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Physical Chemistry > Chromatography and Spectroscopy > Flashcards

Chromatography and Spectroscopy Flashcards

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1
Q

Use of chromatography

A

Used to seperate a mixture into its different components.

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2
Q

TLC stationary and mobile phase

A

Mobile- liquid solvent

Stationary- thin layer of solid adsorbent substance (silica SiO2 or alumina Al2O3) coated on inert solid support.

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3
Q

How does seperation work

Adsorption meaning

A

A solid stationary phase separates by relative adsorption
A liquid stationary phase separates by relative solubility

(tlc is always adsorption as it is always a solid stationary phase)

The attraction between the adsorbant layer and the components in the mixture.

Different components in mixture adsorb differently to surface of stationary phase.

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4
Q

Describe movement of molecule that is highly adsorbed

A
  • Smaller distance travelled
  • Move slowly

(The stronger a component reacts with the stationary phase, the less distance it travels)

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5
Q

Factors affecting movement up plate

A

Polarity*
Temperature
Solvent
Solid adsorbant coating

  • Adsorbent layer is polar, so if component is polar, it can form hydrogen bonds with stationary phase, increasing adsorbance.
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6
Q

Carrying out TLC

A

1) Take a TLC plate and using a pencil to draw a base line (2cm above bottom)
2) Use a capillary tube and drop a small volume of sample onto base line
3) Prepare chromatography tank- solvent in a beaker with watch glass placed on top
4) place TLC plate into beaker with watch glass on top. Base line must be above solvent level
5) Allows solvent to rise up plate then mark solvent front with a pencil.

Compounds can then be identified using an Rf value

Chromatography paper sprayed with developing agent e.g., ninhydrin, produces a deep blue or purple stain, which makes it easier to identify spots.

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7
Q

Ways to improve chromatography practical

A

Wear plastic gloves to prevent contamination of hands to plate

pencil line –solvent doesnt interact/ dissolve pencil line

tiny drop – prevents spread of components and overlap

Depth of solvent– if the solvent is above base line it will dissolve/wash the sample spots off the plate.

lid– to prevent evaporation of toxic solvent

dry in a fume cupboard as the solvent is toxic

UV lamp used if the spots are colourless and not visible

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8
Q

Rf value (Retention factor)
- What it shows
- How to calculate
- Limitations

A

Component identified by comparing its Rf value to known Rf values for same solvent and absorbant system.

Rf value: Distance moves by component / distance moved by solvent front

Similar molecules have very similar Rf values- inaccurate
If spots merge, difficult to tell where one ends and one starts

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9
Q

Use of gas chromatography

A

Used to seperate and identify volatile organic compounds in a mixture

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10
Q

Gas chromatography stationary and mobile phase

A

Stationary - thin layer of liquid or solid absorbed onto inert solid support

Mobile phase- inert carrier gas (He,Ne)

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11
Q

Retention time definition

A

Time for component to travel (through capillary component) from injection to detector of chromatograph

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12
Q

Seperation in gas chromatography: solid vs liquid stationary phase

A

Solid stationary phase: seperation by adsorption

Liquid stationary phase: seperation by relative solubility

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13
Q

Gas chromatography with a solid stationary phase

A

Components with strong adsorption dont travel as far/fast and have greater retention time

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14
Q

Gas chromatography with liquid stationary phase

A

Components with greater relative solubility (dissolve more) have longer retention time.

“like dissolve like”
Liquid Stationary phase dissolves most similar
e.g.
Liquid alkane stationary phase dissolves propane (alkane) most readily as london forces form between them, slowing component

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15
Q

How does gas chromatography work

A
  • Sample injected into gas chromatograph.
  • Mobile inert carrier gas carries it through the capillary column which contains the liquid stationary phase absorbed onto the solid support.
  • components slow down as they interact with the liquid stationary phase
  • more soluble, the slower it moves, the longer the retention time
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16
Q

Limitations of gas chromatography

A
  • some chemicals have same r.t so can’t tell them apart
  • unknown substances don’t have reference peaks

THEREFORE, often used alongside mass spectroscopy

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17
Q

TLC and Gas chromatography stationary and mobile phase

A

TLC
mobile- liquid solvent
Stationary- layer of solid adsorbent (silicon) on inert solid support

Gas
mobile- inert carrier gas (Ne, He)
stationary- h.b.p liquid absorbed or solid on inert solid support

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18
Q

Chromatogram and chromatogrph difference

A

Chromatogram- the data from the experiment
Chromatograph- the equiptment

19
Q

Factors affecting retention time

A
  • solubility (greater solubility, greater retention)
  • Boiling point (Greater boiling point, greater retention)
  • Temperature of instrument (greater temperature, shorter retention)

Faster as a gas than liquid, so if it has a high boiling point, will spend less time as gas, so slower retention.
The higher the instrument temp, more are gases, so shorter retention time.

20
Q

Interpreting a gas Chromatogram- it shows 2 things

A

Retention time (identifies components by comparing to retention times of known components)
Peak integrations (tells concentration of components in sample)

21
Q

Finding concentration of component through integration

A

Concentration found by comparing peak integration with values from standard solutions of component.
(compare using a **external calibration curve)

1) Use serial dilutions of different known concentrations of compound being investigated
2) Obtain gas chromatogram and calculate integration of each peak
3) Plot graph of concentration (x) against peak area (y)
4) Obtain gas chromatogram of compound being investigated and compare with graph to find concentration.

