orgo lab final

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orgo lab final
2013-04-10 01:02:05
orgo lab

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  1. what is the purpose of carrying out a melting point analysis?
    • 1. characterize a known compound if choosing between several known
    • 2.record physical characteristic for new compound
    • 3.establish purity of a known compound
  2. melting point of a pure compound
    if a compound melts over a narrow temp range, usually less than one degree.
  3. what is the criteria for impurity?
    must be soluble in a given compound to depress the purity
  4. what forces affect melting point?
    • ionic 
    • van der waals 
    • dipole-dipole
    • h-bonding (more = higher temp)
    • size of molecules
  5. eutectic point
    point at which two solids of liquid solution are in equilibrium and at what temperature they melt
  6. how to find unknown?
    if you have 3 knowns and must choose between the 3, mix unknown with all different, one with the least/no change is the known
  7. what if you heat the sample too quickly?
    incorrect temperature reading, sample at a diff temp than thermometer
  8. melting range
    from first drop of liquid until complete liquefication
  9. eutectic temperature
    lowest temperature at which a mixture of a given composition will start melting
  10. why carry out a recrystallization?
    to purify a product
  11. picking solvent for recrystallization
    like dissolves like
  12. requirements of solvents for solvent pairs?
    • dissolves solvent completely at solvents melting point
    • precipitates solvent completely when solvent is cold
    • does not react with solute
    • does not dissolve impurities
  13. why cool slowly?
    if cooled too quickly solid will ppt with impurities crashing out
  14. remedies for unsaturated solution
    boil off excess solvent in respirator tube
  15. remedies for supersaturation?
    use seed crystals to induce crystallization and scratch bottom of test tube with glass rod
  16. superheating
    • above boiling point without boiling
    • to prevent, use boiling stick
  17. oiling out.. remedy? prevention?
    • boiling point of solvent is higher than melting point of solute
    • add more solvent
  18. why remove boiling stick during crystallization?
    so that you dont lose crystals that form on the boiling stick
  19. two methods to filtering out solid impurities from hot solution are
    • decantation
    • gravity filtration
  20. 7 steps of recrystallization
    • 1.choose solvent pair
    • 2.dissolve solute
    • 3.decolorize the solution
    • 4.filter out suspended solute
    • 5.crystallizing the solute
    • 6.collecting and washing crystals
    • 7.drying
  21. why do we carry out distillations?
    • purify liquids
    • separate based on boiling points
  22. why wrap apparatus with aluminum foil?
    insulate and minimize heat loss
  23. why wrap sidearm with wet towel?
    to ensure that vapor condenses in side arm
  24. when do you use simple distillation?
    when boiling point difference between the two are large and one liquid composition is less than 10%
  25. when do you use fractional distillation?
    • when liquid composition is substantial
    • boiling point difference between the 2 is small
  26. azeotrope
    • mixture with a fixed composition that can't be altered by simple distillation
    • has a constant boiling point
  27. when is steam distillation used?
    • for immiscible liquids such as water
    • used when substances decompose before their melting points
  28. when do you use vacuum distillation?
    • for compounds that decompose before their boiling points
    • slowly ascending to obtain equilibrium
  29. never distill to ______? because...
    dryness, dry residue of liquids can explode
  30. never distill in a _________? because..
    closed, apparatus can rupture
  31. sources of error for distillation?
    • 1. loose connectors
    • 2. temp variation from airflow in hoods
    • 3. large apparatus, lose heat
  32. boiling point of a liquid
    temperature at which vapor pressure of the liquid equals the force exerted on it
  33. what to avoid in tlc?
    • smeared spots will make it hard to obtain Rf
    • flaked adsorbent won't allow solvent to rise
  34. why should solvent be below the spotting line?
    so that solvent doesnt dissolve samples
  35. make sure the top is on the container for tlc
    so that solvent doesnt evaporate and make a messy plate
  36. filter paper must be in tlc chamber to..
    make sure chamber is saturated with solvent
  37. solvents with _________ migrate rapidly.
    low boiling points and low viscosity
  38. silica is good for
    polar compounds
  39. alumina is good for
    nonpolar compounds
  40. uses of tlc?
    • 1. see if a solid is pure
    • 2. see what compounds are in sample
    • 3. determine identity of compound
    • 4. monitor progress of reaction
    • 5. determine effectiveness of purification
    • 6. determine appropriate conditions for column chroma
    • 7. monitor column chroma
  41. too polar solvent in tlc
    spots will move towards solvent frnt
  42. too nonpolar a solvent in tlc
    spots will stay at bottom
  43. reasons to carry out extractions?
    separate a mixture
  44. order of carrying out extractions? why?
