Chemistry: Atoms First 2e · Organic Chemistry
Amines and Amides
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Amines and amides are the two major nitrogen-containing functional families. An Amine Organic derivative of ammonia; N with 1–3 carbon groups Full entry → is an organic derivative of ammonia in which one, two, or three hydrogens are replaced by carbon groups: \(\mathrm{RNH_2}\), \(\mathrm{R_2NH}\), or \(\mathrm{R_3N}\). An Amide Carbonyl carbon bonded to nitrogen Full entry → has a nitrogen bonded to a carbonyl carbon: \(\mathrm{R{-}CO{-}NR'_2}\). Despite both containing nitrogen, they are profoundly different: amines are weak bases (the nitrogen carries a Lone pair Two unshared electrons on an atom Full entry → and a partial negative charge), while amides are essentially neutral because the lone pair is delocalized into the carbonyl. Amines smell fishy and are found in neurotransmitters, alkaloids, and drugs; amides include the peptide bonds that link amino acids into proteins, plus nylon and many pharmaceuticals. This topic covers structure, classification, naming, basicity, hydrogen bonding, and the everyday chemistry of these two families.
Why this matters
Nitrogen chemistry is life chemistry. Amino acids are joined by amide (peptide) bonds to build every protein in your body — insulin, hemoglobin, antibodies, and enzymes are all amide polymers. Amines appear in neurotransmitters (dopamine, serotonin, epinephrine), in the caffeine in coffee, in nicotine, and in countless medications (antihistamines, local anesthetics, antidepressants). In the clinic, the basicity of amines matters: many drugs are amines that are absorbed and distributed differently depending on whether they are protonated (charged) or unprotonated (neutral), which depends on pH. Amides, by contrast, are the stable backbone of proteins and of synthetic polymers like nylon and Kevlar. Recognizing an amine vs. an amide on sight tells you whether a molecule is basic, how it hydrogen-bonds, and how it will behave in water and in the body.
The college version
Core Concepts
Structure and classification of amines
An amine is ammonia with one or more H replaced by carbon groups:
- Primary (1°) amine: one carbon group, \(\mathrm{RNH_2}\) (e.g., methylamine, \(\mathrm{CH_3NH_2}\)).
- Secondary (2°) amine: two carbon groups, \(\mathrm{R_2NH}\) (e.g., dimethylamine).
- Tertiary (3°) amine: three carbon groups, \(\mathrm{R_3N}\) (e.g., trimethylamine, which gives rotting fish its smell).
The nitrogen in an amine has five valence electrons: three are used in bonds, and two remain as a lone pair. That lone pair makes the nitrogen electron-rich and basic. Amines with an –NH or –NH₂ group can donate and accept hydrogen bonds (N–H is polar), so primary and secondary amines hydrogen-bond like alcohols; tertiary amines have no N–H and can only accept.
Naming amines
Common names are widely used: methylamine, ethylamine, dimethylamine. In IUPAC naming, the parent chain is the longest carbon chain, and the amine is named with the suffix -amine (the amino group gets the lowest locant: 1-propanamine) or with the prefix amino- when the amine is a substituent on a chain with a higher-priority group (e.g., 2-aminopropanoic acid). The nitrogen's substituents in secondary/tertiary amines are designated with N-: N-methyl methanamine, etc.
Structure and naming of amides
An amide is a carbonyl carbon bonded to a nitrogen: \(\mathrm{R{-}CO{-}NR'_2}\). The key structural fact: the nitrogen's lone pair is delocalized into the carbonyl π system (a resonance interaction), so the C–N bond has partial double-bond character, the amide group is planar, and the nitrogen is a very poor base — amides do not accept protons under ordinary conditions. Naming: drop "-oic acid" and add -amide (ethanamide from ethanoic acid; common: acetamide). Substituents on the nitrogen use N- prefixes (N-methylacetamide). The amide linkage between amino acids is the Peptide bond The amide linkage between amino acids Full entry →.
Basicity: amines yes, amides no
This single contrast is the most tested idea in the topic. An amine's lone pair is available to grab a proton:
\[ \mathrm{RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-} \]
Amines are weak bases (methylamine has \(\mathrm{K_b \approx 4.4 \times 10^{-4}}\)). Adding acid converts an amine to an Ammonium salt \(\mathrm{RNH_3^+}\) formed when an amine grabs a proton Full entry → (\(\mathrm{RNH_3^+}\)), which is water-soluble and odorless — this is why amine salts are common drug forms. An amide's lone pair is tied up in resonance with the carbonyl, so amides are neutral (non-basic). Compare: methylamine (a 1° amine) is a base; acetamide (an amide) is not.
