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Which of the following represents a +R (positive resonance) effect?
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.'
In the $\text{O}_3$ molecule, the oxygen-oxygen bond lengths are all 128 pm. What does this observation imply about its structure?
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.
For the carbonate ion ($\text{CO}_3^{2-}$), all carbon-to-oxygen bonds are equivalent. This is best explained by:
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.'
What is 'resonance energy' or 'resonance stabilization energy'?
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.'
Which of the following conditions is NOT necessarily true for canonical (resonance) structures?
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.
In nitromethane ($\text{CH}_3\text{NO}_2$), the two N-O bonds are of the same length, intermediate between a N-O single bond and a N=O double bond. This indicates that nitromethane is:
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.'
What is the consequence of forming a resonance hybrid in terms of its energy compared to its canonical structures?
The context states: 'Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single canonical structure.'
A negative resonance effect (-R effect) is observed when:
The context states: 'This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system. For example in nitrobenzene this electron...'
Who provided the experimental verification of the chromosomal theory of inheritance?
The context states: 'Following this synthesis of ideas, experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced.'
The concept of 'factors' regulating characters, later named genes, was proposed by:
The summary states: 'Mendel proposed the principles of inheritance, which are today referred to as ‘Mendel’s Laws of Inheritance’. He proposed that the ‘factors’ (later named as genes) regulating the characters are found in pairs known as alleles.'
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The bank mixes NEET previous year questions (PYQs) with practice questions, each tagged with its exam appearances where applicable.