Chemistry MCQs for NEET — Practice Questions with Answers

The Lucas test is used to differentiate between primary, secondary, and tertiary alcohols. It involves the reaction of the alcohol with Lucas reagent (a mixture of concentrated HCl and anhydrous ZnCl_2). The test is based on the reactivity of different alcohols with the Lucas reagent, where tertiary alcohols react immediately, secondary alcohols take a few minutes, and primary alcohols do not react at room temperature.

2-Methylpropan-2-ol (a tertiary alcohol) reacts immediately with anhydrous $ ZnCl_2 + HCl $ in the Lucas test and forms an insoluble chloride. This is because tertiary alcohols form stable carbocations quickly, leading to the formation of the insoluble chloride product.

Glycerol is more viscous than ethanol because it forms many hydrogen bonds per molecule. Glycerol has three hydroxyl (OH) groups, each capable of forming hydrogen bonds, creating a more interconnected and cohesive liquid structure compared to ethanol, which has only one hydroxyl group.

When 4.6 grams of ethanol reacts with sodium metal, 1.12 liters of hydrogen gas ($H_2$) is formed at STP. The balanced chemical equation for this reaction is: $$ 2C_2H_5OH + 2Na ightarrow 2C_2H_5ONa + H_2 ext{(gas)} $$ For every 46 grams (1 mole) of ethanol, 1 mole (22.4 liters at STP) of hydrogen gas is produced. Therefore, for 4.6 grams of ethanol, 0.1 mole of hydrogen gas is produced, which corresponds to 1.12 liters at STP.

In the Lucas test, alcohols react with hydrogen chloride in the presence of zinc chloride to form alkyl chlorides. The cloudiness observed is due to the formation of the insoluble alkyl chloride. This test is particularly useful for distinguishing between primary, secondary, and tertiary alcohols based on the time it takes for the cloudiness to appear.

When ethyl alcohol (ethanol) is treated with chlorine, a series of substitution reactions can occur. Under controlled conditions, the final product is chloral (trichloroacetaldehyde), which has the formula $CCl_3CHO$. The reaction proceeds through intermediate stages where ethyl alcohol is progressively chlorinated.

Salicylaldehyde and o-nitrophenol are less soluble in water due to the presence of intra-molecular hydrogen bonding. Intra-molecular hydrogen bonding occurs when a hydrogen atom is shared within the same molecule, reducing the molecule's ability to form hydrogen bonds with water molecules. This leads to decreased solubility in water.