Q4. Identify the reagent in each reaction as a nucleophile or an electrophile (NCERT Exercise 8.13): (a) CH3COOH + HO− → CH3COO− + H2O; (b) CH3COCH3 + −CN → (CH3)2C(CN)(OH) (after protonation); (c) C6H6 + CH3C+O → C6H5COCH3 + H+.
(a) HO− is the nucleophile: it donates an electron pair, here to the acidic hydrogen. Strictly, this step is a proton transfer in which HO− acts as a Brønsted base, not a substitution at carbon (reasoned). (b) −CN is a nucleophile: it attacks the electron-poor carbonyl carbon. (c) CH3C+O (the acylium ion) is an electrophile: it takes a pair of π electrons from the benzene ring.
Q5. Why do alkyl groups act as electron donors when attached to a π system (NCERT Exercise 8.10)?
By hyperconjugation: the σ electrons of the C–H bonds of the alkyl group next to the π system are partly delocalised into it. In propene, CH3–CH=CH2, the three C–H bonds of the methyl group can each overlap with the π orbital. Alkyl groups are also weakly +I.
Q6. Arrange the free radicals CH3•, (CH3)3C•, CH3CH2• and (CH3)2CH• in order of increasing stability.
CH3• < CH3CH2• < (CH3)2CH• < (CH3)3C•: methyl < primary < secondary < tertiary, as for carbocations.
Q7. State whether each group shows the +R or the −R effect: –OCH3, –CHO, –NH2, –CN, –Cl.
+R: –OCH3, –NH2 and –Cl (a lone pair on the atom next to the π system). −R: –CHO and –CN (a π bond to a more electronegative atom draws electrons in).
Q8. Are H–C(=O+H)–OH and H–C+(OH)–OH structural isomers, geometrical isomers or resonance contributors (NCERT Exercise 8.15 (c))?
Resonance contributors of protonated formic acid: the nuclei are in the same positions and only the electrons move (the C=O π pair moves onto oxygen). Of the two, the structure with the C=O+ bond, in which every atom has an octet, contributes more.
Q9. How many C–H bonds can hyperconjugate with the positive carbon in (CH3)2CH+ and in CH3CH2C+(CH3)2?
(CH3)2CH+: two CH3 groups, 6. CH3CH2C+(CH3)2: two CH3 groups (6) and one CH2 group (2) on the positive carbon, 8.