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maverick280857
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Hi
I have a few questions in organic chemistry. I would be very grateful if someone can answer them:
1. (cf Morrison & Boyd 6th ed page 523). "Strongly activating groups generally win out over deactivating or weakly activating groups."
When the bromination of 3-hydroxybenzaldehyde (m-hydroxybenzaldehyde) is carried out using ferric bromide, the chief product is 2-bromo-5-hydroxybenzaldehyde and not 4-bromo-3-hydroxybenzaldehyde. Why?
My reasoning: There are three possible products:
2-bromo-5-hydroxybenzaldehyde (the one listed as the chief product)
4-bromo-3-hydroxybenzaldehyde (I think this should be formed in appreciable quantity as well)
2-bromo-3-hydroxybenzaldehyde (is this not formed due to steric hindrance? I mean if three groups are at 1,2,3 positions would the steric inhibition be responsible for trace formation?)
As -CHO is a meta director and deactivator and -OH is a strongly activating ortho and para director, the effect of -OH will be more in terms of directive influence. However, I have been unable to answer the questions rasied above.
2. I believe the mesomeric effect (+M effect) of -OH, -NH2, -OMe and -NHMe is valid only at the ortho and para positions. That is, when there is a group G attached to a benzene ring, if these groups are at ortho or para positions with respect to G, then the +M effect occurs and causes electron donation to the ring by resonance. However, when they are at the meta position with respect to G, the +M effect does not occur. Why?
I discovered also that most organic chemistry textbooks have the phrase "electron donation/withdrawl by resonance" written as such. So in case you have not encountered the term Mesomeric Effect, it is essentially the same thing: +M effect means electron donation to a conjugated system by resonance and -M effect means electron withdrawl from a conjugated system by resonance.
I am unable to convince myself that the +M effect does not occur when a +M group is at a meta position with respect to a group G already attached to the benzene ring, by drawing resonance structures (which I believe is due to the fact that delocalization-based stabilization of these structures due to +M effect does not occur. But why?)
3. This is related to question 2; I have to compare the stabilities of 9 carbocations among which are the following two carbocations:
m-methoxy benzyl carbocation
p-methoxy benzyl carbocation
If the fact mentioned in question 2 (above) indeed be true, then the p-methoxy benzyl carbocation is more stable as the methoxy group can donate electrons to the benzene ring by resonance and the positive charge on the benzylic carbon can be decreased. On the other hand, there is no such donation possible when the methoxy group is meta with respect to the benzylic carbon (or rather the benzylic group). In both cases, the methoxy group withdraws electrons inductively, but this effect is somewhat less in comparison to the (possible) resonance-based donation as the methoxy group is known to be a strongly activating group. So the para substituted cation should be more stable.
Again the question is: why is this so?
I am sorry for the long post but I thought I should mention my reasoning here as well.
Thanks and cheers,
Vivek
I have a few questions in organic chemistry. I would be very grateful if someone can answer them:
1. (cf Morrison & Boyd 6th ed page 523). "Strongly activating groups generally win out over deactivating or weakly activating groups."
When the bromination of 3-hydroxybenzaldehyde (m-hydroxybenzaldehyde) is carried out using ferric bromide, the chief product is 2-bromo-5-hydroxybenzaldehyde and not 4-bromo-3-hydroxybenzaldehyde. Why?
My reasoning: There are three possible products:
2-bromo-5-hydroxybenzaldehyde (the one listed as the chief product)
4-bromo-3-hydroxybenzaldehyde (I think this should be formed in appreciable quantity as well)
2-bromo-3-hydroxybenzaldehyde (is this not formed due to steric hindrance? I mean if three groups are at 1,2,3 positions would the steric inhibition be responsible for trace formation?)
As -CHO is a meta director and deactivator and -OH is a strongly activating ortho and para director, the effect of -OH will be more in terms of directive influence. However, I have been unable to answer the questions rasied above.
2. I believe the mesomeric effect (+M effect) of -OH, -NH2, -OMe and -NHMe is valid only at the ortho and para positions. That is, when there is a group G attached to a benzene ring, if these groups are at ortho or para positions with respect to G, then the +M effect occurs and causes electron donation to the ring by resonance. However, when they are at the meta position with respect to G, the +M effect does not occur. Why?
I discovered also that most organic chemistry textbooks have the phrase "electron donation/withdrawl by resonance" written as such. So in case you have not encountered the term Mesomeric Effect, it is essentially the same thing: +M effect means electron donation to a conjugated system by resonance and -M effect means electron withdrawl from a conjugated system by resonance.
I am unable to convince myself that the +M effect does not occur when a +M group is at a meta position with respect to a group G already attached to the benzene ring, by drawing resonance structures (which I believe is due to the fact that delocalization-based stabilization of these structures due to +M effect does not occur. But why?)
3. This is related to question 2; I have to compare the stabilities of 9 carbocations among which are the following two carbocations:
m-methoxy benzyl carbocation
p-methoxy benzyl carbocation
If the fact mentioned in question 2 (above) indeed be true, then the p-methoxy benzyl carbocation is more stable as the methoxy group can donate electrons to the benzene ring by resonance and the positive charge on the benzylic carbon can be decreased. On the other hand, there is no such donation possible when the methoxy group is meta with respect to the benzylic carbon (or rather the benzylic group). In both cases, the methoxy group withdraws electrons inductively, but this effect is somewhat less in comparison to the (possible) resonance-based donation as the methoxy group is known to be a strongly activating group. So the para substituted cation should be more stable.
Again the question is: why is this so?
I am sorry for the long post but I thought I should mention my reasoning here as well.
Thanks and cheers,
Vivek
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