Q5. Which reacts faster by SN1, CH2=CHCH2Cl or CH3CH2CH2Cl? Why?
CH2=CHCH2Cl, an allylic halide: its carbocation, CH2=CH–CH2+, is stabilised by resonance, which spreads the positive charge over both end carbons (NCERT page 174).
Q6. Give the main product of 1-bromopropane with (i) AgNO2, (ii) KNO2.
(i) CH3CH2CH2NO2, 1-nitropropane (a nitroalkane, bonded through N). (ii) CH3CH2CH2–O–N=O, propyl nitrite (bonded through O) (NCERT Table 6.4, page 170).
Q7. What does the Wurtz reaction give with (i) 1-bromopropane alone, (ii) a mixture of bromomethane and bromoethane? What does (ii) tell you about the method?
(i) Hexane, CH3(CH2)4CH3: two propyl groups join. (ii) Ethane, propane and butane together, from CH3–CH3, CH3–C2H5 and C2H5–C2H5. So the Wurtz reaction is used with a single halide, to make a symmetrical alkane R–R, which always has an even number of carbon atoms; two different halides give a mixture that is hard to separate (reasoned from NCERT page 181).
Q8. Which gives a phenol under milder conditions with NaOH: 1-chloro-4-nitrobenzene or 1-chloro-3-nitrobenzene? Explain.
1-Chloro-4-nitrobenzene (NCERT: 443 K). With –NO2 para to Cl, the negative charge of the intermediate can sit on the carbon bearing –NO2 and is stabilised. A meta nitro group cannot do this; NCERT says it has no effect, and at most it helps a little through its −I effect (NCERT pages 182–183; reasoned).
Q9. What is the major product when chlorobenzene reacts with CH3COCl and anhydrous AlCl3, and why is the reaction slower than with benzene?
4-Chloroacetophenone (with 2-chloroacetophenone as the minor product). Chlorine directs to ortho and para by resonance but deactivates the ring by its −I effect, so the reaction is slower than for benzene (NCERT pages 184–185).
Q10. A pure enantiomer rotates plane polarised light by +12.0° under certain conditions. What rotation does a mixture of 75% of it and 25% of its mirror image give under the same conditions?
The 25% of the mirror image cancels an equal 25% of the (+) form, as in a racemic mixture, leaving a 50% excess of the (+) form: 0.50 × 12.0° = +6.0° (reasoned from NCERT page 177).
Q11. Arrange C6H5CH2Cl, (C6H5)2CHCl, C6H5CH(CH3)Cl and CH3CH2CH2Cl in order of decreasing SN1 reactivity.
(C6H5)2CHCl > C6H5CH(CH3)Cl > C6H5CH2Cl > CH3CH2CH2Cl. Two phenyl rings spread the positive charge of the carbocation by resonance more than one ring does; of the one-ring cations, the secondary beats the primary; and the propyl cation has no resonance at all. For SN2 the three benzylic chlorides run the other way, because the bulkier groups block the backside attack (reasoned from NCERT Example 6.7 (ii), pages 174–175).