CBSE Class 11 Chemistry Organic Chemistry Some Basic Principles and Techniques MCQs Set 07

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Acyclic or open chain compounds are also called aliphatic compounds and consist of straight or branched chains of carbon atoms, e.g. ethane CH₃CH₃, iso-butane (CH₃)₂CHCH₃, acetaldehyde CH₃CHO and acetic acid CH₃COOH.
2. Alicyclic or closed chain (ring) compounds contain carbon atoms joined in the form of a ring (homocyclic), e.g. cyclopropane, cyclohexane and cyclohexene.
3. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Both statements are definitions of two different classes, so one cannot explain the other.
2. Alicyclic compounds behave like aliphatic compounds even though they are cyclic.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond.
2. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds.
3. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. R describes a consequence of A, not its cause; the cause is the difference in electronegativity.
2. This bond polarity is the origin of the inductive effect.

Answer: (d) A is false, but R is true.

Explanation:
1. Hybridisation can be determined by counting σ-bonds: the terminal carbons form 3σ bonds (sp²) while the central carbon forms 2σ bonds (sp).
2. Correct assertion: in H₂C=C=CH₂, the central carbon is sp-hybridised whereas the terminal carbons are sp²-hybridised.
3. Reason is correct as all carbon atoms are joined by double bonds. So, Assertion is incorrect but Reason is correct.

Teacher's Note:
1. A carbon with two double bonds (cumulated) is sp-hybridised and linear.
2. Count σ bonds (plus lone pairs): 4 → sp³, 3 → sp², 2 → sp.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. In naming cycloalkenes, number the ring to specify the position of the double bonded carbons and choose the direction of numbering so that the substituents get the lowest number.
2. The position of the double bond is not indicated because it is known to be between C-1 and C-2.
3. So, the given compound is 3-methylcyclohexene.
4. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. In a cycloalkene, C-1 and C-2 are always the double bond carbons.
2. The methyl group then gets the lowest possible locant, 3.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. In bond-line structural representations, only atoms such as oxygen, chlorine, nitrogen etc. are specifically written.
2. The terminal positions denote methyl (—CH₃) groups, while line junctions denote carbon atoms bonded to the number of hydrogens required to satisfy the valency of the carbon atoms.
3. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. In bond-line formulas C and H are not written; every corner and line end is a carbon.
2. Hetero atoms and the H attached to them (as in —OH) are always written.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. The names of straight chain compounds are based on their chain structure and end with the suffix ‘-ane’, carrying a prefix indicating the number of carbon atoms present in the chain.
2. Except from CH₄ to C₄H₁₀, where the prefixes (meth-, eth-, prop-, but-) are derived from trivial names.
3. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. From C₅ onwards, the prefixes come from Greek numbers: pent-, hex-, hept-, oct- and so on.
2. R is an exception to the rule stated in A, so it cannot explain A.

Answer: (d) A is false, but R is true.

Explanation:
1. In numbering the carbon chain, the parent carbon chain is numbered so as to give the lowest number to the carbon atom linked by a double or triple bond, even if it violates the rule for saturated hydrocarbons.
2. Numbered from the end nearer the double bond: CH₃(1)—CH(2)=CH(3)—CH(4)(CH₃)—CH₂(5)—CH(6)(CH₃)—CH₃(7).
3. So, the compound is 4,6-dimethylhept-2-ene and the Assertion is incorrect. Reason is correct.

