CourseCourseQ BankQuestion BankDocsDocuments

34.1 Primary and secondary amines

Syllabus
9701–2028–2029
Topic
34.1
Level
A2

Learning objectives

34.1.1Preparation of primary and secondary amines• Recall the reactions (reagents/conditions) by which primary and secondary amines are produced:- (a) reaction of halogenoalkanes with NH₃ in ethanol heated under pressure- (b) reaction of halogenoalkanes with primary amines in ethanol, heated in a sealed tube / under pressure- (c) the reduction of amides with LiAl H₄- (d) the reduction of nitriles with LiAl H₄ or H₂ / Ni34.1.2Amide formation from amines• Describe the condensation reaction of ammonia or an amine with an acyl chloride at room temp. to give an amide34.1.3Basicity of aq. solutions of amines• Describe/explain: the basicity of aq. solutions of amines

Choose an amine preparation route from the required carbon skeleton

Primary and secondary amines can be formed either by adding an alkyl group to nitrogen through nucleophilic substitution or by reducing a C=O/CN nitrogen compound. Choose the route by tracking both the carbon skeleton and the number of carbon groups bonded to N.

Starting material Reagents and conditions Amine product Structural accounting
halogenoalkane, RX excess NH3 in ethanol, heat under pressure primary amine, RNH2 halogen is replaced by NH2; excess NH3 limits further alkylation
halogenoalkane, RX + primary amine, R'NH2 ethanol, heat in a sealed tube/under pressure secondary amine, RR'NH the alkyl group R is added to nitrogen already bearing R'
primary or N-substituted amide LiAlH4 (commonly in dry ether), then work-up primary or secondary amine amide C=O becomes CH2; groups already on N are retained
nitrile, RCN LiAlH4, or H2/Ni primary amine, RCH2NH2 the nitrile carbon becomes the CH2 carbon bonded to NH2

RX+2 NHX3→RNHX2+NHX4X\ce{RX + 2NH3 -> RNH2 + NH4X}

RCONHX2→LiAlHX4RCHX2NHX2RCN→LiAlHX4 or HX2/NiRCHX2NHX2\ce{RCONH2 ->[LiAlH4] RCH2NH2}\qquad\ce{RCN ->[LiAlH4\ or\ H2/Ni] RCH2NH2}

Do not lose the carbonyl or nitrile carbon when counting the product chain: both become the carbon directly attached to N. Ammonia substitution can continue to secondary/tertiary products unless excess ammonia favours the primary amine.

Ammonia and amines form progressively N-substituted amides

At room temperature, ammonia or an amine attacks an acyl chloride and forms an amide by condensation, with HCl eliminated. The groups already bonded to the attacking nitrogen determine the substitution level of the amide product.

Nitrogen nucleophile Amide formed from RCOCl Amide class
NH3 RCONH2 primary (unsubstituted) amide
primary amine, R'NH2 RCONHR' secondary (N-substituted) amide
secondary amine, R'2NH RCONR'2 tertiary (N,N-disubstituted) amide

The first ammonia/amine molecule forms the C-N bond. A second equivalent can neutralise the HCl: for ammonia, RCOCl + 2NH3 -> RCONH2 + NH4Cl. With an amine, the corresponding alkylammonium chloride salt forms.

Ethanoyl chloride plus methylamine gives N-methylethanamide. The methyl group remains bonded to N, while the CH3CO- acyl fragment supplies the ethanamide carbonyl skeleton.

Do not call the product an amine: nitrogen is bonded directly to a carbonyl carbon, so the functional group is an amide. Ammonia, primary amines and secondary amines give different amide classes.

Amines are weak bases because the nitrogen lone pair accepts H+

An amine is a Bronsted-Lowry base: its nitrogen lone pair accepts a proton from water, producing an alkylammonium ion and hydroxide.

The equilibrium is partial, so aqueous amines are weak bases. The position depends on how available the lone pair is and on electron-donating or withdrawing groups.

Ethylamine + H2O <=> ethylammonium ion + OH-. The solution is alkaline even though most ethylamine molecules remain unprotonated.

A weak base is not a base that cannot react; it is one whose protonation equilibrium is incomplete.