lab

Question 1

general IR patterns

Answer 1

• stronger bonds will vibrate at a higher frequency
• smaller atoms will vibrate at a higher frequency

***based on Hooke’s law

Question 2

what to look for in melting point data

Answer 2

• accurate melting points are small range close to lit value
• inaccurate melting points from impurities are large range and lower than lit value

Question 3

crystals in a pure sample are…

Answer 3

• stable
• well-ordered
• low Gibbs energy

Question 4

reflux

Answer 4

• solution is heated to its boiling point
• vapor produced is constantly cooled back into liquid form
• allows reaction to run while being heated, but no reactants or products are lost from evaporation!!

Question 5

extraction

Answer 5

• separating aqueous and organic layer (2 immiscible liquids)
• more small extractions are better

Question 6

sublimation

Answer 6

solid goes to gas without melting–purification process!

Question 7

using a cold finger to purify

Answer 7

• crystals sublime (go straight to gas)
• gas product then re-solidifies on the cold finger

Question 8

percent recovery vs percent yield

Answer 8

• percent recovery is for isolations
• percent yield from synthesis

Question 9

general principles of chromatography

Answer 9

• components are either more in stationary or mobile phase
• phase depends on polarity of components (will be in phase that matches their polarity)

Question 10

TLC specifics

Answer 10

• stationary phase (adsorbent): usually silica on a plate, does not move
• mobile phase (eluent): liquid that moves across plate

*stationary phase is usually the more polar phase!

Question 11

Rf values

Answer 11

• for TLC; distance traveled by compound of interest divided by distance traveled by solvent front
• values closer to 0 indicate compound is more polar
• values closer to 1 indicate compound is less polar

Question 12

shielding in NMR

Answer 12

• shielding: more electrons near the nucleus decrease the magnetic field sensed by the nucleus
• deshielding: alternatively, electron-withdrawing atoms will increase magnetic field sensed by nucleus
• more deshielded nuclei come into resonance at a higher frequency (more downfield, or left)