Teacher's Note:
1. In alkenes, the double bond decides the direction of numbering, not the alkyl groups.
2. Check the name by numbering from both ends before choosing.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Heterolytic fission occurs when the two atoms differ considerably in their electronegativities and the shared pair of electrons is carried away by the more electronegative atom.
2. It occurs readily in polar covalent bonds.
3. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Heterolysis gives ions (carbocations / carbanions); homolysis gives free radicals.
2. R tells where heterolysis is easy, not what heterolysis is.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A covalent bond may be cleaved either by heterolytic cleavage or by homolytic cleavage.
2. In heterolytic cleavage, the bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments: H₃C—Br → H₃C^⊕ + Br^⊖.
3. In homolytic cleavage, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms: R—X → R^∙ + X^∙ (heat or light).
4. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Br is more electronegative than C, so on heterolysis Br takes the electron pair.
2. Curved full arrows show movement of an electron pair; fish-hook arrows show single electrons.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. A carbanion has eight electrons (three bond pairs and one lone pair) in the valence shell of the negatively charged carbon.
2. The negatively charged carbon is sp³-hybridised and the carbanion is pyramidal.
3. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Carbanion: octet, sp³, pyramidal; carbocation: sextet, sp², planar.
2. Hybridisation describes shape, but does not explain the octet.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Free radicals contain odd (unpaired) electrons, so they are short lived and readily try to pair up the odd electrons to form neutral molecules.
2. That is why they are highly reactive.
3. Both statements are correct, but the actual reason (presence of an unpaired electron) is not stated in R. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. The key feature of a free radical is its unpaired electron.
2. Free radical reactions are usually started by heat or light (e.g. halogenation of alkanes).

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The electron displacements in an organic molecule due to the influence of an atom or a substituted group present in the molecule cause permanent polarisation of the bond.
2. Inductive effect and resonance effect are examples of this type of electronic displacement.
3. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. Inductive and resonance effects are permanent; the electromeric effect is temporary.
2. Permanent effects are present even without an attacking reagent.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The presence of alternate single and double bonds in an open chain or cyclic system is termed a conjugated system.
2. These systems often show abnormal behaviour as in them the π-electrons are delocalised and the system develops polarity. Examples are 1,3-butadiene, aniline and nitrobenzene.
3. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. Delocalisation of π-electrons gives extra stability and unusual properties.
2. Conjugation is required for the resonance (mesomeric) effect to operate.

Answer: (d) A is false, but R is true.

Explanation:
1. Resonance hybrids are always more stable than any of the canonical structures would be, if they existed.
2. The delocalisation of electrons lowers the orbital energies, imparting stability. The gain in stability of the resonance hybrid over the most stable of the canonical structures is called resonance energy.
3. So, the energy of the resonance hybrid is lower than that of any canonical form, not the average. Assertion is incorrect but Reason is correct.

Teacher's Note:
1. Resonance energy = energy of most stable canonical form − energy of resonance hybrid.
2. A canonical structure that is lower in energy makes a greater contribution to the hybrid.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. When a covalent bond is formed between atoms of different electronegativity, the bonding electrons shift towards the more electronegative atom; this unequal sharing results in a polar covalent bond.
2. The polarisation of one σ bond induces polarisation of the adjacent σ bonds; this is known as the inductive effect.
3. Classification of substituents as electron withdrawing (−I) or donating (+I) relative to H is true but does not explain the inductive effect.
4. Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Inductive effect fades quickly and is negligible after about three bonds.
2. Hydrogen is taken as the reference (zero) for comparing −I and +I groups.

Answer: (b) CH₃CH₂CH₂—NO₂

Explanation:
1. The NO₂ group is the most strongly electron-withdrawing among the given groups (Cl, NO₂, OH, NH₂).
2. Maximum polarity is present in the C—NO₂ bond, so the dipole moment of nitropropane is the maximum amongst the given molecules (about 3.7 D, compared with about 2.0 D for 1-chloropropane, 1.7 D for propan-1-ol and 1.2 D for propylamine).

Teacher's Note:
1. The more electron-withdrawing the group attached to the chain, the larger the dipole moment.
2. The nitro group has both a strong −I and −R character.

Answer: (a)

Explanation:
1. Electron donor tendency: —OCH₃ > —O—CO—CH₃ (lone pair of O is partly pulled by the C=O group).
2. —COOCH₃ and —CHO are electron-withdrawing (−R) groups and decrease the electron density of the ring.
3. So, the electron density of the benzene ring is maximum in anisole (a).

Teacher's Note:
1. —OCH₃ donates its lone pair to the ring (+R), making anisole activated.
2. In phenyl acetate the O lone pair is shared between the ring and the C=O, so donation is weaker.

