9701 · 19.1
Primary amines flashcards
Revision flashcards for Cambridge 9701 Primary amines (syllabus 19.1). Flip, recall, then mark a real past-paper question.
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What is a primary amine?
An organic compound where one hydrogen atom in an ammonia molecule ($NH_3$) has been replaced by an alkyl or aryl group. It has the general formula $R-NH_2$.
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Why are primary aliphatic amines stronger bases than ammonia?
The attached alkyl group ($R$) exerts a positive inductive effect (+I effect), pushing electron density onto the nitrogen atom. This increases the electron density of the lone pair, making it more available to accept a proton ($H^+$).
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Why is phenylamine a much weaker base than ammonia?
The lone pair of electrons on the nitrogen atom is delocalised into the pi-electron system of the benzene ring. This makes the lone pair less available to accept a proton.
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What is the product when an amine reacts with an acid, e.g., $CH_3NH_2 + HCl$?
An ammonium salt is formed. Methylamine reacts with hydrochloric acid to form methylammonium chloride: $CH_3NH_2 + HCl \rightarrow CH_3NH_3^+Cl^-$.
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What reagents and conditions are used to reduce a nitrile to a primary amine?
1. Lithium tetrahydridoaluminate ($LiAlH_4$) in dry ether, followed by hydrolysis. OR 2. Catalytic hydrogenation using hydrogen gas ($H_2$) with a nickel (Ni) catalyst at high temperature and pressure.
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How is phenylamine prepared in the laboratory?
By the reduction of nitrobenzene using tin (Sn) and concentrated hydrochloric acid (HCl) as the reducing agent, under reflux. An alkali (like NaOH) is then added to liberate the free amine.
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What type of reaction occurs between a primary amine and an acyl chloride?
Nucleophilic addition-elimination. The amine acts as a nucleophile, attacking the electron-deficient carbonyl carbon of the acyl chloride.
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What are the products of the reaction between propylamine and ethanoyl chloride?
An N-substituted amide (N-propylethanamide) and hydrogen chloride gas. $CH_3COCl + CH_3CH_2CH_2NH_2 \rightarrow CH_3CONHCH_2CH_2CH_3 + HCl$.
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What are the reagents and conditions for reacting a primary aliphatic amine with nitrous acid?
Sodium nitrite ($NaNO_2$) and a strong acid (e.g., HCl) are reacted in situ to form nitrous acid ($HNO_2$). The reaction must be kept cold, below 10 °C.
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What are the products when a primary aliphatic amine reacts with cold nitrous acid?
An alcohol, nitrogen gas (seen as effervescence), and water. For example, $CH_3CH_2NH_2 + HNO_2 \rightarrow CH_3CH_2OH + N_2 + H_2O$.
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What is the key difference in the reaction of primary AROMATIC amines with cold nitrous acid?
Instead of forming an alcohol, primary aromatic amines (like phenylamine) form a relatively stable diazonium salt, e.g., benzenediazonium chloride ($C_6H_5N_2^+Cl^-$). This reaction is the basis for forming azo dyes.