N-Phenylethanamide (Acetanilide): Structure, Preparation and Uses

N-phenylethanamide is the IUPAC name for acetanilide, an aromatic amide with the molecular formula C₈H₉NO and a molar mass of 135.16 g/mol. It is a white, odourless crystalline solid that melts at about 114 °C and boils at about 304 °C. The compound is made by acetylating aniline, usually with acetic anhydride, and in organic chemistry its main job is as a protecting group for the amino group of aniline and as an intermediate for dyes and other chemicals.

The same substance appears in textbooks under three names, which is what confuses most students: N-phenylethanamide = acetanilide = N-phenylacetamide. All three describe one structure, C₆H₅NHCOCH₃.

How the name N-phenylethanamide is built

IUPAC names for amides are built from the acid that the amide comes from. Take the naming apart piece by piece:

• ethanamide is the parent. It is the amide of ethanoic acid (acetic acid), CH₃CONH₂. Two carbons, hence “ethan-“, and the amide ending “-amide”.
• N-phenyl tells you a phenyl group (C₆H₅-) has replaced one hydrogen on the amide. The italic N locant is the important part: it says the substituent sits on the nitrogen, not on the carbon chain and not on a ring position.

So N-phenylethanamide is ethanamide with a phenyl group on nitrogen. Read the older name the other way round and you get the same molecule: acetanilide is aniline whose -NH₂ hydrogen has been swapped for an acetyl group, CH₃CO-. Chemists writing in the common style call that N-acetylaniline or N-phenylacetamide.

A caution worth remembering in an exam: 2-phenylethanamide is a different compound. There the phenyl group sits on carbon 2 of the chain (C₆H₅CH₂CONH₂, phenylacetamide). The single letter N versus the number 2 changes the substance completely.

Structure and the amide linkage

Acetanilide is a secondary amide: the nitrogen carries one hydrogen, one acetyl group and one phenyl group. The heart of the molecule is the amide linkage, -NH-CO-.

Two features of that linkage explain nearly all of the compound’s behaviour:

• The nitrogen lone pair is delocalised into the carbonyl. The pair on N overlaps with the C=O pi system, giving the C-N bond partial double bond character and making the amide group flat. Because the lone pair is tied up, acetanilide is not basic the way aniline is; it will not form a stable salt with dilute HCl the way aniline does.
• The N-H bond hydrogen bonds strongly. Neighbouring molecules link N-H to O=C in the crystal, which is why a small molecule of 135 g/mol melts as high as 114 °C while aniline, of similar size, is a liquid at room temperature.

With the lone pair split between the ring and the carbonyl, the -NHCOCH₃ group is still an electron-donating, ortho/para-directing group on the benzene ring, but a much milder one than -NH₂. That single fact is the reason acetanilide exists as a synthetic tool at all.

Physical properties of N-phenylethanamide

Working out the molar mass is a common one-mark question. Using C = 12.011, H = 1.008, N = 14.007 and O = 15.999:

(8 × 12.011) + (9 × 1.008) + 14.007 + 15.999 = 96.088 + 9.072 + 14.007 + 15.999 = 135.17 g/mol, rounded in tables to 135.16 g/mol.

The gap between cold and hot water solubility, about ten-fold, is not a trivia item. It is exactly what makes water the right recrystallisation solvent for acetanilide in the laboratory.

Preparation of acetanilide from aniline

All three standard routes do the same thing: they put an acetyl group on the nitrogen of aniline. They differ in how fast and how cleanly they do it.

1. Acetic anhydride route (the standard method)

This is the route used in almost every college laboratory because the reaction is fast, goes nearly to completion and is easy to control.

C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH

Aniline plus acetic anhydride gives acetanilide plus acetic acid. The equation is already balanced as written, one mole of each reactant giving one mole of each product.

Role of glacial acetic acid. It is added as the reaction medium, not as a reagent. It dissolves the aniline, keeps the mixture homogeneous, and moderates what is otherwise a sharply exothermic reaction so the flask does not run away or char. It also keeps the aniline from being protonated by any strong acid, since an anilinium ion has no lone pair to attack the anhydride.

Role of zinc dust. A pinch of zinc dust is added at the start and it takes no part in the acetylation. Aniline oxidises very easily in air, especially when hot, giving coloured quinone-type and azo-type products that turn the mixture brown or purple. Zinc dust keeps the conditions reducing and reduces any coloured oxidation products already present, so the acetanilide that separates is white instead of dirty brown. Only a small quantity is used, because too much leaves grey specks in the crystals.

2. Acetyl chloride route

C₆H₅NH₂ + CH₃COCl → C₆H₅NHCOCH₃ + HCl

Acetyl chloride is the most reactive of the three acetylating agents, so the reaction is vigorous and needs cooling. The trouble is the hydrogen chloride released: it protonates unreacted aniline to anilinium chloride, which cannot react further, so half the aniline is wasted. The fix is to work with a base that mops up HCl, such as pyridine, or with excess aniline, or with aqueous sodium hydroxide in the Schotten-Baumann arrangement.

