chromotography.

Question 1

on a TLC paper there are 2 spots even though the sample has 3 diff amino acids.explain why there are only 2 spots + explai how chromotogram can be used to identify the amino acids.

Answer 1

1) measure the distance moved by the spot divided by the distance moved by the solvent.
2) compare Rf value with the data book values.
3) one of the spots cotains 2 amino acids with the same RF value.

Question 2

how does GC separate the compounds in a mixture?

Answer 2

by the relative SOLUBILITY in the stationary phase.

Question 3

name the process that TLC separates the amino acids?

Answer 3

adsorbtion

Question 4

state in general terms what determines the distance travelled in a spot by TLC

Answer 4

the balance between the solubility/affinity in the mobile phase and retention of compound by the mobile phase.

Question 5

breifly explain how the peaks in NMR spectra and the absorbtions in IR spectra are formed.

Answer 5

NMR + IR both involve absorbtion of energy
NMR involves the proton spin aligned with or against the magnetic feild (1)
IR —> bonds vibrate.

Question 6

what info can be derived from

• TLC
• GC

Answer 6

• TLC shows number of components in the mixture

- GC shows conc of each component.

Question 7

what are some limitations of TLC?

Answer 7

• difficult to measure hte exact center of the spot
• similar compounds may have similar Rf values
• reference chromotogram may not exist in the data base
• one solvent may not dissolve all the components in the sample.
• a compound may be TOO soluble and thus washes off in the solvent.

Question 8

what is GC used for?

Answer 8

• to separate VOLATILE , compounds in a mixture

Question 9

what is GC useful for?

Answer 9

• analysing org compounds with low BPs

Question 10

what is the mobile phase of the GC?

Answer 10

the inert carrier gas

Question 11

what is the stationary phase of the GC?

Answer 11

• the high BP liquid that is ADSORBED onto the solid support

Question 12

what is the separation in GC based on?

Answer 12

• relative solubility

Question 13

what is the retention time?

Answer 13

• the time taken for the sample to travel through the column

Question 14

a greater solubility b/w the sample and the stationary phase (ie the liquid) the ________ the retention time .
Why?

Answer 14

• the longer

- bc the compound spends more time condensed as a liquid + moves slowly through the column.

Question 15

if the liquid stationary phase is NON POLAR, which type of compounds would have shorter/ faster retention times + why?

Answer 15

• non polar compud would have LONGER retention times as they will interact the MOST.
• POLAR molecules will interact the LEAST and thus will pass through quickly and have shorter retention times.

Question 16

what is the area of the peak in the chromotomagram relative to?

Answer 16

• concentration of each compoud.

Question 17

discuss the limitations of the GC? (3)

Answer 17

• some compounds may have the SAME retention times.
• small amount of one component may HIDE behind a component with a higher concentration.
• unknown compounds have NO retention data to compare with.

Question 18

what does an NMR spectometer measure?

Answer 18

• the amount of (radio wave ) energy that is ABSORBED and able to resonate the nuclei (ie flip the spin states in the nuclei of the atoms) ( of c13/H1 (both have uneven no of nucleons thus can spin)

Question 19

what is the solvent that the sample is dissolved in?

Answer 19

CDCl3

Question 20

what is the reference molecule used

Answer 20

TMS

Question 21

how do deuterated solvents remove OH/NH signals on HMNR spec?

Answer 21

• they have NO spin and thus will NOT absorb any energy.
• they thus replace the OH/NH bond and bc they have no spin and thus dont absorb any energy, they will NOT show any peaks.