Chemistry MCQs for NEET — Practice Questions with Answers

Williamson's synthesis is a well-known method for the preparation of ethers. It involves the reaction of an alkoxide ion with a primary alkyl halide or tosylate in an S_N2 reaction mechanism. This method is widely used in organic chemistry to form symmetrical and asymmetrical ethers.

Phenols are more acidic than alcohols because the phenoxide ion formed after losing a hydrogen ion (H⁺) is stabilized by resonance. The negative charge on the oxygen atom is delocalized over the aromatic ring, which makes the phenoxide ion more stable. On the other hand, the alkoxide ion formed from alcohols does not have such resonance stabilization, making alcohols less acidic.

When benzyl phenyl ether is heated with HI, it undergoes cleavage to form phenol and benzyl iodide. The reaction proceeds as follows: C₆H₅CH₂O-C₆H₅ + HI → C₆H₅OH + C₆H₅CH₂I. Therefore, the products obtained are phenol (option 1) and benzyl iodide (option 3).

The organic compound X with molecular formula $C_5H_{10}O$ is described as yielding phenyl hydrazone and giving a negative response to both the iodoform test and Tollen's test. This means that it is not a methyl ketone or an aldehyde, as these would give positive results with iodoform and Tollen's tests, respectively. Additionally, the compound produces n-pentane on reduction, indicating that the keto group is in the middle of the chain. Therefore, the compound is most likely pentan-3-one.

When $CH_3CHCl_2$ (1,1-dichloroethane) is heated with aqueous KOH, it undergoes a hydrolysis reaction to form acetaldehyde ($CH_3CHO$). The reaction proceeds through the formation of a geminal diol intermediate, which then loses water to form the aldehyde.

The formation of cyanohydrin from a ketone is an example of nucleophilic addition. In this reaction, the nucleophile (cyanide ion, $CN^-$) attacks the electrophilic carbonyl carbon of the ketone, forming a new carbon-carbon bond and resulting in the formation of cyanohydrin.

The C-O-C bond angle is influenced by steric hindrance around the oxygen atom. In the case of $ (CH_3)_2 CH - O - CH (CH_3)_2 $, the two bulky isopropyl groups will create more steric repulsion, causing the bond angle to increase to minimize this repulsion. Therefore, the C-O-C bond angle will be maximum in $ (CH_3)_2 CH - O - CH (CH_3)_2 $.

The reaction between sodium ethoxide and bromocyclopropane does not yield an ether. This is because bromocyclopropane undergoes a nucleophilic substitution reaction where the strain in the three-membered ring leads to the formation of a ring-opened product rather than an ether.

The reagent $ NaOH I_2 /H^+ $ is used for the iodoform test, which specifically converts methyl ketones (or secondary alcohols adjacent to a methyl group) into carboxylic acids with one carbon less. In the case of butan-2-one, it will be converted into propanoic acid using this reagent.