Card Set Information

2011-12-05 22:32:01
organic chemistry

lab final
Show Answers:

  1. T/F - The melting point range of a pure crystalline solid is broad (more than 5 *)
  2. Biphenyl has a known melting point of 70-71 *C. You measure the melting point of Unknown X to be 70-71 *C. The biphenyl Unknown X mixture melts at 60-66 *C. Unknown X_____(is/ is not) Biphenyl.
    is not
  3. T/F – Wet samples give erroneously low melting points
  4. What is the solution you filter the crystals away from called?
    Mother Liquor
  5. T/F – A recrystallizing solvent should be readily removable from the purified product
  6. T/F – Impure substances should be dissolved in copious amounts of recrystallizing solvent in order to get the best purification results
  7. The _________ of a liquid is that temp at which the vapor pressure of the liquid becomes equal to the pressure exerted by its surroundings
    boiling point
  8. Where should the thermometer be positioned in the distillation apparatus
    just below the side arm
  9. Which component should be isolated by distillation first, the component with a lower boiling pt or the component with a higher bp?
    lower bp
  10. What does TLC stand for?
    Thin Layer Chromatography
  11. What does Rf stand for?
    rate of flow
  12. What is the Rf value if a compound travels 2 cm up the TLC plate and the solvent travels 4 cm?
  13. Cis-trans isomerism is a consequence of
    restricted rotation at the carbon-carbon double bond
  14. Lachrymators
    Cause eye irritation, produce tears, and should be used in the hood only
  15. Sn1 parallels
  16. T/F – Low molecular weight alcohols are readily soluble in water
  17. T/F – Phenols are basic
  18. Combining substances to create a large amount of diverse, but similar substances
    combinatorial chemistry
  19. T/F – aldehydes are oxidized by Tollens reagent, ketones are not
  20. Which is less soluble in water, 1-pentanol or 1-heptanol?
  21. How fast do primary, secondary and tertiary alcohol reactions occur with the Lucas reagent?
    Primary – no rxn, sec- slower than tert.
  22. Are tertiary alcohols oxidized with the Bordwell-Wellman test? Why or why not?
    No, will not form a double bond
  23. Are phenols more or less acidic than alcohols? Why?
    More acidic – because the negative charge in the phenoxide ion can be delocalized around the aromatic ring, so it is more likely to donate a proton
  24. Why was the alcohol used in excess during the ester synthesis?
    The alcohol shifts the equilibrium to favor ester synthesis
  25. What would happen if you left sulfuric acid out of the ester synthesis?
    The reaction wouldn’t go or would be very slow because the alcohol must be protonated
  26. Explain the unusually reaction of the allyl chloride with Ag+ in view of the fact that it is a primary halide.
    The double bond stabilizes the carbocation with resonance
  27. Explain the low reactivity of chlorobenzene toward Ag+
    It is sp2 hybridized
  28. What is the purpose of recrystallization
    To remove any impurities within a substance
  29. How are soluble impurities removed during recrystallization?
    Using a vacuum filtration device which removes the solvent from the recrystallized crystals
  30. How does scratching the inside of the vessel with a glass rod induce crystallization?
    It might: create glass particles which provide nuclei for crystals to form, or small amounts of solution drawn on sides of the vessel evaporate to produce dry solutes that are pushed back into the solution
  31. What properties are desirable in a recrystallization solvent?
  32. Dissolve a moderate amount of substance at higher temps, but a small quantity at lower temps
    • Not react with substance being purified
    • Dissolve impurities at a low temp or not at all
    • Be easily removable from purified substance
  33. Give two reasons why vacuum filtration is sometimes preferable to ordinary gravity filtration
  34. Vacuum filtration increases the filtration rate-
    helps dry purified crystals for an more accurate mass measurement
  35. Why is water a good solvent for the recrystallization of acetanilide
  36. Acetanilide completely dissolves in hot water and begins to precipitate out as the water cools
  37. Why must the funnel be heated before the hot acetanilide solution is filtered?
    If the funnel isn’t hot, the acetanilide solution will cool when it hits the funnel resulting in premature crystallization
  38. Assuming charcoal and sugar are the main impurities in a sample of crude acetanilide, explain how recrystallization from water would remove each
    The charcoal is caught by the filtration paper, and the sugar remains dissolved in the solution when the acetanilide precipitates out
  39. Why is it essential to pack the sample tightly in the melting point tube and heat the sample slowly?
    The tightly packed tube allows for heat transfer, and the slow heating prevents too high of a melting point
  40. What is the effect of using too large a sample when determining melting point
    Larger amounts take longer to heat completely which results in a larger mp range
  41. Research chemists when determining melting points of newly synthesized compounds often fill a second tube with a known substance of similar melting point range and co-determine the mps of the two substances using the same apparatus. What purpose does this serve?
    It allows the experimenter to check the accuracy of the thermometer
  42. For what two purposes are melting points routinely used?
    • -to determine purity of a substance
    • -identify an unknown compound
  43. A pure liquid has a constant boiling point, but a liquid with a constant boiling point is not necessarily pure. Explain.
    Constant bp is indicative of an equilibrium between molecules in gas and liquid
  44. What effect does a reduction in the atmospheric pressure have on the boiling point of a liquid?
    It lowers it
  45. Why doesn’t a pure liquid in a distilling flask vaporize all at once when the boiling temperature is reached?
    The molecules towards the top of the liquid are subjected to the pressure before the rest of the liquid
  46. Why is it important that the cooling water in a distillation apparatus enter the condenser jacket at the lower end and exit at the upper end and not vice versa
    If it were switched, the substance would condense and fall back into the flask. It is cooled on the lower end so it will condense and drip into the graduated cylinder
  47. Why should a distilling flask be filled to not more than 2/3 of its capacity at the beginning of a distillation procedure?
    It could boil over