JEE Chemistry Organic Chemistry Some Basic Principles and Techniques MCQs Set 10

Answer: (c) (CH₃)₂CH–CH=CH–CH₃ : 2-Methylpent-3-ene

Explanation:
1. In (CH₃)₂CH–CH=CH–CH₃ the numbering must give the double bond the lowest locant, so the name is 4-methylpent-2-ene, not 2-methylpent-3-ene.
2. The other three names are correct.

Teacher's Note:
1. The double bond gets priority over substituents for the lowest locant.
2. The given name violates this rule.

Answer: (c) 2-Methylcyclopent-2-en-1-ol

Explanation:
1. The OH is the principal group, so its carbon is C1.
2. The double bond must then be C2=C3, and the methyl group is on C2, the ring carbon next to C1.
3. Name: 2-methylcyclopent-2-en-1-ol.

Teacher's Note:
1. Number from the carbon bearing the principal group, then give the double bond the lowest locant.
2. Check the direction that matches the drawing.

Answer: (d) Ethanoic methanoic anhydride

Explanation:
1. The compound has the –CO–O–CO– anhydride group with a CH₃ on one side and H on the other.
2. A mixed anhydride is named by citing the two acids alphabetically followed by anhydride.
3. Name: ethanoic methanoic anhydride.

Teacher's Note:
1. Anhydrides are named from the parent acids.
2. Acetic anhydride would have two acetyl groups.

Answer: (c) 3,3-Dimethylcyclohexan-1-ol

Explanation:
1. The alcohol is the principal group, so the carbon with OH is C1 and the name ends in -ol.
2. The gem-dimethyl carbon is the third carbon of the ring, so both methyls are on C3.
3. Name: 3,3-dimethylcyclohexan-1-ol.

Teacher's Note:
1. A hydroxy-prefix name is wrong when OH is the principal group.
2. Give the principal-group carbon locant 1.

Answer: (b) 2,3-Dimethylbutane

Explanation:
1. The longest chain has four carbons.
2. The two methyl branches sit on C2 and C3, which is the lowest locant set.
3. Name: 2,3-dimethylbutane.

Teacher's Note:
1. 2,4-Dimethylbutane cannot exist because the chain would then be longer.
2. Always check that the chosen chain is the longest.

Answer: (c) 3-Chloroprop-1-ene

Explanation:
1. Number the chain from the end nearer the double bond, so the double bond is at C1.
2. The chlorine is then at C3.
3. Name: 3-chloroprop-1-ene (allyl chloride is the common name).

Teacher's Note:
1. The double bond gets the lowest locant before the halogen substituent.
2. Vinyl chloride is CH₂=CHCl.

Answer: (b) 1-Methylethenyl

Explanation:
1. The group is an ethene unit attached through C1, which carries the methyl group.
2. Name: 1-methylethenyl (the common name is isopropenyl).

Teacher's Note:
1. Number a substituent group from the carbon bearing the free valence.
2. The double bond must be shown by ethenyl, not ethyl.

Answer: (d) Pent-3-en-1-yne

Explanation:
1. Number so that the multiple bonds get the lowest locants together: from the triple bond end they are {1,3}; from the other end {2,4}.
2. Name: pent-3-en-1-yne (the ene is written before yne).

Teacher's Note:
1. Lowest locants go to the multiple bonds taken together.
2. If there were a tie, the double bond would get the lower number.

Answer: (c) 3,4-Diethylhexane

Explanation:
1. The longest chain is six carbons: CH₃CH₂–CH–CH–CH₂CH₃.
2. There is an ethyl group on each of the two middle carbons.
3. Name: 3,4-diethylhexane.

Teacher's Note:
1. Count the longest chain, not the horizontal chain as drawn.
2. An ethyl group means two carbons, not a methyl.

Answer: (c) 3-Ethyl-2-methylpentane

Explanation:
1. Names with an ethyl group on C2 of a pentane mean the longest chain is really a hexane, so options 1, 2 and 4 pick the wrong parent chain.
2. 3-Ethyl-2-methylpentane describes (CH₃)₂CH–CH(C₂H₅)–CH₂CH₃ correctly, with locants in alphabetical order.

Teacher's Note:
1. An ethyl group cannot be on C2 of any chain, because the chain would be longer through the ethyl.
2. List substituents alphabetically.

Answer: (c) All members have the same physical properties

Explanation:
1. Members of a homologous series have the same general formula, the same functional group and similar chemical properties.
2. Their physical properties (boiling point, density) change gradually with molar mass, so they are not the same.

Teacher's Note:
1. Physical properties show a regular gradation along a series.
2. Chemical properties are similar because the functional group is the same.

