I am attempting to answer a question which asks to explain the sequence of pKaH values for a series of substituted guanidines (NH2)2CNR where R varies.
The order is R = H , Ph , CH3CO , MeO , CN , NO2
pKaH = 14.5 , 10.8 , 8.3 , 7.5 , -0.4 , -0.9
I'm not quite sure how to explain this. I think pKaH refers to the pKa of the conjugate acid. I'm guessing CN and NO2 are both very electron withdrawing and allow H+ to be lost easily. However I thought that Ph was electron withdrawing as well, so I can't understand why it has such a high pKa. I'm also not really sure how the ketone and MeO groups would affect the pKa. From what I have found, methoxy is electron releasing so I am quite confused.
To explain the sequence of pKaH values for the series of substituted guanidines (NH2)2CNR where R varies, we need to consider the electron-withdrawing or electron-donating effects of the substituents.
First, let's understand the concept of pKaH. It represents the measure of acidity and indicates the tendency of a compound to lose a proton (H+). The lower the pKaH value, the stronger the acid.
Now, let's analyze the given pKaH values:
1. R = H (pKaH = 14.5): This compound has no substituent (R = H). Since it does not have any electron-withdrawing or electron-donating groups, the guanidine with no substituent will have the highest pKaH value. This means it is the least acidic.
2. R = Ph (pKaH = 10.8): The phenyl group (Ph) is electron-withdrawing due to the presence of delocalized π-electrons in the aromatic system. The electron-withdrawing effect stabilizes the conjugate base after losing a proton, making it less acidic than the guanidine without a substituent (H).
3. R = CH3CO (pKaH = 8.3): The acetyl group (CH3CO) contains a carbonyl group (C=O) which has an electron-withdrawing effect. However, the carbonyl group is also attached to an alkyl group (CH3), which has a slight electron-donating effect. The overall effect of the CH3CO group is weaker electron withdrawal compared to the phenyl group. Hence, the pKaH value is lower than that of Ph.
4. R = MeO (pKaH = 7.5): The methoxy group (MeO) is electron-donating due to the presence of an oxygen atom that can donate electron density. This electron-donating effect decreases the acidity of guanidine compared to compounds with stronger electron-withdrawing groups like Ph and CH3CO.
5. R = CN (pKaH = -0.4): The cyano group (CN) is a strong electron-withdrawing group due to the presence of the highly electronegative nitrogen atom. This electron-withdrawing effect greatly increases the acidity of guanidine and results in a significantly lower pKaH value.
6. R = NO2 (pKaH = -0.9): The nitro group (NO2) is an even stronger electron-withdrawing group compared to CN. The presence of multiple electronegative atoms (nitrogen and oxygen) in the nitro group further enhances its electron-withdrawing effect, causing an even lower pKaH value.
Therefore, the sequence of pKaH values for the series of substituted guanidines is as follows (from highest to lowest pKaH): H > Ph > CH3CO > MeO > CN > NO2.
Remember that electron-withdrawing groups decrease the pKaH value by stabilizing the conjugate base after losing a proton, while electron-donating groups increase the pKaH value by destabilizing the conjugate base.