    • weak base then strong base
    • to avoid strong acid reacting and neutralizing weak base
  45. requirements for extraction solvents
    • 1.readily dissolve others to be extracted
    • 2. not miscible with water
    • 3. not reacting with solute
    • 4.low boiling point
    • 5. non flammable
    • 6. non toxic
    • 7. cheap
  46. emulsions.. how do we fix it?
    • suspension of one liquid as droplets in another caused by too much shaking
    • fix by adding nacl to aq layer
  47. how do you decrease solubility of a subtance in organic layer?
    add nacl or cacl2
  48. what indicates that the organic layer is dry?
    its clear
  49. choosing drying agent?
    • 1.must not react with solute
    • 2.must remove water quickly
    • 3.easy to recover solute from agent
  50. advantages of t-butyl ether
    • forms azeotrope with water
    • doesnt need trying
    • doesnt form peroxides
    • cheaper
  51. disadvantages of diethyl ether
    • flammable
    • explosive
    • forms peroxides
  52. in extractions, why not 100% recovery?
    solute is nearly never completely transferred to organic layer
  53. sn1 reactions
    • polar protic solvent- carbocation stabilized
    • rearrangements can occur
    • tertiary>secondary
    • racemic mixture
  54. sn2 reaction
    • aprotic polar solvent
    • no rearrangements concerted
    • all in one step- fast
    • inverted configuration
  55. why add same amount of alkyl halide and nuc
    for sn2 amount of nucleophile affects rate and adds another variable
  56. possibility of a false sn1sn2?
    too much alkyl halide added can make halide act as a solvent in which nuc isnt soluble
  57. vinyl and aryl halides dont undergo sn1 because
    they are unstable carbocations
  58. vinyl and aryl halides dont undergo sn2 because
    electrons in double bonds repel nucleophile
  59. why dont the br- and cl- attack alkyl iodides that form in sn2?
    • because they are taken up to form nacl and nabr
    • none free
  60. esterification lab.. how could le chatelier principle be used in the reaction?
    • 1. use of excess cheap reagent: boiling points are close and excess of starting material would be an impurity
    • 2. remove product water by drying with molecular sieves of azeotropic distillation
  61. how to remove water?
    • 1. molecular sieves
    • 2. azeotropic distillation
  62. why was the esterification apparatus set up at an angle?
    so that water could be removed as distillation was happening
  63. purpose of IR?
    to identify the products
  64. how could we have used tlc in esterification lab?
    identify what was in the product
  65. role of resin in esterification?
    • acid catalyst
    • easily removed by filtration or pipetting out of flask
  66. disadvantage of using traditional acid catalyst?
    difficult to remove, would need to dilute with ether then dry then evaporate
  67. reason reduction may not go to completion?
    entire reaction is reversible
  68. how is a reduction characterized in orgo?
    • increase in the number of bonds to H
    • decrease of number of bonds bonded to electronegative atoms
  69. why is nabh4 used rather than lialh4?
    • milder
    • more selective in reducing aldehydes and ketones
    • does not react violently with alcohols and water
    • lialh4 produces hydrogen gas
  70. why was ethanol used in stead of ether?
    its insoluble and reduces ketones and aldehyde
  71. why was nabh4 used in excess?
    • to ensure the reaction goes to completion
    • account for nabh4 with solvent
    • account for decomposition
  72. role of each addition of water
    • to hydrolyze ester
    • saturate solution to start recrystallization
  73. role of ice bath?
    to slow down rxn with sodium hydroboride so that its not explosive
  74. how do you minimize getting a mixture of products in aldol lab?
    • use excess of reactant with no hydrogen
    • use more reactive aldehyde in presence of unreactive ketone
    • deprotonate carbonyl with lda to convert the enolate then add second carbonyl slowly
  75. role of sodium hydroxide in dibenzalacetone synthesis?
  76. reason for washing crystals with water?
    to remove base catalyst that can cause oiling out during recrystallization
  77. requirements for starting materials for aldol addition and condensation
    • carbonyl must have one alpha hydrogen for it to form enolate
    • carbonyl that forms enolate must have at least 2 aplha hydrogens to form condensation product
  78. why does the enolate form
    because carbon is deprotonated easily (acidic alpha hydrogen because of resonance when removed)