Unit 2.8 - Instrumental Analysis Flashcards
What does infrared spectroscopy correspond to?
Heat
What can infrared spectroscopy do?
Detect the functional groups present in organic molecules
Describe atoms in a molecule
Not static
Constantly vibrating
What condition must apply for a bond to vibrate as opposed to break?
The energy put into the bond must be less than the bond enthalpy
What are molecular vibrations stimulated by?
Absorbing radiation of the same frequency as the natural vibrational frequency of the bond
Where are the frequencies that correspond to the natural vibrating frequency of most bonds?
In the infrared region of the electro magnetic spectrum
Why does infrared radiation cause bonds to vibrate?
It has frequencies that are the same as the natural vibrational frequency of the bond
Name for the frequencies at which bonds vibrate
Absorptions
Absorptions
The frequencies at which bonds vibrate
What does each bond do at a particular frequency?
Absorbs
What does each bond absorb at?
A particular frequency
At a greater mass, describe the frequency needed for the bond to absorb
Lower frequency
With a weaker bond, describe the energy needed for the bond to absorb
Lower frequency
What can different frequencies of infrared radiation cause bonds to do?
Vibrate in different ways
What causes bonds to vibrate in different ways?
Different frequencies of infrared radiation
2 ways in which bonds can vibrate
Stretching
Bending
Diagram to represent bond stretching
(See notes)
Diagram to represent bond bending
(See notes)
Which 2 conditions must apply for infrared radiation to be absorbed?
Only vibrations…..
Resulting in a change in the dipole of the molecule
Having frequencies in the infrared region
….will absorb infrared radiation
What is absorption stated in terms of and why?
Wave numbers
The numerical values of frequency and wavelength in the infrared region are not easy to handle
Wave number
The reciprocal of wavelength in cm
Wave number unit
cm^-1
The absorption of what occurs in the 4000-2500cm^-1 region of the infrared spectrum?
Single bonds to hydrogen
O-H, N-H, C-H
The absorption of what occurs in the 2500-2000cm^-1 region of the infrared spectrum?
Triple bonds
C=N
The absorption of what occurs in the 2000-1500cm^-1 region of the infrared spectrum?
Double bonds
C=C, C=O
The absorption of what occurs in the 1500-400cm^-1 region of the infrared spectrum?
Other absorptions
C-Cl
What can also affect absorptions?
Neighbouring groups
Give an example of neighbouring groups affecting absorptions
The OH group in an alcohol reacts different to the OH group in a carboxylic acid, where it’s next to a C=O group
Functional group and absorbance of the C=O bond
Carbonyl
Strong (>80%) and sharp
Functional group and absorbance of the O-H bond
Alcohol
Strong (>80%) and broad
Also, carboxylic acid
Medium (50-75%) and very broad
Functional group and absorbance of the C-O bond
Alcohol, ester, carboxylic acid
Generally strong and sharp
Functional group and absorbance of the C-H bond
Any molecule
Depends upon the molecule
How can we differentiate between an alcohol and carboxylic acid in the infrared spectrum?
Alcohol absorbance = strong (>80%) and broad
Carboxylic acid = medium (50-75%) and very broad
Describe the absorptions occurring that are strong and sharp
Occur over a small range of frequencies
Describe the absorption occurring that are medium and very broad
Occur over a large range of frequencies
What can infrared plectra give information about?
Which functional groups are present in a molecule
Can we rely on the infrared spectrum alone to determine the structure of a molecule?
No, other information is required
Name 4 types of molecules that contain the carbonyl (C=O) functional group
Aldehydes
Ketones
Carboxylic acids
Esters
C=O bond nam
Carbonyl
Carbonyl functional group bond
C=O
How does the % transmittance axis appear on an infrared spectrum?
100% at the top, 0% at the bottom
Why does the % Transmittance axis on an infrared spectrum have 100% at the top and 0% at the bottom?
Appears this way when the detector is set to calculate how much energy passed through the molecule at each frequency
How does the % absorbance appear on an infrared spectrum?
0% at the top, 100% at the bottom
Why does the % absorbance axis on an infrared spectrum have 0% at the top and 100% at the bottom?
