The structure of isobutyl group in an organic compound is a) CH3-CH2-CH2-CH2- b) CH3-C(CH3)(CH3)- [tert-butyl,

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Q. The structure of isobutyl group in an organic compound is

a) CH3-CH2-CH2-CH2-

b) CH3-C(CH3)(CH3)- [tert-butyl, i.e. CH3 with two more CH3 branches on the same carbon]

c) CH3-CH(CH3)-CH2-

d) CH3-CH(-)-CH2-CH3 [attachment at the CH]

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Concept understanding — IUPAC Nomenclature of Organic Compounds

Every IUPAC name is built from exactly three pieces, always assembled in the same order: prefix + root word + suffix. The root word encodes how many carbons sit in the compound's longest continuous chain (meth- for 1, eth- for 2, prop- for 3, but- for 4, pent- for 5, hex- for 6, hept- for 7, oct- for 8, non- for 9, dec- for 10, and so on up to hect- for 100). The prefix names whatever substituent groups are attached to that chain -- the halogens (fluoro-, chloro-, bromo-, iodo-), nitro-, alkoxy groups (methoxy-, ethoxy-), and the branched alkyl substituents (methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl -- $CH_3CH(CH_3)CH_2-$ --, tert-butyl -- $(CH_3)_3C-$ --, n-pentyl, neopentyl). The suffix comes in two layers: a primary suffix recording the chain's saturation (-ane for all-single-bonded, -ene/-diene/-triene for one/two/three C=C bonds, -yne/-diyne for one/two C≡C bonds), and, layered on top of it by dropping its terminal 'e', a secondary suffix recording the actual functional group (-ol for alcohols, -al for aldehydes, -one for ketones, -oic acid for carboxylic acids, -oate for esters, -oyl chloride for acid chlorides, -amide for amides, -amine for amines, -nitrile for nitriles, -sulphonic acid for sulphonic acids, -thiol for thiols).

The naming procedure runs in five steps. (1) Pick the longest chain -- but if the compound carries a recognised functional group, the chosen chain MUST include that group's carbon even if a longer chain exists that skips it; with more than one functional group present, the chain must include whichever sits highest in this precedence order: $-COOH > -SO_3H > -COOR/-COX > -CONH_2 > -CN > -CHO > -CO- > -OH > -SH/-NH_2 > -C{\equiv}C- > -C{=}C- > -C{-}C- > -O- > -X > -NO_2$ ; with the SAME group appearing more than once, the chain must include the maximum possible count of it. Ties in chain length are broken by preferring the chain with more substituents. (2) Number the chain from whichever end gives the lowest locant, in priority order: first to the principal (suffix) functional group, then to unsaturation, and only then to substituents as a set; a genuine remaining tie goes to whichever substituent is alphabetically first. (3) Name every substituent. (4) List multiple substituents alphabetically before the root word (ignoring di-/tri-/tetra- for the alphabetising, though they stay in the written name). (5) Assemble: prefix + root + primary suffix + secondary suffix.

Formatting conventions finish the system: the name is written as one word (except salts/acids/derivatives); commas separate adjacent locants, hyphens separate a locant from a letter (2,2-Dimethyl-3-hexene); italicised descriptors (cis-, trans-, meso-) are hyphen-joined and ignored when alphabetising; multiplying prefixes (di-, tri-, tetra-) are plain (no italics/extra hyphen) and likewise ignored when alphabetising. …

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Step 1. The four common butyl-group isomers are: n-butyl, CH3-CH2-CH2-CH2- (attached at a terminal CH2); isobutyl, (CH3)2CH-CH2- (attached at a CH2 that is itself next to a branch point); sec-butyl, CH3-CH2-CH(-)-CH3 (attached at an internal CH); and tert-butyl, (CH3)3C- (attached at the fully-branched quaternary-type carbon).

Step 2. Option (a) is n-butyl (straight chain, attached at the end) -- not isobutyl.

Step 3. Option (b) is tert-butyl (three methyls on one carbon) -- not isobutyl. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 14 on this concept.

CBSE 2026Set ANNUAL1 mark
$Q. Write the IUPAC name of (CH3)2CHNH2.$
›Reveal solutionSolution

(CH3)2CHNH2 is a three-carbon primary amine with the amine group on the middle (second) carbon, named by adding the suffix '-amine' to the parent alkane chain.

The structure (CH3)2CH-NH2 has a propane backbone (3 carbons) with the -NH2 group attached to the middle carbon, C2.

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. IUPAC name of Neopentane is -$
(a) 2,2-dimethylpropane
(b) 2-Methylbutane
(c) 3-Methylbutane
(d) 2-Ethylbutane
›Reveal solutionSolution

Neopentane's IUPAC name is 2,2-dimethylpropane.

