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Home › AS & A Level Chemistry › Unit 19: Organic Chemistry

Nitrogen Compounds

Organic Chemistry Weightage: AS · Papers 1 and 2 (assumed in Paper 4) Topic 19
“Welcome! At AS this topic is short but it links three topics together. A halogenoalkane gives a primary amine with excess ammonia, and a nitrile with KCN. An aldehyde or ketone gives a hydroxynitrile with HCN. Every nitrile can then be hydrolysed to a carboxylic acid. Learn each change as reagent, conditions, type of reaction and what happens to the carbon chain.”
— SCORECHEM ACADEMIC TEAM

0. What Examiners Want

How marks are earned in Nitrogen Compounds
  • Reagents and conditions: NH3 in ethanol, heated under pressure (amine); KCN in ethanol, heat under reflux (nitrile); HCN with KCN as catalyst, heat (hydroxynitrile); dilute acid or alkali under reflux, then acidify (hydrolysis).
  • Type of reaction: nucleophilic substitution for halogenoalkane with NH3 or CN−; nucleophilic addition for HCN with a carbonyl compound.
  • Naming and formulae: primary amines (-amine), nitriles (-nitrile, counting the C of CN), 2-hydroxynitriles and the acids they give.
  • Equations: amine formation (2NH3 and NH4Br), nitrile formation, nitrile hydrolysis (NH4Cl or NH3), balanced.
  • Linking: multi-step routes that use Topics 15 to 19, and counting how the carbon chain changes at each step. Use the Periodic Table in the Data Booklet for any Mr or percentage calculation.

Where this topic appears. Recent AS papers have asked for: the reagents and conditions that give a named nitrile (choosing between a halogenoalkane with KCN, a halogenoalkane with ammonia, and a carbonyl compound with HCN); the starting alcohol that gives a given 2-hydroxynitrile after mild oxidation and reaction with HCN; a synthesis from a halogenoalkane through KCN in ethanol to a nitrile and then, with H+(aq), a carboxylic acid, with the stereoisomerism of each compound; the type of mechanism in the cyanide step; and the completion of the equation for hydrolysis of C4H9CN. The infrared absorptions of C≡N and N–H in the Data Booklet are used with Topic 22.

1. Amines, Nitriles and Hydroxynitriles

PRIMARY AMINEDiagram showing: PRIMARY AMINE; R–NH2​; N has a lone pair of electrons; name: alkyl + -amine; CH3​CH2​NH2​; ethylamine; CH3​CH2​CH2​NH2​; propylamine; NITRILE; R–C≡N; C≡N is linear; C is in the chain; name: alkane + -nitrile; CH3​CN; ethanenitrile (2… PRIMARY AMINE R–NH2​ N has a lone pair of electrons name: alkyl + -amine CH3​CH2​NH2​ ethylamine CH3​CH2​CH2​NH2​ propylamine NITRILE R–C≡N C≡N is linear; C is in the chain name: alkane + -nitrile CH3​CN ethanenitrile (2 C) CH3​CH2​CN propanenitrile (3 C) 2-HYDROXYNITRILE R2​C(OH)–CN OH is on the carbon next to CN name: 2-hydroxy…nitrile CH3​CH(OH)CN 2-hydroxypropanenitrile (CH3​)2​C(OH)CN 2-hydroxy-2-methylpropanenitrile Only PRIMARY amines are needed at AS (classification of amines is not tested). Count the carbon of the CN group: CH3​CH2​CN has THREE carbons, so it is propanenitrile.
The three nitrogen-containing groups of this topic. A primary amine has the group –NH2 on a carbon chain. A nitrile has –C≡N, and its carbon is counted in the name. A 2-hydroxynitrile carries OH and CN on the same carbon.

A primary amine has the group –NH2 on a carbon chain and is named alkyl + -amine: CH3CH2NH2 is ethylamine. The nitrogen atom has a lone pair of electrons, so amines (like ammonia) can act as nucleophiles. At AS only primary amines are needed; classification into primary, secondary and tertiary is not tested.

A nitrile contains the group –C≡N. The carbon of the CN group is part of the carbon chain, so the name is the alkane name with -nitrile added: CH3CN is ethanenitrile (two carbons) and CH3CH2CN is propanenitrile (three). The C≡N group is linear.

A 2-hydroxynitrile (hydroxynitrile) has OH and CN on the same carbon, and the carbon of CN is carbon 1: CH3CH(OH)CN is 2-hydroxypropanenitrile. For methanal the product, HOCH2CN, is hydroxyethanenitrile and needs no number.

