3.9 - Further Organic Analysis (Chromatography + NMR)

What is the Rf value?

The ratio for how far a substance travels up the stationary phase relative to the solvent front

What are the 3 factors of Rf?

1. Solubility of sample in solvent

2. Retention of sample in the Stationary phase

3. Relative affinities between stationary and mobile phases

How do we observe invisible components of a sample? (2)

1. Spray ninhydrin to dye the samples to a purple hue to make them visible

2. Mix fluorescent substance with stationary phase then shine UV on the dried plate to see invisible spots

Describe the features of Column Chromatography

• Mobile phase (Eluent)

• Sample

• Stationary phase (at the bottom)

What 2 conditions are needed for the movement of the sample(s)?

1. Low retention 

2. High solubility

What type of Stationary + Mobile phases are needed to a polar sample?

1. High solubility = Polar solvent

2. Low retention = Non polar stationary phase

Gas Chromatography

Gas Chromatography

What are the 3 types of Chromatography? (uses)

1. TLC = Identification

2. Column = Separation

3. Gas = Both

Describe a Gas Chromatography setup

• Phases

A thin tube of 0.5 mm width and 100m length is coiled and heated in an oven, where the sample is fed through the tube and detected via chromatography:

• Stationary phase = Powder coated with viscous liquid (like oil) in the tube

• Mobile phase = Non-dissolved Inert gas (He or N)

Why is the Mobile phase an inert gas? (2 reasons)

No affinity, we only consider the affinity of samples + stationary phase:

1. Ensures complete separation of sample ONLY

2. Avoids separation interference from samples which contain various substances with different retention times

Describe the process of Gas Chromatography

1. Sample is vaporised + injected into the tube

2. The sample then separates in the tube within the oven and travels to the detector

3. Samples then contact the detector and forms a gas chromatogram once all samples have reached the end of the tube

How do we determine which is the most abundant in a Gas Chromatogram?

• Axis on the graph

The peak with the greatest area under it (not just height or width but a mixture of both):

• x-axis = Signal from detector

• y-axis = Time

What is Retention Time?

• 2 factors

• What is this for Gas + Column methods

The time taken for how long each component was held back by the stationary phase:

1. Mobile phase solubility

2. Retention in the stationary phase

• Gas = Time taken for sample(s) to reach the detector
  Column = Time taken for sample(s) to elute

What is acts as the Detector for Gas chromatography?

HRMS (High-Res Mass Spectrometer)

NMR Environments

NMR Environments

What are the 2 rules when finding the Number of Carbon environments?

1. Look for symmetry (when single bonds are involved you can rotate then to make a molecule symmetrical)

2. Identical surroundings for atoms

In NMR spectra what do the following indicate:

1. Number of Peaks

2. Peak height

3. Peak shape

1. Number of Peaks = No of Carbon environments

2. Peak height = No relevance

3. Peak shape = No relevance

Define Nuclear Spin (cause)

The overall spin of a nucleus in a magnetic field due to a difference in the number of protons and neutrons, causing a difference in energy:

• The difference in energy forms an energy gap in the field

What are the 2 factors of Nuclear Spin?

1. Magnetic field strength

2. Atoms bonded to the Carbons

Comment on the difference in spin between atoms in the same Carbon environment

No difference, atoms in the same Carbon environment have the same spin

What is the effect on ionising on Nuclear spin? (how)

Increases Spin = Ionising an atom increases spin, decreases shielding effect which decreases the effect of a magnetic field on nuclei

What is the relation between Charge density and Energy gap?

Charge density ∝ 1/Energy gap

Give 2 factors which increase Energy split

1. Increase number of identical atoms in the environment

2. Increase number of different electronetgative atoms

What is the physical process where we determine a substances different environments?

Cast radio waves in a magnetic field + record the frequency that interacts with the sample causing it to switch spin states from lower to higher

What is the spin of C-12?

0, since the number of protons = neutrons

How can spin be detected despite C-12 being abundant?

The few C-13 (1000s) are detected amongst the abundant C-12 (1 million), since C-13 has more protons than neutrons it interacts with radio waves to switch spin states from lower to higher

What are the Graph axis for NMR spec?

