The steric factor, or steric effect, is the influence on a reaction rate that arises from the size and spatial arrangement of the groups around a reactive site. It matters because bulky groups physically block a reagent's approach. In nucleophilic substitution, for example, large groups around the carbon hinder the nucleophile and so slow the SN2 reaction, which is why crowded (tertiary) substrates react poorly by that route. It is distinguished from the electronic factor, which concerns charge rather than size.

空间因素(位阻效应)是指由反应活性部位周围基团的大小和空间排列引起的、对反应速率的影响。它之所以重要,是因为体积大的基团会从物理上阻挡试剂的接近。例如在亲核取代中,碳周围的大基团妨碍亲核试剂,从而使 SN2 反应减慢,这正是拥挤的(叔)底物经此途径反应不佳的原因。它与电子因素相区别,后者关乎电荷而非大小。