MS MS in Chemistry 3

Question 1

In an SN2 reaction, which substrate undergoes the fastest substitution?

Accepted Answer

Primary alkyl halide

Answer

SN2 reactions are bimolecular and sterically sensitive; primary alkyl halides have the least steric hindrance around the electrophilic carbon, allowing fastest backside nucleophilic attack.

Question 2

Which reagent converts a primary alcohol to an aldehyde without further oxidation to a carboxylic acid?

Accepted Answer

PCC (pyridinium chlorochromate) in CH2Cl2

Answer

PCC is a mild oxidant in anhydrous conditions that stops at the aldehyde stage; aqueous chromate reagents continue to oxidize aldehydes to carboxylic acids.

Question 3

The Diels-Alder reaction is classified as a [4+2] cycloaddition because it involves:

Accepted Answer

A 4π diene and a 2π dienophile

Answer

In the Diels-Alder reaction, the 4π electron diene reacts with the 2π electron dienophile in a concerted pericyclic [4+2] cycloaddition to form a six-membered ring.

Question 4

Which amino acid side chain can form a disulfide bond, contributing to protein tertiary structure?

Accepted Answer

Cysteine

Answer

Cysteine contains a thiol (–SH) group that is oxidized to form covalent disulfide (S–S) bonds between protein chains or within the same chain, stabilizing tertiary structure.

Question 5

In enzyme kinetics, the Michaelis constant Km represents:

Accepted Answer

The substrate concentration at which reaction velocity is half of Vmax

Answer

Km is defined as the substrate concentration [S] at which v = Vmax/2; a lower Km indicates higher enzyme affinity for its substrate.

MS - Master of Science Practice Test

MS - Master of Science Practice Test

MS MS in Chemistry 3