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Z A Level Chemistry 2 > Chromatography and Spectroscopy > Flashcards

Chromatography and Spectroscopy Flashcards

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1
Q

What is the chemical test and observation for the functional group alkene?

A
  1. Add bromine water drop-wise

2. Bromine water decolourised from orange to colourless

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2
Q

What is adsorption?

A
  • Adsorption is the the process by which the solid silica holds the different substances in the mixture to its surface
  • Separation is achieved by the relative adsorptions of substances with the stationary phase
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3
Q

What is nuclear spin?

A
  • Electrons have a property called spin and the nucleus also has a nuclear spin, that is significant if there is an ODD number of nucleons (protons and neutrons)
  • Almsot all organic molecules content carbon and hydrogen, mostly as the 1H and 12C isotopes with a small proportion (1.1%) of the 13C isotope, so for the organic chemist, NMR is relevant for 1H and 13C, the isotopes with an odd number of nucleus
  • NMR spectroscopy can be used to detect isotopes of the relents with odd numbers of nucleons, such as 19F and 31P, however 13C and especially 1H NMR spectroscopy are the components dorms of analysis used
  • As a 1H nucleus consists of just a proton, 1H NMR is usually referred to as proton NMR
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4
Q

What is stationary phase in gas chromatography?

A
  • Gas chromatography is useful for separating and identifying volatile organic compounds present in a mixture
    1. The stationary phase is a solid or liquid adsorbed onto the surface of an inert solid on a long (1-30m) chromatography column is (a high boiling liquid adsorbed onto an inert solid support)
    2. Works by relative solubility, the polarity of the liquid on the column can be changed and hence changes the solubility of the the solutes in the stationary phase. This can be used to achieve better separation
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5
Q

How does an NMR spectrometer work?

A
  1. Radio frequency radiation has much less energy than the infrared radiation used in IR spectroscopy
  2. The frequency required for resonance is proportional to the magnetic field strength and it is only in strong and uniform magnetic fields that this small quantity of energy can be detected
  3. Typically a very strong super-conducting electromagnet is used, cooled to 4K by liquid helium
    - For organic chemistry, most routine spectrometer operate at radio frequencies of 100,200 or 400MHz
    - They are also found in hospitals as MRI (magnetic resonance imaging) body scanners, a technique that uses the same technology
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6
Q

What is splitting like in aromatic compounds?

A
  • From Figure 1, aromatic protons are expected to absorb in the range delta=6.2-8.0 ppm
  • Splitting does occur but this can be difficult to interpret
  • You are only expected to interpret aromatic proton as groups of protons often one or more multiplet
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7
Q

What is another splitting?

A
  1. In CH(CH)2 above, the two CH3 groups are in the same environment
  2. Sometimes, adjacent protons may have different environments
  3. In molecules such as CH2CH2CH2COOH, the central -CH2- would be split differently by the CH3 and CH2 protons
  4. The resulting splitting would then be shown as a multiplet
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8
Q

What is the chemical test and observation for the functional group carbonyl?

A
  1. Add 2,4-dinitrophenylhydrazine

2. Orange precipitate

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9
Q

What is the stationary phase in thin layer chromatography (TLC)?

A
  1. Dry silica or alumina coated onto a glass or plastic surface
  2. If the solid is absolutely dry, then the chromatography proceeds via adsorption. The solids are polar and therefore their speed at which the solute move up the solid depends on their polarity
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10
Q

What happens in a NMR spectrometer?

A
  1. In an NMR spectrometer, the sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS
  2. The tube is placed inside the NMR spectrometer, where it is spun to even out any imperfections in the magnetic field within the sample
  3. The spectrometer is zeroed against the TMS standard ad the sample is given a pulse of radiation containing a range of radio frequencies, whilst maintaining a constant magnetic field
  4. Any absorptions of energy resulting from resonance are detected and displayed on a counter screen
  5. After analysis the sample can be recovered by evaporation of the solvent
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11
Q

What is the mobile phase?