⚠️ Examiner Trap: Name the nitrile from the whole chain. Count the carbon in the CN group. Butanenitrile is CH3CH2CH2CN (four carbons) and is made from 1-bromopropane, not 1-bromobutane. Do not confuse the name of an amine (ethylamine) with the name of a nitrile (propanenitrile).

2. Making Primary Amines

halogenoalkaneDiagram showing: halogenoalkane; CH3​CH2​Br; bromoethane; +; ammonia; excess NH3​; in ethanol; heat under pressure; (sealed tube); primary amine + NH4​Br; CH3​CH2​NH2​; ethylamine; CH3​CH2​Br + 2NH3​ → CH3​CH2​NH2​ + NH4​Br (nucleophilic… halogenoalkane CH3​CH2​Br bromoethane + ammonia excess NH3​ in ethanol heat under pressure (sealed tube) primary amine + NH4​Br CH3​CH2​NH2​ ethylamine CH3​CH2​Br + 2NH3​ → CH3​CH2​NH2​ + NH4​Br (nucleophilic substitution) EXCESS NH3​: mainly the primary amine NH3​ molecules far outnumber the amine, so the halogenoalkane meets NH3​ first one C–N bond formed, no extra carbon: bromoethane (2 C) gives ethylamine (2 C) LIMITED NH3​: a mixture of amines the amine also has an N lone pair, so it acts as a nucleophile and attacks more RBr giving (CH3​CH2​)2​NH and so on a mixture is formed: low yield of RNH2​
Forming a primary amine. A halogenoalkane is heated with an excess of ammonia dissolved in ethanol, under pressure. Ammonia is the nucleophile (it uses the lone pair on nitrogen) and the Br is replaced by –NH2. The excess of ammonia prevents the amine reacting further.

A halogenoalkane is heated with an excess of ammonia dissolved in ethanol, in a sealed tube under pressure. The reaction is a nucleophilic substitution: the lone pair on the nitrogen of NH3 attacks the Cδ+ atom and the halogen is replaced.

CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br

CH3CH2CH2Br + 2NH3 → CH3CH2CH2NH2 + NH4Br

Two moles of ammonia are used: one forms the C–N bond and the second removes H+ to form the ammonium salt NH4Br. The carbon chain is unchanged in length (bromoethane has two carbons and ethylamine has two).

Why excess ammonia? The primary amine product also has a lone pair on nitrogen, so it is a nucleophile as well. If ammonia is not in excess, the amine attacks further halogenoalkane molecules, giving (CH3CH2)2NH and so on, and the product is a mixture. A large excess of NH3 makes it much more likely that each halogenoalkane molecule meets ammonia, so the primary amine is the main product.

⚠️ Examiner Trap: Conditions for the amine. Give all three: excess ammonia, in ethanol (not just aqueous), and heated under pressure. Name the reaction as nucleophilic substitution. The halogenoalkane and the amine have the same number of carbon atoms.

3. Making Nitriles

halogenoalkaneDiagram showing: halogenoalkane; CH3​CH2​Br; bromoethane (2 C); +; potassium cyanide; KCN; in ethanol; heat under reflux; (ethanol solvent); nitrile + KBr; CH3​CH2​CN; propanenitrile (3 C); CH3​CH2​Br + KCN → CH3​CH2​CN + KBr (ionic: CN−​ replaces… halogenoalkane CH3​CH2​Br bromoethane (2 C) + potassium cyanide KCN in ethanol heat under reflux (ethanol solvent) nitrile + KBr CH3​CH2​CN propanenitrile (3 C) CH3​CH2​Br + KCN → CH3​CH2​CN + KBr (ionic: CN−​ replaces Br−​) NUCLEOPHILIC SUBSTITUTION CN−​ is the nucleophile: its lone pair (on carbon) attacks the Cδ+ bonded to Br Br−​ leaves as the halide ion CHAIN LENGTHENED BY ONE CARBON the carbon of CN joins the chain C2​H5​Br (2 C) gives C2​H5​CN (3 C) use ethanol, not water, as the solvent
Forming a nitrile. A halogenoalkane is heated under reflux with potassium cyanide dissolved in ethanol. The cyanide ion is the nucleophile in a nucleophilic substitution, and the product has one more carbon than the halogenoalkane.

A halogenoalkane is heated under reflux with potassium cyanide dissolved in ethanol (Topic 15). The cyanide ion is a nucleophile that uses the lone pair on its carbon atom, so the reaction is another nucleophilic substitution:

CH3CH2Br + KCN → CH3CH2CN + KBr

Ionic form: CH3CH2Br + CN− → CH3CH2CN + Br−

The product has one more carbon than the halogenoalkane, so this is a standard way of lengthening a carbon chain. The solvent is ethanol: in water, hydroxide ions would compete with cyanide and give some alcohol.

