An analyst finds that a saturated, open-chain primary amine contains 23.7 % nitrogen by mass…
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An analyst finds that a saturated, open-chain primary amine contains nitrogen by mass. The nitrogen content is a common first step in identifying an unknown amine, as in the Kjeldahl method. Assume that the sample is pure and holds only carbon, hydrogen and nitrogen. The amine is a gas that dissolves well in water, and a solution of it turns red litmus blue. Use C = 12, H = 1 and N = 14 in g/mol. The amine is made from an amide by the Hofmann bromamide degradation. Find the molar mass of the amine and the number of carbon atoms in it. Then find the number of carbon atoms in the amide that gives it. Last, find the number of primary amines that share the formula. Use the fact that a saturated amine has the formula , and the numbers of alkyl groups 1, 1, 2 and 4 for one to four carbon atoms.
Need a reminder? The key ideaShow
Primary and secondary amines make N-H hydrogen bonds between their molecules, so they boil above alkanes of similar molar mass. Alcohols boil higher still. Every amine can accept a hydrogen bond from water, so the smaller ones dissolve. The solubility of a solid or liquid in water is a mass in a fixed volume.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Molar mass. There is one nitrogen atom, so .
Step 2. Carbon atoms. , so . The amine is .
Step 3. The amide. The Hofmann degradation loses the carbonyl carbon, so the amide has carbon atoms. It is butanamide or 2-methylpropanamide.
Step 4. Primary isomers. One propyl group on the nitrogen: there are 2 propyl groups, so 2 primary amines. They are propan-1-amine and propan-2-amine.
Answer: , 3 carbon atoms, 4 carbon atoms in the amide, and 2 primary amines.
Check. The molar mass fits, and the nitrogen is .
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