Physical properties and hydrogen bonding
Primary and secondary amines hydrogen-bond through N–H, so they boil higher than alkanes but lower than alcohols of the same size (N–H bonds are less polar than O–H). Small amines (methylamine, ethylamine) are gases at room temperature and dissolve in water. Amides have no free N–H lone pair available for protonation, but primary and secondary amides do hydrogen-bond through N–H; because amides are polar and often crystalline, small amides (like acetamide, m.p. ~82 °C) are solids at room temperature — very different from their amine counterparts.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Amine (basic) | Amide (neutral) | Amine lone pair is free; amide lone pair is delocalized into the carbonyl |
| 1° amine (RNH₂) | 1° carbon (RCH₂OH etc.) | "1°" counts groups on the functional atom (N vs. C); they are different classification systems |
| Trimethylamine (3° amine, fishy) | Ammonia | Ammonia has no carbon groups; trimethylamine is its fully substituted relative |
| Amine salt (RNH₃⁺) | Free amine (RNH₂) | Salt is charged, water-soluble, nonvolatile, odorless; free amine is the base |
| Amide (R–CO–NR₂) | Ester (R–CO–OR') | Amide has N–R on the carbonyl; ester has O–R |
| N-methyl label | C-methyl label | N-methyl attaches to nitrogen; C-methyl attaches to a carbon — different isomers |
| Aniline (weak base) | Alkyl amines (stronger bases) | The aromatic ring delocalizes/withdraws the lone pair, lowering basicity |
| Peptide bond | "Peptide" molecule | The bond is the amide linkage; a peptide is the chain it forms |
Eli explains
The same idea, in plain words
Explain it like I’m 10
An amine is like ammonia with some of its hands replaced by carbon arms — it still has a free pair of electrons, so it acts like a sponge that grabs protons, making it a weak base (fish smell!). An amide is what happens when that nitrogen holds hands with a carbonyl carbon; its free electrons get shared with the oxygen next door, so it has nothing left to grab protons with — it's neutral. Amides are the snap-links that chain amino acids into proteins. Same element, different neighbors: one grabs protons, the other builds your body.
Worked example
Example 1: Amine or amide? Predicting basicity and hydrogen bonding
Classify each compound and predict whether it is basic and whether it can donate hydrogen bonds: (a) \(\mathrm{CH_3CH_2NH_2}\), (b) \(\mathrm{CH_3CONH_2}\), (c) \(\mathrm{(CH_3)_3N}\).
Step 1 — Find the functional group. (a) ethylamine: nitrogen bonded to one carbon group — a 1° amine, lone pair available. (b) acetamide: nitrogen bonded to a carbonyl carbon — an amide. (c) trimethylamine: nitrogen bonded to three methyl groups — a 3° amine.
Step 2 — Predict basicity. (a) Basic (weak base; lone pair grabs H⁺). (b) Non-basic (lone pair delocalized into C=O). (c) Basic (lone pair available) — actually a stronger base than ethylamine in water because the three methyl groups push electron density onto nitrogen.
Step 3 — Predict hydrogen-bond donation. (a) Has N–H → donates and accepts H-bonds. (b) Has N–H → donates H-bonds, but is not basic. (c) No N–H → cannot donate H-bonds (accepts only).
Step 4 — Check against reality. Ethylamine is a water-soluble gas (b.p. 17 °C); acetamide is a crystalline solid (m.p. ~82 °C); trimethylamine is the fishy-smelling gas (b.p. 3 °C). Structure predicts all three behaviors.
Example 2: Making an ammonium salt — the amine–acid reaction
Methylamine, \(\mathrm{CH_3NH_2}\) (molar mass 31.06 g/mol), reacts with hydrochloric acid:
\[ \mathrm{CH_3NH_2(aq) + HCl(aq) \rightarrow CH_3NH_3^+Cl^-(aq)} \]
How many grams of HCl (molar mass 36.46 g/mol) are required to react completely with 10.0 g of methylamine?
Step 1 — Grams of methylamine to moles:
\[ 10.0\ \text{g CH}_3\mathrm{NH}_2 \times \frac{1\ \text{mol CH}_3\mathrm{NH}_2}{31.06\ \text{g}} = 0.322\ \text{mol CH}_3\mathrm{NH}_2 \]
Step 2 — Mole ratio (1:1 from the balanced equation): 0.322 mol HCl needed.
