CHEM UNIT 4: SPECTROSCOPY AND CHROMATOGRAPHY

UV Radiation can initiate reactions

• wavelengths between visible light and X-rays - 400nm to 10nm
• has enough energy to split molecules + produce free radicals - e.g with a chlorine molecule, each chlorine atom takes one electron from the covalent bond - homolytic fission a chlorine free radical is the same as a chorine atom and is very reactive

UV radiation can start the reaction between chlorine and methane

• production of chlorine radicals - initiation step
• chlorine radicals go on to react with the methane in a series of propagation steps - where they get used up and recreated in a chain reaction - new free radicals can react with more methane and continue the cycle
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• the 2 reactions continue very rapidly until 2 free radicals combine in a termination step

UV radiation can create chlorine free radicals from CFCs

-outer edge of the atmosphere - UV from sunlight reacts with chlorine molecules and CFCs to produce chlorine free radicals

the ozone (O3) layer that protects the earth's surface from harmful UV radiation is broken down by the chlorine free radicals

1 chlorine free radical causes the destruction of 2 ozone molecules and makes another chlorine radical which continues the process - cycle of reactions is repeated many times before the chlorine free radical is removed from the system when it combines with another free radical. so one chlorine free radical can end up destroying thousands of ozone molecules

microwaves are used for heating things

• longer wavelenth than IR - between 1m and 1m ; widely used in communication as well as heating stuff up

microwave oven - wavelength of 12.24cm

• in a covalent between 2 atoms of different electronegativities, the bonding electrons are pulled towards the more electronegative atom - makes bond polar
• in a water molecule the oxygen ato is more electronegative than the hydrogens so it pulls the electrons from both the O-H bonds towards it, making itself slightly negative and both hydrgens slightly positive - makes water a polar molecule
• the fats and sugars found in food are also polar, but less so than water
• the microwave radiation creates an electric field and any polar molecules in the food try to line up with the field by rotating - makes them collide with other molecules - generating heat energy
• speed of heating depends on the thickness and density of the food - will penetrate food several cm thick depending on their water content, surface is often dryer than the inside so doesn't heat up too quickly and frozen water molecules heat up slower too

heating effect of microwaves is used in lots of other applications:

• surgeons can use narrow beams of microwaves to kill cancer cells
• in chemical industry - used to heat reactants directly without having to waste energy on heating the container the chemicals are in
• used to dry wood, paper and textiles - more efficient method of heating than conventional ovens

NMR gives you info about a molecule's structure

• any atomic nucleus wit an odd no. of nucleons (protons and neutrons) has a nuclear spin- causes it to have a weak magnetic field
• nuclear magnetic resonance (NMR) spectroscopy looks at how this tiny bit of magnetic field reacts when you put it in a much larger external magnetic field
• Hydrogen nuclei are single protons - they do not have a spin. So you can use proton NMR to find out how many hydrogens there are in an organic molecule and how they're arranged

protons align in 2 directions in an external magnetic field

• normally protons are spinning in random directions so their magnetic fields cancel out
• but when a strong magnetic field is applied, protons align themselves with the direction of the field or against it
• aligned protons are at a lower energy level than the opposing protons, if they absorb radio waves of the right frequency they can flip to te higher energy level, the opposing protons can emit radio waves and flip to the lower energy level
• there tends to be more aligned protons - so there's an absorption of energy overall. NMR measure this absorption of energy  

protons in different environments absorb different amounts of energy

• protons are partly shielded from the effects of external magnetic fields by surrounding electrons
• any other atoms and groups of atoms that are near a nucleus will also affect is amound of electron shielding
• how the protons in a olecule interact with magnetic fields depends on their environments. they will absorb different amounts of energy at different frequencies - it's these differences in absorption between environments that you're looking for in NMR spectroscopy
• an atom's environment depends on all the groups it's connected to, going right along the molecules - not just the atoms it's actually bonded to. to be in the same environment - 2 atoms must be joined to exactly the same things

chemical shift is measured relative to tetramethylsilane

• peaks on spectrum show the frequencies at which the protons in a olecule absorb energy. these differences in absorption are measured relative to a standard substance, like tetramethylsilane.
• tetramethylsilane or TMS has the formula Si(CH3)4. it has 12 protons in identical environments, so it gives a single peak that's well away from most peaks produced by porotons in other molecules .
• The difference in absorption of a proton relative to TMS is called its chemical shift. the TMS peak is given a chemical shift value of 0. spectra often have a peak at 0 because TMS is added to the test sample for calibration purposes.

proton NMR tells you about a molecule's hydrogen environments

• each peak on a proton NMR spectrum is due to one or more protons in a particular proton environment. the relative area under each peak = how many protons are in the enviroment

Example: the proton NMR spectrum of ethanoic acid, CH3COOH

• spectrum has 2 peask - 2 environments
• area ratio is 1:3 - so there must be 1 H environment to every 3 Hs in another environment
• this fits the structure of ethanoic acid

