2.6 Chemical Analysis Flashcards
What did chemical analysis start of with?
Chemical tests - can show if a functional group or ion is present
Hard to identify the whole structure
How is elemental composition of a substance used in chemical analysis?
Tells us molecular formula but doesn’t tell us 3d arrangements of atoms structure
Substance is burned in oxygen and then the gas emitted is accurately weighed
What does Mass spectrometry tell us?
What the formula of the molecule is
What does chromatography show?
Purity and…
Retention time - can be used to identify sample using a standard
UV-VIS spectroscopy
Can quantify the amount of material present
Based on how different functional groups absorb UV/visible light from the spectrum
What other type of spectroscopy is infrared similar to? And what is the difference?
UV-VIS
Infrared - inspects how different bonds in a molecule absorb light as the molecule moves
What do lines on the IR spectrum show?
Different functional groups
What is scissoring movement in a water molecule?
When the hydrogens move closer to one another - both at the same time
What are the strongest bonds in IR spectrum?
Asymmetric tretches
What is a symmetric stretch
When the bonds are moving in the same direction at the same time (bonds are cheering)
What is asymmetric stretch?
When one bond gets longer whilst other gets shorter (seen in water, primary amines etc)
What is the IR spectrum below 1500 referred as?
Fingerprint region
What is the intensity of an IR absorption dependant on?
Whether there is a dipole or not
And number of the bonds
Where is an alkane seen on IR spectrum?
C-H bonds between 2800 and 3000
No other functional groups/peaks
What does a terminal alkyne IR look like?
- normal C-H stretches 2800-3000
- C-H bond at the end of terminal alkyne ~3300
- C≡C seen at ~2200
What would be different in an internal alkyne IR to terminal one?
We wouldn’t see terminal C-H at ~ 3300
C≡C peak would be smaller due to increased symmetry
What is the characteristics of an Alcohol IR spectrum?
Very large broad O-H peak, above the C-H region @ ~3400
What is the characteristics of a primary amine?
Signal with 2 peaks above the C-H region
NH2 produces 2 stretches
How is a secondary amine IR spectra different to a primary one?
Only one signal/peak above the C-H region and a bit to the right (less polar)
What can be seen in the IR of a carboxylic acid?
Very strong C=O peak @ ~1700
Combined with O-H broad signal
What distinguishes a carboxylic acid IR spectrum from other carbonyls?
The OH stretch
Aldehydes and ketones dont have OH broad signal
What is the difference between peaks on an IR and NMR spectrum?
IR - peaks are upside down
NMR - peaks are right way up
What does the simplest form of NMR look at?
The invironment of protons (¹H) in an individual molecule
What does the number of signals on an nmr spectrum tell us?
The different environments that hydrogen has within the molecule
Why is hydrogen good to be looked at on nmr?
¹H isotope is very abundant - 99.98%
Makes it easy to get a good signal to see hydrogen
Which type, apart from hydrogen nmr, is used to analyse structure?
¹³C - 1% abundant
nmr signal is weaker due to lower abundance
Longer time and stronger magnets required to produce good signal
What 4 factors are used to identify a molecule using nmr?
- Number of signals
- Chemical shift
- Integration
- Coupling
Which nucleus spin is easiest to work with?
1/2 as it is nmr active
What value of spin do both an individual proton and neutron carry?
1/2
What values of overall (proton and neutron combined) spins give a nmr active nuclei?
Half integers (1/2, 3/2….)
Combination of odd and even proton and neutron or vise versa
What are the tow possible energies when nucli with an overall spin 1/2 are in presence of an external magnetic field
m = 1/2 (aligned with magnetic field)
m = -1/2 (against applied magnetic field) - higher in energy
What happens when a magnetic field is applied during nmr?
Nucli are split into energy levels (half and minus half)
Which state are nucli more abundant in when magnetic field is applied?
m = 1/2 (the lower energy state)
What does the addition of a radio-frequency pulse do in nmr?
Flips the spin on some of the lower energy nucli so they are in higher energy state (m=-1/2)
When can the radio frequency signal be no longer increased in nmr?
When all the low energy nucli’s spins are flipped to the higher energy level
This is referred to as sample saturation
The system in higher energy level is more….
Unstable
What do the nucli do as a result of the instability?
Interact with their surroundings in different forms to lose energy and go back to lower energy state
What happens as the nucli go back to their original energy state?
They emit energy which is picked up a detector in the nmr machine and then processed through computer to generate a nmr spectrum
What does the position of signals on the x axis of an nmr spectrum tell you?
The environment. Eg. electrons around the, orbitals electrons are in
What does each line on a carbon spectrum indicate?
Each unique carbon environment
What is the order of ¹H chemical shifts from upfield to downfield?
- Hydrogens which aren’t near anything electronegative (0-2)
- Then it’s the H on a carbon that is bonded to something electronegative (3-5)
- H on double bonds or aromatic rings (6-8)
- H of aldehydes (9-10)
What is the order of ¹³C Chemical Shifts from upfield to downfield?
- Carbons just bonded to H and other C (0-30)
- C attached to electronegative atoms eg. O, N and halogens (40-80) goes further left with increasing electronegativity
- Sp2 hybridised carbons - double bonds and aromatic rings (100-160)
- Carbons in carboxylic acids (160-180)
- Carbons in aldehydes and ketons (200-220)
Why does carbons have a larger effect on the magnetic field experienced by the nucleus?
Carbon has p orbitals for its electrons - wider range on x axis
(0-200+)
What does the number of signals depend on?
Number of unique environments in the molecule
What is rotation possible in?
Single/sigma bonds
What does area under the curve indicate?
Integral - number of atoms producing that signal
Describe Toluene
Symmetrical molecule
5 carbon environments
4 hydrogen environments
What is coupling?
When nuclei interact with one another
Learn coupling names
1-7
Singlet, doublet, triplet, quartet, quintet, sextet, spetet
How is coupling constant calculated?
Distance between 2 lines (Hz)
Bigger coupling constant = bigger interaction
What is the general formula to work out the number of coupling lines?
(N+1)
Where N = number of identical neighbours
What 2 factors cause splitting?
- Interaction with neighbours magnetic field
- External magnetic field
What is the distance between lines with 2, 3 and 4 bond coupling?
2 = 12-15Hz
3 = ~ 7Hz
4 = 1-3Hz
2 identical neighbours =
Triplet (3 lines)
Does ¹³C spectrometry have coupling?
ABSOLUTELY NOT
What is the maximum number of bonds away a H can be to be a neighbour?
3 bonds