Neopentane has the structure (CH3)4C — a central carbon attached to four -CH3 groups. To name it: find the longest continuous carbon chain. Picking any two of the outer methyls plus the central carbon plus one more methyl gives a 3-carbon chain (propane: C1-C2-C3), with the remaining two methyl groups both attached to the central carbon, C2. So the name is built as propane with two methyl substituents both at position 2: 2,2-dimethylpropane.

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. The IUPAC name of the compound is : CH2=CH-CH(CH3)-CH3$
(a) 1-Isopropyl ethene
(b) 3-Methylbut-1-ene
(c) 2-Vinyl propene
(d) 1,1-Dimethyl-2-propene
›Reveal solutionSolution

The IUPAC name of CH2=CH-CH(CH3)-CH3 is 3-Methylbut-1-ene.

Step 1 — Identify the longest carbon chain containing the double bond:

CH2=CH-CH(CH3)-CH3 has a 4-carbon main chain (but-ene) with one CH3 branch coming off the third carbon.

Step 2 — Number the chain to give the double bond the lowest locant:

C1(=CH2) = C2(H) − C3(H)(CH3) − C4(H3)

The double bond is between C1 and C2, so it is but-1-ene, and the methyl substituent is on C3.

Step 3 — Assemble the name:

3-Methylbut-1-ene.

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. What is the IUPAC name of the following compound? Structure shown: CH3-CH=CH-CH2Br, drawn atom-by-atom as H-C(CH3)=CH-CH(H)(Br)$
(a) 1-Bromo-3-methyl prop-2-ene
(b) 3-Bromo-1-methyl propene
(c) 1-Bromobut-2-ene
(d) 4-Bromobut-2-ene
›Reveal solutionSolution

For CH3-CH=CH-CH2Br, numbering the 4-carbon chain from the bromine end gives the double bond and the substituent the lowest possible locant set, so the correct IUPAC name is 1-bromobut-2-ene.

The structure is CH3-CH=CH-CH2Br (a straight 4-carbon chain with one C=C double bond and one Br substituent).

Numbering from the CH3 end: C1(CH3)-C2=C3-C4(CH2Br) gives the double bond at position 2 and Br at position 4 -> locants {2,4}.

Numbering from the CH2Br end: C1(CH2Br)-C2=C3-C4(CH3) gives Br at position 1 and the double bond at position 2 -> locants {1,2}. …

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CBSE 2025Set ANNUAL1 mark
$Q. The IUPAC name of CH2=CH-CH=CH2 is ______.$
›Reveal solutionSolution

CH2=CH-CH=CH2 is a 4-carbon chain with two C=C double bonds at positions 1 and 3, so its IUPAC name is buta-1,3-diene.

Number the four carbons: C1(H2)=C2(H)-C3(H)=C4(H2). The parent chain has 4 carbons, so the base name is 'but-'. There are two carbon-carbon double bonds, so the suffix is '-die …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. The IUPAC name of the compound CH3-CH(OH)-CH2-CH=CH-CH3 :$
(a) hex-2-en-4-ol
(b) hex-4-en-2-ol
(c) hex-2-en-4-al
(d) hex-4-en-2-al
›Reveal solutionSolution

The compound is a hexene bearing an -OH group; numbering the chain from the end closest to the -OH group (the senior/principal characteristic group) gives hex-4-en-2-ol.

The given structure is CH3-CH(OH)-CH2-CH=CH-CH3, a straight chain of six carbons containing one C=C double bond and one -OH group. For IUPAC naming of an alcohol, the principal characteristic group (here -OH, named with the suffix -ol) must get the lowest possible locant when there is a choice, ahead of the locant for the double bond.

Numbering from the left end (the end nearer -OH):

C1(CH3) - C2(CH-OH) - C3(CH2) - C4(CH=) - C5(=CH) - C6(CH3)

-OH is on C2, the double bond is between C4 and C5.

…

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CBSE 2023Set ANNUAL1 mark
$Q. Give the IUPAC name of the following compound.$
›Reveal solutionSolution

The compound is named 1-chloro-2,4-dinitrobenzene.

The ring carries three substituents: one Cl and two NO2 groups. To assign the IUPAC name, the ring carbons are numbered so that the substituents get the lowest possible set of locants.

Starting numbering at the carbon bearing Cl (position 1) and proceeding around the ring in the direction that reaches the two NO2 groups earliest, the positions come out as 1 (Cl), 2 (NO2), and 4 (NO2) — locant set {1,2,4}, which is lower than the {1,4,6} obtained numbering the …

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CBSE 2022Set TERM11 markMCQ
$Q. The IUPAC name of the following compound CH3-CH=C-CH2-CH3, with -CH2-CH2-CH3 attached to the C (the doubly-bonded carbon), is$
(a) 3-ethyl-2-hexene
(b) 3-propyl-2-hexene
(c) 3-propyl-2-pentene
(d) 4-ethyl-4-hexene
›Reveal solutionSolution
IUPAC naming picks the LONGEST chain that includes the double bond -- here that means extending through the propyl branch (6 carbons) rather than staying on the drawn backbone (5 carbons), leaving the remaining 2 carbons as an ethyl substituent.