Appears this way when the detector is set to calculate how much energy was absorbed at each frequency
What goes along the axes on an infrared spectrum?
Wave number against % transmittance/absorbance
What does the “fingerprint” region of an infrared spectrum contain?
Lots of peaks caused by the many C-C and C-H absorption that occur within a hydrocarbon
Why does the fingerprint region of the infrared region contain lots of peaks?
Caused by the many C-C and C-H absorptions that occur within a hydrocarbon
Which region of the infrared spectrum needs a lot of analysis to be useful?
The fingerprint region
Where do most organic compounds peak on an infrared spectrum and why?
Approximately 3000cm^-1 due to absorption by C-H bonds
Why do most organic compounds peak at approximately 3000cm^-1?
Due to absorption by C-H bonds
What can be identified using infrared spectrums and how?
Functional groups
By determining whether or not the peaks characteristic of the bonds present in that functional group are present in the infrared spectrum
What’s an indication of an O-H bond in an alcohol on the infrared spectrum?
Broad and sharp peak in the region of 3200-3550cm^-1
What’s an indication of an O-H bond in a carboxylic acid on the infrared spectrum?
A very broad and medium peak in the region of 2500-3500cm^-1
Conditions for ethanol (alcohol) and ethanoic acid (carboxylic acid) to react as an esterification
Presence of concentrated sulfuric acid
Mixture is warmed
What features would be present on the infrared spectrum of an ester?
Peak in the C=O and C-O region
Very broad OH peak of a carboxylic acid is now missing (no O-H bonds are present)
What would we have to do first before using IR spectroscopy to determine if an unknown alcohol is a primary or secondary alcohol?
Oxidise the alcohol under reflux
Take the IR spectrum of the product of this
What would a primary alcohol’s IR spectrum show?
Carboxylic acid O-H peak
Peak corresponding to a carbonyl group
What would a secondary alcohol’s IR spectrum show?
Only a carbonyl peak
Difference between the IR spectrum’s of primary and secondary alcohols
Primary
Carboxylic acid O-H peak
Peak corresponding to a carbonyl group
Secondary
Only a carbonyl peak
What’s an example of a practical application of IR spectroscopy
Modern breathalysers in police stations
How do breathalysers use infrared spectroscopy?
When a sample of breath is given, it is tested for the ethanol OH peak, as there is water vapour in the breath sample as well that can interfere with the alcohol OH peak
So, the amount of alcohol is determined by looking at the relative % composition of the C-H peak and some computer calculations
What can mass spectroscopy be used for?
Identifying different elements
Finding the relative isotopic masses of elements
Identify the molecular mass of a molecule and fragment ions from the molecule
What is used in conjunction to other information to determine the molecular formula of an unknown molecule?
The molecular mass of a molecule and fragment ions from the molecule
What other information as opposed to the molecular mass of a molecule and fragment ions from the molecule is required to work out the molecular formula of an unknown molecule?
Combustion analysis or infrared spectroscopy
What’s the order of the events in the mass spectrometer?
Vapourisation
Ionisation
Acceleration
Deflection
Detection
What type of compound’s mass spectrum contains a number of peaks?
A carbon compound
What is the molecular mass of a compound on a mass spectrometer equal to?
The peak with the highest numerical value
Where is the molecular mass of a compound found on the mass spectrometer?
The peak furthest to the right
What’s the peak furthest to the right of the mass spectrometer known as and why?
The molecular ion peak
It represents the Mr of the compound
What does the peak furthest to the right on the mass spectrometer represent?
The Mr of the compound
Under which circumstance will there be 2 peaks as the molecular ion peaks on the mass spectrum and why?
When the compound contains chlorine
Due to the 2 isotopes 35Cl and 37Cl
The height of the 2 peaks, 35Cl:37Cl will be in the ratio 3:1
What will the ratio between peaks 35Cl and 37Cl be?
3:1
Structural elucidation
The process of determining the chemical structure of a compound
The process of determining the chemical structure of a compound
Structural elucidation
What type of compounds may be used in the exam for us to determine its chemical structure using the mass spectrometer?
