NEET Practice Questions (MCQs) with Answers & Solutions

Similar to phenols, where '-OH group directs the incoming group to ortho and para positions in the ring as these positions become electron rich due to the resonance effect caused by –OH group', the alkoxy group in ethers also shows a strong positive resonance effect, donating electron density to the ortho and para positions, thus activating them for electrophilic attack.

While the NCERT example shows specific cleavage at the less hindered side for $S_N2$, for tertiary alkyl groups, the reaction with HI often proceeds via an $S_N1$ mechanism. The tertiary carbocation formed is highly stable, leading to the formation of the tertiary alkyl iodide. The context implies the general principle of bond cleavage with HI.

The text states: 'Phenols do not react further to give halides because the $sp^2$ hybridised carbon of phenol cannot undergo nucleophilic substitution reaction needed for conversion to the halide.' This directly explains why iodobenzene is not formed from the phenyl group.

The NCERT text explicitly states: 'Para isomer is obtained in 90% yield' for the bromination of anisole.

The text states, 'The alkoxy group (-OR) ... activates the aromatic ring towards electrophilic substitution in the same way as in phenol.'

When symmetrical ethers react with HI, the C-O bond breaks, forming an alcohol and an alkyl iodide. If excess HI is present and heated, the alcohol can further react to form alkyl iodide. However, with limited HI, ethanol and iodoethane are the primary products. The provided example 7.7 (i) shows $CH_3CH_2 – O – CH_2CH_3$ reacting with HI to give $CH_3CH_2OH$ and $CH_3CH_2I$.

As per the NCERT text, 'Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis.' This directly refers to the total biomass produced.

The NCERT states, 'Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers).' It is also defined as GPP minus respiration losses (GPP - R = NPP).

According to the NCERT, 'The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter.'

The NCERT states that 'Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons. Rest of course, is on land.' Since land occupies only 30% of the surface but accounts for $170 - 55 = 115$ billion tons, its productivity per unit area is higher.