Explain why benzaldehyde is less reactive than cyclohexanecarbaldehyde towards nucleophilic attack.

The reactivity of aldehydes depends upon the electrophillic character of carbonyl carbon. Greater the electrophillicity of carbonyl carbon greater is the reactivity of aldehyde. Those aldehydes in which carbonyl group is directly attached to electron donating group has less electrophillicity as the electron donating group increases the electron density at carbonyl carbon. Whereas, in case of electron withdrawing group the reactivity is enhanced.

Benzaldehyde and Cyclohexanecarbaldehyde:
                                                                          In benzaldehyde the carbonyl group is attached to benzne ring. The benzene ring being electron rich delocalizes its electron through resonance and forms a double bond between ipso carbon of benzene ring and carbonyl carbon as shown below. This resonance decreases the electrophillic character of carbonyl carbon, hence, making it less reactive. While, in case of Cyclohexanecarbaldehyde no such resonance is possible, hence it is more reactive as compared to Benzaldehyde.