Acetamide (ethanamide): structure, formula, physical and chemical prop

Article 79 · Chemistry

Acetamide is the first amide most students meet, in Class 12 (amines and carboxylic acid derivatives), A-level organic chemistry and AP Chemistry, and the questions on it are always the same five: draw the structure, give the properties, prepare it, react it, and explain why it is a weaker base than an amine. This page answers those five in the order the exam asks them, with the numbers you are expected to quote.

Structure of acetamide

Acetamide, CH₃CONH₂: the amide group is the C=O and the NH₂ on the same carbon


IUPAC name	Ethanamide
Common name	Acetamide
Molecular formula	C₂H₅NO
Condensed formula	CH₃CONH₂
Molar mass	59.07 g/mol
Functional group	Amide (–CONH₂), primary
Parent acid	Acetic (ethanoic) acid, CH₃COOH

The carbonyl carbon is sp², trigonal planar, bond angles close to 120°. The nitrogen is also close to planar, not pyramidal as in an amine, because its lone pair is delocalised into the C=O. Two resonance structures describe this: the usual one with C=O and a lone pair on N, and a second with C–O⁻ and C=N⁺. The real molecule sits between them, which is why the C–N bond (about 1.33 Å) is shorter than a normal C–N single bond (1.47 Å) and why rotation about it is restricted.

Physical properties

Property	Value	What the examiner wants you to say
Appearance	Colourless, deliquescent crystals	Pure acetamide is odourless; the mousy smell is from impurities
Melting point	79 to 81 °C	Unusually high for a small molecule: strong hydrogen bonding between N–H and C=O of neighbouring molecules
Boiling point	221 °C	Same reason; compare ethylamine at 17 °C and acetic acid at 118 °C
Density	1.16 g/cm³
Solubility	Very soluble in water and ethanol; soluble in chloroform; almost insoluble in ether	Hydrogen bonds with water through both N–H and C=O
Nature	Neutral to litmus	Amides are very weak bases and very weak acids: amphoteric but practically neutral

Chemical character: why acetamide is not a base like ethylamine

An amine is basic because the nitrogen lone pair is free to accept a proton. In acetamide the same lone pair is shared with the carbonyl group by resonance, so it is much less available: the pK_a of the conjugate acid is about −0.5, against 10.7 for ethylamine. Protonation, when it happens in strong acid, is on the oxygen, not the nitrogen. The N–H hydrogens are weakly acidic (pK_a about 15 to 17) and are removed by sodium metal or sodamide to give the amide anion. Write "amphoteric, but far weaker as a base than an amine, because of delocalisation of the lone pair" and you have the two-mark answer.

Preparation of acetamide

From ammonium acetate, by heating (the laboratory method). CH₃COONH₄ → CH₃CONH₂ + H₂O. The salt is heated with a little glacial acetic acid to suppress its dissociation into acid and ammonia, and the water is distilled off; acetamide is collected at 210 to 220 °C.
From acetyl chloride and ammonia. CH₃COCl + 2 NH₃ → CH₃CONH₂ + NH₄Cl. Fast and vigorous; the second ammonia mops up the HCl. Acetic anhydride works the same way and gives ammonium acetate as the by-product.
From ethyl acetate and ammonia (ammonolysis of an ester). CH₃COOC₂H₅ + NH₃ → CH₃CONH₂ + C₂H₅OH. Slow at room temperature, done with concentrated aqueous ammonia.

A fourth route, the partial hydrolysis of acetonitrile (CH₃CN + H₂O → CH₃CONH₂) with acid or base, is the industrial method and the reverse of the dehydration below.

