Synthesis and Analysis

Question 1

How does mass spectrometry work?

Answer 1

Electrons in the spectrometer bombard the sample molecules and break electrons off, forming ions with different mass to charge ratios.

Question 2

How do find the RMM of a compound from it’s mass spectrum?

Answer 2

Look at molecular ion peak (M peak), usually the one with the second highest mass charge ratio.

Question 3

What is the M+1 peak and why is it there?

Answer 3

The peak 1 to the right of the M peak. Mostly due to the carbon-13 isotope which exists naturally and makes up about 1.1% of the carbon.

Question 4

What can you use the M+1 peak for? What is the formula?

Answer 4

To find out how many carbon atoms there are in the molecule.
number of C atoms in organic compound= height of M+1 peak/height of M peak X 100

Question 5

What is the M+2 peak and when does it occur?

Answer 5

If a molecule’s got either chlorine or bromine in it you get an M+2 peak.
It’s because chlorine and bromine have natural isotopes with different masses and they all show up on the spectrum.

Question 6

Describe the peaks of compounds containing chlorine.

Answer 6

Chlorine’s two isotopes, Cl-35 and Cl-37, occur in the ratio of 3:1. So if a molecule contains chlorine, it will give an M peak and an M+2 peak with heights in the ratio 3:1

Question 7

Describe the peaks of compounds containing Bromine.

Answer 7

Bromine’s got two isotopes, Br-79 and Br-81, that occur in equal amounts. So if a molecule contains bromine, the M peak and M+2 peak will both have the same height.

Question 8

When would you see an M+4 peak?

Answer 8

When a molecule contains both halogens.

Question 9

What is fragmentation?

Answer 9

In the mass spectrometer, the bombarding electrons make some of the molecular ions break up into fragments. The fragments that are ions show up on the mass spectrum , making a fragmentation pattern. Fragmentation patterns can be used to identify molecules and even their structure.

Question 10

How do you work out the structural formula from mass spectra?

Answer 10

Got to work out what ion could have made each peal from its m/z value.

Question 11

When is a fragment ion more abundant? What are two examples of stable fragment ions?

Answer 11

More stable.

Carbocations and acylium ions.

Question 12

What is a carbocation?

Answer 12

An ion with a positively charged carbon.

Question 13

Why are carbocations relatively stable?

Answer 13

Because alkyl groups feed electrons towards the +ve charge. You can show that an alkyl group is donating electrons by drawing an arrow on the bond that points to where the electrons are donated.

R–>–C+H2

Question 14

What carbocation is the most stable?

Answer 14

Tertiary- 3 alkyl groups. More electrons donated to stabilise carbocation.

Question 15

What is the acylium ion (RCO+) often formed from?

Answer 15

Aliphatic ketones.

Question 16

What are the two forms of the acylium ion and what are they called?

Answer 16

RESONANCE FORMS

R-C+=O

R-C=O+ (triple C=O bond)

Actual structure somewhere in middle.

Question 17

What does resonance in an ion do?

Answer 17

Helps stabilise what would otherwise be an unstable structure.

Question 18

What are the two types of nuclear magnetic resonance (NMR) spectroscopy?

Answer 18

13C NMR- information about carbon atoms in a molecule are arranged.

1H(proton) NMR- How hydrogen atoms in a molecule are arranged.

Question 19

When does an atomic nucleus have spin?

Answer 19

When it has an odd number of nucleons (protons and neutrons) in its nucleus. Causes it to have a weak magnetic spin.

Question 20

What does NMR spectroscopy look at?

Answer 20

How the small magnetic field created by nuclear spin reacts when you put in a much larger external magnetic field.

Question 21

What is the effect of the external magnetic field on the nuclei?

Answer 21

Normally the nuclei are spinning in random directions- so their magnetic fields cancel out.
When a strong external magnetic field is applied, the nuclei will all align either with the field or opposed to it. The nuclei aligned with the external field are at a slightly lower energy level than the opposed nuclei.

Question 22

How do radio waves affect the energy level of the nuclei?

Answer 22

Radio waves of the right frequency can give the nuclei that are aligned with the external magnetic field enough energy to flip up to the higher energy level.

The nuclei opposed to the external field, can emit radio waves and flip down to the lower energy level.

Question 23

Why is there an overall absorption of energy?

Answer 23

To start with, there are more nuclei aligned with the external field. NMR spectroscopy measures this absorption.

Question 24

How do different environments affect the nuclei?

Answer 24

A nucleus is partly shielded from the effects of an external magnetic field by its surrounding electrons.

Any other atoms and groups of atoms that are around a nucleus will also affect the amount of electron shielding.

So the nuclei in a molecule feel different magnetic fields depending on their environments. This means that they absorb different amounts of energy at different frequencies.

Question 25

Nuclei in different environments absorb energy of different frequencies, NMR spectroscopy measures these differences relative to a standard substance. What is the difference called?

Answer 25

Chemical Shift

Question 26

What is the standard substance used to measure the differences in frequencies? Why is it used?

Answer 26

Tetramethylsilane (TMS)
Si(CH3)4
This molecule has 12 hydrogen atoms all in identical environments and 4 carbon atoms all in identical environments. This means that, in both 1H NMR and 13C NMR, it will produce a single absorption peak, well away from most other absorption peaks.
Given a chemical shift value of 0

INERT so doesn’t react with sample, non toxic and volatile (easy to remove)

Question 27

What is chemical shift measured in?

