E-StoreActivating and deactivating groups
28 May 2009 21:02:45 IST
Activating and deactivating groups
This shortcut will help you identify a lot of them:
*groups with multiple bonds on the atom adjacent to the benzene ring are deactivating groups (e.g. cyano group, carbonyl groups).
*groups with lone pairs on the atom adjacent to the benzene ring are activating groups (e.g. OH, NH2).
This is because groups with lone pairs can contribute to resonance stabilization of benzene when a group is added to the ortho/para positions. (Prove it to yourself by drawing phenol and adding a random group (doesn't matter) to the o-, m-, and p- positions on the ring and see how you can draw resonance structures that involve the lone pair on the alcohol. With the new group places meta, you cannot do this.)
It's important to note that the atom adjacent to benzene is the important one... acyl groups have a double bond in them (being an ether bonded to a carbonyl), but the oxygen that is bonded to benzene is the factor that matters, and they are thus activating.
The two factors that contribute to making a group activating or deactivating are resonance and the neighboring group/inductive effect. I already explained how resonance can play a role... As for induction, electronegative groups draw the electron density away from the ring, which means that addition of new groups is unfavored, and some groups contribute electron density, making electrophilic addition easier.
There are a couple of things to remember that aren't accounted for just looking at bonds/lone pairs:
*alkyl and aryl groups are activating groups (weakly, though) because they contribute electron density through inductive effects.
*though halogens are very electronegative, they have many lone pairs, and resonance is a bigger factor than the inductive effect when it comes to electrophilic addition... so they are WEAKLY DEACTIVATING, but STILL ORTHO-PARA DIRECTORS.
*NO2 is strongly deactivating because there is a charge separation with a positive charge (and a negative one, but that one is stabilized by resonance) within the group. It is the prototypical deactivator in my mind. You can see another example of a positive charge making a group strongly deactivating in NH3+.
*groups with multiple bonds on the atom adjacent to the benzene ring are deactivating groups (e.g. cyano group, carbonyl groups).
*groups with lone pairs on the atom adjacent to the benzene ring are activating groups (e.g. OH, NH2).
This is because groups with lone pairs can contribute to resonance stabilization of benzene when a group is added to the ortho/para positions. (Prove it to yourself by drawing phenol and adding a random group (doesn't matter) to the o-, m-, and p- positions on the ring and see how you can draw resonance structures that involve the lone pair on the alcohol. With the new group places meta, you cannot do this.)
It's important to note that the atom adjacent to benzene is the important one... acyl groups have a double bond in them (being an ether bonded to a carbonyl), but the oxygen that is bonded to benzene is the factor that matters, and they are thus activating.
The two factors that contribute to making a group activating or deactivating are resonance and the neighboring group/inductive effect. I already explained how resonance can play a role... As for induction, electronegative groups draw the electron density away from the ring, which means that addition of new groups is unfavored, and some groups contribute electron density, making electrophilic addition easier.
There are a couple of things to remember that aren't accounted for just looking at bonds/lone pairs:
*alkyl and aryl groups are activating groups (weakly, though) because they contribute electron density through inductive effects.
*though halogens are very electronegative, they have many lone pairs, and resonance is a bigger factor than the inductive effect when it comes to electrophilic addition... so they are WEAKLY DEACTIVATING, but STILL ORTHO-PARA DIRECTORS.
*NO2 is strongly deactivating because there is a charge separation with a positive charge (and a negative one, but that one is stabilized by resonance) within the group. It is the prototypical deactivator in my mind. You can see another example of a positive charge making a group strongly deactivating in NH3+.
TO REALLY REMEMBER:
Activating groups are electron donating: -CH3, -R, -NH2, -NR2, -OH, -OR.
Deactivating groups are electron withdrawing: -NO2, -COOH, -COOR, -SO3H, -CN.
Halogens are deactivating by induction, but ortho-para directing by resonance.
Source:
yahooanswers.com (sometimes they beat wiki)
Comments (6)
NugoRama
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Joined: 11 Mar 2009 20:15:19 IST
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28 May 2009 21:17:10 IST
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