The effect of the nitro group on acidity in aromatic systems is dependent upon its location though regardless of its relative position to an acidic group (e.g., phenolic OH) it always increases acidity.
The effects that need to be considered: para position, the main effect is resonance, perhaps the only effect in the para position.
In the meta position, resonance (with the OH group) is not an option and only inductive effects are important. Inductive effects are through bond and fall off with distance, are minimal for the para-position (hence only resonance) and may play a role in the ortho position though other factors may override. Inductive effects tend to be less than resonance effects.
In the ortho position, both resonance and inductive effects may play a role though other effects such as steric inhibition of resonance or internal hydrogen bonding may counter the resonance effect.
Based on this, the order of acidity is predicted to be (considering only resonance and inductive effects), ortho > para > meta. The actual pKas for these phenols is para > ortho > meta (pKas 7.15, 7.2, 8.4, respectively). Note that the pKa of phenol is 10 so the nitro group increases acidity (smaller pKa corresponds to stronger acid).
So, why isn’t the ortho > para. First, they are close (difference is 0.05 pKa units). Second, the phenolic H may internally H-bond to a nitro O and be less available to act as an acidic proton. Alternatively, there may be some steric inhibition of resonance, decreasing the acidity of the ortho derivative. Most people think the reason is the former.
The effect of the nitro group on acidity in aromatic systems is dependent upon its location though regardless of its relative position to an acidic group (e.g., phenolic OH) it always increases acidity.
The effects that need to be considered: para position, the main effect is resonance, perhaps the only effect in the para position.
In the meta position, resonance (with the OH group) is not an option and only inductive effects are important. Inductive effects are through bond and fall off with distance, are minimal for the para-position (hence only resonance) and may play a role in the ortho position though other factors may override. Inductive effects tend to be less than resonance effects.
In the ortho position, both resonance and inductive effects may play a role though other effects such as steric inhibition of resonance or internal hydrogen bonding may counter the resonance effect.
Based on this, the order of acidity is predicted to be (considering only resonance and inductive effects), ortho > para > meta. The actual pKas for these phenols is para > ortho > meta (pKas 7.15, 7.2, 8.4, respectively). Note that the pKa of phenol is 10 so the nitro group increases acidity (smaller pKa corresponds to stronger acid).
So, why isn’t the ortho > para. First, they are close (difference is 0.05 pKa units). Second, the phenolic H may internally H-bond to a nitro O and be less available to act as an acidic proton. Alternatively, there may be some steric inhibition of resonance, decreasing the acidity of the ortho derivative. Most people think the reason is the former.
Increases the acidic strength with a -M effect dominating on ortho and para positions.
Even on the meta position, the -I Effect stabilises the conjugate base.
Increases the acidic strength with a -M effect dominating on ortho and para positions.
Even on the meta position, the -I Effect stabilises the conjugate base.
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The effect of the nitro group on acidity in aromatic systems is dependent upon its location though regardless of its relative position to an acidic group (e.g., phenolic OH) it always increases acidity.
The effects that need to be considered: para position, the main effect is resonance, perhaps the only effect in the para position.
In the meta position, resonance (with the OH group) is not an option and only inductive effects are important. Inductive effects are through bond and fall off with distance, are minimal for the para-position (hence only resonance) and may play a role in the ortho position though other factors may override. Inductive effects tend to be less than resonance effects.
In the ortho position, both resonance and inductive effects may play a role though other effects such as steric inhibition of resonance or internal hydrogen bonding may counter the resonance effect.
Based on this, the order of acidity is predicted to be (considering only resonance and inductive effects), ortho > para > meta. The actual pKas for these phenols is para > ortho > meta (pKas 7.15, 7.2, 8.4, respectively). Note that the pKa of phenol is 10 so the nitro group increases acidity (smaller pKa corresponds to stronger acid).
So, why isn’t the ortho > para. First, they are close (difference is 0.05 pKa units). Second, the phenolic H may internally H-bond to a nitro O and be less available to act as an acidic proton. Alternatively, there may be some steric inhibition of resonance, decreasing the acidity of the ortho derivative. Most people think the reason is the former.
The effect of the nitro group on acidity in aromatic systems is dependent upon its location though regardless of its relative position to an acidic group (e.g., phenolic OH) it always increases acidity.
The effects that need to be considered: para position, the main effect is resonance, perhaps the only effect in the para position.
In the meta position, resonance (with the OH group) is not an option and only inductive effects are important. Inductive effects are through bond and fall off with distance, are minimal for the para-position (hence only resonance) and may play a role in the ortho position though other factors may override. Inductive effects tend to be less than resonance effects.
In the ortho position, both resonance and inductive effects may play a role though other effects such as steric inhibition of resonance or internal hydrogen bonding may counter the resonance effect.
Based on this, the order of acidity is predicted to be (considering only resonance and inductive effects), ortho > para > meta. The actual pKas for these phenols is para > ortho > meta (pKas 7.15, 7.2, 8.4, respectively). Note that the pKa of phenol is 10 so the nitro group increases acidity (smaller pKa corresponds to stronger acid).
So, why isn’t the ortho > para. First, they are close (difference is 0.05 pKa units). Second, the phenolic H may internally H-bond to a nitro O and be less available to act as an acidic proton. Alternatively, there may be some steric inhibition of resonance, decreasing the acidity of the ortho derivative. Most people think the reason is the former.
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VOTE