Chemistry (9701) | Primary and secondary amines

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, \(\text{(CH}_3\text{)}_2\text{NH}\)
• Tertiary Amine (\(3^\circ\)): The nitrogen atom is bonded to three alkyl or aryl groups and no hydrogen atoms.
  Example: Triethylamine, \(\text{(CH}_3\text{CH}_2\text{)}_3\text{N}\)

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.

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 four main ways 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 even quaternary ammonium salts. To maximise the yield of the desired primary amine, we must use a large excess of ammonia.

Don't worry about the Quaternary Salt.

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. You must again use a sealed tube/pressure and ethanol solvent.

Reaction:

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

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, \(-C\equiv\text{N}\), contains a carbon atom).

Reagents and Conditions:

• Reagent: Lithium aluminium hydride (\(\text{LiAlH}_4\)) (a powerful reducing agent) 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):

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

3. The Basic Nature of Aqueous Amine Solutions

This is arguably the most important 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 the water molecule, producing hydroxide ions (\(\text{OH}^-\)).

General Equation for Basic Behaviour in Water:

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

Since \(\text{OH}^-\) ions are produced, the resulting solution is alkaline, meaning it has a pH greater than 7.

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 the sigma bond (the positive inductive effect).
• By pushing electrons onto the N atom, the lone pair becomes more concentrated and thus more available to attract and bond with an incoming proton (\(\text{H}^+\)).
• Furthermore, the resulting positive ion (\(\text{R-NH}_3^+\)) is stabilized by the electron-donating effect of the R group(s), making the formation of this ion more favorable.

Analogy: Imagine the nitrogen lone pair is money. Alkyl groups are friends giving the nitrogen more money (electron density). The richer the nitrogen is, the more easily it can "pay" (share electrons) to bond with a proton.

Basicity Trend:

$$\text{Primary Amine} > \text{Ammonia} > \text{Phenylamine}$$

(The phenylamine case involves delocalization, which you cover in Topic 34.2, making it a much weaker base.)

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 stabilize 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 (or acid 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 Hydrogen Chloride (\(\text{HCl}\))

General Equation (Ammonia):

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

Note: We use 2 moles of \(\text{NH}_3\) because one mole is needed to react with the \(\text{HCl}\) side product.

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.

(While the syllabus focuses specifically on the reaction with ammonia or "an amine," understanding the general mechanism applies to secondary amines as well.)

Why is this reaction so fast?

Acyl chlorides are highly reactive because the chlorine atom is a good leaving group, and the carbonyl carbon is strongly positive (\(\text{C}^\delta+\)) due to the pulling power of both the oxygen and chlorine atoms. The reaction proceeds easily without heating.

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.