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My guess is that, since the relative electronegativities are similar, therefore the difference would arise only due to magnitudes of difference in electronegativities. And hence the strength should be more for benzoic acid since it has more electronegative oxygen which boosts the individual dipole strengths.

But the answer in my textbook is the opposite given without a reason. What is the fault in my guess, is the lone pair that makes a difference?

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If you compare formic acid (1.415 D) and formamide (3.71 D) you note that the amide has the larger dipole moment, ie there appears to be a trend. If you add a ring it has a secondary (minor) effect of enhancing the dipole moments, more for formic acid (by about 0.3 D) than formamide (by about 0.05D), but we are motivated to ignore the ring and focus on the amide and carboxylic acid groups.

The planar structure of an amide can be explained by invoking resonance between two forms, one neutral with double bonded C and O, another with double bonded C and N and resulting partial charges on O and N. That separation of charges accounts for the large dipole moment of amides.

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In this case, the lone pairs of both Nitrogen(Benzamide) and Oxygen(Benzoic acid) are in conjugation. Hence, when delocalization of electrons take place, a partial positive charge develops on both of them. As you stated, oxygen is more electronegative. Hence, relative to the Nitrogen of Benzamide, its ability to handle that positive charge is lesser. So, the tendency of delocalization of electrons is greater due to which it is actually feasible for it to exist with all of that partial charges as compared to the Oxygen of Benzoic acid. Then again, this is all an assumption since dipole moment is mostly calculated experimentally

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  • $\begingroup$ Partial positive charge develops on both of them? Not at all. It should be clear from the second resonance structure of amide group. $\endgroup$
    – Mithoron
    Mar 29 at 20:40
  • $\begingroup$ @Mithoron second resonance structure shows positive charge on Nitrogen, what are you trying to say? $\endgroup$
    – Baksish
    Mar 30 at 4:54
  • $\begingroup$ See Buck's answer. $\endgroup$
    – Mithoron
    Mar 30 at 13:19
  • $\begingroup$ no i was talking about the nitrogen of amide and oxygen of benzoic acid @Mithoron $\endgroup$
    – Baksish
    Mar 30 at 15:35

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