Potassium cyanide is highly toxic. It can kill if swallowed, and it is harmful if its dust is breathed in or it gets on the skin. Cyanide ions stop cells from using oxygen in respiration (they inhibit a respiratory enzyme).Hydrogen cyanide is released with acid. The reaction uses KCN followed by dilute acid. Cyanide ions and acid form hydrogen cyanide, HCN, which is also highly toxic and very volatile (it boils at about 26 °C), so it escapes as a gas.Why use KCN rather than HCN?
• HCN is so volatile and toxic that it is very hard to handle safely; solid KCN is easier to measure and contain.
• HCN is a weak acid, so it supplies only a small concentration of CN− ions. KCN is ionic and fully provides CN−, the actual nucleophile, so the reaction is faster.Precautions:
• work in a fume cupboard, so any HCN gas is removed;
• wear gloves and eye protection and avoid contact with the solid;
• use small quantities;
• never add acid to cyanide outside the fume cupboard;
• dispose of cyanide waste safely, never mixed with acid waste.These precautions reduce the risk, but KCN is still hazardous: a risk assessment is needed before the reaction is carried out.Example: a student plans to make 2-hydroxypropanenitrile from ethanal. The safest plan uses KCN (not HCN) in a fume cupboard, adds the dilute acid slowly inside the fume cupboard, and uses the smallest amounts that will work.
• HCN is so volatile and toxic that it is very hard to handle safely; solid KCN is easier to measure and contain.
• HCN is a weak acid, so it supplies only a small concentration of CN− ions. KCN is ionic and fully provides CN−, the actual nucleophile, so the reaction is faster.Precautions:
• work in a fume cupboard, so any HCN gas is removed;
• wear gloves and eye protection and avoid contact with the solid;
• use small quantities;
• never add acid to cyanide outside the fume cupboard;
• dispose of cyanide waste safely, never mixed with acid waste.These precautions reduce the risk, but KCN is still hazardous: a risk assessment is needed before the reaction is carried out.Example: a student plans to make 2-hydroxypropanenitrile from ethanal. The safest plan uses KCN (not HCN) in a fume cupboard, adds the dilute acid slowly inside the fume cupboard, and uses the smallest amounts that will work.