Cyclic compounds
Compounds with up to 6 carbon atoms
Elements C, H, O, N and Cl
What does each peak represent on the mass spectrometer?
An ion
What does the height of the peak represent on a mass spectrum?
The abundance of the ion
What forms when an atom is bombarded with high energy electrons in the ionisation chamber?
Molecular ions
What is a molecular ion?
A carbocation formed when an atom is bombarded with high energy electrons in the ionisation chamber of the mass spectrometer
How is a molecular ion produced in the ionisation chamber of a mass spectrometer?
The compound will lose 1 electron totform a positive carbocation(known as a radical ion as it has both positive charge and an unpaired electron
This carbocation formed is called the molecular ion
What is the significance of the molecular ion formed in the ionisation chamber of the mass spectrum?
Has a peak on the far right of the mass spectrum, allowing us to identify the molecular mass of the compound
What is the value of m/e of the two chlorine isotopes as their molecular ion peaks?
92 and 94
Describe the molecular ion formed in the ionisation chamber and why is this significant?
Energetically unstable
Will fragment, breaking into smaller fragment ions and radicals
What fragment to form smaller fragment ions and radicals and why?
Molecular ions
Energetically unstable
Can we see the smaller fragments that result from the energetically unstable molecular ions? Why?
Yes
They can be accelerated and deflected by the magnetic field (radicals do not appear)
Why do radicals formed following the fragmentation of the energetically unstable molecular ion not appear as peaks on the mass spectrometer?
They can’t be accelerated and deflected by the magnetic field
How are ions produced during the fragmentation of the energetically unstable molecular ions?
Produced by breaking C-C bonds
What is each peak to the molecule?
A unique fragment of it
What does the set of peaks make up for a molecule?
Energy the fingerprint of the molecule
How can we identify a molecule using a mass spectrometer?
Using the value of m/z corresponding to that of the molecular ion and a spectral database that lists all known fragment ions
What does adding new compounds to the database increase the opportunity for?
Fingerprinting
Which carbocations have the most abundant peaks in the mass spectrometer?
The more stable carbocations
What do more stable carbocations have on the mass spectrometer?
More abundant peaks
Is the molecular ion the most abundant ion in a compound why?
No
It will fragment and there are other more stable carbocations with more abundant peaks in the spectrum
NMR spectroscopy
Nuclear magnetic resonance
What causes resonance?
Absorption of energy
What does NMR spectroscopy correspond to?
Energy being absorbed to cause a change within molecules
What is the energy absorbed that causes a change in molecules in NMR spectroscopy for?
To reverse the spin of the nucleus of the atom within a magnetic field
What does the chemical shift show?
The energy absorbed to cause a change within molecules
What is NMR spectroscopy the basis for?
MRI cscans in medicine
What technique is the basis of MRI cscans in medicine?
NMR spectroscopy
Wha is an MRI scan sensitive to?
Hydrogen in water to give information about internal organs
What are protons and neutrons known as?
Nucleons
What is a property that nucleons have?
Spin, which can be in 2 opposite directions
When does a nucleus behave like a tiny magnetic?
When there is an unpaired electron (any nucleus with an odd atomic number or odd atomic mass)
What’s significant about protons (1H) and (13C)?
They’re nuclei with unpaired electrons, so they behave like tiny magnets
What does the fact that 1H and 13C behave like tiny magnets mean for them?
In an applied magnetic field, these nuclei can either line up in the same direction or in the opposite direction to the applied field
What’s significant about the way that the nuclei of 1H and 13C align themselves when they align themselves either in the same direction or in the opposite direction to an applied field?
There is a small energy difference between these 2 alignments
How is energy supplied in an NMR spectrometer?
As low energy radio frequency radiation
What happens to nuclei in the presence o the low energy radio frequency radiation supplied during NMR spectroscopy?
Absorbed - excitation a they go from the lower energy state to the higher energy state
Exactly the same energy is lost later (relaxation)
What’s the name for the process of excitation and relaxation of nuclei with energy?
Resonance
What does the magnetic field felt by a nucleus also depend upon?
Its enviornment
What do we mean by the “enviornment” of a nucleus?