A

The mobile phase does move, and is normally a liquid or gas

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12
Q

What are the four main types of proton that absorb over different chemical shift ranges?

A
  1. H bonded to C=O or part of COOH
  2. H bonded to C=C or aromatic ring
  3. HC bonded to electronegative atom (e.g. O,N,Cl,Br or benze ring or C=O)
  4. HC bonded to an alkyl chain
    - Factors psych as solvent, concentration, and substituents may move a peak outside of these ranges
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13
Q

What are hydroxyl and amino protons like?

A
  1. Organic compounds may contain protons that are not bonded to carbon atoms, for example, organic compounds often contain -OH and -NH protons
    - The functional groups involved include:
  2. Alcohols, ROH, phenols ArPH, and carboxylic acids, RCOOH
  3. Amides, RNH2, amides, RCONH2, and amino acids, RC(NH2)COOH
    - In solution NH and OH protons may be involved in hydrogen bonding and the the NMR peaks are often broad and of variable chemical shift
    - Example look at textbook
    - The broadening of the peaks also means that OH and NH protons are not usually involved in spin-spin coupling and all this can make assigning OH and NH protons difficult
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14
Q

What is the standard reference chemical in NMR?

A
  • Tetramethylsilane (TMS), (CH3)4Si, is used as the standard reference chemical against which all chemical shifts are measured
  • TMS is given a chemical shift value of 0 ppm
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15
Q

How do you work out the relative numbers of each type of proton?

A
  1. In a carbon-13 NMR spectrum, the peak area is not directly related to the number of carbon atoms responsible for the peak, and this all change for proton NMR- the rate of the relative areas under each peak gives the ratio of the number of protons responsible for each peak
  2. The NMR spectrometer measures the area under each peak as an integration trace, and the integration trace is shown wither as na extra line on the spectrum or as a printed number of the relative peaks areas and this provides invaluable information for identifying an unknown compound
    - Examples in textbooks and notes
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16
Q

Describe the process of gas chromatography

A
  1. A small amount of the volatile mixture is injected into the apparatus, called a gas chromatograph
  2. The mobile carrier gas carries the components in the sample through the capillary column which contains the liquid stationary phase absorbed onto the solid support
  3. The components slow downs they interact with the liquid stationary paste inside the column
  4. The more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column
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17
Q

What is the chemical shift?

A
  1. In an organic molecule, every carbon and hydrogen atom is bonded to other atoms, and all atoms have electrons surrounding the nucleus, which shift the energy and radio frequency needed for nuclear magnetic resonance to takes place
  2. The frequency shift is measured on a scale called chemical shift, delta, in parts per million (ppm)
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18
Q

What affect does the deuterium oxide have on proton exchange?

A
  1. Deuterium exchanges and replaced the OH and NH protons in the sample with deuterium atoms
    - For example the following equilibrium is set up with methanol CH3OH + D2O (equilibrium arrow) CH3OD + HOD
  2. So the second spectrum is essentially being run on CH3OD and as deuterium does not absorb in this chemical shift range, the OH peak disappears
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19
Q

How do you analyse a gas chromatogram?

A
  1. As each component of the mixture leaves the column, its relative concentration can be measured by a recorder
  2. The relative amounts of each component in a mixture can be measured by the real of their respective peaks
    % of each component = (area of peak/total area of peaks) x 100%
  3. Linking the column to a mass spectrometer means that as the unknown wave the column, it can be identified by comparing its fragmentation pattern with that of known compounds, and it can be identified almost instantly
  4. This technique is called gas chromatography mass spectrometry (GC-MS)
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20
Q

What is a stationary phase?

A

The stationary phase does not move, and is normally a solid or a liquid supported on a solid

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21
Q

What is the chemical test and observation for the functional group carboxylic acid?

A
  1. Add aqueous sodium carbonate

2. Effervescence

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22
Q

What is proton exchange?

