Blocking Group Definition:
Blocking groups, in aromatic substitution reactions, is the functional group that can be readily installed and uninstalled.
Blocking Group Explanation:
Most organic molecules contain more than one functional group, and most functional groups can react in more than one way (Referring to attachment of the functional group in the ortho/para/meta positions of the aromatic compound). This idea is called selectivity, which comprises questions as to which functional groups can react, where will it react and how will they react.
Selectivity comes in three sorts – chemoselectivity, regioselectivity and stereoselectivity – and blocking groups come under ‘regioselectivity’ which implies that a functional group can react in two different places and a reagent must choose where to react.
For Example – In electrophilic aromatic substitution, regioselectivity, comes into question because an electron donating substituent favors ortho/para while electron withdrawing substituent favors meta substitution. Hence, for regiochemical control (choosing ortho or para) we use a blocking group that combines with the aromatic compound in the position where we do not wish the substituent to bond.
For example, consider the bromination of tert-butylbenzene. Here we notice that direct bromination produces the para product as the major product, while the desired ortho product is produced as the minor product. So, in order to achieve the ortho product only, we use a blocking group. In this case, sulphonation is used for blocking.
Therefore, in order for a group to function as blocking group, it must be easily removable after the desired reaction is achieved.