3. In the nucleobase, guanine, a six-membered pyrimidine ring is fused to five-m
ID: 142988 • Letter: 3
Question
3. In the nucleobase, guanine, a six-membered pyrimidine ring is fused to five-membered imidazole ring, which has two types of nitrogen: a pyr idine- type nitrogen (N2) and a pyrrole-type nitrogen (No). Which best describes e antterence in the nucleophilicity of the pyridine-type N7 over the pyrrole-type N9? NH deoxyribose-5'-phosphate-DNA Guanine (a) The pyridine-type N7 has a lone pair in an sp2 hybrid orbital. (b) The pyrrole-type N9 has a lone pair in an sp3 hybrid orbital. (d) The pyrrole-type N9 has a lone pair in an sp2 hybrid orbital. . The biological activity of chemical mustards depends on the relative The pyridine-type N7 has a lone pair in a p orbital in resonance in its ring (c) nucleophilicity of the attacking atom, either nitrogen or sulfur. Which order of decreasing nucleophilicity is correct? (a) melphalan > sulfur mustard> mechlorethamine (b) sulfur mustard> melphalan > mechlorethamine (c) sulfur mustard> mechlorethamine> melphalan d) mechlorethamine> sulfur mustard> melphalanExplanation / Answer
3 . a) the pyridine type nitrogen N7 has a lone pair in the sp2 hybrid orbital .
Both pyridine and pyrrole , nitrogen have the sp2 hybridization.
But in pyridine , the lone pair of electrons are involved in the sp2 hybridization . Out of the three hybrid orbitals , two of them are used up in sigma bond formation with the neighbouring atoms , while the third unhybridized p orbital is occupied in the formation of 6pi electron system .
In pyrrole on the other hand , the lone pair is involved in the aromatic sextet . It is sp2 hybridized not sp3 because sp3 will make the structure unstable .
4. The less electronegative the attacking atom is , the more easily it can give away the electron to the electrophile and become a nucleophile .
The electronegativity trend between nitrogen ,Sulphur and chlorine is -
Nitrogen >chlorine >sulphur
Therefore the correct answer should be
C) sulphur mustard > mechlorethamine > melphalan.
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