Analytical techniques

Often used by organic chemists to help them identify the type of bonds in a compound

• All covalent bonds act like springs, not rigid bars 2. Like springs, the bonds can vibrate in various ways 3. The frequency of vibration occurs in the infrared region of the electromagnetic spectrum 4. If an organic molecule is irradiated with an in infrared frequency which matches the vibration frequency of that particular bond, it will absorb some of that energy and cause that covalent bond to vibrate more 5. Observing which frequencies of IR radiation are absorbed allows bonds to be identified

• A beam of IR radiation containing a spread of frequencies is passed through a sample 2. Bonds in a molecule absorb IR radiation at characteristic wave numbers 3. The radiation that emerges is missing the frequencies that are absorbed by the bonds found in the sample 4. The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation 5. The frequency is expressed as a wave number, measured in cm^-1

It will be given to you in the exam

Approx 3000cm^-1 due to absorption by C-H bonds and so does not tend to be identified

O-H and C to O double bonds. You could be asked to identify other bonds as part of an application of knowledge question

Water impurities

To identify impurities. These may be revealed in absorptions that should not be there in the pure compound

The area of an IR spectrum below 1500cm^-1. This shape is unique for any particular substance and can be used to identify the compound

Can use a computer to match the fingerprint region of a sample with those on a database of known compounds. An exact match confirms the identification of the sample

• Radiation from the sun is absorbed by the Earth and eventually re-emit it as longer wavelength IR radiation (heat) 2. Water vapour, carbon dioxide and methane are all greenhouse gases which are found in the atmosphere. They absorb this IR radiation and in turn, vibrate more 3. These gases then re-radiate some heat back to the Earth, increasing the surface temperature and leading to global warming 4. The best greenhouse gas absorbs the most IR radiation and traps the heat in the Earth’s atmosphere. The more polar the bonds, the more IR radiation them bonds absorb. They will in turn, vibrate more

Sensors detect and measure the amount of pollutants such as CO, NO and unburnt hydrocarbons. This commonly occurs on motorways and in busy town centres to monitor localised pollution

A ray of IR radiation is passed through the breath that is exhaled into the breathalyser chamber. The characteristic bonds of ethanol are detected and measured - the higher the absorbance of IR radiation, the more ethanol in the person’s breath

An organic molecule is hit with a beam of high energy electrons which causes the molecule to lose an electron and form a positive ion, the molecular ion. It then detects the mass to charge ratio (m/z) of the molecular ion. This is used to give the Mr of the compound

It is the peak with the highest m/z value on the spectrum i.e. it is the peak furthest to the right. It is also known as the M+ peak

The collision with the beam of high energy electrons is so violent that, for some molecules, the molecular ion is then further broken apart or fragmented

Simplified versions, you will try to identify fragments that give you clues to the whole structure

Find the Mr by finding the highest m/z value

The presence of C-13. Present in natural carbon at 1.1% so may affect one in a hundred carbons and forms a tiny molecular ion peak one mass higher than usual. Ignore this when determining Mr

Fragment ions

For larger molecules, there will be many fragments and therefore many fragment peaks

To distinguish between isomers

• Divide percentage or mass of each element in a molecule by it’s Ar 2. Divide all answers by the smallest answer 3. This is the empirical formula 4. Discover empirical mass 5. Divide Mr by empirical mass 6. Multiply empirical formula by this answer to find molecular formula

To identify an unknown organic molecule