"Why" Questions (High-Yield Reasoning)

CHEExam Practice & Reasoning

These short-answer reasoning questions reward a one-line cause stated in the right vocabulary: hydrogen bonding, resonance/delocalisation, $+I$ / $\text{-}I$ effects, solvation, and activation of the ring. Identify the dominant factor and state it crisply rather than listing everything.

$1^\circ$ amines boil higher than $3^\circ$ amines of similar mass: $1^\circ$ (and $2^\circ$ ) amines have N-H bonds and form intermolecular H-bonds, while $3^\circ$ amines have no N-H and cannot.
Amines boil lower than alcohols of comparable mass because N is less electronegative than O, so N-H...N hydrogen bonds are weaker than O-H...O bonds.
Diazonium salts are kept at $273\text{-}278\,K$ and used at once because they are thermally unstable; on warming they decompose, releasing $N_2$ and forming phenol.
Aniline resists clean direct nitration: $\text{-}NH_2$ is strongly activating so the ring is over-substituted, and the strongly acidic medium protonates N to $\text{-}NH_3^+$ (a meta-directing deactivator), giving a mixture.
Aromatic $1^\circ$ amines are made by reducing nitro compounds, not by Gabriel synthesis (fails for aryl halides) or ammonolysis (aryl halides are unreactive to $NH_3$ ).
Trimethylamine is weaker than dimethylamine in water because solvation of the bulky $(CH_3)_3NH^+$ cation is poor (steric crowding around N reduces H-bonding stabilisation).
Aniline is basic but phenol is not: N is less electronegative than O and shares/donates its lone pair more readily, so the anilinium ion is more stable than protonated phenol.
$p$ -nitroaniline is weaker than aniline because the $\text{-}NO_2$ group withdraws electron density ( $\text{-}M$ / $\text{-}I$ ), depleting N's lone pair and destabilising the cation.
Aniline gives $2,4,6$ -tribromoaniline with bromine water because $\text{-}NH_2$ is so strongly activating ( $o$ / $p$ -directing) that all three available positions brominate readily, with no catalyst.
Lower amines are water-soluble because N-H...O (and N...H-O) hydrogen bonds form with water; solubility falls as the alkyl/aryl part grows and dominates.
Aniline does not undergo Friedel-Crafts reactions because its basic N forms a salt with the Lewis acid $AlCl_3$ , deactivating the ring (the lone pair is no longer available to activate it).

Where the marks go

Answering a 'why basic' question with the $+I$ effect alone when solvation or resonance is actually the deciding factor (e.g. $3^\circ$ amine in water, or aniline).
Explaining the boiling-point order by molecular mass instead of the real cause — presence or absence of N-H hydrogen bonding.
Saying aniline is non-basic — it IS basic, just weaker than $NH_3$ ; the contrast is with phenol, which is essentially non-basic.
Giving a long list of factors instead of naming the single dominant cause the examiner wants in a one-mark 'why' question.
Stating diazonium salts decompose simply because they are 'reactive' — specify the cause (thermal instability) and the products ( $N_2$ + phenol).

How the board asks it

Give reasonsbasicity order and the role of solvation of the cation
Give reasons: $(CH_3)_3N$ is a weaker base than $(CH_3)_2NH$ in aqueous solution.
Give reasonsn-h intermolecular hydrogen bonding and boiling-point trends
Account for the fact that primary amines have higher boiling points than tertiary amines of comparable molecular mass.
Give reasonssalt formation of the basic nitrogen with the lewis acid $AlCl_3$
Why does aniline not undergo the Friedel-Crafts reaction?
Give reasonsthermal instability of the diazonium ion
Give a reason: benzenediazonium salts are prepared and used at $273\text{-}278\,K$ and not at room temperature.
Distinguishazo-dye test separating an aromatic amine from an aliphatic amine
How will you distinguish between aniline and ethylamine by a simple chemical test, and explain why the test works?
Assertion–Reasonavailability of nitrogen's lone pair contrasting aniline with phenol
Assertion: Aniline is basic but phenol is essentially non-basic. Reason: Nitrogen is less electronegative than oxygen and donates its lone pair more readily, so the anilinium ion is more stable than protonated phenol. State whether both are correct and whether the reason explains the assertion.

PreviousConversions (Roadmaps through Diazonium Salts)

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.