Step 3 — Moles of HCl to grams:
\[ 0.322\ \text{mol HCl} \times \frac{36.46\ \text{g HCl}}{1\ \text{mol HCl}} = 11.7\ \text{g HCl} \]
Check: the salt \(\mathrm{CH_3NH_3^+Cl^-}\) (methylammonium chloride) is a water-soluble, crystalline, essentially odorless solid — exactly why pharmaceutical companies convert amine drugs into ammonium salts for stable, soluble tablet formulations.
Example 3: Counting atoms — the percent nitrogen in methylamine
What percentage of methylamine's mass is nitrogen (N = 14.01 g/mol)? Molar mass of \(\mathrm{CH_3NH_2}\): C (12.01) + 5 H (5 × 1.008 = 5.04) + N (14.01) = 31.06 g/mol.
Step 1 — Write the formula for percent composition:
\[ \%\ \text{N} = \frac{\text{mass of N in 1 mol}}{\text{molar mass}} \times 100\% \]
Step 2 — Substitute:
\[ \%\ \text{N} = \frac{14.01\ \text{g/mol}}{31.06\ \text{g/mol}} \times 100\% = 45.1\% \]
Step 3 — Interpret. Nearly half of methylamine's mass is nitrogen. The same calculation applied to proteins (amide polymers) is how nutrition labels and biochemists estimate nitrogen/protein content from elemental analysis.
Key takeaways
- Amine: nitrogen with 1, 2, or 3 carbon groups (1°, 2°, 3°) and a lone pair → weak base.
- Amide: nitrogen bonded to a carbonyl carbon → lone pair delocalized → non-basic, planar, partial double-bond C–N.
- Basicity contrast: \(\mathrm{CH_3NH_2}\) (amine) is basic; \(\mathrm{CH_3CONH_2}\) (acetamide) is not.
- Amines react with acids to form water-soluble ammonium salts: \(\mathrm{RNH_2 + HCl \rightarrow RNH_3^+Cl^-}\).
- Naming: amines use -amine (or amino- as prefix); amides use -amide; N- prefixes label nitrogen substituents.
- Peptide bonds (protein backbones) are amide bonds.
- 1° and 2° amines hydrogen-bond (N–H); 3° amines and amides have modified H-bonding roles; small amines are water-soluble.
- Amines smell fishy/ammoniacal; trimethylamine is the classic fish-odor molecule.
- Aromatic amines (aniline, \(\mathrm{C_6H_5NH_2}\)) are weaker bases than aliphatic amines because the ring withdraws electron density from the lone pair.
- Quaternary ammonium salts (\(\mathrm{R_4N^+}\)) are permanently charged cations — used in disinfectants and fabric softeners.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why are amines weak bases but amides are not?
Show answer
Amines have a lone pair on nitrogen that is available to accept a proton. In amides, that lone pair is delocalized into the adjacent carbonyl (resonance), so it is not available for protonation.
Classify \(\mathrm{(CH_3)_2NH}\) (dimethylamine) as primary, secondary, or tertiary, and predict whether it can hydrogen-bond.
Show answer
Secondary (2°) amine — two carbon groups on nitrogen. It has an N–H bond, so it can donate and accept hydrogen bonds.
What product forms when ethylamine reacts with HCl? Name it and state two of its properties.
Show answer
Ethylammonium chloride, \(\mathrm{CH_3CH_2NH_3^+Cl^-}\): a water-soluble, crystalline salt. (Free amine + HCl → ammonium salt.)
What is the functional-group name of the bond linking amino acids in proteins?
Show answer
Amide bond (peptide bond).
Which is the stronger base in water: methylamine or Aniline Aminobenzene, \(\mathrm{C_6H_5NH_2}\) Full entry →? Why?
Show answer
Methylamine — alkyl groups push electron density onto nitrogen, making the lone pair more available; in aniline, the aromatic ring withdraws electron density, weakening basicity.
Name the amide derived from ethanoic acid, and write its structure.
Show answer
Ethanamide (common name acetamide), \(\mathrm{CH_3CONH_2}\).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Amine
- Organic derivative of ammonia; N with 1–3 carbon groups
- Amide
- Carbonyl carbon bonded to nitrogen
- Lone pair
- Two unshared electrons on an atom
- Ammonium salt
- \(\mathrm{RNH_3^+}\) formed when an amine grabs a proton
- Peptide bond
- The amide linkage between amino acids
- Primary/secondary/tertiary amine
- 1, 2, or 3 carbon groups on nitrogen
- N- prefix
- Marks substituents attached to nitrogen in naming
- Resonance delocalization
- Lone pair spread over multiple atoms (N into C=O)
- Quaternary ammonium
- \(\mathrm{R_4N^+}\) — four carbon groups, positive charge
- Aniline
- Aminobenzene, \(\mathrm{C_6H_5NH_2}\)
Sources & references
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