Test: SN1 & SN2 Reactions (January 28) - NEET MCQ

(III) forms tertiary carbocation, hence most reactive. (I) is least reactive as highly unstable carbocation is formed at bridge head carbon of bicyclic compound.

Tertiary halide would react at fastest rate with AgNO₃ as reaction will proceed by S_N1 mechanism.

Stronger the nucleophile, faster the S_N2 reaction. Polar aprotic solvent solvate cations, makes anionic nucleophile more available for reaction, hence faster reaction. A better leaving group lowers the activation energy increasing rate of S_N2 reaction.

Allyl bromide (I) is most reactive among the given halides as pi bonds from allylic position stabilises the transition state. Vinyl iodide (II) is least reactive due to partial double bond character. Electron withdrawing inductive effect of CH₃O^- increases reactivity of (IV) over (III)

(II) is most reactive as it forms aromatic carbocation (III) is next most reactive as it forms allylic carbocation. (I) is least reactive as it forms least stable cyclopropyl carbocation.

The transition state in S_N2 reaction is pentavalent as indicated here.
 

H₂O has higher solvating power than CH₃CH₂OH,hence faster S_N1 reaction occur in H₂O. Reaction proceeds via carbocation intermediate, it passes through more than one transition states. Due to the presence of some intimate ion pair, S_N1 reaction occur resulting in partial racemisation and net inversion of configuration.
Since, nucleophile is not involved in rate determining step, reaction occur at same rate with both CH₃— ¹⁸OH and CH₃OH.

Both nucleophile and leaving group are electron rich species, nucleophile attacks from backside (most remote position) to have minimum repulsion with leaving group. Also, due to the presence of leaving group on front side there is less physical space for attack of nucleophile.

Stronger bond formation at α-carbon affects the stability of product (thermodynamics) not the rate (kinetics) of reaction. Change of isotope at α-position or in halogens affect the rate, hence show kinetic isotopic effect. Presence of electron withdrawing group in substrate increases electrophilicity of α-carbon hence, increases its reactivity towards nucleophiles.

ZnCI₂ is a Lewis acid, helps in formation of carbocation. AgNO₃ form s AgCI precipitate, derive heterolysis reaction belo w in forward direction:

From the given information structure of A can be derived as: