Cyanides, isocyanides and nitro compounds

One chain, two ways to attach nitrogen

The cyanide ion, $\text{CN}^{-}$, has a lone pair on carbon and a lone pair on nitrogen. The nitrite ion, $\text{NO}_{2}^{-}$, has a lone pair on nitrogen and lone pairs on each oxygen. Each ion can attack an alkyl halide through more than one atom. This gives two families of compounds from one pair of reactants. The choice depends on the salt.

Cyanides (nitriles)

An alkyl cyanide, $\text{R}\text{-}\text{C}\equiv \text{N}$, is also called a nitrile. The IUPAC name takes the alkane and adds nitrile: ethanenitrile for $\text{CH}_{3}\text{CN}$ and benzonitrile for $\text{C}_{6}\text{H}_{5}\text{CN}$.

Preparation.

• Alkyl halide with alcoholic potassium cyanide: $\text{R}\text{-}\text{X}\ +\ \text{KCN}\ \rightarrow \ \text{R}\text{-}\text{C}\equiv \text{N}\ +\ \text{KX}$. The ionic cyanide attacks through carbon.
• Dehydration of an amide by $\text{P}_{2}\text{O}_{5}$.
• For an aryl nitrile, the Sandmeyer reaction with $\text{CuCN}/\text{KCN}$.

Reactions.

• Hydrolysis. Boil with dilute acid: $\text{R}\text{-}\text{C}\equiv \text{N}\ +\ 2\text{H}_{2}\text{O}\ \rightarrow \ \text{R}\text{-}\text{COOH}\ +\ \text{NH}_{3}$. Ammonia stays as the ammonium salt. Partial hydrolysis stops at the amide.
• Reduction. $\text{LiAlH}_{4}$ or $\text{H}_{2}/\text{Ni}$ gives the primary amine $\text{R}\text{-}\text{CH}_{2}\text{-}\text{NH}_{2}$.

So a halide can be taken one carbon up the chain, to the acid or to the amine.

Isocyanides (carbylamines)

An isocyanide is $\text{R}\text{-}\text{N}\equiv \text{C}$. Here nitrogen is bonded to the alkyl group, and carbon is at the end. The group is best drawn as $\text{R}\text{-}\text{N}^{+}\equiv \text{C}^{-}$.

Preparation.

• Alkyl halide with silver cyanide: $\text{R}\text{-}\text{X}\ +\ \text{AgCN}\ \rightarrow \ \text{R}\text{-}\text{N}\equiv \text{C}\ +\ \text{AgX}$. Silver cyanide has a covalent character, and the ion is held so that the nitrogen lone pair attacks.
• The carbylamine reaction of a primary amine with chloroform and alcoholic potassium hydroxide, from the previous course.

Properties. Isocyanides have an extremely disagreeable smell and are toxic.

• Acid hydrolysis. Dilute acid gives the primary amine and formic acid: $\text{R}\text{-}\text{N}\equiv \text{C}\ +\ 2\text{H}_{2}\text{O}\ \rightarrow \ \text{R}\text{-}\text{NH}_{2}\ +\ \text{HCOOH}$.
• Alkali does not hydrolyse them. This tells a nitrile from an isocyanide. A nitrile is hydrolysed by both acid and alkali.
• Reduction by $\text{H}_{2}/\text{Ni}$ gives a secondary amine, $\text{R}\text{-}\text{NH}\text{-}\text{CH}_{3}$, with a methyl group on the nitrogen.
• Heating at about 523 K changes the isocyanide into the nitrile.

Nitro compounds

A nitro compound has the group $\text{-}\text{NO}_{2}$ on carbon. In the nitro group, nitrogen is bonded to carbon, and the two oxygen atoms share the negative charge equally.

Nitroalkanes come from an alkyl halide: with silver nitrite, $\text{R}\text{-}\text{X}\ +\ \text{AgNO}_{2}\ \rightarrow \ \text{R}\text{-}\text{NO}_{2}\ +\ \text{AgX}$ is the main reaction. With potassium nitrite, the main product is the alkyl nitrite, $\text{R}\text{-}\text{O}\text{-}\text{N}=\text{O}$, where the oxygen has attacked. As for the cyanides, the covalent silver salt reacts through nitrogen, and the ionic potassium salt through the negative end, here an oxygen. The two isomers differ greatly. Nitromethane boils at 101 °C. Methyl nitrite is a gas, with a boiling point near −12 °C.