Reactions of acetamide

Reaction	Reagent and condition	Product	Where it appears
Hydrolysis	Dilute acid or alkali, heat	Acetic acid (or its salt) + ammonia (or ammonium salt)	Every board; the test for an amide is the ammonia given off with NaOH
Hofmann bromamide degradation	Br₂ + concentrated NaOH (or KOH), warm	Methylamine, CH₃NH₂, with one carbon fewer	Class 12, A-level: the name reaction that shortens the chain by one carbon
Dehydration	P₂O₅ (or P₄O₁₀), heat	Acetonitrile, CH₃CN	Class 12: amide to nitrile
Reduction	LiAlH₄ then water (or Na/ethanol)	Ethylamine, C₂H₅NH₂	Class 12, A-level: C=O reduced to CH₂, nitrogen kept
With nitrous acid	HNO₂ (NaNO₂ + HCl), cold	Acetic acid + N₂ + H₂O	Distinguishes a primary amide from an amine (no diazonium salt)
With sodium	Na metal, heat	Sodium acetamide, CH₃CONHNa + H₂	Shows the weakly acidic N–H

The Hofmann bromamide degradation is the one to be able to write mechanistically at A-level and to write as an equation at Class 12: CH₃CONH₂ + Br₂ + 4 NaOH → CH₃NH₂ + Na₂CO₃ + 2 NaBr + 2 H₂O. The carbonyl carbon leaves as carbonate; that is why the amine has one carbon fewer.

Uses of acetamide

Acetamide is a plasticiser (it softens cellulose materials and some lacquers), a solvent for many organic and inorganic compounds because it is polar and hydrogen-bonding, a stabiliser in peroxide solutions, a laboratory reagent for preparing methylamine and acetonitrile, and an intermediate in the manufacture of some pharmaceuticals and dyes. It is also used to lower the freezing point of water in some antifreeze-type applications. It is classified as a possible human carcinogen (IARC group 2B) on the basis of animal studies, so it is handled with gloves in the school lab.

The five-mark answer, in order

Structure: CH₃CONH₂, planar amide group, resonance between C=O and C–N.
Properties: high melting and boiling point for its size (hydrogen bonding), very soluble in water, neutral to litmus.
Weaker base than ethylamine: lone pair on N delocalised into C=O.
Preparation: heat ammonium acetate; or acetyl chloride + ammonia.
Reactions: hydrolysis, Hofmann bromamide (→ methylamine), P₂O₅ dehydration (→ acetonitrile), LiAlH₄ reduction (→ ethylamine).

How we teach Class 12 and A-level organic chemistry

One tutor, one student, the functional groups in the order the board sets them, and every name reaction written out with the reason it works, not memorised as a line. Amides come after amines and before the polymers; a student who can do this page in 20 minutes on a blank sheet is ready for the paper. The first 30-minute class is free: send the chapter and the last test on WhatsApp or the contact page. The revision method that makes organic chemistry stay is in active recall for chemistry; the subject page is online chemistry tutoring.

Questions parents ask

1What is the structure of acetamide?

Acetamide is CH3–CO–NH2: a methyl group bonded to a carbonyl carbon (C=O) that also carries an amino group (NH2). The C=O and the NH2 on the same carbon make it an amide, the simplest amide after formamide. The carbonyl carbon and the nitrogen are both planar (the lone pair on nitrogen is delocalised into the C=O), so the C, O, N and the two H atoms of NH2 lie in one plane.

2Is acetamide acidic or basic?

It is amphoteric but very weakly so: the nitrogen lone pair is delocalised into the carbonyl group, so it is a far weaker base than ethylamine (aqueous amides do not turn litmus blue), and the N–H can be deprotonated only by a strong base such as sodium metal or sodamide. In an exam, the point to make is that amides are much less basic than amines because of resonance.

3What is the difference between acetamide and acetanilide?

Acetamide is CH3CONH2, an amide of acetic acid with ammonia. Acetanilide is CH3CONHC6H5, the amide of acetic acid with aniline: one hydrogen of the NH2 is replaced by a phenyl ring. Acetanilide is the one used in the aniline protection question in Class 12 organic chemistry; acetamide is the one used for Hofmann bromamide and dehydration to nitrile.

4Why does acetamide smell of mice?

Pure acetamide is odourless. The mousy smell in bottles of laboratory acetamide comes from traces of impurities, chiefly acetonitrile and other nitrogen compounds formed as it slowly decomposes. It is a common exam-lab observation and not a property of the pure compound.

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Acetamide (ethanamide): structure, formula, physical and chemical properties, how it is prepared, its reactions and uses, for Class 12, A-level and AP chemistry