19.2 Nitriles and hydroxynitriles
- Syllabus
- 9701–2028–2029
- Topic
- 19.2
- Level
- AS
Learning objectives
19.2.1Preparation of nitriles• Recall preparation of nitriles: (a) reaction of a halogenoalkane with KCN in ethanol and heat19.2.2Preparation of hydroxynitriles• Recall preparation of hydroxynitriles: (a) the reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat19.2.3Hydrolysis of nitriles with dil. acid or dil.• Describe the hydrolysis of nitriles with dil. acid or dil. alkali followed by acidification to produce a carboxylic acid
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+KCNethanol, heatRCN+KX
CHX3CHX2Br+KCNCHX3CHX2CN+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+HCNKCN, heatCHX3CH(OH)CN
CHX3COCHX3+HCNKCN, heat(CHX3)X2C(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+2HX2O+HX+RCOOH+NHX4X+
RCN+HX2O+OHX−RCOX2X−+NHX3
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.