structural determination

Question 1

How NMR works

Answer 1

Atomic nuclei with odd number of protons / odd mass number have an overall spin (in even number it cancels).
A nucleus with spin can be detected using Nuclear Magnetic Resonance (NMR) spectroscopy. A spinning
nucleus, such as ‘H, behaves as a moving charge and generates a magnetic field, like a bar magnet.
When this nucleus is placed in an externally applied magnetic field it can align with, or against, the field. The flips between these two states can be identified through the absorption / emission of em radiation - as specific amount of energy to flip alignment which corresponds to a specific frequency of EM radiation (e=hf)

Question 2

C-13 NMR spectroscopy

Answer 2

About 1% of all carbon atoms are the C-13 (?3C) isotope (which has nuclear spin); the rest is mostly carbon12, which does not have nuclear spin. Carbon NMR relies on the magnetic properties of the few C-13 nuclei.

Question 3

effect of carbon’s environment on frequency at which resonance is achieved

Answer 3

The more electronegative the attached elements, the further electrons are pulled away from the carbon, the more the atom is deshielded, and hence the atom will require a larger amount of energy (higher frequency of EM radiation) to achieve resonance.

Question 4

(butan-1-ol) spectrum

Answer 4

Question 5

ethyl ethanoate spectrum

Answer 5

Question 6

TMS

Answer 6

This zero point is where you would find a peak due to the carbon-13 atoms ina
compound called tetramethylsilane - usually called TMS. Everything else is compared with this.

You will find that some NMR spectra show the peak due to TMS (at zero), and others leave it out. If you need to analyse a spectrum which has a peak at zero, you can ignore it because that’s the TMS peak.

Question 7

why is TMS chosen as the standard

Answer 7

° All carbon atoms are in exactly the same environment. This produces a single, strong peak.
° Silicon is less electronegative than carbon. This means that the electrons in the C-Si bonds are closer
to the carbons in this compound than in almost any “normal” organic molecule. As a consequence,
the signal from TMS is furthest to the right, out of the way of your sample signals.
° TMS is inert — so doesn’t react with sample.
° It is non-toxic, and it is volatile — so sample can be reclaimed after NMR.

Question 8

ho w NMR scale is defined in chemical shift (delta)

Answer 8

Question 9

Solvents for NMR spectroscopy

Answer 9

A commonly used solvent is CDCl3. This is a trichloromethane (chloroform) molecule in which the normal hydrogen has been replaced by its heavier isotope, deuterium.
This is used as there are no protons to confuse with the H NMR spectra

Question 10

Proton NMR

Answer 10

Proton NMR is similar in principle to carbon-13 NMR but looks at hydrogen instead. The integration trace says the number of hydrogens in the environment

Question 11

spin spin splitting

Answer 11

Question 12

Proton NMR - Some common signais to look out for:

Answer 12

Question 13

Answer 13

Question 14

Chromatography - what is a stationary and mobile phase?

Answer 14

mobile phase is where molecules can move
stationary phase is where molecules don’t move

Question 15

Paper chromatography

Answer 15

Question 16

relative affinity

Answer 16

How much of a substance is likely to be in either the mobile or stationary phase.

Can be affected by the solvent used (mobile phase) or the materials used for stationary phase

substances with similar polarity have high affinity for each other.

The substance with the highest R; value was attracted most strongly to the Mobile phase but was only
weakly attracted to the stationary phase. Therefore, it travelled a substantial distance up the paper. e.g. if both solvent and substance is polar and stationary phase is non polar.

It is important to note that it is the relative affinity for the two phases that is important. A compound with
an R¢ value of 0.5 might be weakly attracted to both phases, or strongly attracted to both phases.

Question 17

Thin layer chromatography (TLC)

Answer 17

Thin layer chromatography is very similar to paper chromatography. The main difference is that the
stationary phase is a sheet of glass or plastic that has been coated in a thin layer of a solid such as silica
(silicon dioxide) or alumina (aluminium oxide). This makes the chromatograms easier to set up and handle.

Unlike the dyes used at GCSE, most organic compounds are colourless. Chromatography can still be used, but the resulting spots must be visualised using ultraviolet light (for aromatic compounds) or by spraying with a chemical (e.g. ninhydrin), which reacts
with the organic compounds to form coloured products.

Question 18

column chromatography

Answer 18

Question 19

Gas chromatography

Answer 19

Question 20

Gas chromatography — mass spectrometry (GC-MS)

Answer 20