A Level Chemistry (9701): Primary and Secondary Amines (Topic 34.1)

Welcome to the fascinating world of Nitrogen compounds! This section covers amines, which are essentially organic versions of ammonia. They are incredibly important in biology (think amino acids and DNA) and industrial chemistry. Don't worry if organic synthesis seems daunting; we will break down the structures, synthesis routes, and core properties of primary and secondary amines step-by-step!

1. Structure and Classification of Amines

Amines are organic compounds derived from ammonia, \(\text{NH}_3\), where one or more hydrogen atoms are replaced by alkyl (R) or aryl groups. The key feature of any amine is the Nitrogen atom (N), which has a lone pair of electrons. This lone pair is crucial because it dictates almost all the chemical reactions of amines.

How to Classify Amines

We classify amines based on how many R groups are attached directly to the Nitrogen atom:

  • Primary Amine (\(1^\circ\)): The nitrogen atom is bonded to one alkyl or aryl group and two hydrogen atoms. (Functional group: \(-\text{NH}_2\))
    Example: Ethylamine, \(\text{CH}_3\text{CH}_2\text{NH}_2\)
  • Secondary Amine (\(2^\circ\)): The nitrogen atom is bonded to two alkyl or aryl groups and one hydrogen atom. (Functional group: \(-\text{NH}-\))
    Example: Dimethylamine, \((\text{CH}_3)_2\text{NH}\)
  • Tertiary Amine (\(3^\circ\)): The nitrogen atom is bonded to three alkyl or aryl groups and no hydrogen atoms.
    Example: Triethylamine, \((\text{CH}_3\text{CH}_2)_3\text{N}\)

Note: The syllabus (19.1) states that classification is not tested at AS Level, but at A Level (34.1) you must be able to work with primary and secondary amines.

Quick Tip: Classification depends on what's attached to the Nitrogen, not the number of carbons in the chain (unlike alcohols or halogenoalkanes).
Key Takeaway for Section 1

Amines are classified by the number of carbon chains (R groups) attached to the lone-pair bearing Nitrogen atom. Primary amines have one R group (\(\text{RNH}_2\)) and secondary amines have two (\(\text{R}_2\text{NH}\)).

2. Preparation (Synthesis) of Primary and Secondary Amines

The syllabus requires you to recall methods to produce primary and secondary amines. These methods involve either substitution, or more commonly, reduction.

Method 2.1: Halogenoalkanes with Ammonia or Primary Amines

This is a classic nucleophilic substitution reaction. Since the N atom in ammonia (\(\text{NH}_3\)) has a lone pair, it acts as a nucleophile, attacking the partially positive carbon atom (\(\text{C}^\delta+\)) in the halogenoalkane (\(\text{R-X}\)).

A) Producing a Primary Amine (\(1^\circ\)):

Reagents and Conditions:

  • Halogenoalkane (e.g., bromoethane, \(\text{R-X}\))
  • Excess concentrated ammonia (\(\text{NH}_3\))
  • Solvent: Ethanol (to dissolve both reactants)
  • Conditions: Heated under pressure (in a sealed tube)

The Problem: Polysubstitution

The newly formed primary amine (\(\text{RNH}_2\)) is also a nucleophile (it still has an N lone pair!) and can react further with the halogenoalkane, leading to a mixture of secondary, tertiary, and quaternary ammonium salts. To maximise the yield of the desired primary amine, we must use a large excess of ammonia.

Reaction (producing the salt first, which is then neutralised by excess \(\text{NH}_3\)):

\(\text{R-X} + 2\text{NH}_3 \rightarrow \text{R-NH}_2 + \text{NH}_4\text{X}\)

B) Producing a Secondary Amine (\(2^\circ\)):

If you want to make a secondary amine, you use a primary amine as the nucleophile instead of ammonia, reacting it with a halogenoalkane in ethanol under pressure in a sealed container.

Reaction:

\(\text{R-X} + 2\text{R}'\text{-NH}_2 \rightarrow \text{R-NH-R}' + \text{R}'\text{-NH}_3\text{X}\)

Method 2.2: Reduction of Nitriles

This is a much 'cleaner' method because it only yields the primary amine product and, crucially, increases the carbon chain length by one atom (since the nitrile group, \(-\text{C}\equiv\text{N}\), contains a carbon atom).

Reagents and Conditions:

  • Reagent: Lithium aluminium hydride (\(\text{LiAlH}_4\)) in dry ether, followed by reaction with water OR
  • Reagent: Hydrogen gas (\(\text{H}_2\)) over a metal catalyst (Nickel, Ni) and heat.

General Equation (Reduction of Nitrile):

\(\text{R-C}\equiv\text{N} + 4[\text{H}] \rightarrow \text{R-CH}_2\text{NH}_2\)

Did you know? You often see this method used in multi-step synthesis where a halogenoalkane is first converted to a nitrile using \(\text{KCN}\) in ethanol, which then undergoes reduction. This is how you "step up" the chain length.

Method 2.3: Reduction of Amides

Amides can also be reduced to form amines.

Reagents and Conditions:

  • Reagent: Lithium aluminium hydride (\(\text{LiAlH}_4\)) in dry ether, followed by reaction with water.

