Basicity of Amines

Why Amines Are Basic

Like ammonia, the nitrogen atom of an amine is $sp^3$ hybridised and pyramidal, with three bonds to hydrogen or carbon and a fourth $sp^3$ orbital holding an unshared pair of electrons. It is this lone pair that defines the chemistry of amines. NCERT states it plainly: amines "have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base." A Lewis base donates an electron pair; a Brønsted base accepts a proton. The same lone pair does both.

When an amine meets a mineral acid it forms an ammonium salt, and treatment of that salt with a strong base such as $\ce{NaOH}$ regenerates the free amine. The acid–base equilibrium in water is the reference reaction for measuring strength:

$$\ce{R-NH2 + H2O <=> R-NH3^+ + OH^-}$$

The position of this equilibrium is what we quantify. The more readily the lone pair is donated — and the more stable the resulting cation — the stronger the base.

Kb, pKb and Base Strength

NCERT defines the basicity constant from the equilibrium above. For the reaction of an amine with water,

$$K_b = \frac{[\ce{R-NH3^+}][\ce{OH^-}]}{[\ce{R-NH2}]} \qquad\text{and}\qquad pK_b = -\log K_b$$

The textbook statement to memorise is exact: "Larger the value of $K_b$ or smaller the value of $pK_b$, stronger is the base." Because $pK_b$ is a negative logarithm, the two scales run in opposite directions. When a question asks you to rank amines, translate it immediately: lowest $pK_b$ = strongest base; highest $pK_b$ = weakest base.

NCERT pins the reference points. Ammonia has $pK_b = 4.75$. Aliphatic amines fall below it (range roughly 3 to 4.22), so they are stronger bases. Aromatic amines like aniline rise far above it. Table 9.3 lists the aqueous-phase values:

Two patterns leap out of this table. The aliphatic amines cluster near $pK_b \approx 3$, well below ammonia. The aniline family sits near $pK_b \approx 9$, far above ammonia. Everything that follows is the explanation of those two clusters — and of the small, irregular spread within the aliphatic cluster.

The Inductive Effect of Alkyl Groups

NCERT frames structure-basicity as a competition: "Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine."

Alkyl groups are electron-releasing — they exert a positive inductive effect ($+I$). They "push electrons towards nitrogen and thus make the unshared electron pair more available for sharing with the proton." After protonation, the same $+I$ effect helps to disperse the positive charge on the substituted ammonium cation, stabilising it. Both halves of the argument — a more available lone pair and a more stabilised cation — point the same way: alkyl substitution should raise basicity.

Carried to its conclusion, this predicts that basic strength rises with the number of alkyl groups: $\ce{3^\circ > 2^\circ > 1^\circ > NH3}$. NCERT confirms that this expectation is genuinely realised — but only under specific conditions, which the next section makes precise.

Gas Phase vs Aqueous Phase

This is the single most examined subtlety in the topic. NCERT is explicit that the inductive prediction holds in one phase and breaks in the other.

In the gaseous phase, the inductive effect acts alone, with no solvent to interfere. The order is exactly the one $+I$ predicts:

$$\text{gas phase: } \ce{(CH3)3N > (CH3)2NH > CH3NH2 > NH3}$$

In the aqueous phase, a second factor enters. After the amine accepts a proton, the substituted ammonium cation is stabilised "not only by electron releasing effect of the alkyl group ($+I$) but also by solvation with water molecules." Solvation works through hydrogen bonds from the N–H hydrogens of the cation to surrounding water. NCERT gives the governing rule: "The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion."

A primary ammonium ion $\ce{R-NH3^+}$ has three N–H bonds available for hydrogen bonding to water; a secondary $\ce{R2NH2^+}$ has two; a tertiary $\ce{R3NH^+}$ has only one. So the order of stabilisation by solvation runs opposite to the inductive order — it favours the primary cation. Steric bulk reinforces this: small groups like $\ce{-CH3}$ pose no obstruction, but bigger groups crowd the nitrogen and hinder both protonation and H-bonding.

The NCERT Aqueous Orders

NCERT does not leave the outcome to inference — it states the resolved aqueous orders verbatim. These two lines are worth memorising exactly, because they differ between the methyl and ethyl series:

$$\ce{(CH3)2NH > CH3NH2 > (CH3)3N > NH3}$$ $$\ce{(C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3}$$

In both aqueous series the secondary amine is the strongest base, because it strikes the best balance — enough alkyl groups for a useful $+I$ push, yet still two N–H bonds for solvation and not too much steric crowding. The placement of the tertiary amine is what shifts between the series. The change of the alkyl group "from $\ce{-CH3}$ to $\ce{-C2H5}$ results in change of the order of basic strength", as NCERT puts it. Do not assume one fixed order across both series.

Aromatic Amines and the Aniline Anomaly

Aniline ($pK_b = 9.38$) is a far weaker base than ammonia ($pK_b = 4.75$), let alone methylamine ($pK_b = 3.38$). NCERT asks the question directly — "pKb value of aniline is quite high. Why is it so?" — and answers with resonance. Because the $\ce{-NH2}$ is bonded directly to the ring, the nitrogen lone pair "is in conjugation with the benzene ring and thus making it less available for protonation."

The accounting is by resonance structures. Aniline is a resonance hybrid of five structures (the neutral form plus four that push the lone pair into the ortho and para positions). The anilinium ion formed on protonation has only two (the Kekulé pair). "Greater the number of resonating structures, greater is the stability." So the free base is more stabilised than its conjugate acid; protonation costs the molecule its delocalisation energy, and "the proton acceptability or the basic nature of aniline ... would be less than that of ammonia."

The contrast with benzylamine ($\ce{C6H5CH2NH2}$, $pK_b = 4.70$) makes the point. Here a $\ce{-CH2-}$ spacer separates the nitrogen from the ring, so there is no conjugation; the lone pair stays localised and available, and benzylamine is comparable to ammonia and far more basic than aniline. NCERT's increasing order is $\ce{C6H5NH2 < C6H5CH2NH2}$.

Substituents on Aniline

Once aniline is the baseline, ring substituents shift basicity in a predictable direction. NCERT states it directly: in substituted aniline "electron releasing groups like $\ce{-OCH3}$, $\ce{-CH3}$ increase basic strength whereas electron withdrawing groups like $\ce{-NO2}$, $\ce{-SO3H}$, $\ce{-COOH}$, $\ce{-X}$ decrease it."

The headline case is the nitro group. A $\ce{-NO2}$ group withdraws electrons by both $-I$ and $-M$ (resonance), so p-nitroaniline is markedly weaker than aniline, which is in turn weaker than p-toluidine ($p$-$\ce{CH3}$-aniline). The para position lets the $-NO2$ conjugate directly with the nitrogen lone pair, deepening the effect. The NCERT-style increasing order of basic strength is therefore:

$$\text{p-nitroaniline} < \text{aniline} < \text{p-toluidine}$$

Worked Basicity-Ordering Examples

Ordering questions reward a fixed routine: separate aromatic from aliphatic (aliphatic wins, ammonia sits between them), then rank within each block using the rules above.