Answer:
The activation strength of substituents in electrophilic aromatic substitution reactions refers to their ability to increase the reactivity of an aromatic ring towards electrophilic attack. Substituents that are electron-donating or have a positive inductive effect are considered activating, while those that are electron-withdrawing or have a negative inductive effect are considered deactivating. Here is the ranking of the given substituents by increasing activation strength:
-Br
-CH2CH3
-CN
-N(CH3)2
Step-by-step explanation:
-Br (bromine) is a deactivating substituent. It has a negative inductive effect, which withdraws electron density from the ring, making it less reactive towards electrophilic aromatic substitution reactions. Hence, it has the lowest activation strength among the given substituents.
-CH2CH3 (ethyl) is a weakly activating substituent. It has a slight electron-donating effect due to the +I (inductive) effect of the alkyl group, which can increase the electron density on the aromatic ring and make it more reactive towards electrophilic attack. However, the effect is relatively weak compared to other activating groups, so it has a moderate activation strength.
-CN (cyano) is a moderately activating substituent. It has both electron-donating (+I) and electron-withdrawing (-M) effects. The electron-donating effect dominates over the electron-withdrawing effect, making it an activating group overall. It can increase the electron density on the aromatic ring and enhance its reactivity towards electrophilic substitution reactions.
-N(CH3)2 (dimethylamino) is a strongly activating substituent. It has a strong electron-donating effect (+I), which can significantly increase the electron density on the aromatic ring and make it highly reactive towards electrophilic attack. Hence, it has the highest activation strength among the given substituents.
In summary, the ranking of the given substituents by increasing activation strength towards electrophilic aromatic substitution reactions is: -Br < -CH2CH3 < -CN < -N(CH3)2.