Question

HNCO (isocyanic acid) has following resonating structures:
$\underset{I}{H-N=C=O}\leftrightarrow \underset{II}{H-\stackrel{\ominus }{N}-C\equiv \stackrel{\oplus }{O}}\leftrightarrow \underset{III}{H-\stackrel{\oplus }{N}\equiv C-\stackrel{\ominus }{O}}$
The order of stability is:

• A. $I>III>II$
• B. $I>II>III$
• C. $II>III>I$
• D. $II>I>III$

Answer 1

The correct option is A $I>III>II$

$\underset{\underset{\text{(Uncharged species}\text{is most stable)}}{I}}{H-N=C=O}\underset{\underset{\left(\text{-ve charge is more}\text{stable on high E.N. atom}\right)}{III}}{H-\stackrel{\oplus }{N}\equiv C=\stackrel{\ominus }{O}}\underset{\underset{(\text{-ve charge is less}\text{stable on less E.N. atom)}}{II}}{H-\stackrel{\ominus }{N}=C=\stackrel{\oplus }{O}}$

Negative charge on the more electronegative element and positive charge on more electropositive element is comparatively more stable.