General Equation (Reduction of Amide):

\(\text{R-CONH}_2 + 4[\text{H}] \rightarrow \text{R-CH}_2\text{NH}_2 + \text{H}_2\text{O}\)

Quick Review: Synthesis Routes

  • Start with R-X + \(\text{NH}_3\): Good for making primary amines, but requires excess \(\text{NH}_3\) and heat/pressure.
  • Start with R-C\(\equiv\)N (Nitriles): Good for increasing chain length. Requires \(\text{LiAlH}_4\) or \(\text{H}_2/\text{Ni}\).
  • Start with R-CONH\(_2\) (Amides): Requires \(\text{LiAlH}_4\).

3. The Basic Nature of Aqueous Amine Solutions

This is arguably the most important chemical property of simple amines.

3.1 Why are Amines Basic?

The basicity of amines is entirely due to the presence of the lone pair of electrons on the nitrogen atom. According to the Brønsted-Lowry theory, a base is a proton (\(\text{H}^+\)) acceptor.

When an amine dissolves in water, it acts as a base by accepting a proton from water molecules, producing hydroxide ions (\(\text{OH}^-\)):

\(\text{R-NH}_2(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{R-NH}_3^+(aq) + \text{OH}^-(aq)\)

Since \(\text{OH}^-\) ions are produced, the resulting solution is alkaline (pH > 7).

Reaction with Acids: Amines also react directly with dilute mineral acids to form water-soluble ammonium salts:

\(\text{R-NH}_2(aq) + \text{HCl}(aq) \rightarrow \text{R-NH}_3^+\text{Cl}^-(aq)\)

3.2 Comparing Basicity (Ammonia vs. Primary Alkyl Amines)

Primary alkyl amines (like ethylamine) are stronger bases than ammonia.

The Explanation: The Inductive Effect

  • Alkyl groups (R groups like \(\text{CH}_3\) or \(\text{C}_2\text{H}_5\)) are electron-donating groups.
  • These groups push electron density towards the nitrogen atom via sigma bonds (the positive inductive effect).
  • This makes the lone pair on nitrogen more available to attract and bond with an incoming proton (\(\text{H}^+\)).
  • Furthermore, the resulting alkylammonium ion (\(\text{R-NH}_3^+\)) is stabilised by the electron-donating effect of the R group, facilitating ion formation.

Basicity Trend:

\(\text{Primary Amine} > \text{Ammonia} > \text{Phenylamine}\)

(In phenylamine, the nitrogen lone pair delocalises into the benzene ring, making it much less available and thus a weaker base, as covered in Topic 34.2.)

Common Mistake to Avoid

When explaining basicity, ensure you reference the availability of the lone pair and the stability of the conjugate acid (\(\text{RNH}_3^+\)). It is not enough just to say "alkyl groups are electron-donating"; you must link this to the N atom's ability to accept a proton.

Key Takeaway for Section 3

Amines are basic due to the N lone pair. Primary alkyl amines are stronger bases than ammonia because the electron-donating alkyl groups stabilise the resultant ammonium ion and make the lone pair more available for proton acceptance.

4. Reactions of Amines with Acyl Chlorides

Primary and secondary amines react quickly and vigorously with acyl chlorides at room temperature.

4.1 Amide Formation (Condensation Reaction)

This reaction is a type of condensation or addition-elimination reaction. The amine acts as a nucleophile, attacking the carbonyl carbon (\(\text{C=O}\)).

A) Reaction with Ammonia: Forms an unsubstituted amide.

  • Reagent: Ammonia (\(\text{NH}_3\))
  • Conditions: Room temperature
  • Product: Amide and Ammonium Chloride (\(\text{NH}_4\text{Cl}\))

General Equation (Ammonia):

\(\text{R-COCl} + 2\text{NH}_3 \rightarrow \text{R-CONH}_2 + \text{NH}_4\text{Cl}\)

Note: Two moles of \(\text{NH}_3\) are used because one mole reacts with the acyl chloride and the second reacts with the \(\text{HCl}\) produced.

B) Reaction with a Primary Amine: Forms an N-substituted amide.

  • Reagent: Primary amine (\(\text{R}'\text{NH}_2\))
  • Conditions: Room temperature
  • Product: N-substituted Amide

General Equation (Primary Amine):

\(\text{R-COCl} + 2\text{R}'\text{NH}_2 \rightarrow \text{R-CONHR}' + \text{R}'\text{NH}_3\text{Cl}\)

C) Reaction with a Secondary Amine: Forms an N,N-disubstituted amide.

Why is this reaction so fast?

Acyl chlorides are highly reactive because the chlorine atom is an effective leaving group, and the carbonyl carbon is strongly electrophilic (\(\text{C}^\delta+\)) due to the electron-withdrawing nature of both the oxygen and chlorine atoms. The reaction proceeds easily without heating.

Memory Aid: Product Names
When you react an acyl chloride with an amine, you always get an amide (plus the alkylammonium salt). This is a key reaction for building up complex organic structures, including polymers and proteins.
Key Takeaway for Section 4

Primary and secondary amines (and ammonia) react readily with acyl chlorides via condensation at room temperature to form amides. The reaction is fast because acyl chlorides are very reactive due to two electron-withdrawing groups attached to the carbonyl carbon.