Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. When atoms or groups of atoms are bulky or large, they can obstruct the approach of other reactants, making it difficult for them to interact. This phenomenon is particularly important in organic chemistry, where the size and shape of molecules can significantly influence their reactivity. In organic reactions, steric hindrance can affect the rate and outcome of reactions. For example, in substitution reactions, larger groups can block access to reactive sites, leading to slower reactions or favoring alternative pathways. Understanding steric hindrance helps chemists design more effective reactions and predict the behavior of complex molecules.