The cyano group (CN) is an electron-withdrawing group (EWG). The pKa for meta-cy
ID: 474534 • Letter: T
Question
The cyano group (CN) is an electron-withdrawing group (EWG). The pKa for meta-cyanophenol (A) is 8.61 and the pKa for para-cyanophenol (B) is 7.95. Use this data to explain the effects of the cyano group on the acidity of phenol. Resonance effects should be considered. Use complete drawings to support your answer (i.e., draw out the cyano group and ALL relevant resonance forms of the conjugate bases).
Consider the following guiding questions as you prepare your explanation: 1) What is the relationship between pKa and acidity? Which is the stronger acid, A or B? 2) What do the conjugate bases of these phenols look like? (please refer to them as CB-A and CB-B) 3) Are the cyano groups involved in the resonance of CB-A and/or CB-B?
OH CN CB-A meta-cyanophenol pKa 8.61 OH CN CB-B para-cyanophenol pKa 7.95 Explain the difference in acidity:Explanation / Answer
A stronger acid will have higher dissociation in order to produce more H+ ions.
This means the stronger acid has a larger Ka value.
Since pKa = -log(Ka), this means that larger Ka value leads to a lesser pKa value.
So to conclude, the lesser the pKa value, the stronger is the acid.
Thus, B is stronger acid because it has a lower pKa value.
The conjugate bases of these compounds are formed by the elimination of H+ ion from the OH group attached to the ring. Thus their conjugate bases contain a O- attached to the benzene ring instead of the OH group.
No, the cyano groups are not incolved in resonace because the lone pair on the N-atom is not able to participate in resonance because C-atom, not the N-atom, is in direct contact with the ring. If the cyano group had been attached to the ring via the N-atom, then it would have participated in resonance.
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