Worked example. Which reagents and conditions form butanenitrile, CH3CH2CH2CN? The nitrile has four carbons, so the starting compound must have three carbons in the chain before the CN is added. 1-bromopropane heated with KCN in ethanol fits. 1-Bromobutane with ammonia gives butylamine (an amine). Propanal with HCN gives 2-hydroxybutanenitrile, and propanone with HCN gives a hydroxynitrile with four carbons in a different structure, so neither gives butanenitrile.

4. Making Hydroxynitriles

An aldehyde or ketone reacts with HCN with KCN as catalyst, with heating (Topic 17). The reaction is nucleophilic addition: CN− adds to the Cδ+ of the C=O and the oxygen then takes H+.

CH3CHO + HCN → CH3CH(OH)CN (2-hydroxypropanenitrile)

CH3CH2COCH3 + HCN → CH3CH2C(CH3)(OH)CN (2-hydroxy-2-methylbutanenitrile)

Every aldehyde and ketone gives a 2-hydroxynitrile, because the OH is formed on the carbonyl carbon and the CN joins the same carbon. The chain is lengthened by one carbon. HCN is extremely toxic, so the cyanide is normally generated in the reaction mixture from KCN and acid. The full curly-arrow mechanism is in Topic 17.

Worked example (working back to the alcohol). Compound Y is heated with a mild oxidising agent and distilled. One of the products reacts with HCN to give 2-hydroxybutanenitrile. The hydroxynitrile CH3CH2CH(OH)CN comes from the aldehyde propanal, CH3CH2CHO. A mild oxidant turns a primary alcohol into an aldehyde, so Y is propan-1-ol. (A secondary alcohol would give a ketone and a different hydroxynitrile.)

5. Hydrolysis of Nitriles

DILUTE ACID, heat under refluxDiagram showing: DILUTE ACID, heat under reflux; CH3​CH2​CN + 2H2​O + HCl; → CH3​CH2​COOH + NH4​Cl; acid is formed directly; N ends up as NH4​+​ (ammonium salt); DILUTE NaOH(aq), heat under reflux; CH3​CH2​CN + NaOH + H2​O; → CH3​CH2​COONa +… DILUTE ACID, heat under reflux CH3​CH2​CN + 2H2​O + HCl → CH3​CH2​COOH + NH4​Cl acid is formed directly N ends up as NH4​+​ (ammonium salt) DILUTE NaOH(aq), heat under reflux CH3​CH2​CN + NaOH + H2​O → CH3​CH2​COONa + NH3​ the SALT forms, NH3​ is given off then ADD DILUTE ACID: CH3​CH2​COONa + HCl → CH3​CH2​COOH + NaCl The nitrile carbon becomes the COOH carbon: the chain length is UNCHANGED. propanenitrile (3 C) → propanoic acid (3 C). 2-hydroxynitrile → 2-hydroxy carboxylic acid. Total from the halogenoalkane: bromoethane (2 C) → propanoic acid (3 C).
Hydrolysis of a nitrile to a carboxylic acid. With dilute acid the acid forms directly. With dilute alkali the sodium salt forms first, so the mixture is acidified afterwards to release the free acid.

Hydrolysing the –CN group gives a carboxylic acid (Topic 18). Heat under reflux with:

The carbon of the nitrile group becomes the carbon of the COOH group, so the chain length does not change. A 2-hydroxynitrile gives a 2-hydroxy carboxylic acid: CH3CH(OH)CN + 2H2O + HCl → CH3CH(OH)COOH + NH4Cl. In an exam equation for an unfamiliar nitrile such as C4H9CN the pattern is the same: C4H9CN + 2H2O + HCl → C4H9COOH + NH4Cl.

Percentage composition (using the Data Booklet). Butanenitrile, C4H7N, has Mr = 4 × 12.0 + 7 × 1.0 + 14.0 = 69.0, so the percentage of nitrogen = 14.0 / 69.0 × 100 = 20.3%. Use Ar values from the Periodic Table in the Data Booklet.

⚠️ Examiner Trap: Alkali gives the salt. Heating a nitrile with NaOH(aq) gives the sodium salt and ammonia, not the acid, so a second step (acidification) is needed. Do not forget that the product of the acid hydrolysis includes ammonium chloride (or ammonium sulfate), and that propanenitrile gives propanoic acid (three carbons), not butanoic acid.