Answer: (b)

Explanation:
1. Aniline is a conjugated system (lone pair of —NH₂ in conjugation with the π-electrons of the ring).
2. It is more polar than expected because the NH₂ group releases electron density towards the benzene nucleus rather than attracting it from benzene.
3. Cyclohexylamine, cyclohexa-1,4-diene and 3-methoxycyclohexene are not conjugated systems.

Teacher's Note:
1. Abnormal stability and polarity are shown by conjugated systems where electrons are delocalised.
2. Isolated double bonds (as in cyclohexa-1,4-diene) do not show resonance.

Answer: (a) inductive effect

Explanation:
1. The permanent displacement of electrons through a chain involving only σ-bonds is called the inductive effect.
2. It involves the electron displacement along the chain of saturated carbon atoms due to the presence of a polar covalent bond at one end of the chain.

Teacher's Note:
1. Mesomeric (resonance) effect involves π-electrons; electromeric effect is temporary.
2. The inductive effect is represented by an arrow head in the middle of the bond (→).

Answer: (b) —NR₂ < —OR < —F

Explanation:
1. −I-effect is related to the ability of a substituent to attract electrons from the attached carbon atom, i.e. it is based on the electronegativity of the atom.
2. This effect increases with increase in the electronegativity of the atom: N < O < F.
3. So, —NR₂ < —OR < —F (−I-effect); option (b) is correct.

Teacher's Note:
1. Across a period the −I effect increases with electronegativity of the atom attached to carbon.
2. Fluorine shows the strongest −I effect among the halogens.

Answer: (a) C > B > A

Explanation:
1. The dispersal of charge stabilises the carbocation. More the number of alkyl groups, greater is the dispersal of positive charge and more is the stability (B > A).
2. In C^⊕H₂OCH₃ (C), the lone pair on O is directly attached to the positive carbon and delocalises the charge by resonance, so it is the most stable.
3. Thus, the correct order is C^⊕H₂OCH₃ > CH₃C^⊕H₂ > C^⊕H₃, i.e. C > B > A.

Teacher's Note:
1. An O atom with a lone pair next to C⁺ stabilises it strongly by +R effect.
2. CH₃⁺ has no stabilising group and is the least stable.

Answer: (a) CH₃C^∙H₂ and Cl^∙

Explanation:
1. Homolytic cleavage results in the formation of free radicals.
2. CH₃CH₂—Cl → CH₃C^∙H₂ + Cl^∙ (homolytic fission).

Teacher's Note:
1. In homolysis each atom takes back one electron of the bond.
2. Homolysis is favoured by heat or light and by non-polar bonds.

Answer: (b) trigonal planar

Explanation:
1. The shape of a carbocation is trigonal planar, with the positively charged carbon being sp²-hybridised.
2. The unhybridised empty p-orbital lies perpendicular to the plane of the three σ bonds (bond angle 120°).

Teacher's Note:
1. Carbocations and free radicals are planar; carbanions are pyramidal.
2. The empty p-orbital is where a nucleophile attacks.

Answer: (b) II > I > IV > III

Explanation:
1. Among the given free radicals, the benzyl radical (II) is the most stable due to resonance.
2. In the remaining free radicals, the more the alkyl groups (α-H atoms) bonded to the electron deficient carbon, the more stable is the radical: I is 2°; IV and III are 1°.
3. ^∙CH₂—CH₃ (IV) has 3 α-H atoms while ^∙CH₂—CH(CH₃)₂ (III) has only 1, so IV is more stable than III.
4. Order: II > I > IV > III.

Teacher's Note:
1. Resonance stabilisation comes first, then the degree (3°, 2°, 1°), then the number of α-H atoms.
2. Note that III here is a primary radical, not tertiary.

Answer: (c) 7 p-orbitals and 7 unpaired electrons

Explanation:
1. In the benzyl free radical, all seven carbons (six ring carbons and the CH₂ carbon) are sp²-hybridised and each has one unhybridised p-orbital.
2. These 7 parallel p-orbitals contain 7 electrons (one each, shown as unpaired electrons), which delocalise over the ring and the CH₂ carbon.
3. So the odd electron is spread over the system, making the benzyl radical highly stable.

