Question

what is steeric hindrance of protonoation

Answer 1

Steric effects arise from a fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Exchange interaction, or Born repulsion), and this may affect the molecule's preferred shape (conformation) and reactivity.
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Answer 2

steric hindrance is the phenomenon of avoiding the bonding between the molecules or atom and a molecule, due to the large size of the one of the molecules which avoid the attack of the other reactant.