Q.Write IUPAC names of the following compounds:

The key idea is IUPAC nomenclature — find the longest chain (or, for a substituted ring, the ring system) containing the principal feature (a multiple bond, or the ring itself for (e)), then number to give the lowest locants.

(a) $CH_3CH=C(CH_3)_2$ — a 4-carbon chain with the double bond at C-2 and one methyl branch on C-2.

Name: 2-Methylbut-2-ene

(b) $CH_2=CH-C\equiv C-CH_3$ — a 5-carbon chain carrying both a double bond (C-1) and a triple bond (C-3); number from the end nearer the first multiple bond.

Name: Pent-1-en-3-yne

(c) $CH_2=CH-CH=CH_2$ — the textbook draws this as a zig-zag chain of four carbons with a double bond at each end. Numbering from either end gives the double bonds locants 1 and 3.

Name: Buta-1,3-diene

(d) $C_6H_5-CH_2-CH_2-CH=CH_2$ — a benzene ring attached to a 4-carbon but-3-enyl chain. Named as a substituted benzene, the chain substituent is but-3-en-1-yl.

Name: (But-3-en-1-yl)benzene

(e) A benzene ring bearing a $-CH_3$ and an $-OH$ on adjacent (ortho) carbons — named as a substituted phenol, with the ring numbered from the $-OH$ carbon (C-1) so the methyl gets the lowest locant, C-2.

Name: 2-Methylphenol (common name: o-cresol)

(f) $CH_3(CH_2)_4CH[(CH_2)_3CH_3]CH_2CH(CH_3)_2$ — the longest chain runs through both arms of the central CH: 10 carbons (decane), with a 2-methylpropyl branch on that carbon. Numbering from the end nearer the branch gives it locant 5.

Name: 5-(2-Methylpropyl)decane

(g) $CH_3-CH=CH-CH_2-CH=CH-CH(C_2H_5)-CH_2-CH=CH_2$ — a 10-carbon chain carrying three double bonds and one ethyl branch. Numbering from the right end gives the double bonds the lower locant set {1,5,8} (versus {2,5,9} from the left), placing the ethyl group at C-4.

Name: 4-Ethyldeca-1,5,8-triene

(a) $CH_3-CH=C(CH_3)_2$

The longest chain through the double bond is four carbons: $(CH_3)_2C=CH-CH_3$. One of the two methyls on the doubly-bonded carbon is the fourth chain carbon; the other is a substituent. Numbering to give that substituent the lowest locant puts the methyl on C-2 and the double bond at C-2.

(a) 2-Methylbut-2-ene.

(b) $CH_2=CH-C\equiv C-CH_3$

Five-carbon chain: double bond between C-1 and C-2, triple bond between C-3 and C-4. Numbering from the left gives the lowest locants (double bond at 1, triple bond at 3); "ene" is cited before "yne".

(b) Pent-1-en-3-yne.

(c) $CH_2=CH-CH=CH_2$

The textbook draws this as a zig-zag skeletal chain of four carbons, with a double bond at each end and a single bond in the middle -- a conjugated diene. The parent chain is the whole 4-carbon chain; numbering from either end gives the double bonds the locants 1 and 3 (the two ends are equivalent by symmetry).

(c) Buta-1,3-diene.

(d) $C_6H_5-CH_2-CH_2-CH=CH_2$

The real figure shows a benzene ring attached to a $-CH_2-CH_2-CH=CH_2$ chain (the ring was dropped in an earlier extraction pass, leaving only the chain fragment). With the ring as the parent (benzene) and the chain as a substituent, the substituent is a 4-carbon alkenyl group attached through C-1, with its own double bond at C-3: a but-3-en-1-yl group.

(d) (But-3-en-1-yl)benzene (equivalently, 4-phenylbut-1-ene).

(e) 2-methyl-substituted phenol

The real figure shows a benzene ring carrying a $-CH_3$ group and an $-OH$ group on two adjacent (ortho) ring carbons -- a substituted phenol, not a plain hydrocarbon (this specific part tests general ring-numbering rules, not just hydrocarbon naming). The principal characteristic group, $-OH$, must get the lowest locant, so the ring is numbered with the $-OH$ carbon as C-1; the adjacent methyl then falls at C-2.

(e) 2-Methylphenol (common name: o-cresol).

(f) $CH_3(CH_2)_4-CH[(CH_2)_3CH_3]-CH_2-CH(CH_3)_2$

The central CH bears three carbon chains. The longest chain runs from the pentyl end, through the central carbon, into the butyl chain: $5 + 1 + 4 = 10$ carbons (decane). Numbering from the butyl end places the branch point at C-5; the remaining branch, $-CH_2-CH(CH_3)_2$, is a 2-methylpropyl (isobutyl) group.

(f) 5-(2-Methylpropyl)decane (= 5-isobutyldecane).

(g) $CH_3-CH=CH-CH_2-CH=CH-CH(C_2H_5)-CH_2-CH=CH_2$

The parent chain must contain the maximum number of double bonds, so it is the ten-carbon chain holding all three C=C bonds; the ethyl remains a substituent. Numbering from the right (the terminal $=CH_2$) gives the lower set of double-bond locants, $\{1,5,8\}$ against $\{2,5,9\}$ from the left, placing the ethyl on C-4.

(g) 4-Ethyldeca-1,5,8-triene.