Welcome to Amides: Organic Nitrogen Chemistry
Welcome to one of the most interesting chapters in your A2 Chemistry journey! If you have ever wondered what holds the proteins in your muscles together, what gives Kevlar vests their bullet-stopping strength, or what makes paracetamol work, you are looking at the chemistry of amides.
Don't worry if organic nitrogen mechanisms have felt intimidating in the past. We will break this topic down into simple, manageable building blocks. By the end of this guide, you will master the structures, physical properties, synthesis, and reactions of amides with confidence.
1. What is an Amide? Structure and Naming
The Functional Group
An amide is a derivative of a carboxylic acid where the \(-OH\) group has been replaced by an amino group (\(-NH_2\)). The characteristic functional group of a primary amide is:
Amide Functional Group: \(-\text{CONH}_2\)
Analogy time: Think of a carboxylic acid (\(R-\text{COOH}\)) as a plug socket. If you unplug the \(-OH\) group and plug in an \(-NH_2\) group, you get an amide (\(R-\text{CONH}_2\)).
Naming Primary Amides
Naming primary amides is straightforward! Follow these two simple rules:
• Find the longest carbon chain containing the carbonyl carbon (\(C=O\)).
• Take the name of the parent alkane, remove the final -e, and add -amide.
Let's look at key examples:
• \(H\text{CONH}_2\): Methanamide (1 carbon)
• \(CH_3\text{CONH}_2\): Ethanamide (2 carbons)
• \(CH_3CH_2\text{CONH}_2\): Propanamide (3 carbons)
• \(C_6H_5\text{CONH}_2\): Benzamide (benzene ring attached to \(-\text{CONH}_2\))
Did You Know?
In biochemistry, the bond connecting amino acids together in proteins is called a peptide link. In organic chemistry, that very same bond is simply an amide group!
Key Takeaway
Primary amides contain the carbonyl group directly bonded to an \(-NH_2\) group (\(-CONH_2\)) and are named by adding -amide to the parent carbon chain stem.
2. The Basicity Puzzle: Why Aren't Amides Basic?
You already know that amines (like ethylamine, \(CH_3CH_2NH_2\)) act as weak bases because the nitrogen atom has a lone pair of electrons ready to accept a proton (\(H^+\)).
So, you might expect amides to be basic too, right? Surprisingly, amides are essentially neutral in aqueous solution!
Why is this?
• In an amide, the lone pair of electrons on the nitrogen atom is delocalised into the adjacent carbonyl \(\pi\)-system (\(C=O\)).
• Because this electron pair is spread out over the \(O-C-N\) system, it is not readily available to form a dative covalent bond with an incoming \(H^+\) ion.
• Therefore, primary amides do not act as bases.
Memory Trick: The greedy, electronegative oxygen in \(C=O\) pulls the nitrogen's lone pair close to itself, locking it away so protons cannot grab it!
Key Takeaway
Unlike amines, amides are neutral because the nitrogen lone pair is delocalised across the carbonyl group, making it unavailable to accept protons.
3. Physical Properties of Amides
Melting and Boiling Points
Primary amides have remarkably high melting and boiling points compared to alkanes and even similar-sized carboxylic acids. In fact, except for methanamide (which is a liquid), all other primary amides are crystalline solids at room temperature.
The Reason: Extensive Hydrogen Bonding!
• Each primary amide molecule contains a strongly electronegative oxygen atom (\(C=O\)) and two partially positive hydrogen atoms attached to nitrogen (\(N-H\)).
• This setup allows molecules to form multiple intermolecular hydrogen bonds with one another, forming strong, stable networks that require significant thermal energy to break.
Solubility in Water
• Small amides (e.g., methanamide, ethanamide): Highly soluble in water because they can form hydrogen bonds with polar water molecules.
• Larger amides: As the non-polar hydrocarbon tail gets longer, it disrupts water's hydrogen bonding network, causing water solubility to drop rapidly.
Key Takeaway
Extensive hydrogen bonding gives primary amides unusually high melting/boiling points and makes short-chain amides readily soluble in water.
4. Preparation of Primary Amides
How do we make an amide in the laboratory? The standard method in your CCEA specification is the reaction between an acyl chloride and concentrated ammonia.
The Reaction: Acyl Chloride + Ammonia
When an acyl chloride reacts vigorously with concentrated aqueous ammonia at room temperature, a primary amide and ammonium chloride are produced:
\(R\text{COCl} + 2\text{NH}_3 \rightarrow R\text{CONH}_2 + \text{NH}_4\text{Cl}\)
For example, making ethanamide from ethanoyl chloride:
\(CH_3\text{COCl} + 2\text{NH}_3 \rightarrow CH_3\text{CONH}_2 + \text{NH}_4\text{Cl}\)
Why do we need 2 moles of \(NH_3\)?