A
  • Chemists have decided a technique called proton exchange for identifying -OH and -NH protons
    1. A proton NMR spectrum is run as normal
    2. A small volume of deuterium oxide, D2O, is added, the mixture is shaken and a second spectrum is run
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23
Q

What is adsorption?

A
  • Stationary Phase: A polar solid that has to be dry
  • Mobile Phase: A liquid
  • How does it work: The components in the mixture are usually polar compounds and are attracted to the solid phase to varying degrees. The more the solute is attracted to the solid phase, the more slowly it will move
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24
Q

What is the amount of chemical shift determined by?

A
  1. The amount of chemical shift is determined by chemical environment, especially the presence of nearby electronegative atoms
  2. So depending on the chemical environment, NMR requires a different energy and frequency, producing absorption peaks at chemical shifts
  3. This means that the carbon and hydrogen arrangement in a molecule ca be mapped out without needing to carry out conventional chemical test and without destroying the organic compound under test
25
Q

What is the chemical test and observation for the functional group aldehyde?

A
  1. Add Tollens’ reagent and warm

2. Silver mirror

26
Q

What are the four main types of carbon atom that absorb over different chemical shift ranges?

A
  • There are four main types of carbon atom that absorb over different chemical shift ranges
    1. Carbon as part of a C=O
    2. Carbon as part of C=C or aromatic ring
    3. Carbon bonded to an electronegative atom e.g. O,N,Cl,Br
    4. Carbon bonded to carbon
  • The chemical shift may also be outside of these ranges, depending on the solvent, concentration and substituents
27
Q

What is resonance?

A
  1. An electron has two different spin states, and the nucleus also has two different spin states and these have different energies
  2. With the right combination of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flips between the two spin states
  3. This is called resonance and the whole process gives the name ‘nuclear magnetic resonance’
28
Q

What is the chemical test and observation for the functional group primary and secondary alcohols, and aldehyde?

A
  1. Add acidified potassium dichromate (VI) and warm in a water bath
  2. Colour change from orange to green
29
Q

What is spin-spin coupling?

A
  1. A proton NMR peak can be split into sub-peaks or splitting patterns
  2. These are caused by the proton’s spin interacting with the spin states of nearby protons that are in different environments
  3. This proves information about the number of protons bonded to adjacent carbon atoms
30
Q

What is relative solubility?

A
  • Stationary Phase: a solid or a viscous liquid adsorbed onto a solid
  • Mobile Phase: A gas or a liquid
  • How does it work: The solute (in the mixture) is more soluble in one phase than another. The components are separated according to their difference solubilities in the two phases. If a solute is relatively more soluble in the mobile phase than another solute, then it will pass more quickly
31
Q

How do you determine the concentration of components in a sample?

A
  • The concentrations of. a component in a sample is determined by comparing its peak integration (peak area) with values obtained from standard solutions of the component
    1. Prepare standard solutions of known concentrations of the compound being investigated
    2. Obtain gas chromatograms for each standard solution
    3. Plot a calibration curve of peak area against concentration. This is called external calibration and offers a method for converting a peak area into a concentration
    4. Obtain a gas chromatogram of the compounding investigated under the same conditions
    5. Use the calibration curve to measure the concentration of the compound
32
Q

What is chromatography in general?

A
  • A mixture carried by a mobile phase is moved over a stationary phase
  • The stationary phase tends hold back the components of the mixture to differing extents, so they move through the stationary phase at different rates and therefore separate
  • The components of the mixture to be separated are called solutes
33
Q

What is NMR spectroscopy?

A
  1. Nuclear Magnetic Resonance (NMR) spectroscopy is used in the analysis of organic compounds
  2. The technique uses a combination of a very strong magnetic field and radio frequency radiation
  3. With the right combination of magnetic field strength and frequency, the nuclei of some atoms absorb this radiation
  4. The energy for the absorption can be measured and recorded as an NMR spectrum
34
Q

What are retention times on a gas chromatogram?

