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Nitromethane has one carbon. Dimethylamine has two, both on the nitrogen. So the job is to add one carbon to the nitrogen, cleanly. The expected route uses three reactions from the amines chapter, each doing one thing.
What are the three steps from nitromethane to dimethylamine?
Reduce nitromethane to methylamine.
Convert methylamine to methyl isocyanide with the carbylamine reaction.
Reduce methyl isocyanide to dimethylamine.
The molecular formulas show the bookkeeping: nitromethane is CH3NO2, methylamine is CH5N and dimethylamine is C2H7N, with the IUPAC name N-methylmethanamine (PubChem: nitromethane, methylamine, dimethylamine). The extra carbon comes from chloroform in step 2.
How do you reduce nitromethane to methylamine?
One laboratory method for methylamine is reducing nitromethane with zinc and hydrochloric acid (Wikipedia: Methylamine). Aliphatic nitro compounds can also be reduced to amines by catalytic hydrogenation over platinum(IV) oxide or Raney nickel, or by iron in acetic acid (Wikipedia: Reduction of nitro compounds). Exam-prep notes list the same family of reagents for nitroalkanes: Sn or Zn with concentrated HCl, or H2 over Pt, Pd or Raney nickel (Aakash concept notes on amines). Sn/HCl is the usual one to write above the arrow.
CH3NO2 + 6[H] → CH3NH2 + 2H2O
Count it: the left side has 9 hydrogens (3 + 6), the right side has 5 + 4. The two oxygens leave as water.
What does the carbylamine reaction do here?
The carbylamine reaction (also called the Hofmann carbylamine reaction or Hofmann isocyanide synthesis) converts a primary amine into an isocyanide using chloroform and a base; the base turns chloroform into dichlorocarbene, which the amine attacks (Wikipedia: Carbylamine reaction; procedure in Organic Syntheses, vol. 55, p. 96). Only primary amines give it, which is why it doubles as a test for them. Methylamine is primary, so it works.
CH3NH2 + CHCl3 + 3KOH (alc.) → CH3NC + 3KCl + 3H2O
The carbon of chloroform becomes the terminal carbon of the isocyanide, so the chain now has two carbons, joined through nitrogen: CH3–N≡C. If you want the test side of this reaction in more detail, we covered it in the carbylamine test for primary amines. Keep this product on paper: methyl isocyanide is very endothermic, with a gas-phase formation enthalpy of +150.2 kJ/mol (Wikipedia: Methyl isocyanide).
Why does reducing methyl isocyanide give dimethylamine?
Reducing an isocyanide gives a secondary amine, and one of the two groups on nitrogen is always methyl (Aakash: Reduction of isocyanides). The terminal carbon picks up three hydrogens and becomes CH3; the nitrogen picks up one.
CH3–N≡C + 4[H] → CH3–NH–CH3
LiAlH4 is the reagent to write above the arrow. Since the starting group was already methyl, both groups end up as methyl, and the product is dimethylamine. Compare this with a nitrile: reducing CH3C≡N puts the new CH2 next to carbon and gives a primary amine, ethylamine. Same atoms, opposite connection, different class of amine. Our hexanenitrile to 1-aminopentane walkthrough shows the nitrile side of this contrast.
Why not just methylate methylamine?
It is tempting to write CH3NH2 + CH3I → (CH3)2NH + HI. The trouble is that the product amine can itself react with the alkylating agent, so direct alkylation tends to run on to more substituted amines (Wikipedia: Amine alkylation). With methyl iodide you risk a mixture of secondary, tertiary and quaternary products. The isocyanide route adds exactly one carbon, at exactly one place, and stops. That is the point to make in your answer.
A checklist for writing this answer in an exam
Write all three steps with reagents above the arrows: Sn/HCl (or H2, Ni), then CHCl3 with alcoholic KOH, then LiAlH4.
Name the intermediates: methylamine and methyl isocyanide (methyl carbylamine).
Show where the new carbon comes from: chloroform.
Say why the product is secondary: isocyanide reduction always gives an N-methyl secondary amine.
If asked, give the reason you avoided direct methylation: over-alkylation.
Practise on nitroethane to N-methylethanamine: the same three steps work, because the ethyl group carries through unchanged. For basicity questions, how the basicity order of methylamines differs in water and gas is the next read.
If you get one of these conversions wrong and cannot see which step broke, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer. For the ideas behind each reaction, the Fermi Concept Library covers every concept in chemistry, each with its own lesson and its place in the map.