Nitrobenzene is made by nitrating benzene with concentrated nitric and concentrated sulphuric acids at 323 to 333 K. Sulphuric acid helps nitric acid to give the nitronium ion, $\text{NO}_{2}^{+}$, which attacks the ring. Nitrobenzene is a pale yellow oil with a smell like bitter almonds. It is toxic. It does not dissolve in water, boils at about 484 K and is denser than water. A second nitro group enters the meta position, since the nitro group directs meta.

Reduction. The product depends on the medium.

• Acidic ($\text{Sn}/\text{HCl}$ or $\text{Fe}/\text{HCl}$, or $\text{H}_{2}/\text{Pd}$): aniline. Six electrons are taken by each nitrobenzene molecule.
• Neutral ($\text{Zn}/\text{NH}_{4}\text{Cl}$): N-phenylhydroxylamine, $\text{C}_{6}\text{H}_{5}\text{NHOH}$. Four electrons.
• Alkaline ($\text{Zn}/\text{NaOH}$): hydrazobenzene, $\text{C}_{6}\text{H}_{5}\text{NH}\text{-}\text{NHC}_{6}\text{H}_{5}$, formed through nitrosobenzene, azoxybenzene and azobenzene. Five electrons for each nitrobenzene, since two molecules take ten.

A hydrogen on the carbon next to a nitro group is acidic, as in nitromethane, with a $\text{p}K_a$ of about 10. Nitroalkanes dissolve in alkali.

Choose 12 g of nitrobenzene in an acidic medium. You need 0.293 mol of metal, which is 19 g of zinc. Now choose the neutral medium. The metal falls to two-thirds, since four electrons are taken instead of six.

Practise with new numbers

Try it in Python

The program works out the metal and the product for 12.3 g of nitrobenzene in each of the three media.

Common mistakes

• Mixing up the two cyanides. Potassium cyanide gives the nitrile. Silver cyanide gives the isocyanide.
• Mixing up the two nitrites. Silver nitrite gives the nitroalkane. Potassium nitrite gives the alkyl nitrite.
• Saying alkali hydrolyses an isocyanide. It does not. Acid does.
• Expecting aniline from every reduction of nitrobenzene. Only in acid.
• Reducing an isocyanide and expecting a primary amine. It gives a secondary amine, $\text{R}\text{-}\text{NH}\text{-}\text{CH}_{3}$.

Remember these

• $\text{KCN}$ gives $\text{R}\text{-}\text{CN}$, and $\text{AgCN}$ gives $\text{R}\text{-}\text{NC}$. $\text{AgNO}_{2}$ gives $\text{R}\text{-}\text{NO}_{2}$, and $\text{KNO}_{2}$ gives $\text{R}\text{-}\text{ONO}$.
• Nitrile: acid or alkali hydrolysis to the acid, and reduction to $\text{RCH}_{2}\text{NH}_{2}$. Isocyanide: acid hydrolysis to $\text{RNH}_{2}$ and $\text{HCOOH}$, and reduction to $\text{RNHCH}_{3}$.
• Nitrobenzene: aniline in acid, N-phenylhydroxylamine when neutral, hydrazobenzene in alkali.

References

• NCERT Chemistry Class 12, the chapter Amines (cyanides, isocyanides and nitro compounds appear in the older edition and in several state syllabi).
• OpenStax Organic Chemistry, Chapter 24 Amines and Heterocycles, section on the synthesis of amines.
• Standard texts, such as Morrison and Boyd, Organic Chemistry, on nitro compounds and the reduction of nitrobenzene.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which reagent turns bromoethane into the isocyanide?

2. How can you tell a nitrile from an isocyanide by hydrolysis?

3. What does the reduction of nitrobenzene with zinc and ammonium chloride give?

4. Why does silver nitrite give a nitroalkane but potassium nitrite an alkyl nitrite?