JEE Chemistry Organic Chemistry Some Basic Principles and Techniques MCQs Set 41

Answer: (a) 1.39 angstrom

Explanation:
1. The bonds are equal because of resonance and lie between 1.54 (single) and 1.34 (double)
2. The value is 1.39 angstrom

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (d) One cation and one anion

Explanation:
1. In heterolysis the bonding pair goes to one atom
2. Carbon-chlorine fission gives a carbocation and \( \ce{Cl^-} \) (the shared pair goes to the more electronegative Cl)

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (b) Carbanion

Explanation:
1. A carbanion has three bonds and one lone pair, giving eight electrons
2. The other three have six or seven electrons

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (d) Carbocation

Explanation:
1. \( \ce{SbCl5} \) is a Lewis acid and removes \( \ce{Cl^-} \)
2. The planar carbocation formed can be attacked from either face, giving the racemic mixture

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (b) \( \ce{RCOCl} \)

Explanation:
1. An acyl chloride with anhydrous \( \ce{AlCl3} \) gives the acylium ion, which acylates benzene
2. The other three are not Friedel-Crafts reagents

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (a) 1/2

Explanation:
1. By Huckel's rule the number of pi electrons is 4n' + 2 where n' is a whole number
2. The count must be an integer, so 1/2 is impossible

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (b) \( \ce{SO3} \)

Explanation:
1. An electrophile is electron-deficient
2. \( \ce{SO3} \) has a very electron-poor sulphur, while water, ammonia and ether have lone pairs and are nucleophiles

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (b) Triphenylmethyl cation

Explanation:
1. Three phenyl rings delocalise the positive charge extensively
2. This outweighs the stabilisation from alkyl groups in the other options

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (d) Bimolecular nucleophilic substitution

Explanation:
1. The nucleophile \( \ce{B^-} \) attacks carbon and replaces the leaving group \( \ce{A^-} \)
2. This is nucleophilic substitution, and it is bimolecular when both species are in the rate-determining step

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (c) Substitution

Explanation:
1. Aqueous alkali replaces the halogen with OH
2. This is nucleophilic substitution

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (b) Planar

Explanation:
1. The SN1 intermediate is a carbocation with an sp2 carbon
2. Its geometry is trigonal planar

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (a) meta-directing

Explanation:
1. The nitro group withdraws electrons by -I and -M effects
2. It deactivates the ring and directs the incoming group to the meta position

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (c) Electrophilic substitution reaction

Explanation:
1. The delocalised pi cloud attracts electrophiles
2. Substitution keeps the aromatic ring intact, so electrophilic substitution is typical

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (d) To produce electrophile

Explanation:
1. \( \ce{AlCl3} \) is a Lewis acid that removes \( \ce{Cl^-} \) from the alkyl or acyl halide
2. This generates a carbocation or acylium ion electrophile

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (d) NaCl

Explanation:
1. A Friedel-Crafts catalyst must be a Lewis acid
2. NaCl is a neutral ionic salt and has no accepting orbital

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (b) Elimination reaction

Explanation:
1. Loss of HX from adjacent carbons forms a C=C bond
2. This is an elimination reaction

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (a) Mesomeric effect of Cl

Explanation:
1. The lone pair of Cl donates electrons into the double bond (+M)
2. This stabilises the carbocation at the carbon bearing Cl, so H adds to the terminal carbon

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (c) Addition reaction

Explanation:
1. Acetylene adds \( \ce{H2} \) to become ethylene
2. Adding atoms across a multiple bond is addition

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (d) Free radical substitution

Explanation:
1. Chlorination with \( \ce{Cl2} \) and light goes through chlorine radicals
2. A chlorine atom replaces hydrogen, so it is free radical substitution

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (c) Nucleophilic substitution

Explanation:
1. Water acts as the nucleophile and replaces Br
2. This is nucleophilic substitution (SN1 here)

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (d) Phenol

Explanation:
1. The OH group of phenol donates electrons to the ring by its +M effect
2. The ring is activated, whereas -NO2, -COOH and -CHO deactivate it

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (c) Reaction of NaOH with 2,4-dinitrofluorobenzene

Explanation:
1. The two nitro groups activate the ring towards attack by \( \ce{OH^-} \), which replaces F
2. The others are addition, SN at a side-chain carbon, and electrophilic substitution

Teacher's Note:
1. Decide the hybridisation, charge or electronic effect first, then compare the options on that basis.
2. Remember the trend: more s-character means shorter bonds and higher acidity; more alkyl groups mean more stable carbocations and radicals.

Answer: (a) \( \ce{AlCl3} \)

Explanation:
1. \( \ce{AlCl3} \) has an electron-deficient aluminium and accepts a lone pair
2. The other three are electron-rich nucleophiles

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (d) \( \ce{CH3O^-} \)

Explanation:
1. A negatively charged oxygen is a stronger nucleophile than a neutral one, and methoxide is a stronger base than phenoxide (which is resonance stabilised)
2. So \( \ce{CH3O^-} \) is the strongest

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.

Answer: (a) Cl

Explanation:
1. The rate depends on the leaving group ability, and \( \ce{Cl^-} \) is the weakest base and best leaving group of the four
2. So the acid chloride reacts the fastest

Teacher's Note:
1. Draw the species with all lone pairs and charges before answering.
2. For stability questions, ask whether the charge can be delocalised by resonance or hyperconjugation.

Answer: (b) Predominantly 2-butene

Explanation:
1. Saytzeff's rule: the more highly substituted alkene is the major product
2. 2-Butene (disubstituted) is favoured over 1-butene

Teacher's Note:
1. Link each reaction to its intermediate: carbocation (SN1, E1), planar and racemising; concerted (SN2), inversion.
2. Directing effects follow from electron donation or withdrawal by the substituent.