3.3.6 Organic Analysis Knowledge Flashcards
(53 cards)
1
Q
Why can’t a mass spectrometer which measures m/z ratios to 1 d.p. tell the difference between C10H16O4 and C11H4O4?
A
they have the same molecular mass to 1 d.p.
2
Q
why can’t high resolution mass spectrometry tell the difference between propan-1-ol and propan-2-ol?
A
they have the same molecular formula, so their molecular mass will be identical
3
Q
why does C have a relative molecular mass of 12.00000?
A
12C is a standard reference
4
Q
what is the test for alkenes?
A
shake with bromine water
5
Q
what is the positive result for the test for alkenes?
A
Bromine water turns from orange to colourless if alkene present
6
Q
what is the test for haloalkanes?
A
- add NaOH(aq) and warm to substitute the halide ion
- Acidify with HNO3
- Add AgNO3
- Once a precipitate is formed, add dilute ammonia
- Then add concentrated ammonia
7
Q
what are the positive results of the test for haloalkanes?
A
- a white, cream, or yellow precipitate will form of AgCl, AgBr, or AgI respectively if a haloalkane was present
- On addition of dilute ammonia, any AgCl precipitate will dissolve
- On addition of concentrated ammonia, any AgBr precipitate will dissolve, but any AgI precipitate will remain
8
Q
what is the test for primary or secondary alcohols?
A
- add acidified K2Cr2O7
- Warm
9
Q
what is the positive result of the test for primary or secondary alcohols?
A
solution will turn from orange to green if primary or secondary alcohol present (also for aldehyde)
10
Q
what are the tests for aldehydes?
A
warm with Fehling’s solution or warm with Tollen’s solution
11
Q
what are the positive results of the tests for aldehydes?
A
Blue solution turn to brick red precipitate or silver mirror forms, if aldehyde present
12
Q
what is the test for carboxylic acids?
A
Add Na2CO3
13
Q
what is the positive result of the test for carboxylic acids?
A
effervescence as CO2 given off
14
Q
what is the test for h2 gas?
A
Apply a lit splint
15
Q
what is the positive result of the test for h2 gas?
A
a squeaky ‘pop’ occurs
16
Q
what is the test for co2 gas?
A
Bubble through limewater
17
Q
what is the positive result of the test for co2 gas?
A
Solution turns cloudy
18
Q
what is the test for o2 gas?
A
apply a glowing splint
19
Q
what is the positivie result of the test for o2 gas?
A
The splint will relight
20
Q
what is the test for so4^2- ions?
A
Add BaCl2 solution
21
Q
what is the positive result for the test for so4^2- ions?
A
a white precipitate will form
22
Q
what is the test for halide ions?
A
Add HNO3 then AgNO3
23
Q
what is the positive result for the test for halide ions?
A
- a white, cream, or yellow precipitate will form of AgCl, AgBr, or AgI respectively halide ions were present
- On addition of dilute ammonia, any AgCl precipitate will dissolve
- On addition of concentrated ammonia, any AgBr precipitate will dissolve, but any AgI precipitate will remain
24
Q
what is the test for co3^2- ions?
A
Add any acid
25
what is the positive result of the test for co3^2- ions?
effervescence as CO2 is given off
26
what is infrared spectroscopy used for?
Used to identify types of bonds present in an organic molecule
27
how does infrared spectroscopy work?
- A pair of atoms bonded together are constantly vibrating
- Some bonds are very strong so they vibrate with a very high frequency. Some bonds are very weak so they vibrate with a very low frequency.
- Every different type of bond has its own 'natural frequency' which falls within the infrared region of the EM spectrum
- When IR radiation is shone through a solution containing a molecule, if the natural frequency of any of the bonds match the frequency of the IR light, that light will be absorbed by the molecule
- The IR radiation which passes through the solution will be missing some frequencies, and these can be used to indicate which bonds are present in the molecule
28
how are IR spectrums produced?
- IR energy is absorbed by bonds which makes the bonds vibrate
- The bonds vibrate at the same frequency as the IR energy that is absorbed
29
what is the process of IR spectroscopy?
- A beam of IR light containing a wide range of frequencies is passed through a sample
- Some of the frequencies of IR light are absorbed by the molecule. Many of the frequencies are not absorbed, and so pass straight through the sample. The light that emerges through the other side of the sample is missing the frequencies which are absorbed.
- Once detected, a computer plots a graph of the intensity of the light which emerges from the sample - transmittance
- The frequency is plotted on the x-axis with the units 'wavenumbers' which are cm^-1
30
how do you tell OH peaks in acids and alcohols apart?
- OH bond in alcohols: 3230-3550cm^-1, 'test-tube' shaped, smooth edges
- OH bond in acids: 2500-3000cm^-1, more jagged edges
If an OH acid peak is present, there must be a peak for C=O in the range 1680-1750cm^-1
31
what is the fingerprint region?