• x-axis = Frequency

• y-axis = Intensity

What are the differences in NMR graphs produced from:

• Strong magnets

• Weak magnets

• Conclusion

• Strong = Higher frequencies of interaction

• Weak = Lower frequencies of interaction

• The same sample can product different graphs from magnet strength

How do we get Weak + Strong magnets to give the same values?

We use a standard + plot x values as PPM to calculate the different in shift → making the percentage difference the same

What are Peaks measured in?

PPM (parts per million)

What standard is used for NMR?

• Structure

TMS (Tetramethylsilane):

• Central Silicon atom bonded to 4 methyl molecules

Give 5 reasons why TMS is used as a standard

1. Uncreative with samples

2. Contains one Carbon environment with a low shifted peak

3. Doesn’t interfere with graph due to very low shift value

4. Low BP → Easy to remove

5. Non toxic

When making a structural formula from the following info what 2 things should we remember?

• Number of peaks < Carbon in the molecular formula

1. Branching is in the molecule

2. The molecule may be cyclical (ringed)

In Carbon NMR what do we look for in a molecule’s structure?

We look for “complete” units which are shown in the data booklet

***READ NOTES ON UNKNOWN MOLECULE IDENTIFICATION IN CHEM NOTES 1***

***READ NOTES ON UNKNOWN MOLECULE IDENTIFICATION IN CHEM NOTES 1***

Hydrogen NMR

Hydrogen NMR

What isotope(s) of Hydrogen are detected by NMR?

¹H

How is ¹H detected in Hydrogen-NMR?

When in a magnetic field, Hydrogen-1 nuclei aligns with or against the magnetic field lines giving different alignments causing an energy gap where radio waves promote nuclei to higher energy levels where the frequencies of the waves are absorbed by different hydrogen environments

Give 3 reasons why we use Hydrogen NMR

1. ¹H is abundant

2. Greater analysis of molecule via splitting

3. Less of a sample needed due to high abundance of Hydrogen-1 isotope

What are the 4 rules of Hydrogen-NMR?

1. Hydrogens bonded to the same carbon are in the same environment

2. Hydrogens bonded to carbon in the same carbon environments are in the same environment

3. Hydrogens bonded to Carbon in different Carbon environments are in different environments

4. Hydrogen bonded to O-H once is in its own environment

For Low Res Hydrogen-NMR, what are the following shown from by NMR spectograms:

• No of peaks

• No of Hydrogen atoms in an environment

• No of peaks = No of hydrogen environments

• No of H-atoms = Ratio of areas under the peaks

***PREDICTING ENVIRONMENTS, NO OF PEAK ANALYSIS, AREAS (BOTH) IN CHEM NOTES 1***

***PREDICTING ENVIRONMENTS, NO OF PEAK ANALYSIS, AREAS (BOTH) IN CHEM NOTES 1***

What is the difference between High + Low Res NMR?

High res gives lines not peaks with an area under it

What are the peak names, shapes + ratios for High Res Mass Spec?

• Singlet = 1 line

• Doublet = 2 lines (1:1)

• Triplet = 3 lines (1:2:1)

• Quartet = 4 lines (1:3:3:1)

• Multiplet = 5 lines (N/A)

***IDENTIFYING SPLITTING + H-NEIGHBOURS IN NOTES 1***

***IDENTIFYING SPLITTING + H-NEIGHBOURS IN NOTES 1***

What is the n+1 rule?

Where the n= number of neighbouring H and we add 1 to get the number of peaks shown for that hydrogen environment

How many Hydrogens are in the following:

• Singlet

• Doublet

• Triplet

• Quartet

• Multiplet

• Singlet = 0

• Doublet = 1

• Triplet = 2

• Quartet = 3

• Multiplet = 4+

***SPLITTING WITH EQUIV H IN CHEM NOTES 1***

***SPLITTING WITH EQUIV H IN CHEM NOTES 1***

What type of Splitting do N-H and O-H observe?

Singlets

***EXAM TECH IN CHEM NOTES 1***

***EXAM TECH IN CHEM NOTES 1***

What is the Standard used in H-NMR?

• How is it processed to be useable? (2)

TMS:

• Dissolve the TMS in a solvent to:
  1. Give clear peaks
  2. Use less of the TMS

What are the 3 conditions when Using TMS in H-NMR?

1. Liquid

2. Inert + not Hydrogen atoms in sample

3. Solvent can have deuterium (Hydrogen-2 isotope ²H) which is non spin active