Chemistry MCQs for NEET — Practice Questions with Answers

Actually, ${\mathrm{Na}}_2\mathrm{S}$ & NaCN are removed as ${\mathrm{H}}_2\mathrm{S}$ & HCN by conc. ${\mathrm{HNO}}_3$

$$\begin{gathered} {\mathrm{Na}}_2\mathrm{S}+2{\mathrm{HNO}}_3\to 2{\mathrm{NaNO}}_3+{\mathrm{H}}_2\mathrm{S} \\ \mathrm{NaCN}+{\mathrm{HNO}}_3\to {\mathrm{NaNO}}_3+\mathrm{HCN} \end{gathered}$$

(d) Key Idea Lesser the electron density of acyl carbon atom, more will be the susceptibility of nucleophile to attack it.

The Cl atom has strong -I effect and weakest +R effect because of the weak p-bond between the small sized C-atom and large sized Cl atom.

Thus, in CH₃COCl, acyl carbon has least electron density and hence, more susceptible for nucleophilic attack.

(d) Double bond have preference over triple bond while naming CH$\equiv$C-CH=CH₂
                                                                                       1-butene 3-yne

Key Idea: The carbon which is more electrophilic in nature, loses an $\mathrm{\alpha }$-H with a base more readily. Among the given compounds, carbonyl carbon (>C=O) ie, ketone is more electrophilic in nature, thus loses an $\mathrm{\alpha }$

(d) The acidity of halogenated acid increases almost proportionately with the increase in electronegativity of the halogen present.

So, the correct order is:

FCH₂COOH > CICH₂COOH > BrCH₂COOH > CH₃ COOH

(d) An optically active compound rotate plane polarized light. But due to some external compensation (heat, light, catalyst, solvent etc), the optically active compound lose their optical activity. The compound then exists as racemic mixture i.e, both d and l form are present in solution due to some intramolecular rearrangement.

So, there is no rotation of plane polarised light and compound does not have an asymmetric centre and when it again forms the symmetric centre both d- and l forms are obtained in equal amount.

Halogens are the most reactive elements. High reactivity is due to their low dissociation energy. Further among halogens reactivity decreases as we move down the group. 

In case of different nucleophiles, but present in the same group in the periodic table, then larger is the atomic mass, higher is the nucleophilicity. Hence the decreasing order of nucleophilicity of the halide ions is I- > Br- > Cl- > F- 

When the Lassaigne's extract is boiled with concentrated HNO3, it decomposes Na2S and NaCN (if formed during the test) to prevent their interference in the detection of halogens. This step ensures that the precipitation of AgCl is due to the presence of halogens in the given organic compound.

Ether Boiling point = 34.5° C

Benzene boiling point = 80°C 

Both can be separated by Distillation.