Answer: (b) sp, sp, sp², sp², sp³

Explanation:
1. The first two carbons share a triple bond, so both are sp.
2. The next two share a double bond, so both are sp².
3. The last carbon (CH₃) is sp³.

Teacher's Note:
1. Read the bonds carbon by carbon.
2. Triple bond: sp, double bond: sp², single bonds only: sp³.

Answer: (c) Benzene

Explanation:
1. A heterocyclic compound has at least one atom other than carbon in its ring.
2. Thiophene has S, furan has O and pyridine has N in the ring.
3. Benzene has only carbon atoms in the ring, so it is carbocyclic.

Teacher's Note:
1. Carbocyclic means a ring of carbon atoms only.
2. Hetero atoms commonly met: N, O, S.

Answer: (a) 1

Explanation:
1. A hetero atom is any atom other than carbon and hydrogen in the structure.
2. Maleic anhydride, C₄H₂O₃, has one ring oxygen.
3. The two carbonyl oxygens are not counted as hetero atoms of the ring.

Teacher's Note:
1. Hetero atoms in a ring are counted among the ring atoms.
2. The exocyclic C=O oxygens are not part of the ring.

Answer: (c) Neobutyl

Explanation:
1. Isopropyl, tert-butyl and neopentyl are accepted common names of alkyl groups.
2. There is no common alkyl group called neobutyl.

Teacher's Note:
1. The prefix neo is used for groups with a quaternary carbon, such as neopentyl.
2. Do not invent neobutyl.

Answer: (c) C₆H₅–

Explanation:
1. The phenyl group is attached through a ring carbon that is bonded to two other ring carbons, so it is a secondary radical.
2. Vinyl and n-butyl are primary and tert-butyl is tertiary.

Teacher's Note:
1. Count carbon neighbours of the carbon with the free valence.
2. In the phenyl group two ring carbons are attached.

Answer: (a) CH₃–CH(CH₃)–CH₂–C(CH₃)₂–CH₃

Explanation:
1. Isooctane is the common name of 2,2,4-trimethylpentane, a standard for octane rating.
2. Its structure is CH₃–CH(CH₃)–CH₂–C(CH₃)₂–CH₃.
3. The other structures are 2,3,4-trimethylpentane and 2-methylheptane.

Teacher's Note:
1. Draw the longest chain of five carbons and place three methyl groups at 2, 2 and 4.
2. Isooctane has an octane number of 100.

Answer: (c) Tertiary butyl alcohol

Explanation:
1. The OH group is on a carbon attached to three other carbons, so the compound is a tertiary alcohol.
2. Its common name is tertiary butyl alcohol (tert-butyl alcohol).

Teacher's Note:
1. Isobutyl alcohol is (CH₃)₂CHCH₂OH.
2. sec-Butyl alcohol is CH₃CH(OH)CH₂CH₃.

Answer: (c) Tertiary butyl carbinol

Explanation:
1. Neopentyl alcohol is (CH₃)₃C–CH₂OH.
2. In carbinol nomenclature CH₃OH is the parent, and the groups replacing the hydrogens are named as substituents.
3. The substituent here is the tertiary butyl group: tertiary butyl carbinol.

Teacher's Note:
1. Carbinol is another name for the CH₂OH carbon.
2. Write the attached group in front of the word carbinol.

Answer: (a) –COOR

Explanation:
1. The order of decreasing priority is carboxylic acid > sulphonic acid > anhydride > ester > acid halide > amide > nitrile > aldehyde > ketone > alcohol > amine.
2. Among the choices, the ester –COOR is highest.

Teacher's Note:
1. Memorise the priority order as a list.
2. Priority decides which group becomes the suffix.

Answer: (d) 3-Methylpent-4-enoic acid

Explanation:
1. The longest chain containing the COOH carbon and the double bond is five carbons: HOOC–CH₂–CH(CH₃)–CH=CH₂.
2. The double bond is at C4 and the methyl at C3.
3. Name: 3-methylpent-4-enoic acid.

Teacher's Note:
1. Include the double bond in the main chain rather than naming it as ethenyl when the chain is longer.
2. Number from the COOH carbon.

Answer: (c) Ethyl 2-bromopropanoate

Explanation:
1. An ester is named as alkyl alkanoate: the alkyl group of the alcohol part (ethyl) comes first as a separate word.
2. The acid part is propanoic acid with Br on C2.
3. Name: ethyl 2-bromopropanoate.

Teacher's Note:
1. The alkyl group attached to the oxygen is a separate word.
2. The substituent locants apply to the acid chain.