 10, the substitution reactions always took place at the trigonal (sp²) carbon atom of a carbonyl group. In this chapter we shall be looking at reactions such as the second reaction in the margin. These are substitution reactions, because the Cl group is replaced by the PhS group. But the CH2 group at which the reaction takes place is a tetrahedral (sp3), or saturated, carbon atom, rather than a C=O group. This reaction and the one above may look superficially the same but they are quite different in mechanism. The requirements of good reagents are also different in substitutions at carbonyl groups and at saturated carbon—that’s why we changed the nucleophile from NH3 to PhS−: ammonia would not give a good yield of PhCH2NH2 in the second reaction. Let’s have a look at why the mechanisms of the two substitutions must be different. Here’s a summary of the mechanism of the first reaction.

mechanism of nucleophilic substitution at the carbonyl group

In the first step the nucleophile attacks the C=O π bond. It’s immediately obvious that the fi rst step is no longer possible at a saturated carbon atom. The electrons cannot be added to a π bond as the CH₂ group is fully saturated. In fact, there is no way for the nucleophile to add before the leaving group departs (as it did in the reaction above) because this would give an impossible five-valent carbon atom. Instead, two new and different mechanisms become possible. Either the leaving group goes first and the nucleophile comes in later, or the two events happen at the same time. The first of these possibilities you will learn to call the SN1 mechanism. The second mechanism, which shows how the neutral carbon atom can accept electrons provided it loses some at the same time, you will learn to call the SN2 mechanism. You will see later that both mechanisms are possible with this molecule, benzyl chloride.

مواضيع ذات صلة

Significance of the SN2 rate equation

Resolutions using diastereoisomeric salts

Separating enantiomers is called resolution

Chiral compounds with no stereogenic centres

Diastereoisomers can arise when structures have more than one stereogenic centre

Absolute and relative stereochemistry

Diastereoisomers can be chiral or achiral

Dia stereoisomers are stereoisomers that are not enantiomers

The rotation of plane-polarized light is known as optical activity

R and S can be used to describe the configuration of a chiral centre

Many chiral molecules are present in nature as single enantiomers

Stereogenic centres