1 Molecular shape affects molecular reactivity

To start your exploration of the effect of the shape of molecules on how they react, let us start with two simple examples: methane, CH₄, and bromomethane, CH₃Br.

In both cases, the carbon atoms form four single bonds and it turns out that these are directed towards the corners of a tetrahedron (you will see why this is later).

The resulting molecular shapes are shown in Figure 2 as ball-and-stick representations. The grey sphere is the carbon atom, the white spheres represent the hydrogen atoms and the red sphere is bromine.

Figure 2 The structures of (a) methane, CH₄; (b) bromomethane, CH₃Br

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Two ball-and-stick representations of methane CH₄ (labelled (a) in the diagram) and bromomethane CH₃Br (labelled (b) in the diagram). Both diagrams have a central carbon atom (shown as a grey sphere). Methane then has 4 white spheres connected to it (the hydrogen atoms). Bromomethane has 3 white spheres (hydrogen) and one red sphere (representing bromine).

Figure 2 The structures of (a) methane, CH₄; (b) bromomethane, CH₃Br

Ball-and-stick representations of each of these molecules are shown at the top of Figure 3. Again at each carbon atom, there are four bonds directed towards the corners of a tetrahedron, and the complexity of the molecular shape increases from left to right, in line with an increase in the number of carbon atoms.

Figure 3 The molecules (a) bromomethane, CH₃Br; (b) bromoethane, CH₃CH₂Br; (c) 2-bromopropane, (CH₃)₂CHBr; (d) 2-bromo-2-methylpropane, (CH₃)₃CBr, shown in both ball-and-stick (top) and space-filling representations (bottom)

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Two diagrams of each molecule are displayed, both ball-and-stick (top) and space-filling representations (bottom). Labels explain that they are arranged so the leftmost molecules are 'least crowded/most reactive' and the rightmost molecules are 'most crowded/least reactive'. From left to right, the molecules are (a) bromomethane, CH₃Br; (b) bromoethane, CH₃CH₂Br; (c) 2-bromopropane, (CH₃)₂CHBr; (d) 2-bromo-2-methylpropane, (CH₃)₃CBr,

Figure 3 The molecules (a) bromomethane, CH₃Br; (b) bromoethane, CH₃CH₂Br; (c) 2-bromopropane, (CH₃)₂CHBr; (d) 2-bromo-2-methylpropane, ...

Ball-and-stick representations are a natural three-dimensional development of structural formulae, and they show the disposition of the atoms in space. But by emphasising the bonds, they fail to reveal the subtleties of the molecular shape created by the different sizes of atoms. In this respect, so-called space-filling models are better. These appear at the bottom of Figure 3. In each case, the viewing direction is the same as the ball-and-stick model above.

Note: also shown in Figure 3 are the relative rates of reaction of the four compounds when they are treated with lithium iodide in acetone (propanone) solution – you will be looking at this in the next section.