In Friedel-Crafts alkylation, the reactivity of alkyl halides follows:
Answer: B
F-C alkylation proceeds via carbocation formation. Tertiary carbocations are most stable, followed by secondary, then primary. Reactivity: Tertiary > Secondary > Primary.
Q.2Hard
Which of the following will give a positive iodoform test?
Answer: B
Iodoform test requires CH3CO- group. Pentan-2-ol (CH3CH2CH(OH)CH3) can be oxidized to CH3COCH2CH3 which contains the required methyl ketone group.
Q.3Hard
The compound that will undergo decarboxylation easily is:
Answer: C
β-Ketoacids (RCOCH2COOH) undergo easy decarboxylation through a six-membered transition state, producing ketones. This is more facile than decarboxylation of regular carboxylic acids.
Q.4Hard
A compound with molecular formula C6H12O shows a broad O-H stretch in IR at 3300 cm⁻¹ and no C=O peak. The 1H-NMR shows a singlet at δ 3.3 ppm. The compound is likely:
Answer: B
The singlet at δ 3.3 ppm suggests the OH is on a quaternary carbon (no neighboring H, hence no coupling). 2,3-dimethyl-2-butanol fits: the OH is on a quaternary carbon, explaining the singlet and broad O-H in IR.
Q.5Hard
In the hydroboration-oxidation of alkenes, the reaction is stereospecific because:
Answer: B
Hydroboration occurs through a concerted mechanism involving a four-membered ring transition state, resulting in syn-addition of BH across the double bond.
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Q.6Hard
A compound C5H10O2 with no C=O in IR but shows broad O-H stretch. 1H-NMR exhibits signals at δ 3.8 and δ 4.7 ppm. The compound is most likely:
Answer: B
No C=O in IR rules out ketone/aldehyde. The signals at δ 3.8 (OCH2) and δ 4.7 suggest acetal carbons (characteristic of dioxolane ring). 2,2-dimethyl-1,3-dioxolane fits C5H10O2 with protected diol functionality.
Q.7Hard
Which of the following compounds will show optical isomerism?
Answer: C
2-bromobutane (CH3CHBrCH2CH3) has a carbon with four different groups attached, making it chiral. The bromine-bearing carbon is a stereocenter.
Q.8Hard
The decarboxylation of malonic acid derivatives proceeds through:
Answer: B
Malonic acid derivatives (with two electron-withdrawing groups) undergo decarboxylation through a cyclic transition state mechanism, facilitated by the stabilization of the resulting carbanion by the remaining electron-withdrawing group.
Q.9Hard
In the crossed Cannizzaro reaction, the aldehyde that acts as both reducing and oxidizing agent is:
Answer: B
Formaldehyde is the only aldehyde without an α-hydrogen, so it undergoes the Cannizzaro reaction where it acts as both oxidizing and reducing agent, forming formate and methanol.
Q.10Hard
The rearrangement that occurs during the dehydration of 3-methylbutan-2-ol under acidic conditions is:
Answer: B
The initially formed secondary carbocation rearranges via methyl shift to form a more stable tertiary carbocation, producing 2-methylbut-2-ene as major product.
Q.11Hard
The mechanism of the esterification reaction between a carboxylic acid and alcohol is:
Answer: C
Esterification follows the nucleophilic acyl substitution mechanism: the OH of the alcohol attacks the carbonyl carbon after protonation, forming a tetrahedral intermediate that collapses to form the ester.
Q.12Hard
When propyne undergoes hydration in the presence of Hg2+/H2SO4, the major product is:
Answer: C
Propyne (CH3C≡CH) hydrates to form an unstable enol intermediate. The enol tautomerizes to acetone (CH3COCH3), following Markovnikov's rule with H adding to the carbon with more H atoms.
Q.13Hard
In the reduction of ketones and aldehydes using DIBAL-H, the product is:
Answer: B
DIBAL-H (Diisobutylaluminum hydride) reduces carbonyl compounds to primary alcohols (from aldehydes) or secondary alcohols (from ketones). It's milder than LiAlH4.
Q.14Hard
The directing effect of the amino group (-NH2) in electrophilic aromatic substitution is:
Answer: C
The amino group is a strong electron-donating group (by resonance) that activates the benzene ring and directs incoming electrophiles to ortho/para positions due to resonance stabilization of the intermediate.
Q.15Hard
In the Robinson annulation reaction, the product formed is:
Answer: B
The Robinson annulation combines a Michael addition followed by an aldol condensation, forming a bicyclic ketone product with a new six-membered ring fused to an existing ring system.
Q.16Hard
The enolate ion formed from a ketone in basic conditions attacks an alkyl halide in the:
Answer: A
Although the enolate has negative charge on both oxygen and carbon (resonance), the carbon is more nucleophilic toward alkyl halides. The SN2 attack occurs at the alkyl halide carbon, not the enolate O.
Q.17Hard
In the bromination of toluene, the ortho:para ratio is approximately 2:1 instead of 1:1 due to:
Answer: C
Although para is thermodynamically more stable, ortho is kinetically favored but sterically hindered. The 2:1 ratio reflects kinetic control with steric factors reducing ortho product.
Q.18Hard
The order of reactivity in electrophilic aromatic substitution for halogens is:
Answer: A
Fluorine is most reactive due to the best balance of electronic effects and hyperconjugation, while iodine is least reactive.
Q.19Hard
In the acetoacetic ester synthesis, the initial condensation is between acetoacetic ester and alkyl halide in the presence of:
Answer: A
Sodium ethoxide creates the enolate of acetoacetic ester, which then undergoes SN2 with alkyl halide for alkylation.
Q.20Hard
Which intermediate is formed when acetyl chloride reacts with benzene in the presence of AlCl₃?
Answer: A
In Friedel-Crafts acylation, AlCl₃ facilitates the formation of the acetyl cation (CH₃CO⁺) from acetyl chloride. This electrophilic acylium ion then attacks the benzene ring.