Teacher's Note:
1. The extra p-orbital on CH₂ lies parallel to the ring p-orbitals and overlaps with them.
2. The odd electron of the benzyl radical appears at the CH₂, ortho and para carbons in its resonance structures.

Answer: (c) Both the rules

Explanation:
1. Both the rules are correct for resonating structures.
2. I. The overall charge of the system must remain the same.
3. II. The arrangement of atoms must be identical or almost the same in every resonance structure; only electrons move.

Teacher's Note:
1. Resonance structures differ only in the positions of π-electrons and lone pairs.
2. They must also have the same number of paired and unpaired electrons.

Answer: (b) 5

Explanation:
1. Phenoxide ion has two structures with the negative charge on oxygen (the two Kekulé forms of the ring).
2. It has three more structures in which the negative charge is on the two ortho carbons and the para carbon.
3. So, there are 5 resonance structures of the phenoxide ion:

Teacher's Note:
1. The negative charge of phenoxide can reach only the ortho and para positions.
2. This is why —O⁻ and —OH groups are ortho–para directing.

Answer: (b) Structure II

Explanation:
1. In structure I, the positively charged carbon has only six electrons (an incomplete octet).
2. In structure II, R—C≡O^⊕, every atom (C and O) has a complete octet and there is one more covalent bond.
3. A structure with more covalent bonds and complete octets on all atoms is more stable even though the positive charge is on oxygen; so structure II is more stable (acylium ions are best written as R—C≡O⁺).

Teacher's Note:
1. Complete octets on all atoms is a more important rule than placing a positive charge on the less electronegative atom.
2. The same reasoning explains why CH₃—O—C⁺H₂ is stabilised as CH₃—O⁺=CH₂.

Answer: (a) \( \text{B.O.} = \frac{\text{no. of bonds}}{\text{no. of resonating structure}} \)

Explanation:
1. In compounds exhibiting resonance, bond order can be given by the formula:
2. \( \text{B.O.} = \frac{\text{total no. of bonds between two atoms in all the structures}}{\text{total no. of resonating structures}} \).
3. For example, in the carbonate ion, C—O bond order = \( \frac{4}{3} \) = 1.33.

Teacher's Note:
1. Resonance makes all the equivalent bonds identical in length.
2. In benzene, C—C bond order = 3/2 = 1.5.

Answer: (d) —COOH

Explanation:
1. −R-effect is shown by groups like —COOH, —CHO, >C=O, —CN and —NO₂.
2. +R-effect is shown by halogens, —OH, —OR, —OCOR, —NH₂, —NHR, —NR₂, —NHCOR.
3. So, —COOH shows −R-effect.

Teacher's Note:
1. −R groups have a π-bond to an electronegative atom that pulls electrons out of the conjugated system.
2. +R groups have a lone pair on the atom attached to the system.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. When a covalent bond is formed between atoms of different electronegativity, the bonding electrons are shifted towards the more electronegative atom. This is known as inductive effect and this unequal sharing of electrons results in polar covalent bond.
2. Thus, both (A) and (R) are correct but (R) is not the correct explanation of (A).

Teacher's Note:
1. The inductive effect is a permanent effect transmitted through σ bonds.
2. Its strength falls off rapidly along the chain (δ⁺ > δδ⁺ > δδδ⁺).

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Formic acid (HCOOH) is a stronger acid than benzoic acid (PhCOOH), although the phenyl group is electron attracting (−I-effect).
2. This is due to the +R effect of the phenyl group, which is greater than its −I-effect; it pushes electrons towards the COOH group.
3. Hence, it decreases the acidic nature of benzoic acid. Both (A) and (R) are correct and (R) is the correct explanation of (A).