3. Glacial acetic acid route

C₆H₅NH₂ + CH₃COOH ⇋ C₆H₅NHCOCH₃ + H₂O

Here acetic acid is the reagent rather than the solvent. The reaction is slow and reversible, needing several hours of refluxing, and the yield is only good if the water formed is distilled off to pull the equilibrium to the right. It is cheap and is used industrially, but it is a poor choice for a three-hour practical class.

Laboratory preparation and recrystallisation, step by step

Aim: to prepare acetanilide from aniline and acetic anhydride and purify it by recrystallisation from water.

Quantities (typical): 5 mL aniline (about 5.1 g, 0.055 mol), 7.5 mL glacial acetic acid, 5 mL acetic anhydride, a small pinch (about 0.2 g) of zinc dust, a 100 mL round-bottom flask with an air condenser.

1. Put the aniline, glacial acetic acid and acetic anhydride in the flask and swirl to mix. Add the zinc dust.
2. Fit the air condenser and reflux gently on a sand bath for 20 to 30 minutes.
3. Pour the hot mixture in a thin stream into 150 mL of cold water with constant stirring. Acetanilide is far less soluble in cold water than in the hot acid mixture, so it separates at once as a white solid, and the excess anhydride is destroyed by the water.
4. Cool thoroughly, filter under suction, and wash the solid with a little cold water to remove acetic acid.
5. Recrystallise. Transfer the crude solid to a beaker, add the minimum volume of boiling water needed to dissolve it, and if the solution is coloured add a very small quantity of activated charcoal and boil for a minute. Filter the hot solution through a fluted filter, then let the filtrate cool slowly and undisturbed.
6. Shining white plates separate. Filter, wash with a few millilitres of ice-cold water, press dry and leave on a watch glass or in a desiccator.
7. Record the melting point. Pure acetanilide melts sharply at about 114 °C; a low or broad range means the crystals are still wet or impure.

Yield calculation. 5.1 g of aniline is 5.1 ÷ 93.13 = 0.0548 mol. Since one mole of aniline gives one mole of acetanilide, the theoretical yield is 0.0548 × 135.16 = 7.4 g. A careful student usually recovers 5 to 6 g after recrystallisation, which is a 68 to 81 per cent yield. Some loss is unavoidable, because the mother liquor still holds about 0.5 g of dissolved product for every 100 mL of cold water.

Why water works as the recrystallisation solvent: 100 mL of boiling water dissolves about 5.5 g of acetanilide, while at 0 °C the same 100 mL holds only about 0.53 g. Hot water therefore takes the product into solution along with the impurities, and cooling forces most of the acetanilide out as crystals while the impurities stay behind in the liquid.

Chemical reactions of N-phenylethanamide

Hydrolysis back to aniline

The amide linkage is strong but it can be cut with water under acidic or basic conditions on refluxing.

• Acidic hydrolysis: C₆H₅NHCOCH₃ + H₂O + HCl → C₆H₅NH₃⁺Cl⁻ + CH₃COOH. The aniline comes out as its salt; adding alkali afterwards liberates free aniline.
• Basic hydrolysis: C₆H₅NHCOCH₃ + NaOH → C₆H₅NH₂ + CH₃COONa. Here the free amine is released directly and the acid is trapped as sodium acetate.

Amides hydrolyse far more slowly than esters, so both reactions need prolonged refluxing rather than gentle warming.

Nitration to p-nitroacetanilide

Treating acetanilide with a mixture of concentrated nitric acid and concentrated sulphuric acid below about 10 °C, often in glacial acetic acid, gives mainly p-nitroacetanilide with a small quantity of the ortho isomer.

The para product is separated by recrystallisation and can then be hydrolysed with acid or alkali to give p-nitroaniline, a dye intermediate. The two-step sequence, acetylate then nitrate then hydrolyse, is the standard textbook preparation of p-nitroaniline.

Bromination to p-bromoaniline

Acetanilide treated with bromine in glacial acetic acid gives p-bromoacetanilide, which on hydrolysis gives p-bromoaniline.

Compare that with what happens if you brominate aniline directly. Aniline plus bromine water gives an instant white precipitate of 2,4,6-tribromoaniline, because the free -NH₂ group activates the ring so strongly that all three available positions are attacked. Mono-substitution is simply not achievable that way, which is the clearest demonstration of why the acetyl group is put on first.

Why acetylation is used as a protecting group

Acetylating aniline before an electrophilic substitution solves three separate problems at once:

• It moderates a very strongly activating group. The -NH₂ lone pair pours electron density into the ring, so aniline gives polysubstituted products. Once acetylated, the lone pair is shared with the carbonyl, so -NHCOCH₃ donates less. The ring is still activated and still ortho/para directing, but only enough for clean mono-substitution.
• It prevents oxidation. Nitrating mixture is a powerful oxidising medium and aniline is easily oxidised, giving tarry, dark products and a ruined yield. The amide nitrogen is much harder to oxidise, so acetanilide survives the mixture.
• It avoids the anilinium ion problem. In strongly acidic nitrating mixture, aniline is protonated to C₆H₅NH₃⁺. That cation is deactivating and meta directing, so free aniline under nitration conditions gives an awkward mixture containing a large share of the meta isomer. The neutral amide is not protonated in the same way, so the directing effect stays ortho/para.