A
  1. 5 major peaks show that 5 drugs were present in the sample, identified from their retention time
  2. You can see from the relative sizes of the peaks that there is a different proportion of each drug
35
Q

What is the chemical environment of a carbon atom determined by?

A
  • The chemical environment of a carbon atom is determined by the position of the atom with the molecule
    1. Carbon atoms that are bonded to different atoms or groups of different environment and will absorb at different chemical shifts
    2. If two carbon atoms are positioned symmetrically within a molecule, they they are equivalent and have the SAME chemical shift. They will then absorb radiation at the same chemical shift and contribute to the same peak
36
Q

How is it separated in gas chromatography? What is the retention time?

A
  1. The components of the mixture are separated depending on their solubility in the liquid stationary phase
  2. The compounds in the mixture reach the detect at different times depending on their interactions with the stationary phase in the column
  3. The compound retained in the column for the shortest time has the lowest retention time and is detected first
  4. There retention time is the time taken for each comment to travel through the column
37
Q

What is chromatography used for?

A
  1. Chromatography is used to separate individual components from a mixture of substances
  2. All forms of chromatography have a stationary phase and a mobile phase
  3. Chromatography can be used in the analysis of drugs, plastics, flavourings, air samples, and has applications in forensic science
38
Q

Where is GC used and how?

A
  1. Environmental analysis: organic pollutants in water; pesticides in food
  2. Forensic drug detection; minute amounts of drugs in forensic samples
  3. Security operations explosives in luggage or on people
  4. Space probes: the components of a planet’s atmosphere
39
Q

What does carbon-13 NMR spectroscopy provide?

A
  • A carbon-13 NMR spectrum provides two important pieces of information about the molecule:
    1. The NUMBER of different carbon environments - from the number of peaks
    2. The TYPES of carbon environment present - from the chemical shift
40
Q

What two pieces of information can be obtained from a gas chromatogram?

A
  • Each component is detected as a peak on the gas chromatogram and two pieces of infuriation can be obtained from a gas chromatogram
    1. Retention times can be used to identify the components present in the sample by comparing these to retention times for unknown components
    2. Peak integrations (the areas under each peak) can be sued to determine the concentrations of components in the sample
41
Q

What are equivalent and non-equivalent protons?

A
  • For carbon-13 NMR, you saw that the same chemical environment absorb at the same chemical shift value, these carbon atoms are EQUIVALENT to one another and for proton NMR, the same principle applies:
    1. If two or more protons are EQUIVALENT, they will absorb at the sane chemical shift, increasing the size of the peak
    2. Protons of different types have different chemical environments and are NON-EQUIVALENT - they absorb at different chemical shift
42
Q

What is the n+1 rule?

A
  1. The splitting of a min peak into sub-peaks is called spin-spin coupling or spin-spin splitting and the number of sub peaks is one greater than the number of adjacent protons causing the splitting
  2. This sounds complicated but the pattern is easy to see using the n+1 rule
    - For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1
  3. When analysing spin-spin splitting, you are really seeing the number of hydrogen atoms on the immediately adjacent carbon atoms
43
Q

What are the disadvantages of gas chromatogram?

A
  1. A mixture may have many components and each of these have to have retention times for comparison
  2. Unknown compounds may have no reference times for comparison and hence identification
  3. Different substances may have identical retention times
  4. Substances that have high boiling points cannot be separated
44
Q

What are the the two mechanisms by which chromatography work?

A
  • Relative solubility

- Adsorption

45
Q

How do you interpret a TLC plate?

A

-By comparing its Rf value with known values recorded used the same solvent system, and absorbent Rf value Rf=distance moved by the component/distance moved by the solvent front

46
Q

What does a proton NMR provide?

