Chromatography & Qualitative Analysis

What is chromatography?

An analytical technique used to separate and identify components of a mixture

What does separation in chromatography depend on?

The distribution of substances between a mobile phase and a stationary phase

What is the mobile phase in chromatography?

A liquid or gas that carries substances through or over the stationary phase

How does solubility in the mobile phase affect movement?

More soluble substances move further and faster

What is the stationary phase in chromatography?

A solid or liquid that attracts components of the mixture

How does affinity for the stationary phase affect movement?

Greater affinity means slower movement and shorter distance

What is an Rf value?

A retention factor unique to each substance in a given solvent system

Why does each substance have a unique Rf value?

Different affinities for the mobile and stationary phases

How is Rf value calculated?

Distance moved by substance divided by distance moved by solvent

Does the same compound always have the same Rf value?

No, Rf values change with different solvents

What is thin-layer chromatography (TLC)?

A technique using a silica-coated plate to separate mixture components

What is the stationary phase in TLC?

Silica

What is the mobile phase in TLC?

A solvent

Why must samples be placed above the solvent level in TLC?

To prevent them dissolving directly into the solvent

What is the solvent front in TLC?

The furthest point reached by the solvent

Why is the solvent front marked in pencil?

Pencil does not dissolve in solvent

How are colourless spots visualised in TLC?

Using UV light or a developing agent like iodine

What is gas chromatography (GC)?

A technique used to separate volatile substances

What is the mobile phase in gas chromatography?

A high-pressure carrier gas

What is the stationary phase in gas chromatography?

A solid coated inside a long coiled tube

What property is measured in gas chromatography?

Retention time

What does retention time help identify?

Individual components in a mixture

What types of substances are analysed using GC?

Volatile liquids

What data can be used to identify organic compounds?

NMR, IR, mass spectrometry, elemental analysis, and chemical tests

What is a homologous series?

A group of organic compounds with the same functional group

What is the typical reaction of alkanes?

Combustion and free radical substitution

How are alkenes identified chemically?

They decolourise bromine water

What type of reaction do alkenes undergo?

Electrophilic addition

How are haloalkanes identified?

Reaction with aqueous silver nitrate

What precipitate indicates a chloroalkane?

White AgCl precipitate soluble in dilute ammonia

What precipitate indicates a bromoalkane?

Cream AgBr precipitate soluble in concentrated ammonia

What precipitate indicates an iodoalkane?

Yellow AgI precipitate insoluble in ammonia

How are alcohols oxidised?

Using acidified potassium dichromate

What colour change indicates oxidation of alcohols?

Orange to green

Which alcohols are oxidised by dichromate?

Primary and secondary alcohols

How are aldehydes identified?

Tollens’, Fehling’s, or dichromate tests

What is a positive Tollens’ test result?

A silver mirror

What is a positive Fehling’s test result?

Brick-red precipitate

What does 2,4-DNP test for?

The presence of a carbonyl group

What result does 2,4-DNP give?

Yellow-orange precipitate

How do ketones react with Tollens’ reagent?

No reaction

How are carboxylic acids identified?

Reaction with carbonates producing effervescence

What gas is released when carboxylic acids react with carbonates?

Carbon dioxide

What pH do carboxylic acids have?

Less than 7

How can esters be identified?

By their sweet smell

What reactions do esters undergo?

Acid or alkaline hydrolysis

How do amines react with acids?

They form salts

What reactions do nitriles undergo?

Hydrolysis or reduction

What functional group is present in arenes?

A benzene ring

What reactions do arenes undergo?

Electrophilic substitution

How is phenol identified chemically?

It reacts with bromine water to form a precipitate

Why does phenol not react with carbonates?

It is too weakly acidic

What reactions do acyl chlorides undergo?

Rapid hydrolysis and reactions with alcohols or amines

Why are acyl chlorides very reactive?

The C–Cl bond is highly polar

What reactions do amides undergo?

Hydrolysis or reduction

What is formed when amides are hydrolysed under acidic conditions?

A carboxylic acid and ammonium ions