Teacher's Note:
1. Net electron donation by the ring destabilises the benzoate ion slightly.
2. pK_a: HCOOH ≈ 3.75, C₆H₅COOH ≈ 4.20.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. The stability of carbonium ions is influenced by both resonance and inductive effects.
2. Allyl and benzyl carbonium ions are stabilised by resonance (delocalisation of the positive charge over the π-system).
3. Electron releasing groups do stabilise carbonium ions, but that is not the reason for the extra stability of allyl and benzyl carbonium ions.
4. Thus, both (A) and (R) are correct but (R) is not the correct explanation of (A).

Teacher's Note:
1. Allyl and benzyl cations are 1° yet more stable than many alkyl cations because of resonance.
2. Inductive and hyperconjugative effects are much weaker than resonance.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond.
2. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds.
3. Thus, both Assertion and Reason are correct and Reason is not the correct explanation of Assertion.

Teacher's Note:
1. Bond polarity is the result of the electron shift; electronegativity difference is the cause.
2. Polar bonds are the starting point of inductive effects.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. The electron displacements in an organic molecule due to the influence of an atom or a substituted group present in the molecule cause permanent polarisation of the bond.
2. Inductive effect and resonance effect are examples of this type of electronic displacement.
3. Thus, both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. Inductive and resonance effects are permanent; the electromeric effect is temporary.
2. Permanent effects influence physical properties like dipole moment.

Answer: (c) A is true, but R is false.

Explanation:
1. Less reactivity of saturated hydrocarbons is due to the presence of only strong C—C and C—H single (σ) bonds between atoms, so the Assertion is correct.
2. Isomeric paraffins do not have the same parent name, e.g. butane and 2-methylpropane (isobutane) are isomers of C₄H₁₀ with different parent chains. So, the Reason is incorrect.
3. Assertion is correct, but Reason is incorrect.

Teacher's Note:
1. Paraffin means ‘little affinity’ — alkanes react only under drastic conditions.
2. The parent name depends on the longest chain, which changes from isomer to isomer.

Answer: (c) A is true, but R is false.

Explanation:
1. Olefins (alkenes) have one double bond between two carbon atoms in their molecules.
2. They have the general formula C_nH_2n, i.e. two H atoms less than the corresponding alkane C_nH_2n+2.
3. Assertion is correct but Reason is incorrect.

Teacher's Note:
1. Olefins = alkenes (C=C); acetylenes = alkynes (C≡C).
2. The name olefin means ‘oil forming’ (they form oily dihalides with halogens).

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. When double and triple bonds are present in a single compound, the lowest sum (lowest locant) rule is followed.
2. Numbering from the triple bond end: CH₃(5)CH(4)=CH(3)—C(2)≡CH(1), sum of locants = 1 + 3 = 4.
3. Numbering from the other end: CH₃(1)CH(2)=CH(3)—C(4)≡CH(5), sum of locants = 2 + 4 = 6.
4. So it is named pent-3-en-1-yne. Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

Teacher's Note:
1. Only when the locants are equal from both ends does the double bond get the lower number.
2. In the name, ‘-en’ is written before ‘-yne’.

Answer: (c) A is true, but R is false.

Explanation:
1. The compound contains —CHO as the principal functional group, so numbering is done from the functional group side (CHO carbon = C-1).
2. Cl and CH₃ are substituents at C-4 and C-2; they are written in alphabetical order in the name: 4-chloro-2-methylpentanal.
3. Alphabetical order decides only the order of writing prefixes, not the numbering. Assertion is correct but Reason is incorrect.

Teacher's Note:
1. Numbering is decided by the principal functional group, then multiple bonds, then substituents.
2. Alphabetical order matters for numbering only when there is a tie in locants.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. neo-hydrocarbons contain a quaternary carbon atom, i.e. a (CH₃)₃C— group attached to the chain.
2. A carbon atom bonded to four other carbon atoms is called a quaternary (4°) carbon.
3. Assertion and Reason both are correct statements but Reason is not the correct explanation of Assertion.

Teacher's Note:
1. neo-pentane is (CH₃)₄C, which has one quaternary carbon.
2. The degree of a carbon (1°, 2°, 3°, 4°) is the number of carbons attached to it.