CourseCourseQuestionQuestionDocumentDocument

19. Nitrogen compounds

Syllabus
9701–2028–2029
Section
19
Level
AS

19.1 Primary amines

Syllabus
9701–2028–2029
Topic
19.1
Level
AS

Prepare a primary amine with ammonia in ethanol under pressure

Heat a halogenoalkane with excess NH₃ in ethanol under pressure. Ammonia acts as the nucleophile and the carbon skeleton of the halogenoalkane is retained while X is replaced by NH₂.

The attacking NH₃ first forms an alkylammonium species, RNH₃⁺. A second NH₃ molecule removes a proton, giving the amine RNH₂ and NH₄⁺; the leaving halide X⁻ therefore appears in the ammonium salt.

RX+2 NHX3→ethanol,  heat,  pressureRNHX2+NHX4X\ce{RX + 2NH3 ->[ethanol,\ heat,\ pressure] RNH2 + NH4X}

CHX3CHX2Br+2 NHX3→CHX3CHX2NHX2+NHX4Br\ce{CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br}

Use excess ammonia to favour the primary amine because the amine product can otherwise undergo further alkylation. Detailed classification of amines is outside the stated AS assessment scope.

19.2 Nitriles and hydroxynitriles

Syllabus
9701–2028–2029
Topic
19.2
Level
AS

KCN substitutes X and extends the carbon chain by one

Heat a halogenoalkane with KCN in ethanol. CN⁻ attacks through its carbon end and replaces X, forming a nitrile R–C≡N by nucleophilic substitution.

RX+KCN→ethanol,  heatRCN+KX\ce{RX + KCN ->[ethanol,\ heat] RCN + KX}

CHX3CHX2Br+KCN→CHX3CHX2CN+KBr\ce{CH3CH2Br + KCN -> CH3CH2CN + KBr}

The carbon atom in CN⁻ becomes the nitrile carbon, so bromoethane's two-carbon skeleton gives three-carbon propanenitrile. This one-carbon extension is specific to the KCN substitution route.

Do not attack through nitrogen or write an isocyanide product R–N≡C. HCN belongs to the separate carbonyl-addition route, not this halogenoalkane substitution.

HCN adds CN and OH to the former carbonyl carbon

Heat an aldehyde or ketone with HCN using KCN as catalyst. CN⁻ attacks the δ+ carbonyl carbon, and protonation converts C=O into a carbon bearing both –OH and –C≡N: a hydroxynitrile.

CHX3CHO+HCN→KCN,  heatCHX3CH(OH)CN\ce{CH3CHO + HCN ->[KCN,\ heat] CH3CH(OH)CN}

CHX3COCHX3+HCN→KCN,  heat(CHX3)X2C(OH)CN\ce{CH3COCH3 + HCN ->[KCN,\ heat] (CH3)2C(OH)CN}

The former carbonyl carbon remains in place and gains OH and CN. The separate carbon atom of CN becomes the new nitrile carbon, so the product contains one more carbon than the aldehyde or ketone.

Do not say that the new nitrile carbon bears the OH group: OH is attached to the original carbonyl carbon. Hydroxynitrile preparation is addition, not nitrile hydrolysis.

Hydrolysis turns the nitrile carbon into a carboxyl carbon

Hydrolysis medium Immediate carbon product Nitrogen product Final step for RCOOH
dilute acid, heat carboxylic acid, RCOOH NH₄⁺ already protonated
dilute alkali, heat carboxylate, RCO₂⁻ NH₃ acidify RCO₂⁻

RCN+2 HX2O+HX+→RCOOH+NHX4X+\ce{RCN + 2H2O + H+ -> RCOOH + NH4+}

RCN+HX2O+OHX−→RCOX2X−+NHX3\ce{RCN + H2O + OH- -> RCO2- + NH3}

The carbon in –C≡N becomes the carbon in –COOH, so the total number of carbons is unchanged during hydrolysis. Propanenitrile therefore produces propanoic acid after the correct work-up.

Do not write NH₃ as the final nitrogen species in acidic solution; it is protonated to NH₄⁺. Do not report RCO₂⁻ as the free carboxylic acid before acidification.