22
Q

Qualitative analysis for organic functional groups (chemical test and observation):

Alkene
Haloalkane
Carbonyl
Aldehyde
Primary & secondary alcohol, aldehyde
Carboxylic acid
Phenol

A

Alkene
- Bromine water drop-wise
- Orange to colourless

Haloalkane
- Add AgNO3 and ethanol, warm to 50 degrees in water bath
- chloroalkane white slowest
- bromoalkane cream
- iodoalkane yellow fastest

Carbonyl
- 2,4-DNP (dinitrophenylhydrazine)
- yellow/orange precipitate

Aldehyde
- Add Tollen’s reagent and warm
- Silver mirror

1,2 alcohol and aldehyde
- add potassium dichromate, H2SO4 and warm in water bath
-Orange to green

Carboxylic acid
- Add aq NaCO3
- effervescence

Phenol
- Add NaOH and a carbonate
- solid will dissolve with NaOH (colourless solution) and nothing happens with carbonate (only reacts with strong acids)

23
Q

Use of NMR spectroscopy

A

Used to identify structure of a compound

24
Q

What is nuclear spin

Why does hydrogen and carbon have a nuclear spin.

A

If a nucleus has an odd number of nucleons (protons & neutrons), the nucleus gains magnetic moment and has a nuclear spin (change in orientation)

Hydrogen- 1 proton
Carbon- although has 6 protons and 6 neutrons, 1% of carbons are C13, which have 7 neutrons.

25
Q

What is resonance
(nuclear magnetic resonance)

A

When an external strong magnetic field and strong radio frequency radiation is applied,
the nucleus absorbs the radio waves, increases energy level and rapidly flips between the 2 spin states.

Nuclei that spin in the direction of the magnetic field have a lower energy.

26
Q

Use of magnetic field and radio frequency radiation

A
  • Magnetic field creates different energy levels.
  • When RF radiation at the right frequency is applied, resonance occurs, allowing the nuclei to absorb energy and flip to a higher energy state.
27
Q

Resonance and nuclear spin diff explanation

A

Nuclear Spin: Just a property of nuclei w odd no. , with no energy absorbed.

Resonance: Short process of absorption of energy to switch between different spin states in presence of magnetic field and r.f radiation.

28
Q

What is a chemical shift (δ)

A

How much the field has shifted away from the field of TMS

29
Q

What is TMS and why is it used

A

TMS (tetramethylsilane) (CH3)4Si

Used as a standard reference peak which other chemical shifts are measured against. Helps calibrate the spectrum.

TMS is used because:
*its signal is away from all the others
*it only gives one signal
*gives strong signal so only a small amount needed
*it is non-toxic
*it is inert
*it has a low boiling point so can be removed from sample easily

30
Q

What does NMR measure

A

The amount of radiowave energy absorbed to resonate the nuclei (flip the spin states of nuclei)

The amount of energy a nucleus absorbs depends on the environment that it’s in.

31
Q

Solvent used

A

CDCl3 Deuterated chloroform (polar)
CCl4 (non-polar) good solvent for non polar substances

Deuterated solvents are used as they don’t interfere with spectrum and produce no NMR signal.

Deuterium is an isotope of hydrogen (2H)
- produces no NMR signal.
- H1 replaced with H2 (deuterium)
- which has 1 proton & 1 neutron

32
Q

X and Y axis on graph of spectroscopy

A

x axis- δ chemical shift in parts per million (ppm)
y axis- absorption

33
Q

Running the spectrum

always remember:
- sample
- solvent (deutereum CDCl3, D2O, CCl4)
- reference (TMS)

A
  • Dissolve sample in solvent and place in NMR tube with small amount of TMS
  • Tube placed in spectrometer and spun (evens out imperfections in magnetic field within sample)
  • Spectrometer is zeroed against TMS standard
  • Sample given pulse of radiation containing range of radio frequencies and provided a constant magnetic field
  • Any absorptions of energy during resonance are detected and diplayed on computer screen.
  • After analysis, sample can be recovered by evaporation of solvent
34
Q

When carbon atoms are positioned symmetrically (have same chemical environment)

A

They absorb radiation at same chemical shift.

35
Q

How does polarity affect chemical shift

A

In polar environments, electronegative atoms draw electron density away, shifting it to a higher chemical shift due to deshielding from magnetic field of nuclei.

36
Q

What factors may move a peak outside of the range of the spectrometer

A

Solvent, concentration, substituents

37
Q

Integration values tell us

A

Relative number of H in each environment

38
Q

Splitting of proton NMR is called

What causes splitting

A

Spin-spin coupling or spin-spin splitting

Proton’s spin in one environment creates a small magnetic field which interacts with the magnetic field/spin state of protons in an adjacent environment.

Shorter: Caused when a proton’s spin interacts with spin states of protons from different environments.

39
Q

Splitting patterns pascals triangle

Names of splitting patterns

A

1
1 1
1 2 1
1 3 3 1

singlet
doublet
triplet
quartet
quintet/pentet
sextet
heptet/septet

(anything more than a singlet is a multiplet- but only write it after quintet)

40
Q

Problems with -OH and -NH protons

A
  • Chemical shift occurs over a range of values
  • Broad signals
  • no splitting pattern
41
Q

What are OH and NH known as

Why are OH and NH signals so broad

A

OH and NH protons known as labile (readily changeable)

Rapid proton exchange with spall traces of H2O in solvent

42
Q

How to identify -OH and -NH protons

A

1) A proton NMR spectrum is run as normal
2) Small volume of deuterium oxide is added, the mixture is shaken and second spectrum is run
3) In second spectrum, OH peak disappears.

Deuterium will exchange and replace OH and NH protons
CH3OH + D2O –> CH3OD + HOD

Protons bonded to electronegative atoms are able to hydrogen bond to the deuterium oxide. This weakens OH or NH bond, allowing exchange.

43
Q

What does spectroscopy identify

A

Bonds and functional group

44
Q

What does mass spectroscopy identify

A
  • Mr of molecule
  • Fragments of molecule

Physical Chemistry (7 decks)

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