Answer 27

Part per million (ppm) relative to TMS

Question 28

Describe 13C NMR spectrums.

Answer 28

Spectrum will have one peak on it for each carbon environment in the molecule. The carbon atoms which are attached to more electronegative atoms (e.g. oxygen, nitrogen or chlorine) will be less shielded and so have a higher chemical shift.

Question 29

Describe 1H NMR spectrums

Answer 29

Each peak on a 1H NMR spectrum is due to one or more hydrogen nuclei (protons) in a particular environment. The relative are under each peak also tells you the relative number of H atoms in each environment.

Question 30

What is an integration trace?

Answer 30

1H NMR spectra can get quite cramped and sometimes it’s not easy to see the ratio of the areas- so an integration trace is often shown. The height increases shown on the integration trace are proportional to the areas of the peaks.

Question 31

Describe splitting patterns. (n+1 rule)

Answer 31

The peaks on a 1H NMR spectrum may be split into smaller peaks (spin-spin splitting). Peaks always split into the number of hydrogens on the neighbouring carbon, plus one. Called the n+1 rule.

Question 32

What are deuterated solvents?

Answer 32

Their hydrogen atoms have been replaced by deuterium (D or 2H).

Example- CCl3D OD2

Question 33

Why are proton free solvents used?

Answer 33

If a sample has to be dissolved, then a solvent is needed that doesn’t contain and 1H atoms, because they would show up on spectrum and confuse things.

Question 34

What is Deuterium?

Answer 34

An isotope of hydrogen that’s got two nucleons (a proton and a neutron).

Question 35

Why doesn’t deuterium have nuclear spin and therefore doesn’t have a magnetic field?

Answer 35

Has an even number of nucleons.

Question 36

What do you need to look for when analysing 1H NMR spectra?

Answer 36

• Number of peaks tells you how many different hydrogen environments there are in your compound.
• The ratio of the peak areas tell you about the relative number of hydrogens in each environment. Sometimes these ratios are written above the peaks, otherwise use integration traces.
• You can use the chemical shift of each peak to work out what type of environment the hydrogen is in.
• The splitting pattern of each peak tells you the number of hydrogens on the adjacent carbon. n+1 rule.

Question 37

How does infrared spectroscopy work?

Answer 37

A beam of IR radiation is passed through a sample of a chemical. The IR radiation is absorbed by the covalent bonds in the molecules, increasing their vibrational energy. Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too- so the O-H group in an alcohol and the O-H in a carboxylic acid absorb different frequency.

Question 38

What is wavenumber?

Answer 38

1/wavelength.

Question 39

What is an infrared spectra?

Answer 39

Shows what frequencies of radiation the molecules are absorbing.

Question 40

What is chromatography used for?

Answer 40

To separate out substances in a mixture.

Question 41

What is the mobile phase of chromatography?

Answer 41

Where the molecules can move. Always a liquid or a gas.

Question 42

What is the stationary phase of chromatography?

Answer 42

Where the molecules can’t move. Must be a liquid, or a liquid held in a solid.

Question 43

When is column chromatography mostly used and why is it needed?

Answer 43

For purifying an organic product. This needs to be done to separate it from unreacted chemicals or side products.

Question 44

How does column chromatography work?

Answer 44

It involves packing a glass column with a solid, absorbent material such as aluminium oxide coated with water (slurry). STATIONARY PHASE.
The mixture to be separated is added to the top of the column and allowed to drain down into the slurry. A solvent is then run slowly and continually through the column. This solvent is the mobile phase.

As the mixture is washed through the column, its components separate out accordingly to how soluble they are in the mobile phase and how strongly they are absorbed onto the stationary phase (retention).
Each different component will spend some time absorbed onto (stuck to) the stationary phase and some time dissolved in the mobile phase.

The longer a component spends dissolved in the mobile phase, the quicker it travels down the column. If a component spends a long time absorbed to the stationary phase, it will take a long time to travel down the column. So, the more soluble a component is in the mobile phase, the quicker it’ll pass through the column.

As a component of the mixture reaches the end of the column it is collected. It can then be identified using the time taken to pass through the column (retention time) or another technique.

Question 45

When do you use gas-liquid chromatography?

Answer 45

If you’ve got a mixture of volatile liquids.

Question 46

What is the stationary phase of GLC?

Answer 46

Viscous liquid, such as an oil, which coats the inside of a long tube. This tube is coiled to save space, and built into an oven.

Question 47

What is the mobile phase of GLC?

Answer 47

An unreactive carrier gas such as nitrogen.

Question 48

How does GLC work?

Answer 48

The sample is vaporised and passed through the oven as a gas.
Each component takes a different amount of time from being injected into the tube to being recorded at the other end. This is the retention time.
You can use the retention time to identify the components of the mixture.

Question 49

What are the retention times for components in a mixture shown on?

Answer 49

A chromatogram.

Question 50

What does the retention time depend on?

Answer 50

How much time the component spends moving along with the carrier gas, and how much time it spend stuck to the viscous liquid.

Question 51

How do you read a chromatogram?

Answer 51

The area under each peak tells you the relative amount of each component that’s present in the mixture.

Question 52

What can GLC be used for?

Answer 52

To find the level of alcohol in blood or urine.

Also used to find the proportions of various esters in oils used in paints- picture restoration.