6. Choosing and Linking Routes

SUBSTITUTION (halogenoalkane)Diagram showing: SUBSTITUTION (halogenoalkane); reagent: KCN in ethanol; conditions: heat under reflux; CH3​CH2​Br → CH3​CH2​CN; propanenitrile; Br is replaced by CN; type: nucleophilic substitution; product: a NITRILE; ADDITION (aldehyde or ketone); reagent: HCN with KCN as catalyst; conditions:… SUBSTITUTION (halogenoalkane) reagent: KCN in ethanol conditions: heat under reflux CH3​CH2​Br → CH3​CH2​CN propanenitrile Br is replaced by CN type: nucleophilic substitution product: a NITRILE ADDITION (aldehyde or ketone) reagent: HCN with KCN as catalyst conditions: heat CH3​CHO → CH3​CH(OH)CN 2-hydroxypropanenitrile CN adds to C=O; O becomes OH type: nucleophilic addition product: a 2-HYDROXYNITRILE Both routes add ONE carbon atom to the carbon skeleton. Either product is hydrolysed to a carboxylic acid (nitrile → acid; 2-hydroxynitrile → 2-hydroxy acid).
Two ways to a nitrile. A halogenoalkane gives a plain nitrile by substitution with KCN in ethanol; an aldehyde or ketone gives a 2-hydroxynitrile by addition of HCN with KCN as catalyst. Both lengthen the chain by one carbon.
halogenoalkaneDiagram showing: halogenoalkane; RBr; excess NH3​/ethanol,; heat, under pressure; primary amine; RNH2​; amines: more reactions; at A Level (Topic 34); halogenoalkane; RBr; KCN in ethanol,; heat under reflux; nitrile; RCN halogenoalkane RBr excess NH3​/ethanol, heat, under pressure primary amine RNH2​ amines: more reactions at A Level (Topic 34) halogenoalkane RBr KCN in ethanol, heat under reflux nitrile RCN dilute acid or alkali, reflux, H+​ carboxylic acid RCOOH aldehyde or ketone RCHO RCOR′ HCN, KCN catalyst, heat 2-hydroxynitrile R2​C(OH)CN dilute acid or alkali, reflux, H+​ 2-hydroxy acid R2​C(OH)COOH Substitution (halogenoalkane) and addition (carbonyl) are different reactions but each adds ONE carbon through CN.
The reactions of this topic in one map. A halogenoalkane gives an amine (with NH3) or a nitrile (with KCN); a carbonyl compound gives a 2-hydroxynitrile (with HCN). Both kinds of nitrile hydrolyse to carboxylic acids.
Starting compound Reagents and conditions Product Reaction type Carbons
halogenoalkane excess NH3 in ethanol, heat under pressure primary amine nucleophilic substitution same
halogenoalkane KCN in ethanol, heat under reflux nitrile nucleophilic substitution +1
aldehyde or ketone HCN, KCN catalyst, heat 2-hydroxynitrile nucleophilic addition +1
nitrile (or hydroxynitrile) dilute acid or alkali, reflux, then acidify carboxylic acid hydrolysis same

Worked route. Propan-2-ol to 2-hydroxy-2-methylpropanoic acid: (1) acidified K2Cr2O7, heat, giving propanone; (2) HCN with KCN, heat, giving (CH3)2C(OH)CN; (3) dilute HCl, heat under reflux, giving (CH3)2C(OH)COOH. Three carbons become four. If you instead made 2-bromopropane from propan-2-ol and then used KCN in ethanol, you would obtain (CH3)2CHCN, 2-methylpropanenitrile, which has no OH group.

Stereoisomerism in these products. A carbon atom bonded to four different groups is a chiral centre. 2-Bromobutane, CH3CH2CHBrCH3, and the nitrile made from it, 2-methylbutanenitrile, CH3CH2CH(CH3)CN, each have a chiral carbon and so show optical isomerism. An aldehyde-derived hydroxynitrile such as CH3CH(OH)CN (carbon bonded to H, CH3, OH and CN) is also chiral, whereas (CH3)2C(OH)CN is not.

7. Quick Sheet and Checklist

Topic Key point
Primary amine RX + excess NH3 in ethanol, heat under pressure; nucleophilic substitution; RNH2 + NH4X; same C
Why excess the amine is also a nucleophile; otherwise a mixture of amines
Nitrile RX + KCN in ethanol, heat under reflux; nucleophilic substitution; +1 C
Hydroxynitrile aldehyde or ketone + HCN, KCN catalyst, heat; nucleophilic addition; 2-hydroxynitrile; +1 C
Hydrolysis (acid) dilute HCl or H2SO4, reflux → RCOOH + NH4+
Hydrolysis (alkali) dilute NaOH, reflux → RCOONa + NH3; acidify to get RCOOH
Naming -amine; -nitrile counts the C of CN; 2-hydroxy…nitrile
Not at AS classification of amines, basicity, reduction of nitriles, amides, amino acids (A Level)

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