Activating / Deactivating groups.

[eric275]

I am having a hard time making a connection with activating and deactivating groups. I don't seem to understand how to look at groups and be able to determine if it is an electron donating group, CH3, or an electron withdrawing group, as in SO3H. I know that it has to do with the electroneg. of the group being added but for some of the groups, NH2,OR, I would think they would be deactivating but turn out to activating? I know that I could just memorize them but I would really like to understand how to tell by looking at the group if possible, thanks.

 

[toony]

... of the group being added but for some of the groups, NH2,OR, I would think they would be deactivating but turn out to activating? ...

not only the electronegativity, you also need to consider lone electron pairs. Both NH2 and OR have lone e pair(s) which would help activating the ring system.. The amine has one and the ester group has two.

 

[HBomb]

Draw the resonance forms. Resonance will show you whether or not a substituent group is activating or deactivating. Donaters activate. Withdrawers deactivate. As toony states, the lone pairs of electrons are important. If you cannot visualize them (or if you can't figure out if they exist or not), you'll have to work on understanding formal charges.

The exceptions are alkyl groups (like -CH3) which donate electrons by induction, not resonance. The others are halogens (like -Br) which withdraw by induction and by donate by resonance (so are overall are deactivators but O,P directors).

 

[eric275]

Thanks for the feedback, it helps a lot!