Q4. Give the IUPAC name of isopentane, (CH3)2CHCH2CH3.
Longest chain: four carbons (butane), with a methyl branch on C2 counting from the end nearer the branch: 2-methylbutane.
Q5. Name (CH3)2CHCH(CH2CH3)CH2CH2CH3.
Two chains have six carbons. One carries methyl on C2 and ethyl on C3; the other carries only an isopropyl group on C3. Take the chain with more side chains and number it from the end nearer them (2,3, not 4,5): 3-ethyl-2-methylhexane.
Q6. Write the condensed formula of but-2-enal.
Four carbons with the CHO carbon as C1 and a C=C between C2 and C3: CH3CH=CHCHO.
Q7. Name CH3CH(OH)CH2COOH.
The acid is the principal group, so C1 is the COOH carbon; the chain has four carbons (butanoic acid) and OH on C3 becomes the prefix hydroxy-: 3-hydroxybutanoic acid.
Q8. Write the first five members of the homologous series that begins with HCOOH (NCERT Exercise 8.6 (a)) and name them.
HCOOH (methanoic acid), CH3COOH (ethanoic acid), CH3CH2COOH (propanoic acid), CH3(CH2)2COOH (butanoic acid), CH3(CH2)3COOH (pentanoic acid). Each differs from the next by CH2.
Q9. Why is NCERT’s example named 1-chloro-2,4-dinitrobenzene and not 4-chloro-1,3-dinitrobenzene?
Numbering from Cl towards the neighbouring NO2 gives the locants 1,2,4 (the other way round, 1,4,6); numbering from a nitro group gives 1,3,4 (the name in the question), 1,2,5, 1,3,6 or 1,4,5. Compared term by term, 1,2,4 is lowest: at the second place 2 beats 3 and 4, and against 1,2,5 the third place decides, 4 against 5. So it wins on lowest locants alone. Alphabetical order is not needed.
Q10. Name BrCH2CH2CH=CH2.
The C=C gives the suffix and must get the lowest number, so C1 is the CH2= carbon; bromine is a prefix on C4: 4-bromobut-1-ene.