NEET Practice Questions (MCQs) with Answers & Solutions

The NCERT states, 'In the monocotyledons, the vascular bundles have no cambium present in them. Hence, since they do not form secondary tissues they are referred to as closed.'

According to the NCERT text, 'For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K. At 413 K, ethoxyethane is the main product.' Therefore, $\text{H}_2\text{SO}_4$ at 413 K favors ether formation.

The NCERT states, 'The formation of ether is a nucleophilic bimolecular reaction (SN2) involving the attack of alcohol molecule on a protonated alcohol... The method is suitable for the preparation of ethers having primary alkyl groups only.'

The NCERT states, 'However, the dehydration of secondary and tertiary alcohols to give corresponding ethers is unsuccessful as elimination competes over substitution and as a consequence, alkenes are easily formed.' While bulkiness and $\text{S}_{\text{N}}1$ are related, the primary reason given in the text is the competition from elimination.

The NCERT states, 'Ethers containing substituted alkyl groups (secondary or tertiary) may also be prepared by this method.' It also mentions, 'It is an important laboratory method for the preparation of symmetrical and unsymmetrical ethers. The reaction involves SN2 attack of an alkoxide ion on primary alkyl halide.'

The NCERT specifies, 'The reaction involves SN2 attack of an alkoxide ion on primary alkyl halide.' Using secondary or tertiary alkyl halides in Williamson synthesis often leads to elimination products due to competing reactions.

The NCERT states, 'For example, ethanol is dehydrated to ethene in the presence of sulphuric acid at 443 K.' At higher temperatures, elimination (alkene formation) is favored over substitution (ether formation).

The NCERT explicitly defines Williamson synthesis as, 'In this method, an alkyl halide is allowed to react with sodium alkoxide. R–X + Na+–O’R' → R–O–R + Na+X'.'

If a mixture of ethyl alcohol and methyl alcohol were used for bimolecular dehydration, it would lead to a mixture of three possible ethers: diethyl ether, dimethyl ether, and ethyl methyl ether, making the separation difficult and the yield of the desired unsymmetrical ether low. This is an application of the understanding that SN2 dehydration with a mixture of alcohols is not selective.

This is a direct application of the Williamson synthesis, where a primary alkyl halide ($\text{CH}_3\text{CH}_2\text{Br}$) reacts with a sodium alkoxide ($\text{CH}_3\text{ONa}$) via an $\text{S}_{\text{N}}2$ reaction to form an ether, methyl ethyl ether ($\text{CH}_3\text{CH}_2\text{OCH}_3$).