Basic Principles of Orgainc Chemistry MCQs for NEET — Chemistry Questions with Answers

The context defines the resonance effect as: 'The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two $\pi$-bonds or between a $\pi$-bond and lone pair of electrons present on an adjacent atom’. The effect is transmitted through the chain.'

The solution to Problem 8.17 states regarding structure III: 'does not contribute as oxygen has positive charge and carbon has negative charge, hence least stable'. A negative charge on a more electronegative atom and a positive charge on a more electropositive atom confer stability, so the opposite scenario makes it least stable.

The context mentions: 'The two structures are less important contributors as they involve charge separation.' Separating charges requires energy and generally leads to higher potential energy, thus lower stability, compared to structures without charge separation or with optimal charge placement.

The context defines +R effect as: 'In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities.'

The context states for $\text{O}_3$: 'The normal O–O and O=O bond lengths are 148 pm and 121 pm respectively. Experimentally determined oxygen-oxygen bond lengths in the $\text{O}_3$ molecule are same (128 pm)... The two structures shown above constitute the canonical structures or resonance structures and their hybrid i.e., the III structure represents the structure of $\text{O}_3$ more accurately.' This implies the actual bond is an average.

The context for problem 4.3 states: 'The single Lewis structure based on the presence of two single bonds and one double bond between carbon and oxygen atoms is inadequate to represent the molecule accurately as it represents unequal bonds. According to the experimental findings, all carbon to oxygen bonds in $\text{CO}_3^{2-}$ are equivalent. Therefore the carbonate ion is best described as a resonance hybrid of the canonical forms I, II, and III shown below.'

The context states: 'The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy between the actual structure and the lowest energy resonance structure is called the resonance stabilisation energy or simply the resonance energy.'

The context states: 'The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons.' It also says: 'They contribute to the actual structure in proportion to their stability,' which implies they are not necessarily equally stable (some are more stable contributors than others). For example, in Problem 8.17, stability I > II > III is clearly mentioned.

The context states: 'however, it is known that the two N–O bonds of nitromethane are of the same length (intermediate between a N–O single bond and a N=O double bond). The actual structure of nitromethane is therefore a resonance hybrid of the two canonical forms I and II.'

The context states: 'Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single canonical structure.'