On top of that, the acetyl group is physically bulky. It blocks the two ortho positions next to it, so the incoming electrophile goes predominantly to the para position. The result is a usable, mostly single product. When the substitution is finished, hydrolysis removes the acetyl group and hands back the amine. This put-on, react, take-off sequence is the classic example of a protecting group in an undergraduate organic syllabus.

Uses of acetanilide

• Chemical intermediate. It is the starting point for p-nitroaniline, p-bromoaniline, p-phenylenediamine, sulpha-type intermediates such as 4-acetamidobenzenesulphonyl chloride, and a range of azo dye components. This is its main modern role.
• Rubber accelerator. Used in the rubber industry as an accelerator and in the manufacture of accelerator intermediates during vulcanisation.
• Stabiliser for hydrogen peroxide. Small quantities slow the decomposition of hydrogen peroxide during storage.
• Cellulose ester varnishes. Used as a plasticiser or camphor substitute in cellulose ester lacquers and varnishes.
• Laboratory reference material. Its sharp, well-characterised melting point near 114 °C makes it a standard sample for calibrating melting point apparatus and for teaching recrystallisation technique.

Antifebrin: the historical note

Acetanilide was introduced in 1886 by A. Cahn and P. Hepp under the trade name antifebrin, as the first aniline derivative found to lower fever and dull pain. It was withdrawn from that use because of serious toxicity: it causes methaemoglobinaemia, in which haemoglobin is converted to a form that cannot carry oxygen, producing cyanosis, along with liver and kidney damage on repeated dosing.

The chemistry behind the story is the interesting part for a student. In 1889 one of acetanilide’s urinary metabolites was identified as paracetamol, and by 1948 it was established that most of the effect of acetanilide in the body came from that metabolite rather than from acetanilide itself. Paracetamol was then developed in its own right, and acetanilide was left behind. Acetanilide today is a laboratory and industrial chemical only; it is not a medicine and has no place in any human use. Treat it as a toxic organic solid and handle it with the usual laboratory precautions.

Common mistakes to avoid

• Writing the formula as C₈H₁₀NO. Count again: six ring hydrogens, one N-H and three methyl hydrogens give nine. The formula is C₈H₉NO.
• Calling acetanilide basic. Aniline is basic; acetanilide is a neutral amide because its nitrogen lone pair is delocalised.
• Confusing N-phenylethanamide with 2-phenylethanamide. The N locant means the substituent is on nitrogen.
• Saying the zinc dust reacts with aniline. It does not. Its only job is to stop colour-forming oxidation.
• Claiming nitration gives only the para product. It gives predominantly para, with a minor ortho isomer that has to be separated.

References

• PubChem – Acetanilide (CID 904), National Library of Medicine.
• NIST Chemistry WebBook, acetanilide (N-phenylacetamide), CAS 103-84-4.
• NCERT Chemistry Class 12, Unit on Amines – preparation and reactions of aniline and its acetyl derivative.

FAQs

What is N-phenylethanamide?

N-phenylethanamide is the IUPAC name for acetanilide, an aromatic secondary amide of formula C₈H₉NO and structure C₆H₅NHCOCH₃. The name means ethanamide, CH₃CONH₂, carrying a phenyl group on its nitrogen. It is also written N-phenylacetamide or N-acetylaniline.

What is the molecular formula and molar mass of acetanilide?

The molecular formula is C₈H₉NO and the molar mass is 135.16 g/mol, from (8 × 12.011) + (9 × 1.008) + 14.007 + 15.999. It is a white crystalline solid melting at about 114 °C and boiling at about 304 °C.

Why is zinc dust added during the preparation of acetanilide?

Zinc dust keeps the reaction mixture reducing so that aniline is not oxidised to coloured quinone and azo type products while it is being refluxed, and it reduces any coloured impurity already present. It plays no part in the acetylation itself, so only a pinch is used.

Why is aniline acetylated before nitration?

Three reasons. The -NH₂ group is too strongly activating and causes polysubstitution; nitrating mixture oxidises aniline to tarry products; and in strong acid aniline is protonated to the anilinium ion, which is deactivating and meta directing. Acetylation gives a milder ortho/para directing -NHCOCH₃ group whose bulk pushes substitution to the para position, and it can be removed afterwards by hydrolysis.

Why is acetanilide recrystallised from water?

Because its solubility changes sharply with temperature: about 5.5 g per 100 mL in boiling water against roughly 0.53 g per 100 mL at 0 °C. Hot water dissolves the crude product along with its impurities, and slow cooling drops most of the acetanilide out as pure white crystals while the impurities remain dissolved.

Related Topics on EngineeringHulk

• 👉 Isomers of Pentane
• 👉 Valency of Chlorine
• 👉 NaHCO3 (Sodium Bicarbonate): Formula, Properties & Uses
• 👉 Water Quality Parameters — pH, DO, BOD, COD Explained