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.162Hard
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.163Hard
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.164Hard
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.165Hard
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.
Advertisement
Q.166Hard
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.167Hard
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.168Easy
Which of the following compounds undergoes SN1 reaction most readily?
Answer: A
Tertiary alkyl halides form stable tertiary carbocations, making SN1 mechanism favorable.
Q.169Medium
In the Friedel-Crafts alkylation of benzene with 1-chloropropane and AlCl3, the major product is isopropylbenzene due to:
Answer: A
Primary carbocation rearranges via hydride shift to form more stable secondary carbocation, which then forms isopropylbenzene.
Q.170Easy
Which reagent is used to differentiate between aldehydes and ketones?
Answer: C
Both Tollens and Fehling reagents give positive test with aldehydes (silver mirror and red precipitate respectively) but not with ketones.
Q.171Medium
The pKa of phenol (C6H5OH) is approximately 10, while that of aliphatic alcohol is around 16. This difference is due to:
Answer: B
Phenoxide ion is stabilized by resonance with the benzene ring, making phenol more acidic than aliphatic alcohols.
Q.172Medium
Which of the following compounds will not give a positive Iodoform test?
Answer: D
Iodoform test requires either a methyl ketone (COCH3) or secondary alcohol with CH3 adjacent to CHOH. Ethanol has no such structure.
Q.173Hard
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.174Easy
The oxidation of primary alcohols with chromic acid (H2CrO4) in acidic medium gives:
Answer: C
Chromic acid oxidizes primary alcohols to carboxylic acids. The aldehyde is an intermediate which is further oxidized.
Q.175Easy
Which mechanism best describes the addition of HBr to propene?
Answer: C
Addition to alkenes follows electrophilic addition mechanism with carbocation formation, leading to Markovnikov's product.
Q.176Medium
In the nitration of anisole (C6H5-O-CH3), the major product is:
Answer: A
Methoxy group is ortho-para directing and activating. Due to steric hindrance at para position, 2-nitroanisole is the major product.
Q.177Medium
The reaction of acetyl chloride with aniline preferentially gives N-acetylaniline rather than p-acetylaniline because:
Answer: A
The lone pair on nitrogen of aniline is more nucleophilic than the π electrons of benzene, making N-acylation the dominant pathway.
Q.178Easy
Which of the following compounds exhibits keto-enol tautomerism?
Answer: B
Acetone (CH3-CO-CH3) has α-hydrogen atoms and can exist in equilibrium between keto and enol forms.
Q.179Medium
In the reaction of phenylmagnesium bromide (Grignard reagent) with CO2 followed by hydrolysis, the main product is:
Answer: B
Grignard reagent attacks CO2 to form a salt, which upon hydrolysis gives benzoic acid.
Q.180Hard
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.
