Core Practical 6: Tests for Alcohol, Aldehyde, Alkene and Carboxylic Acid
Function Group Test Procedure Positive Result
Alkenes Add Bromine Water Orange to Colourless
1° / 2° Alcohol Add Acidified Potassium Dichromate Orange to Green
3°Alcohol Remains Orange
Aldehyde Fehling’s test – warm with reagent,
containing blue Cu II ions which will oxidise
aldehydes but not ketones
Blue to brick red precipitate
Tollen’s test – warm aldehyde with Tollen’s
reagent (silver nitrate dissolved in aqueous
ammonia); silver ions are reduced to
metallic silver
Silver mirror forms
Carboxylic Acid Add NaHCO3(aq) CO2 evolved – bubble through
limewater, should turn it cloudy
Mass Spectrometry
Mass spectrometry can be used to determine the molecular formula of a compound. High resolution
mass spectrometry uses four decimal places, providing a precise molecular mass taking into account
the isotopic masses and exact individual atomic masses. Compare calculated values to database values
Infrared Spectroscopy
All bonds vibrate at a characteristic frequency. The frequency depends on the mass of atoms in the
bond, the bond strength, and type of vibration. The frequency of vibrations are in the IR region of the
EM spectrum.
If IR light is passed through a compound, it will absorb some or all of the light at the frequencies at
which the bonds vibrate. Wavenumbers (cm-1
) are used as a
measure of wavelength or frequency of absorption.
Below 1500cm-1 is the fingerprint region – this is complicated and contains many signals. This part is
unique for each compound and can be used to identify a compound using a database.
The wavenumbers and peak descriptions below can be used to identify functional groups present
O-H Alcohol 3200-3550cm-1
– smooth peak
O-H Acid 2500-3000cm-1
– jagged peak
C=O Carbonyl 1700cm-1
– strong peak
C=C Alkene 1650cm-1
– medium peak
N-H Amine 3100-3500 – medium peak
The greenhouse effect is caused by gases in the atmosphere which absorb the IR radiation given off
from the surface of the Earth and would otherwise be radiated into space. The IR radiation is absorbed
by bonds in these gases the same way as in an infrared spectrometer. For example, the C=O bonds in
CO2 and the O-H bonds in water absorb IR. Energy is then re-radiated to Earth, contributing to global
warming
