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OCR A A-Level Organic Chemistry > Chapter 29 > Flashcards

Chapter 29 Flashcards

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1
Q

How does TLC work?

A
  • The stationary phase is silica or alumina (this is then on a glass/ plastic plate)
  • The mobile phase is the solvent
  • Separation occurs due to relative adsorption to the stationary phase, as the components in the mixture would each adsorb differently to the stationary phase
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2
Q

How would you carry out TLC?

A
  • Draw a base line near the bottom in pencil​
  • Using a capillary tube, spot a small sample of the solution to be tested onto the baseline of the plate​
  • Pour some solvent into the chromatography tank, ensuring the depth is lower than the pencil line. ​
  • Place the TLC plate into the solvent, ensuring the solvent does not touch the spot, and that the edges of the TLC plate do not touch the sides of the tank
  • Place a lid on the solvent tank. ​
  • Allow the solvent to rise until it is about 1 cm below the top of the plate​
  • Remove the plate, and mark the solvent level immediately with pencil​
  • Allow the plate to dry​, and use a dye if necessary to show the position of the spots
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3
Q

How are TLC plates analysed?

A
  • The Rf (retention factor) is calculated for each component
  • Rf values are compared to a database to identify components
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4
Q

How are Rf values calculated?

A
  • Distance moved by the component / distance moved by the solvent front
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5
Q

What is gas chromatography used for?

A
  • To separate volatile components in a mixture
  • Useful for analysing compounds with low boiling points
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6
Q

How does gas chromatography work?

A
  • The stationary phase is a high boiling liquid adsorbed onto an inert solid support
  • The mobile phase is an inert carrier gas (e.g. nitrogen)
  • Separation is by relative solubility with the stationary phase
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7
Q

How are gas chromatograms analysed?

A
  • The retention time (the time taken for each component to travel through the column) is compared to a database and is used to identify the component
  • The peak integration (area under the peak) tells you the relative concentrations of the components
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8
Q

How can you find the the concentration of a component using gas chromatography?

A
  • You prepare standard solutions of the component and run gas chromatograms for each
  • Plot a calibration curve of peak area against concentration
  • Run a gas chromatogram for your sample, and find the peak integration of the component
  • Use your calibration curve to find the concentration of the component
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9
Q

How are alkenes tested for?

A
  • Decolourise bromine water
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10
Q

How are haloalkanes tested for?

A
  • Heat with aqueous silver nitrate in ethanol
  • Chloroalkanes will form a white precipitate
  • Bromoalkanes will form a cream precipitate
  • Iodoalkanes will form a yellow precipitate
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11
Q

How are carbonyls tested for?

A
  • Add 2,4-DNP
  • Orange precipitate formed
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12
Q

How are aldehydes tested for?

A
  • Add Tollens’ reagent
  • Silver mirror formed
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13
Q

How are aldehydes tested for?

A
  • Add Tollens’ reagent
  • Silver mirror formed
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14
Q

Which 2 functional groups are tested for in the same way? How are they tested for?

A
  • Primary alcohols
  • Secondary alcohols
  • Aldehydes
  • Warmed with acidified potassium dichromate
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15
Q

How are phenols tested for?

A
  • As weak acids, they can react with strong bases (e.g. NaOH) but not weak bases, such as Na2CO3
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16
Q

How are carboxylic acids tested for?

A
  • They can react with weak bases like Na2CO3
  • Effervescence would be given off
17
Q

How does NMR spectroscopy work?

A
  • In nuclei where there is an odd number of nucleons, the nucleus is either ‘spin up’ or ‘spin down’ (when there is an even number of nucleons equal numbers are spin up and spin down)
  • In the presence of a strong magnetic field and radio waves of the correct frequency, the nucleus can absorb energy and rapidly flip between the 2 states
  • NMR spectrometers measure what frequency of radio waves interact with the sample
18
Q

What 2 substances are needed to run NMR?

A
  • TMS
  • A deuterated solvent
19
Q

What is TMS, and why is it used in NMR spectroscopy?

A
  • Si(CH3)4
  • It is used as the standard reference chemical that all chemical shifts are measured against (it has a chemical shift of 0ppm)
20
Q

What are deuterated solvents, and why are they needed in NMR spectroscopy? Give an example of one.

A
  • Most solvents contain carbon and hydrogen atoms, so would produce a peak in both 13C and 1H NMR spectra
  • A deuterated solvent is used instead as all of the 1H atoms are replaced with 2H atoms, so any hydrogen atoms would not produce a peak
  • CDCl3
21
Q

What 2 pieces of information can be obtained from a 13C NMR spectrum, and how?

A
  • The number of carbon environments from the number of peaks
  • The types of carbon environments present from their chemical shift
22
Q

Why do carbon atoms in the same environment have the same chemical shift?

A
  • They would absorb radiation of the same frequency
23
Q

What limitation is there when finding the type of carbon environment from an NMR spectrum?

A
  • Only the bond with the highest shift value is shown on the spectrum
24
Q

If there are 2 or more peaks in the same region of an NMR spectrum, how can their environments be told apart? What exception is there to this?

A
  • The one with a higher shift value would be closer to the bonds that would have a higher shift value in the molecule
  • This is not true for aromatic compounds

OCR A A-Level Organic Chemistry (22 decks)

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