The stability of intermediates can be compared through various factors, mainly including structure, electronic effects, hybridization effects, and spatial effects.
Structural factors: For example, carbocations are usually more stable in a planar structure, while carboanions are more stable in a pyramidal structure, but when connected to a conjugated system, they can adopt a more stable planar structure.
Electronic effect: Electron withdrawing groups often lead to decreased stability of intermediates, while electron donating groups may increase stability. For example, allyl cations have strong stability due to their electron donating effect.
Hybrid effect: Different types of carbocations (such as benzyl or allyl) are usually more stable, and the stability of carbocations generally follows the rule of 3 °>2 °>1 °.
Space effect: The size and shape of intermediates can also affect their stability. For example, some intermediates with high steric hindrance may be unstable due to their crowded environment.
By comprehensively considering these factors, the stability of different intermediates can be effectively compared.