31 Year NEET Previous Year Questions: Some Basic Principles & Techniques - 1 - NEET MCQ

• Na + C + N → NaCN: This reaction forms sodium cyanide, used to detect nitrogen in the compound. This belongs to Lassaigne’s test.
• 2Na + S → Na₂S: This forms sodium sulfide, used to detect sulfur. This also belongs to Lassaigne’s test.
• Na + X → NaX: This forms sodium halide, used to detect halogens like Cl, Br, and I. This belongs to Lassaigne’s test.
• 2CuO + C → 2Cu + CO₂: This reaction involves the reduction of copper oxide by carbon and is unrelated to Lassaigne’s test for detecting N, S, or halogens.

Therefore, the reaction that does NOT belong to Lassaigne’s test is  2CuO + C → 2Cu + CO₂ 

• A. CHCl₃ + C₆H₅NH₂: These are heat-sensitive liquids. Therefore, Distillation under reduced pressure is used.
• B. Crude oil in petroleum industry: Crude oil is a complex mixture of hydrocarbons that have close boiling points. Therefore, Fractional Distillation is used.
• C. Glycerol from spent-lye: Glycerol is separated using Simple Distillation.
• D. Aniline-water: Aniline and water are separated using Steam Distillation as aniline is volatile in steam.
• A - I (Distillation under reduced pressure)
• B - III (Fractional Distillation)
• C - IV (Simple Distillation)
• D - II (Steam Distillation)

Therefore, the correct answer is A-I, B-III, C-IV, D-II.

The stability of carbocation can be described by the hyperconjugation. Greater the extent of hyperconjugation, more is the stability of carbocation

Stability order of carbocations = (4) > (2) > (1) > (3)

The technique described in the question is known as sublimation. Sublimation is a process where a solid turns directly into a gas without passing through the intermediate liquid state. This physical change occurs under specific conditions of temperature and pressure and is characteristic of certain substances.
Option B: Sublimation is the correct answer because it directly describes the process of solid substances transforming into vapour without becoming liquid first. For example, substances like iodine, naphthalene, and dry ice (solid carbon dioxide) sublimate when heated under normal atmospheric conditions. Sublimation is not only a fascinating chemical process but is also exploited in various scientific and industrial applications, including purification of substances, where impurities do not sublimate and can thus be easily separated from the vaporized material.

The other options provided do not describe this process:

• Crystallization (Option A) involves the formation of solid crystals from a homogeneous solution. It is typically used to purify solids wherein the impurities remain dissolved in the solvent.
• Distillation (Option C) is a process used to separate mixtures based on differences in the conditions required to change the phase (liquid phase) of the components of the mixture. It does not involve direct transition from solid to gas.
• Chromatography (Option D) is a method for separating components of a mixture based on differences in their movement through a stationary medium under the influence of a solvent or carrier gas. It does not involve the phase transition of solid to gas directly.

The correct order of group number of ions is

∴ The correct order is B, A, D, C, E

In case, nitrogen and sulphur both are present in organic compound, sodium thiocyanate is formed. It gives blood red colour and no Prussian blue since there are no free cyanide ions.

can not act as lewis acid because they does not contain vacant orbital
BF₃ → Contains vacant orbital on central atom (Boron).

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

Cl has pronounced −I effect than +R effect due to large size difference between carbon and chlorine, also due to high electronegativity of chlorine. 

• Lesser the steric hinderance on halide carbon, more will be the reactivity of alkyl halide towards S_N2 reaction. 
• So correct order towards S_N2 reactivity is : 

−COOCH₃ has higher priority than − C = C − and − CHO in IUPAC nomenclature. 

C₁ = sp²
C₂ = sp³

Kjeldahl method is not used for the organic compound containing
–NO₂ group, –N = N- group,
And N– containing in the ring.

The correct answer is: Option (c)

The correct answer is: Option (c)

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

Planar, cyclic, conjugated species containing (4n + 2)π electrons will be aromatic in nature (n is an integer)

The blue colour is of  Fe₄ [Fe(CN)₆]₃

In S_NAr reactions, a carbanion is formed as an intermediate, so any substituent that increases the stability of carbanion and hence the transition state leading to its formation will enhance the S_NAr reactions. To compare the rates of substitution in chlorobenzene, chlorobenzene having electron-withdrawing group, and chlorobenzene having electronreleasing group, we compare the structures carbanion I (from chlorobenzene), II (from chlorobenzene containing electron-withdrawing group) and III (from chlorobenzene containing electron-releasing group).

G withdraws electrons, neutralises (disperses) –ve charge of the ring, stabilises carbanion, facilitates S_N reaction (activation effect)

G releases electrons, intensifies –ve charge, destabilizes carbanion, retards S_N reaction (deactivation) NO₂ is activating group and CH₃ and OCH₃ are deactiving group.
Hence, the correct order of nucleophilic substitution reactions

IUPAC name of the structure is 3-ethyl-2-hydroxy-4-methylhex-3-en-5-ynoic acid

Decreasing order of deactivating effect of the given  m-directing group is > NO₂ > – CN > – SO₃H > – COOH
—NO₂ group is most deactivating group due to strong – E, – I and – M effects.

–NO₂ is a powerful electron withdrawing group. Its presence on ring makes the ring less active

  (iso-butyl group)

Stability depends on number of hyperconjugative structures.

Conformers are form of stereoisomers in which isomers can be interconverted by rotations about single bonds. I and II are staggered and eclipsed conformers respectively.

Stability depends on number of hyperconjugative structure.

In the carbonium ion the carbon atom carrying the positive charge is sp² hybridized.

It shows geometrical isomerism but does not show optical isomerism.

According to the IUPAC nomenclature rules, when naming alkenes with substituents, the numbering of the carbon chain should give the multiple bond (double or triple bond) the lowest possible number. In the given compound, the correct numbering starts from the end nearest the double bond, which gives the double bond position 1 instead of 2. Therefore, the correct name is 3-Bromoprop-1-ene, not 1-Bromo-prop-2-ene as stated in option a). Hence, option a) is not according to the IUPAC system.
All other options (b, c, and d) follow correct IUPAC nomenclature rules.

Electrophilic rate order

Since nitration is an electrophilic substitution hence presence of electron releasing group like CH₃ in the nucleus facilitates nitration.