Chemistry MCQs for NEET — Practice Questions with Answers

According to the context provided under 'Many misconceptions are associated with resonance and the same need to be dispelled. You should remember that:', 'The molecule does not exist for a certain fraction of time in one canonical form and for other fractions of time in other canonical forms.' Instead, the resonance hybrid is a single, real structure that is an average of all contributing canonical forms.

The context states: 'Among the resonance structures, the one which has more number of covalent bonds, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others.'

The context mentions: 'The concept of resonance was introduced to deal with the type of difficulty experienced in the depiction of accurate structures of molecules like O3. According to the concept of resonance, whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons are taken as the canonical structures...'

For $\text{CH}_3\text{COO}^{-}$, the two resonance structures involve the negative charge being delocalized between the two oxygen atoms, with the carbon atom having a complete octet in both. These structures are equivalent in energy and thus equally stable, making them significant contributors to the resonance hybrid.

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.'