Amines and other nitrogen-containing compounds

Count substituents on nitrogen rather than carbon

For ordinary neutral amines, primary, secondary and tertiary mean one, two and three organic groups directly attached to N. Methanamine CH₃NH₂, dimethylamine (CH₃)₂NH and trimethylamine (CH₃)₃N illustrate the sequence. Their selected N has three ordinary bonds and one lone pair. The substituted carbon’s own degree is a different label.

Thus tert-butylamine (CH₃)₃CNH₂ is a primary amine: nitrogen has one carbon-group attachment even though that carbon has three carbon neighbours. A quaternary ammonium ion R₄N⁺ has four organic groups, four ordinary bonds and no nitrogen lone pair. Protonated R₃NH⁺ still has three organic groups plus H; it is the conjugate acid of a tertiary amine rather than an R₄N⁺ quaternary organic-substitution example.

A nitrogen symbol alone does not identify an amine

An amide N is directly attached to an acyl carbon, as CH₃CONH₂. A nitrile has C≡N, an imine a C=N motif, and a nitro group has the familiar ordinary contributor C–N⁺(=O)–O⁻. That nitro contributor’s N⁺ and O⁻ charges must both remain visible even though the whole selected nitromethane molecule is neutral.

These motifs have different electronic contexts. A lone pair’s availability depends on conjugation and the relevant orbital system; the mere presence of nitrogen or nonbonding electrons does not fix pK_a, nucleophilicity or a preferred protonation site. Formal charge, atomic oxidation state and a partial-charge distribution remain different quantities.

Ask whether a lone pair belongs to the aromatic system

In the ordinary neutral pyridine model, N supplies one electron to the six-electron aromatic π system while its lone pair is outside that system. For pyrrole, the nitrogen pair supplies two of the six π electrons. Using that pair for a different bond changes its electronic context, so a generic all-nitrogen basicity rule misses the distinction.

A selected neutral imidazole tautomer contains one pyridine-like and one pyrrole-like nitrogen role. Which atom bears the H is part of the tautomer’s identity. Actual protonation and basicity depend on solvent, temperature, substituents and the relevant conjugate-acid equilibrium. Do not transfer one tabulated ranking to every medium or confuse basicity, an equilibrium property, with nucleophilic reaction rate.

A conjugate-acid pKₐ describes the specified equilibrium

For B+H⁺⇌BH⁺, define acid dissociation of BH⁺ as K_a=a_B a_H⁺/a_BH⁺. Under stipulated compatible ideal dilute aqueous activities and externally buffered pH, c_B/c_BH⁺=10^(pH−pK_a,BH⁺). With only these two forms, f_BH⁺=1/[1+10^(pH−pK_a,BH⁺)].

The original toy sets pK_a,BH⁺=9.00 for an unnamed monobasic species at fixed medium and temperature. This is the conjugate acid’s pK_a, not the acidity of an N–H bond in neutral B. At pH 7.00, 9.00 and 11.00, the protonated fractions are about 0.9901, 0.5000 and 0.009901. Additional ionization, binding, nonideal activities or an unbuffered solution would need further balances.

Combine acid constants only in matched conventions

For an invented matched pair HA and BH⁺, the transfer HA+B⇌A⁻+BH⁺ has K=K_a,HA/K_a,BH⁺=10^(pK_a,BH⁺−pK_a,HA). A larger value favours the products at equilibrium under those reference conventions; it supplies no reaction speed.

With the stipulated pK_a values 4.50 and 9.00, K≈31622.8. In a separate ideal equal-initial HA/B model with no initial products and neglected other equilibria, K=x²/(c−x)², so the converted fraction x/c=√K/(1+√K)≈0.994408. Those invented constants are not assigned to the separately recognizable named net CH₃COOH+CH₃NH₂⇌CH₃COO⁻+CH₃NH₃⁺; that equation illustrates nuclear and charge accounting, not claimed measured values for its species.

Worked examples

Try it yourself

1. How many ordinary N lone pairs are present in the selected R₄N⁺ quaternary ammonium representation?

   Show the answer and reasoning

   Zero.

   Four ordinary σ bonds and net +1 at nitrogen leave no N lone pair in that representation.

2. Can the ordinary nitro contributor C–N(=O)–O be drawn with both shown charges omitted while preserving its Lewis claim?

   Show the answer and reasoning

   No.

   The stated contributor requires N⁺ and singly bonded O⁻, even when its whole molecule is neutral.

3. Does pK_a,BH⁺=9.00 refer to dissociation of neutral B’s N–H bond?

   Show the answer and reasoning

   No.

   It refers to BH⁺⇌B+H⁺ in the stipulated acid-equilibrium convention.

4. In the two-form buffered toy, what are the neutral and protonated fractions at pH=pK_a,BH⁺?

   Show the answer and reasoning

   Each is one-half.

   Their concentration ratio is one and the two fractions sum to one.

What to carry forward

• Count organic attachments at nitrogen itself.
• Preserve formal charges in nitrogen-group drawings.
• Identify the relevant lone-pair orbital context.
• Name the conjugate acid when quoting an amine pK_a.
• Equilibrium favourability does not supply a reaction time.