3.3.6 Organic analysis - Chemistry Alevel

Tests for organic compounds - reagents - Alkenes

Bromine water (Br₂(aq))

Tests for organic compounds - reagents - Primary, secondary or tertiary alcohol (does not distinguish between the three)

Sodium

Tests for organic compounds - reagents - Primary and secondary alcohols

Potassium dichromate (VI) acidified K₂Cr₂O₇

Tests for organic compounds - reagents - Tertiary alcohols

Potassium dichromate (VI) acidified K₂Cr₂O₇

Tests for organic compounds - reagents - Aldehydes 1

Fehlings

Tests for organic compounds - reagents - Aldehydes 2

Tollens reagent

Tests for organic compounds - reagents - Carboxylic acids

Sodium hydrogencarbonate NaHCO₃

Tests for organic compounds - reagents -Halogeoaklane

Hydrolyse by warming with aqueous with NaOH(aq)

Add AgNO₃

Tests for organic compounds - observations before the test -Alkenes

Orange/brown

Tests for organic compounds - observations before the test - Primary and secondary alcohols

Orange

Tests for organic compounds - observations before the test - Tertiary alcohols

Orange

Tests for organic compounds - observations before the test - Aldehydes 1

Blue solution

Tests for organic compounds - observations before the test - Aldehydes 2

Colourless solution

Tests for organic compounds - observations after the test - Alkenes

Colourless (it has been decolourised)

Tests for organic compounds - observations after the test - Primary, secondary or tertiary alcohol (does not distinguish between the three)

Bubbles of H₂ gas (effervescence)

Tests for organic compounds - equations - Primary, secondary or tertiary alcohol (does not distinguish between the three)

2R-OH + 2Na → 2R-O⁻Na⁺ + H₂

Tests for organic compounds - observations after the test - Primary and secondary alcohols

Green (alcohol is oxidised & Cr₂O₇²⁻ reduced to Cr³⁺)

Tests for organic compounds - observations after the test - Tertiary alcohols

Orange (no change)

Potassium dichromate

Tests for organic compounds - observations after the test - Aldehydes 1

Brick red precipitate

Tests for organic compounds - observations after the test - Aldehydes 2 (Tollens)

Silver mirror

Tests for organic compounds - observations after the test - Carboxylic acids

Bubbles of CO₂ gas (verify CO₂ by bubbling through limewater → cloudy)

Tests for organic compounds - observations after the test - Halogeoaklane - Iodine

I⁻ = yellow precipitate

Tests for organic compounds - observations after the test - Halogeoaklane - Bromine

Br⁻ = cream precipitate

Tests for organic compounds - observations after the test - Halogeoaklane - Chlorine

Cl⁻ = white precipitate.

Electromagnetic waves - highest frequency to lowest frequency

X-rays, ultra violet, visible, infrared, microwaves, radio waves

Electromagnetic waves - smallest wavelength to longest wavelength

X-rays, ultra violet, visible, infrared, microwaves, radio waves

Electromagnetic waves - what do they consist of

They are electric and magnetic fields which vibrate at the same frequency and are perpendicular to each other and to the direction of travel of the wave.

Forms of spectroscopy - what the energy causes - ultra-violet/visible

Movement of electrons to higher energy levels.

Forms of spectroscopy - what the energy causes - Infra-red

Bonds vibrate.

Forms of spectroscopy - what the energy causes - Radio waves

Changes nuclear spin.

Forms of spectroscopy - what the energy causes - Microwaves

Molecules rotate

Forms of spectroscopy - spectroscopy technique - ultra-violet/visible

Ultra-violet/visible spectroscopy

Forms of spectroscopy - spectroscopy technique - Infra-red

Infra-red spectroscopy

Forms of spectroscopy - spectroscopy technique - Radio waves

NMR spectroscopy (MRI)

Forms of spectroscopy - spectroscopy technique - Microwaves

Microwaves spectroscopy

Using mass spectrometry to find the molecular formula of compounds - what happens

High resolution mass spectrometers measure the m/z values to enough precision to find the molecular formula. Each molecular formula has a different Mr if measured to enough precision.

Using mass spectrometry to find the molecular formula of compounds - problems

More than one compound can have the same molecular formula, and so this alone cannot identify a compound.

What does the frequency of a wave depend on?

The mass of the atoms in the bond, the bond strength, and the type of vibrations.

Finger print region - Use

It can be used to identify a compound and check if a compound is pure (if there are any extra peaks there is an impurity).

What does each trough represent (& name)?

Each trough is called a peak and represents the energy absorbed by a particular bond, causing it to vibrate.

Wavenumber equation

Wavenumber (cm⁻¹) = 1/Wavelength (cm)

What are wave numbers used to measure?

They are used to measure the frequency of IR radiation absorbed.

Where is the region for functional group signals?

Above 1500 cm⁻¹

Wavenumber - C=O

1680 - 1750 cm⁻¹

Wavenumber - O-H (alcohols)

3230 - 3550 cm⁻¹

Wavenumber - O-H (acids)

2500 - 3000 cm⁻¹

Peak description - O-H

It is a much wider peak and a broader absorption than C-H

Peak description - C=O

It is a sharp peak and a sharp absorption.

Peaks - O-H only

Alcohol

Peaks - O-H and C=O

Carboxylic acid

Peaks - C=O only

It is a carbonyl and so it is an aldehyde or a ketone.

Infra red and global warming - which gases and what do they do

Gases like; CO₂ CH₄, H₂O(g) prevent the radiation from escaping. These are green house gasses.

Infra red and global warming - description of the atmosphere

The atmosphere comprises of several layers of gas. Infra red radiation from the sun has to pass through these layers to reach the Earth's surface and then pass back out into space.

What is the importance of the greenhouse effect?

The Greenhouse Effect is important in keeping the Earth warm.

What are the problems of the Greenhouse effect and what is causing them?

The concentration of greenhouse gases in the atmosphere has increased due to the burning of fossil fuels... and so more heat energy is trapped and the temperature of Earth is rising.

Why can these gases absorb infrared radiation?

The bonds in the molecules are efficient at absorbing the infrared radiation as when it hits a molecule it causes the bond to vibrate.