Atom bonded to different atoms or groups
Chemical shift
The place on an NMR spectrum where the nucleus absorbs the radiation
Unit of chemical shift
ppm (parts per million)
What happens to the sample that is investigated during NMR spectroscopy?
Placed inside a large applied magnetic field
Irradiated with radio frequency radiation
When is radiation absorbed (chemical shift)?
If it corresponds to the energy gap between the 2 ways that the nuclei are aligned
What is the chemical shift measured relative to?
A reference chemical …
Tetramethylsilicane Si(CH3)4
What is tetramethylsilicane used as?
A reference chemical for the chemical shift values
Why is tetramethylsilicane used as the reference chemical for chemical shift values?
Has 12 identical protons
So,it Gives a strong signal at 0ppm
Under which condition of NMR usually carried out?
In solution
What’s the issue with regular solvents that would be used for NMR spectroscopy?
Most solvents contain carbon and hydrogen and so would produce signals on an NMR spectrum
What is used to avoid the signals produced by carbon and hydrogen that’s usually in the solvents used for NMR spectroscopy?
Deuterium
Deuterium
Isotope of hydrogen with 1 proton and 1 neutron
Why are solvents containing deuterium instead of hydrogen used for NMR spectroscopy?
Has 1 proton and 1 neutron
Therefore, has an equal number of nucleons = does not show up on an NMR spectrum
What’s the most common solvent used in NMR spectroscopy and why?
CDCL3
Although is does show up on the spectrum, the resulting peak is easily removed
What contains a small % of the 13C isotope?
Naturally occurring carbon
What does the presence of atom of the 13C isotope in naturally occurring carbon mean?
Means that organic compounds will absorb energy and 13C NMR spectra can be produced
What goes along the x-axis for 13C spectroscopy?
Chemical shifts
What goes along the y-axis for 13C spectroscopy?
Absorption
What does the number of peaks on the 13C NMR spectrum represent?
The number of different carbon environments
What does the chemical shift value of a 13C NMR spectrum represent?
The environment of each carbon
What is meant by the environment of an atom?
Atom is bonded to different atoms or groups
What makes a carbon be in a different Enviornment to another?
Next to a different atom, so it’s not chemically equivalent
What is a carbon atom to another one if it’s bonded to a different atom?
Not chemically equivalent
What’s the stages in identifying the structure of a molecule from he 13C NMR spectrum of it?
- Identify the number of carbon environments
- Look at the chemical shifts to find the environments
- Structure - draw all possible structural isomers. Identify the number of 13C environments in each structure.
What’s it important not to ignore when counting hydrogen environments?
The OH group
What’s 1H spectroscopy also known as?
Proton spectroscopy
What information is obtained form 1H NMR spectroscopy that’s similar to 13C NMR spectroscopy?
Number of peaks - number of different proton environments
Chemical shift value - gives the Environment for each proton
What’s the additional information we obtain using 1H Spectroscopy?
Relative peak areas - ratio of the numbers of protons in each environment
Peak splitting - helps determine the position of a proton environment within the molecule (more in A2)
What do relative peak areas/height of peaks show on H1 NMR spectroscopy represent?
Ratio of the numbers of protons in each environment
What’s shows the Ratio of the numbers of protons in each environment in H1 NMR spectroscopy?
Relative peak areas
Where will peak areas information be given?
Above each peak
Given in the information table
Labelled as integration
What does comparing the integrals or areas for each peak on a H1 NMR spectrometer give us information about?
The relative numbers of protons in each environment
What will the relative peak areas be in the ratio of for 1CH3 and 1CH2 in a molecule?
3:2
What are the stages to working out a compound from H1 NMR spectroscopy?
Ratio of area
From molecular formula - how many protons?
How many proton environments (peaks)?
Work out the amount of protons in each environment from the ratio of area
Use the chemical shift values
What do the peak areas when using H1 NMR spectra give us information about?
The numbers of protons in each environment
Which methods should you use as examples when a question asks which “instrumental method to analyse a sample”?
IR spectroscopy
or
Mass spectroscopy
Does the =O bond in a hydrocarbon count as one of the double bonds when working out the structure?
No