A
  • A proton NMR spectrum provides four important pieces of information about a molecule. Firstly the spectrum provides similar information to a carbon-13 NMR spectrum but for protons:
    1. The NUMBER of different proton environments - from the number of peaks
    2. The TYPES of proton environments present - from the chemical shift
  • There are two extra pence of information:
    3. The RELATIVE NUMBERS of each type of proton - form the integration traces or ratio numbers of the relative peak areas
    4. The number of non-equivalent protons ADJACENT to a given proton from the spin-spin splitting pattern
47
Q

Why does spin-spin coupling occur in pairs?

A
  1. In an NMR spectrum, if you see one splitting pattern there must always be another - slitting patterns occur in pair because each proton splits the signal of the other
  2. This can make it very easy to spot a structural feature when analysing a molecule, as there are several very common splitting pairs that you may see in a spectrum
    - Look at notes
48
Q

How do gas chromatograms work 2?

A
  1. A gas chromatogram is a very sensitive way of separating and identifying the components of a mixture of volatile components
  2. First, pure substances are run under identical conditions and their retention times measured
  3. The mixture is then run and the retention times of the components compared with those of the pure substances
49
Q

What are deuterated solvents?

A
  1. Molecules of most common solvents contain carbon and hydrogen atoms, which will produce a single in both 13C and 1H NMR spectra
  2. A deuterated solvent is usually used in which the 1H atoms have been replaced by 2H atoms (deuterium, D)
  3. Deuterium produces no NMR single in the frequency ranges used in 1H and 13C NMR spectroscopy
  4. Deuterated trichloromethane, CDCl3, is commonly used as a solvent in NMR spectroscopy, but this will still produce a peak in a carbon-13 NMR spectrum
  5. The computer usually filters out this peak before displaying the spectrum
50
Q

What is the chemical test and observation for the functional group haloalkane?

A
  1. Add silver nitrate and ethanol and warm to 50degrees C in a water bath
  2. Chloroalkane - white precipitate
    Bromoalkane - cream precipitate
    Iodoalkane - yellow precipitate
51
Q

What is the mobile phase in gas chromatography?

A
  1. The mobile phase is an inert carrier gas such as helium or neon or a volatile liquid. The mobile phase is called the carrier gas
  2. The time taken for the volatile components of the mixture to move through the column is called their retention time
52
Q

What is the mobile phase in thin layer chromatography (TLC)?

A
  1. A pure liquid or a mixture of liquids

2. The chromatography depends on the relative solubilities of the solutes in the mobile and stationary phase

53
Q

Why is using a retention time beneficial?

A
  • It is common to run a TLC of a sample alongside pure samples of compounds that may be present, it is then easy to identify the amino acids in the unknown sample visually, without needing to calculate any Rf values
  • This value means that a solute can be identified even if the chromatogram is run for different times
  • As long as the substances used for the stationary and mobile phases are the same, a comparison is valid
54
Q

State the property of alpha amino acids that allow them to be separated by TLC?

A

Adsorption

55
Q

How could the student analyse the chromatogram to identify the three alpha amino acids that were present?

A
  1. Measure how far the each spot travels relative to the solvent front
  2. Compare Rf values with that of know amino acids
56
Q

Why could the fact that several alpha amino have structures that are very similar cause problems when using TLC to analyse mixtures of alpha amino acids?

A

The amino acids have similar Rf values

57
Q

How could infrared spectroscopy be used to confirm which compound is a carboxylic acid?

A

The carboxylic acid has a peak at 2500-3300 cm^-1 for the O-H in carboxylic acids

58
Q

How would carbon 13 NMR and infrared spectroscopy allow the carboxylic acids to be identified and distinguished from one another?

A
  • The carboxylic acids have a peak 2500-3300 cm^-1 for the O-H in carboxylic acid
  • One has 4 peaks for 4 different carbon environments
  • One has 3 peaks for 3 different carbon environments
59
Q

What would the difference be if the 1HNMR spectra was run but with a few drops of D20 added?

A

The peaks at 10.2-12ppm for two COOH protons would disappear as the deuterium would exchange with the COOH protons

Z A Level Chemistry 2 (17 decks)

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