29 Chromotography And Spectroscopy

Question 1

What is chromatography used for

Answer 1

Used to separate individual components for a mixture of substances
Can be used in analysis of drugs plastics flavourings air samples and has applications in forensic science

Question 2

What does thin-layer chromatography TLC do

Answer 2

Quick and inexpensive analytical technique that indicates how many components are in a mixture
Use are TLC plate
In TLC the adsorbent is the stationary phase, the different components in the mixture have different affinities for the adsorbant and bind with different strengths to its surface

Question 3

How to carry out TLC

Answer 3

Take a TLC plate using the pencil draw a line across the plate about 1 cm from one end of plate this is baseline
Using capillary tube spot a small amount of solution of sample onto baseline on plate
Prepare chromatography tank for TLC plate, Can be made from small beaker with watch glass placed on top, put some solvent into beaker to depth of 0.5 cm
Please prepare TLC plate in beaker, solvent must not cover spot, Cover beaker with watch glass and leave it undisturbed on bench, solvent will rise up TLC plate
Allow solvent to rise up plate until it is about 1 cm below top of plate, Remove the plate from beaker and immediately mark the solvent front with a pencil, allow paint to dry
If there are any visible spots circle them with a pencil ( can use UV light to see them or chemical to locating ageing like iodine)

Question 4

How can the components be identified in TLC

Answer 4

Each component can be identified by comparing its Rf value with known values using same solvent system and adsorbent

Question 5

Gas chromatography diagram

Answer 5

INSERT PIC

SEE TXTBOOK PG 244

Question 6

What occurs in gas chromatography

Answer 6

Small amount of volatile mixture injected into apparatus called gas chromatograph
Mobile carrier gas carries components in sample through capillary column which contains liquid stationary phase absorbed onto solid support
Components of mixture separated depending on solubility in liquid stationary phase compounds in mixture which detect are at different times depending on interactions with stationary phase in column

Question 7

What is a peak integration

Answer 7

Areas under each peak

Question 8

How is the concentration of the components in a sample determined

Answer 8

By comparing its peak integration (peak area) with values obtained from standard solutions of the component
Prepare standard solutions of known concentrations of compounds being investigated
Obtain gas chromatograms for each standard solution
Plot calibration curve of peak area against concentration, called external calibration and offered a method for converting a peak area into concentration
Obtain gas chromatogram of compounds being investigated under same conditions
Use calibration curve to measure concentration of compound

Question 9

Tests for functional groups

Answer 9

INSERT PIC

SEE TXTBOOK PG 246

Question 10

What is nuclear magnetic resonance NMR

Answer 10

Technique uses a combination of very strong magnetic field and radio-frequency radiation, with the right combination of magnetic field strength and frequency, nuclei of some atoms absorb this radiation, energy for absorption can be measured and recorded as an NMR spectrum

Question 11

Nuclear spin

Answer 11

Electrons have a property called spin nucleus also has a nuclear spin, that is significant if there is an odd number of nucleons ( protons and neutrons)
NMR Is relevant for 1H and 13C, the isotopes with an odd number of nucleons

Question 12

The NMR spectrometer

Answer 12

Radiofrequency radiation has much less energy than the infrared radiation used in IR spectroscopy
Frequency required for resonance is proportional to magnetic field strength and it is only in strong and uniform magnetic fields that this small quantity of energy can be detected
Typically very strong superconducting electromagnetic used

Question 13

How is frequency shift measured

Answer 13

All atoms have electrons surrounding the nucleus which shift the energy and radio-frequency needed for nuclear magnetic resonance to take place frequency shift measured on a scale called chemical shift in units of parts per million (ppm)
TMS is given chemical shift value of 0 ppm
Depending on the chemical environment NMR requires different energy and frequency producing absorbtion peaks at chemical shifts

Question 14

How to run the NMR spectrum

Answer 14

In the spectrometer the sample is dissolved in a solvent and placed in a narrow NMR sample tube with a small amount of TMS
Tube is placed inside NMR spectrometer, and spun to even out any imperfections in magnetic field in sample
Spectrometer is zeroed against TMS standard and sample is given pulse of radiation containing a range of radiofrequencies whilst maintaining constant magnetic field
Any absorbtions of energy resulting from resonance are detected and displayed on a computer
After analysis sample can be recovered by evaporation of solvent

Question 15

What is a deuterated solvent

Answer 15

A deuterated solvent is usually used in which 1H atoms have been replaced by 2H atoms
Deuterium produces no NMR signal In the frequency ranges used in 1H and 13C NMR spectroscopy
Deuterated trichloromethane commonly used as solvent but still produces peak in carbon 13 NMR spectrum, computer usually filters out this peak before displaying spectrum

Question 16

What two important pieces of information does carbon 13 NMR spectrum provide

Answer 16

Number of different carbon environments from the number of peaks
Types of carbon environment present from chemical shift

Question 17

How is the chemical environment of a carbon atom determined by the position of the atom within the molecule

Answer 17

Carbon atoms that are bonded to different atoms or groups of atoms have different environments and will absorb at different chemical shifts
If two carbon atoms are positioned symmetrically within a molecule then they are equivalent and have the same chemical environment, they will absorb radiation at the same chemical shift and contribute to the same peak

Question 18

Proton NMR Provides information about

Answer 18

The number of different proton environments from the number of peaks
The types of proton environments present from the chemical shift
The relative numbers of each type of protons from integration traces or ratio numbers of the relative peak areas
The number of non-equivalent protons adjacent to a given proton from the spin spin splitting pattern

Question 19

What factors may move a peek outside of its ranges

Answer 19

Solvent concentrations and substituents

Question 20

Equivalent and non-equivalent protons

Answer 20

If two or more protons are equivalent they will have absorb at the same chemical shift increasing the size of the peak
Protons of different types have different chemical environments and are non equivalent, they absorb at different chemical shifts

Question 21

Difference between C 13 and H1 relative numbers of each type of atom

Answer 21

In c13 NMR spectrum Peak area is not directly related to number of carbon atoms responsible for peak
In proton NMR ratio of relative areas under each peak gives ratio of number of protons responsible for each peak

Question 22

The NMR spectrometer measures the area under each peak as an

Answer 22

Integration trace

Trace is shown as either Extra line on spectrum or as printed number of relative peak areas

Question 23

What is spin spin coupling

Answer 23

A proton NMR peak can also be split into sub peaks or splitting patterns
These are caused by the protons spin interacting with the Spin states of nearby protons that are in different environments, this provides info about the number of protons bonded to adjacent carbon atoms
Splitting of the main peak into sub peaks is called spin spin coupling or spin spin splitting

Question 24

N + 1 rule

Answer 24

For a proton with any protons attached to an adjacent carbon atom the number of sub peaks in a splitting pattern = n + 1

Question 25

Why do splitting patterns occur in pairs

Answer 25

In NMR spectrum If you see one split in pattern there must always be another, splitting patterns occur in pairs because each proton splits the signal of the other

Question 26

What is the technique called proton exchange

Answer 26

Used to identify -OH and -NH protons
Proton NMR spectrum run as normal
Small volume of deuterium oxide D20 added, mixture shaken, 2nd spectrum is run
Deuterium exchanges and replaces OH and NH protons in sample with deuterium atoms
Eg with methanol:
CH3OH + D2O —> CH3OD + HOD