• Molecule 1: Acts as a nucleophile, attacking the carbonyl carbon and displacing the chlorine atom to form the amide and \(HCl\).
• Molecule 2: Acts as a base to neutralise the acidic \(HCl\) byproduct, forming solid white fumes of ammonium chloride (\(NH_4Cl\)).
Observations: A vigorous, exothermic reaction occurs, producing dense white smoke / white solid of \(\text{NH}_4\text{Cl}\).
Key Takeaway
Reacting an acyl chloride with concentrated ammonia (\(1:2\) molar ratio) yields a primary amide along with ammonium chloride.
5. Chemical Reactions of Amides
Amides are relatively stable compounds, but they undergo two crucial types of reactions you must know for your exams: Hydrolysis and Dehydration.
Reaction 1: Acid Hydrolysis
Heating an amide under reflux with dilute hydrochloric acid (\(\text{HCl}_{(aq)}\)) breaks the amide bond.
Products: A carboxylic acid and an ammonium salt.
General Equation:
\(R\text{CONH}_2 + \text{H}_2\text{O} + \text{HCl} \rightarrow R\text{COOH} + \text{NH}_4\text{Cl}\)
Specific Example (Ethanamide):
\(CH_3\text{CONH}_2 + \text{H}_2\text{O} + \text{HCl} \rightarrow CH_3\text{COOH} + \text{NH}_4\text{Cl}\)
Ethanamide + water + hydrochloric acid \(\rightarrow\) ethanoic acid + ammonium chloride
Reaction 2: Alkaline (Base) Hydrolysis
Heating an amide under reflux with dilute sodium hydroxide (\(\text{NaOH}_{(aq)}\)) also cleaves the molecule.
Products: A carboxylate salt (e.g., sodium salt) and ammonia gas (\(\text{NH}_3\)).
General Equation:
\(R\text{CONH}_2 + \text{NaOH} \rightarrow R\text{COONa} + \text{NH}_3\)
Specific Example (Ethanamide):
\(CH_3\text{CONH}_2 + \text{NaOH} \rightarrow CH_3\text{COONa} + \text{NH}_3\)
Ethanamide + sodium hydroxide \(\rightarrow\) sodium ethanoate + ammonia
Exam Tip & Observation: This reaction provides a handy qualitative test for amides! The pungent ammonia gas released turns damp red litmus paper blue.
Quick Comparison: Acid vs Alkaline Hydrolysis
• Acid Hydrolysis: Produces free carboxylic acid (\(R\text{COOH}\)) and ammonium ions (\(\text{NH}_4^+\)).
• Alkaline Hydrolysis: Produces carboxylate ion / salt (\(R\text{COO}^-\)) and free ammonia gas (\(\text{NH}_3\)).
Reaction 3: Dehydration (Making Nitriles)
If water is removed from a primary amide, a nitrile (\(R-\text{C}\equiv\text{N}\)) is formed. This is done by heating the solid amide with a powerful dehydrating agent: phosphorus(V) oxide (\(\text{P}_4\text{O}_{10}\)).
General Equation:
\(R\text{CONH}_2 \xrightarrow{\text{P}_4\text{O}_{10}} R\text{C}\equiv\text{N} + \text{H}_2\text{O}\)
Specific Example (Ethanamide to Ethanenitrile):
\(CH_3\text{CONH}_2 \xrightarrow{\text{P}_4\text{O}_{10}} CH_3\text{CN} + \text{H}_2\text{O}\)
Key Takeaway
• Acid hydrolysis (\(\text{HCl}\)) gives a carboxylic acid + \(\text{NH}_4\text{Cl}\).
• Alkaline hydrolysis (\(\text{NaOH}\)) gives a carboxylate salt + \(\text{NH}_3\) gas.
• Dehydration using \(\text{P}_4\text{O}_{10}\) converts primary amides into nitriles.
6. Summary & Common Exam Pitfalls
Common Mistakes to Avoid
• Calling amides basic: Remember, the lone pair on nitrogen is delocalised into the \(C=O\) group. Amides are neutral!
• Mixing up hydrolysis products: In acid hydrolysis, you get \(R\text{COOH}\) (not \(R\text{COO}^-\)) and \(\text{NH}_4^+\) (not \(\text{NH}_3\)). In alkaline hydrolysis, you get \(R\text{COO}^-\) (not \(R\text{COOH}\)) and \(\text{NH}_3\) (not \(\text{NH}_4^+\)).
• Missing the stoichiometric 2 in amide preparation: Remember that \(1\) mole of acyl chloride requires \(2\) moles of \(\text{NH}_3\).
Quick Review Checklist
• Can you draw and name primary amides up to 4 carbons?
• Can you explain why ethanamide has a higher melting point than propane or chloroethane?
• Can you write balanced equations for the acid and alkaline hydrolysis of propanamide?
• Can you state the reagent and condition needed to convert an amide into a nitrile?