Basics of Organic Chemistry (GOC) — Page 2 | NEET Chemistry

Q11

During the preparation of Mohr's salt solution (Ferrous ammonium sulphate), which of the following acid is added to prevent hydrolysis of

\mathrm{Fe}^{2+}

ion?

A dilute hydrochloric acid

B concentrated sulphuric acid

C dilute nitric acid

D dilute sulphuric acid

Correct Answer

Option D

Solution

Mohr's salt is the ammonium iron(II) sulfate, with the chemical formula

\mathrm{(NH_4)_2Fe(SO_4)_2 \cdot 6H_2O}

.

It is known for its stability compared to the other iron(II) salts which tend to readily oxidize to iron(III) salts when exposed to the air.

To prevent the oxidation and hydrolysis of the

\mathrm{Fe}^{2+}

ion during the preparation of Mohr's salt, an acid is added.

This acid serves several purposes: it maintains the acidic environment necessary to prevent hydrolysis, aids in the solubilization of iron(II) sulfate, and minimizes the oxidation of

\mathrm{Fe}^{2+}

ions to

\mathrm{Fe}^{3+}

.

The options provided give four different types of acids, from which one needs to be selected for the preparation of this salt.

We can evaluate them based on their appropriateness: Dilute Hydrochloric Acid: While capable of maintaining a low pH to prevent oxidation, HCl can potentially introduce chloride ions (

\mathrm{Cl^-}

), which can form complexes with iron, changing the composition of the solution.

Concentrated Sulphuric Acid: This choice is too strong and can lead to excessive acidity as well as potential safety issues during handling and dilution.

It is unnecessary for this application.

Dilute Nitric Acid: Nitric acid is an oxidizing agent and can promote the oxidation of

\mathrm{Fe}^{2+}

to

\mathrm{Fe}^{3+}

, which is undesirable in the preparation of Mohr's salt.

Dilute Sulphuric Acid: This is a very common choice since it helps maintain the stability of the

\mathrm{Fe}^{2+}

ion without contributing extraneous ions that could form undesired complexes or products.

Moreover, it keeps the solution acidic, helping prevent oxidation and hydrolysis, while not introducing any oxidizing characteristics.

Given these considerations, the best choice for preventing hydrolysis of the

\mathrm{Fe}^{2+}

ion during the preparation of Mohr's salt is Dilute Sulphuric Acid (Option D).

This is because it maintains the suitable acidic conditions needed for stabilizing the ferrous ion and does not interfere with the redox stability of the iron by providing an oxidative chemical environment.

Q12

The correct order for the rate of

\alpha,\beta

-dehydrohalogenation for the following compounds is _________.

A (i) < (ii) < (iii)

B (ii) < (i) < (iii)

C (iii) < (ii) < (i)

D (ii) < (iii) < (i)

Correct Answer

Option D

Solution

Rate of Dehydrohalogenation : II < III < I

Q13

In Lassaigne's extract of an organic compound, both nitrogen and sulphur are present, which gives blood red colour with

\mathrm{Fe}^{3+}

due to the formation of-

A NaSCN

B

\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NOS}\right]^{4-}

C

[\mathrm{Fe}(\mathrm{SCN})]^{2+}

D

\mathrm{Fe}_{4}\left[\mathrm{Fe}\left(\mathrm{CN}_{6}\right)\right]_{3} \cdot \mathrm{xH}_{2} \mathrm{O}

Correct Answer

Option C

Solution

Sol.

In case nitrogen and sulphur both are present in an organic compound, sodium thiocyanate is formed, it give blood red colour and no prussian blue since there are no free cyanide Ions

\mathrm{Na}+\mathrm{C}+\mathrm{N}+\mathrm{S} \rightarrow \mathrm{NaSCN}

\mathrm{Fe}^{+3}+\mathrm{SCN}^{\ominus} \longrightarrow[\mathrm{Fe}(\mathrm{SCN})]^{2+}

Blood red

Q14

Amongst the given options which of the following molecules/ion acts as a Lewis acid?

A

\mathrm{H}_{2} \mathrm{O}

B

\mathrm{BF}_{3}

C

\mathrm{OH}^{-}

D

\mathrm{NH}_{3}

Correct Answer

Option B

Solution

can not act as lewis acid because they does not contain vacant orbital BF

_3

$\to$ Contains vacant orbital on central atom (Boron).

Q15

Consider the following compounds/species: The number of compounds/species which obey Huckel's rule is ___________.

A 6

B 2

C 5

D 4

Correct Answer

Option D

Solution

Huckle's rule = (4n + 2) $\pi$ electrons Comp (i), (ii), (v), (vii) obey Huckle's rule

Q16

Predict the order of reactivity of the following four isomers towards S N 2 reaction. (I) CH 3 CH 2 CH 2 CH 2 Cl (II) CH 3 CH 2 CH(Cl)CH 3 (III) (CH 3 ) 2 CHCH 2 Cl (IV) (CH 3 ) 3 CCl

A (IV) > (II) > (III) > (I)

B (IV) > (III) > (II) > (I)

C (I) > (II) > (III) > (IV)

D (I) > (III) > (II) > (IV)

Correct Answer

Option D

Solution

$\bullet$ Lesser the steric hinderance on halide carbon, more will be the reactivity of alkyl halide towards S N 2 reaction.

$\bullet$ So correct order towards S N 2 reactivity is :

Q17

What is the hybridization shown by C 1 and C 2 carbons, respectively in the given compound? OHC

-

CH = CH

-

CH 2 COOCH 3

A sp 3 and sp 3

B sp 2 and sp 3

C sp 2 and sp 2

D sp 3 and sp 2

Correct Answer

Option B

Solution

$-$ COOCH 3 has higher priority than $-$ C = C $-$ and $-$ CHO in IUPAC nomenclature. C 1 = sp 2 C 2 = sp 3

Q18

The incorrect statement regarding chirality is

A S N 1 reaction yields 1 : 1 mixture of both enantiomers

B The product obtained by S N 2 reaction of haloalkane having chirality at the reactive site shows inversion of configuration

C Enantiomers are superimposable mirror images on each other

D A racemic mixture shows zero optical rotation

Correct Answer

Option C

Solution

The stereoisomers related to each other as non-superimposable mirror image are called enantiomers.

Q19

The compound which shows metamerism is :

A C 4 H 10 O

B C 5 H 12

C C 3 H 8 O

D C 3 H 6 O

Correct Answer

Option A

Solution

Metamerism compound which have same molecular formula but different number of carbon atoms on either sides of functional group are known as metamers and this phenomenon is known as metamerism .

Compounds with formula C 4 H 10 O has ether functional group in which following arrangements are possible.

So, it shows metamerism.

For example CH 3 - CH 2 - O - CH 2 - CH 3 , and CH 3 - O - CH 2 - CH 2 - CH 3 are metamers as structure of alkyl chains are different around the functional group.

Q20

Dihedral angle of least stable conformer of ethane is :

A 0

^\circ

B 120

^\circ

C 180

^\circ

D 60

^\circ

Correct Answer

Option A

Solution

Ethane has two conformers (i) Eclipsed (ii) Staggered Eclipsed conformer is least stable while staggered conformer is most stable.

In eclipsed conformer the dihedral angle is 0

^\circ

.