The compound is CH3-CH=C(-CH2-CH2-CH3)-CH2-CH3: a 5-carbon backbone (CH3-CH=C-CH2-CH3) with a propyl group (-CH2-CH2-CH3) attached to the double-bonded carbon (the third carbon).

Step 1 -- find the longest chain THROUGH the double bond:

Staying on the drawn backbone gives 5 carbons (a pentene) with a 3-carbon propyl substituent -- NOT the longest option.

Extending the main chain through the propyl branch instead gives: CH3-CH=C-CH2-CH2-CH3, which is 6 carbons (a hexene), leaving the original CH2-CH3 (2 carbons) as an ethyl substituent on the double-bonded carbon. …
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CBSE 2022Set TERM11 markMCQ
$Q. [hexagon ring structure] is$
(a) Cyclopentane
(b) Cyclohexane
(c) Cyclobutane
(d) none of these
›Reveal solutionSolution

In skeletal (line-angle) structural formulas, a plain unlabelled polygon represents a cyclic saturated hydrocarbon with one carbon at each vertex -- a hexagon means 6 carbons, i.e. cyclohexane.

In skeletal notation for cycloalkanes, each vertex of the polygon represents a carbon atom (with enough hydrogens implied to satisfy its 4 bonds), and each line/edge represents a single C-C bond, with no double bonds shown (a plain hexagon, unlike benzene, has no circle or alternating double bonds drawn inside it).

…

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CBSE 2022Set ANNUAL1 markMCQ
$Q. Match the following:$
(1) -NO2
(2) -OCH3
(3) -CH2-CH2-CH3
(4) -NH2 with
(i) Propyl
(ii) Amino
(iii) Methoxy
(iv) Nitro
(a) (1)-(iii), (2)-(ii), (3)-(iv), (4)-(i)
(b) (1)-(iii), (2)-(iv), (3)-(i), (4)-(ii)
(c) (1)-(iv), (2)-(iii), (3)-(i), (4)-(ii)
(d) (1)-(ii), (2)-(i), (3)-(iv), (4)-(iii)
›Reveal solutionSolution

Each group is matched to its standard IUPAC substituent name: nitro (-NO2), methoxy (-OCH3), propyl (-CH2CH2CH3), amino (-NH2).

Match each numbered group to its lettered name:

(1) -NO2 is the nitro group → (iv) Nitro

(2) -OCH3 is the methoxy group (an -O-CH3 ether-type substituent) → (iii) Methoxy

(3) -CH2-CH2-CH3 is a straight three-carbon chain, the propyl group → (i) Propyl …

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CBSE 2020Set ANNUAL1 mark
$Q. Write IUPAC name of CH3C(Cl)(C2H5)CH2CH3$
›Reveal solutionSolution
The central carbon of CH3-C(Cl)(C2H5)-CH2-CH3 carries a methyl, a chlorine, and two ethyl-type arms. The longest chain through it is 5 carbons (pentane), so the correct IUPAC name is 3-chloro-3-methylpentane -- not a 4-carbon (butane) parent name.

The structure: CH3-C(Cl)(C2H5)-CH2-CH3. The substituted carbon in the middle is bonded to:

a methyl group (CH3),

a chlorine (Cl),

an ethyl group (C2H5 = CH2CH3), and

a -CH2CH3 chain (a second ethyl arm).

Step 1 - longest chain: the IUPAC rule is to pick the LONGEST continuous carbon chain that includes the substituted carbon. Running from the terminal carbon of one ethyl arm, through the central carbon, out to the terminal carbon of the other ethyl arm gives CH3CH2-C-CH2CH3 = 5 carbons. That is a pentane parent, longer than any 4-carbon (butane) chain. The remaining methyl on the central carbon becomes a substituent.

…
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CBSE 2020Set ANNUAL1 markMCQ
$Q. Write the IUPAC name of CH3-CH2-CH(OH)-CHO.$
(a) 1-formyl propanol
(b) 1-hydroxy butanal
(c) 2-hydroxy butanal
(d) 3-hydroxy butanal
›Reveal solutionSolution

CH3-CH2-CH(OH)-CHO is a 4-carbon aldehyde with an -OH group on the second carbon, so its IUPAC name is 2-hydroxybutanal.

To name an aldehyde by IUPAC rules, the longest carbon chain containing the -CHO group is chosen as the parent chain, and numbering always starts from the carbon of the -CHO group (which is always C1, since -CHO can only be at a chain end).

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Q.The structure of isobutyl group in an organic compound is

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