Spin-spin coupling splits the peaks in an NMR Spectrum

• in high resolution NMR spectra, the peaks are often split into smaller peaks - down to the magnetic fields of neighbouring protons interacting with each other - called spin-spin coupling. only protons that are on adjacent carbon atoms will affect each other
• these multiple peaks = multiplets. always split into the number of neighbouring protons plus one - it's called the n+1 rule e.g if there are 2 protons on the next door carbon, the peak will be split into 2 + 1 = 3
• work out the no. of neighbouring protons by looking at how many the peak splits into. into 2 = doublet - one neighbouring single proton, into 3 = triplet - 2 neighbouring single protons, into 4 = quartet - 3 neighbouring single protons
• e.g this is the spectra for 1,1,2-trichloroethane

an NMR spectrum gives you a lot of info

can tell you about:

• different proton environent in the molecule (from the chemical shifts)
• relative number of protons in each environment (from relative peak area)
• numbe rof protons adjacent to a particular proton (from splitting pattern)

example

• 2 sets of peaks = 2 proton environments
• peak at 9.5ppm is likely to be due to an R-CHO group, so compund must be an aldehyde
• peaks at 2.5ppm can also be from a carbonyl compound, has an area of 3 so group must be R-COCH3
• the quartet's got 3 neighbouring protons, and as the doublet's got 1 - likely these 2 groups are next to each other

magnetic resonance can see inside you

MRI - Magnetic resonance imagery uses the same principle as NMR

• when you have an MRI scan, you're put in a large magnet and irradiated with radio waves. the hydrogen nuclei in the water molecules in your body interact with the radio waves
• different frequencies of waves are absorbed depending on what sort of tissue the water molecules are in. this allows an image of the body to be built up without the potential damage caused by X-rays.
• by moving the beam of radio waves, a series of images are produced, which can be added together by a comp to build up a 3-D picture.
• technique is used in cancer treatment, bone and joint treatment and studies of the brain and cardiovascular systems

NMR has other important uses too - pharmaceutical industry - used to monitor the products to make sure they are pure. NMR spectrum is like the fingerprint of a molecule so if there were impurities in the product they'd be easy to spot on the spectrum. 

Has several commercial uses:

• in the chemical industry - reaction can be followed to the point where one functional group changes to another can be measured. e.e in the oxidation of a secondary alcohol to a ketone, the point at which all the OH groups in the the alcohol have gone can be seen as well as the point when the first C=O groups in the ketone appear.
• degree of polymerisation that has occured in polymer manufacture can be measured. Machine is set up so that you just record the absorption at the frequency of the double bond in the monomer. you can watch the number of double bonds change as the polymerisation takes place
• polymers can be attacked by oxygen during the processes used to make them into useful objects - can be identified by IR spectroscopy. absorption as 1700cm-1 is a carbonyl group where the polymer has been oxidised - first step in the formation of cracks in the polymer.

Good for separating and identifying things

need to know about gas and HPLC (high pressure/performance liquid chromatography) which both have 2 phases:

• mobile phase - where molecules can move - always a liquid or a gas
• stationary phase - where molecules can't move - must be a solid or a liquid held in solid

gas chromatography is very high-tech

• the stationary phase is a viscous liquid, e.g. oil which coats the inside of the tube
• tube is coiled to save space and built into an oven
• mobile phase is an unreactive carrier gas e.g nitrogen
• sample gets injected into the carrier gas stream as either a liquid or gas - if it is a liquid the inlet is heated to vaporise it.
• amount that each component absorbs to the stationary phase is different - take a diff amount of time from being injected into the tube to being recorded at the other end = retention time - used to identify the compound.
• at the end of the tube is a detector - most common measures the thermal conductivity of gases leaving the tube - connected to a recorder which gives a peak on the graph

• that shows the retention time
• area under each peak (or height if sharp) = relative amount of each component.

HPLC is a useful alternative to GC

• stationary phase - small particles of a solid packed into a tube - often silica bonded to various hydrocarbons
• liquid mobile phase - often a polar mixture such as methanol and water - forced through the tube under high pressure, mixture to be separated is injected into a stream of solvent and is carried through the tube as a solution. Mass spectrometer can be used to analyse each component as it is collected
• the tube isn't heated unlike in GC - mixture is separated because the different parts are attracted by different amounts to the solid, so they take different lengths of time to travel through the tube.
• retention time measured using a detector that shines UV light through the stream of liquid leaving the tube. UV is absorbed by the parts of the mixture as they come through - graph produced in the same way as GC
• can be use when GC can't - when the sample is heat sensitive or has a high boiling point.

both can be used to check purity of samples

• gas chromatography - used in the chemical industry to routinely check the purity of products in a continuous process: small amount of the product is diverted by valves into a GC apparatus at regular time intervals. The chromatogram produced will show the presence of impurities. The likely impurities (like unused reactants) can be looked for automatically by comp. controlled detectors which can be linked to valved that shut down the process if the level of impurities is too high.
• HPLC - checking that equipment used in drug manufacture is clean. Very strict levels of impurities and residues are permitted, and HPLC can be used to check whether these have exceeded as it is a very sensitive method of analysis.