Distilling a Mixture of Cyclohexane and Toluene

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Distilling a Mixture of Cyclohexane and Toluene

• Organic Chemistry Lab I
• Fall 2009
• Dr. Milkevitch
• September 28 30, 2009

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Steps in a Chemical Synthesis

• Determine target compound
• Design synthesis (research reactions)
• Obtain starting materials equipment
• Assemble apparatus, begin synthesis
• Monitor complete synthesis
• Extract and isolate product
• Purify product
• Identify product (spectroscopic methods, physical
  properties)
• Assess success of synthesis
• Modify synthesis, continue work if necessary

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Most Important Steps??

• Determine target compound
• Design synthesis (research reactions)
• Obtain starting materials equipment
• Assemble apparatus, begin synthesis
• Monitor complete synthesis
• Extract and isolate product
• Purify product
• Identify product (spectroscopic methods, physical
  properties)
• Assess success of synthesis
• Modify synthesis, continue work if necessary

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Introduction to Simple Distillation

• Simple Distillation A purification method for
  liquids, which involves the condensation of
  vapors from a boiling liquid and collection of
  those vapors in a receiving vessel

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What occurs during a distillation
Water outlet/ inlet
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Theory

• A pure liquid exhibits a vapor pressure that is
  dependent on temperature
• Thats why you can smell some fragrant liquids

Snif, snif
Liquid exerts a pressure at ambient temp/ pressure
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When you heat a liquid

• Vapor pressure increases slowly
• Then, more rapidly near the boiling point
• More volatile liquids (e.g., diethyl ether)
  exhibit higher vapor pressures at all temps

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Definition of Boiling Point

• Boiling Point The temperature at which the
  vapor pressure above the solution equals the
  atmospheric pressure.
• Lots of molecules in the air above liquid
• Equals atmospheric pressure
• Solution boils off

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Common to distill mixtures of liquids

• Cyclohexane / toluene are an example
• Solution of cyclohexane / toluene boils when the
  vapor pressure (Ptot) is equal to the atmospheric
  pressure (Patm).
• Contribution of each component to the total
  pressure is called partial pressure
• Patm Ptot Pcyclohexane Ptoluene
• Daltons law of partial pressures

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Raoults Law

• Simple proportionality between solvent mole
  fraction and its partial pressure at a given
  temperature.
• Partial pressure of a solution is the product of
  the vapor pressure of the pure component (P?) at
  a particular temperature and the mole fraction
  (X) of the component.
• Mole fraction the number of moles of a component
  divided by the total number of moles of all
  components

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Raoults Law

• Partial pressure of cyclohexane
• Pcyclohexane P?cyclohexane Xcyclohexane
• Partial pressure of toluene
• Ptoluene P?toluene Xtoluene
• Therefore
• Ptot P?cyclohexane Xcyclohexane
  P?toluene Xtoluene

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Miniscale Apparatus for Distillation
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Important Tips Concerning Miniscale Distillation

• