Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

A Synthesis&Analytic Technique Paper 2 Monday 19th June AM 2h15 > Chapter 29 Key Concepts > Flashcards

Chapter 29 Key Concepts Flashcards

1
Q

Chromatography

A

Used to separate individual components from a mixture of substances. All forms have a stationary phase and a mobile phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is chromatography used for?

A 
Analysis of 
drugs
Plastics
Flavourings 
Air samples 
And has applications in forensic science
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Theory behind TLC?

A

Quick and inexpensive
Tells you how many components are in a mixture. For this method you use a TLC plate that is usually a plastic sheet or glass coaxed in a thin layer of a solid adsorbent substance
*Usually SILICA

Adsorbent is the STATIONARY PHASE
Different components have different affinities for the adsorbent and bind with different strengths to its surface.
Separation happens because of the relative adsorption

Often run the TLC of sample alongside pure samples of what might be present so it’s easier to identify the unknown sample visually without calculating any Rf values

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How to identify the component using the RF value

A

Rf=distance moved by the component/distance moved by solvent front

Compare the Rf value with known values recorded using the same solvent system and adsorbent

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What’s gas chromatography used for?

A

Useful for separating and identifying volatile organic compounds present in a mixture

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe gas chromatography

Stationary phase
Mobile phase

A

Stationary phase
-high boiling liquid absorbed onto an inert solid support

Mobile phase
-inert carrier gas like Helium or Neon

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Describe the stages of the gas chromatograph

A
  1. small amount of volatile mixture is injected into the apparatus.
    * APPARATUS IS CALLED GAS CHROMATOGRAPH
  2. Mobile carrier gas carries the components in the sample through a capillary column
    * COLUMN CONTAINS THE LIQUID STATIONARY PHASE ABSORBED TO THE SOLID SUPPORT
  3. Components slow down as they interact with the liquid stationary phase
    * THE MORE SOLUBLE THE COMPONENT IS IN THE LIQUID STATIONARY PHASE THE SLOWER IT MOVED THROUGH THE CAPILLARY COLOMN
  4. Compound reaches the detector at different times depending on interaction with the stationary phase
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

How does separation occur in gas chromatography

A

Components of the mixture are separated depending on their solubility in the liquid stationary phase.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Two things we can get from the gas chromatogram

A

each component is detected as a peak

  • by comparing with known RF values, you can identify the components present
  • peak integrations/peak area can be used to determine the concentrations if the components
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How do you work out the concentration of a component?

A

Basically Compare the peak area/peak integration with values obtained from the standard solutions of the component

  1. prepare standard solutions of known concentrations of the compounds being investigated
  2. get a gas chromatogram for each of the standard solutions
  3. plot a calibration curve of peak area against concentration in EXTERNAL CALIBRATION
  4. get a gas chrom of the compound being investigated under the same conditions
  5. Use the calibration curve to measure the concentration of the compound.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Identifying alkenes?

A

Add bromine water dropwise

Decolorises from orange to colourless

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Identifying haloalkanes

A

Add silver nitrate and ethanol and warm to to 5O degrees

Chloroalkane white ppt
Bromoalkane cream ppt
Iodoalkane yellow ppt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Identifying carbonyl group

A

2,4-DNP

Orange ppt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Identifying aldehyde

A

Add tollens reagent and warm

Silver mirror

Or add acidified potassium dichromate. Colour change orange to green

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Identifying primary or secondary alcohol

A

Add K2Cr2SO4

Colour change from orange to green

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Identifying Carboxylic Acids

A

Add aq sodium carbonate

You see effervescence

17
Q

How does NMR work

A

Uses a the right combo of very strong magnetic field and radio frequency radiation which some atoms then in turn absorb. The energy for the absorption can be measured and recorded as an NMR spectrum

18
Q

When is nuclear spin significant?

A

When there’s a an odd number of NUCLEONS

19
Q

How do the electrons surrounding the nuclei of atoms affect the NMR

A

Electrons surrounding the nucleus of an atom shift the energy and radio frequency needed for NMR to occur.

Frequency shift is measured on a scale called chemical shift.(delta) in ppm parts per million.

Amount of shift is determined by chemical environment.
*especially the presence of electronegative atoms

20
Q

What is TMS and what is it used for

A

TMS is tetramethylsilane (CH3)4Si is the standard reference chemical against which all chemical shifts are measured on a scale called chemical shift. It’s chemical shift value is 0ppm

21
Q

How does chemical environment affect absorption peaks

A

Depending on chemical environment, NMR requires a different energy and frequency making absorption peaks at chemical shifts.

22
Q

Pros of NMR

A

Allows CH arrangement in a molecule to be mapped out without having to carry out conventional chemical tests

Doesn’t destroy the organic compound that you’re testing

23
Q

Describe the steps of the NMR technique

A
  1. Sample dissolved in solvent and placed in a narrow NMR sample tube with a small amount of TMS
  2. place tube inside the NMR spec where it is spun to even out imperfections
  3. Zero the spec against the TMS standard and give the sample a pulse of radiation containing the range of radio frequencies whilst maintaining a constant magnetic field
  4. any absorptions from energy resulting from resonance are detected and displayed on a computer screen
    * can recover sample by evaporating the solvent
24
Q

Why is Deuterium used

A

Solvents containing deuterium isotopes eg CDCl3 are used as they don’t make signals in the frequency ranges used in H1 and C13 NMR cuz DEUTERIUM HAS AN EVEN NUMBER OF NUCLEONS

25
Q

What does C 13 NMR tell you about a molecule?

A
  • the number of different carbon environments which is the number of peaks
  • the chemical shift tells you the type of carbon environment present
26
Q

Describe how the position of the atom within the molecule determines the chemical environment of a carbon atom

A

Carbon attached to different atoms or group of atoms have different environments and therefore WILL ABSORB AT DIFFERENT CHEMICAL SHIFTS

two Carbons positioned symmetrically within a molecule are EQUIVALENT and will ABSORB AT THE SAME CHEMICAL SHIFT contributing to the same peak

27
Q

What does proton NMR tell us

A

-number of peaks tells us the number of different proton environments

-chemical shift tells us the types of proton environments present just like CNMR
BUT
PNMR also tells us

-relative numbers of each type of protons from integration traces or ratio numbers of the relative peak areas

  • spin spin splitting pattern gives us the number of non equivalent protons adjacent to a given proton
  • chemical shift range of 12ppm but for CNMR its 200ppm
28
Q

Factors that change the range of the chemical shift

A

Solvents
Concentration
Substituents

29
Q

What is proton exchange ?

+think of spot the difference

A

Used to identify OH and NH
-run the pNMR
-add a small volume of deuterium oxide, shake and run a second spectrum .
-The deuterium exchanges and replaces OH and NH protons in the sample with the deut and sets up an equilibrium
eg with methanol
CH3OH + D2O <==> CH3OD + HOD
Deut nah absorb in this chemical shift range so OH peak disappears

30
Q

Structure determination for identification steps

A

Elemental analysis
-Use percentage composition to work out the empirical formula

Mass spectra
-work out molecular formula

Infrared spec
-use absorption peaks to work out functional group

NMR
-work out the order of atoms

31
Q

A small amount of TMS

A

(tetramethylsilane) is added to the sample to calibrate the spectrum
The same calibration compound is used for both H and C NMR
TMS is used because: •its signal is away from all the others •it only gives one signal •it is non-toxic •it is inert •it has a low boiling point and so can be removed from sample easily

A Synthesis&Analytic Technique Paper 2 Monday 19th June AM 2h15 (22 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions