Double-bonded carbon compounds or ‘unsaturated’ carbon bonds

Molecules with double covalent bonds in them, like all other covalent compounds, have characteristic shapes. Carbon dioxide is a linear molecule, meaning that the atoms cannot rotate (or twist) around double bonds. In O C O the O C O bond angle is therefore 180 . Another example of a compound containing double carbon to carbon bonds is the molecule ethene, C2H4 (Figure 2.12). This molecule is flat (or planar), with the H C H bond angles at 120 to the C C bond. The C C bond cannot twist around the double bond and so the arrangement is quite rigid. The double bond, however, gives the molecule a point of ‘vulnerability’ for chemical attack. This is because it is under great strain at the point of C C. The ‘natural angle’ for the H C H bonds would be 109.5 , but it is 120 in ethene and 180 in carbon dioxide. These angles distorted from the natural angle of 109.5 put a great
strain on the C C bonds. It is these positions that are the ‘weakest links’ in the molecule.