The fingerprint region is the area of the spectrum 1500cm^-1 and below
32
how is the fingerprint region formed?
A complicated combination of peaks formed by different twisting and bending vibrations of the molecule as a whole
33
what is the fingerprint region used for?
- Individual peaks cannot be interpreted as coming from specific bonds below 1500cm^-1
- Each molecule has its own unique fingerprint region, so a computer can compare the fingerprint region to a database of samples, to find an exact match
34
how can infrared spectroscopy be used to idenitify impurities?
If a sample of a compound has been obtained, its IR spectrum can be compared to that of the pure spectrum in a databse to indicate if impurities are present
35
what is a mass spectrometer?
machine used to analyse elements or compounds; can accurately determine Ar of atoms or Mr of molecules
36
what factors do mass spectrometers measure?
- relative abundance
- mass/charge (m/z) ratio
37
what are the 4 main steps of TOF mass spectrometry?
- ionisation
- acceleration
- ion drift
- detection
38
describe the full process of TOF mass spectrometry
- Vacuum: the entire machine is a vacuum inside to prevent any of the particles being tested colliding with mollecules from the air
- Ionisation: sample particles gain a positive charge through electron impact or electrospray ionisation
- Acceleration: the positive ions are attracted to a negatively charged plate and accelerate towards it. The amount they accelerate depends on m/z ratio (high m/z ratios will accelerate to lower speeds than low m/z ratios). Once accelerated, all ions have the same kinetric energy
- Ion Drift: some ions will pass through a hole in the negatively charged plate, forming a beam of particles which travel along the 'flight tube' towards the detector. Because they are travelling at different speeds, they start to drift further apart.
- Detection: different m/z ratio ions arrive at the detector at different times due to different velocities. Time of flight is recorded and as each ion hits the detector is gains an electron, generating a current, the size of which is proportional to the number of each type of ion.
- Data analysis: signal from detector passed to computer which generates a mass spectrum
39
what are the 2 methods of ionisation in a mass spectrometer?
- a. electrospray ionisation
- b. electron-impact ionisation
40
describe the process of electrospray ionisation
- sample X is dissolved in a volatile solvent (water or methanol)
- sample X is injected through a hypodermic needle to produce a fine mist (aerosol)
- tip of the needle is attached to the positive terminal of a high voltage power supply
- particles are ionised by gaining a proton (H+ ion) from the solvent as they leave the needle
- this produces XH+ ions (ions with a single positive charge and a mass of Mr + 1
- solvent evaporates while XH+ ions are attracted towards negative plate where they are accelerated
41
what type of molecule is electrospray ionisation used for, and why?
electrospray ionisation is used for substances with higher molecular mass as fragmentation rarely takes place
42
give an equation for the ionisation of calcium through electrospray ionisation
Ca + H+ ⇒ CaH+
43
describe the process of electron impact ionisation
- sample being analysed is vapourised
- high energy electrons are fired at sample from an electron gun (hot wire filament with a current running through it which emits electrons)
- high energy electrons knock one electron off each particle, forming a 1+ ion
- 1+ ions are then attracted towards a negative electric plate where they are accelerated
44
what type of substance is electron impact ionisation used for?
electron impact ionisation is used for elements/substances with low formula mass
45
give an equation for the ionisation of a methane molecule through electron impact ionisation
CH4(g) ⇒ CH4+(g) + e-
46
what are the results from mass spectrometry if a single element is injected?
sample will be separated into different isotopes of that element where each peak represents a different isotope, as each will have a different m/z ratio
47
what is m/z equal to in mass spectrometry?
as m/z is mass / charge ratio, and charge is nearly always +1, m/z = mass of ion
48
what are the results from mass spectrometry if the sample if the sample has undergone electrospray ionisation?
With electrospray ionisation, fragmentation will not occur; there will be one peak that will be equal to the mass of the MH+ ion. Subtract 1 to get the Mr of the molecule.
49
what is low resolution mass spectrometry?
Low resolution mass spectrometry data is measured to 1 d.p
50
why can low resolution mass spectrometry not always be used?
Low resolution mass spectrometry data is measured to 1 d.p - if two molecules have the same Mr to 1 d.p., a mass spectrometer couldn't identify which was present
51
what is high resolution mass spectrometry?
High resolution mass spectrometry can measure the mass of an ion up to 5 d.p.
52
what can high resolution mass spectrometry be used to calculate?
High resolution mass spectrometry can measure the mass of an ion up to 5 d.p. - the molecular formula of the compound can be calculated from the molecular ion peak
53
how are calculation of high resolution mass spectra done?
often done by a computer which can compare the data to a huge database to indicate the molecular formula of the molecule
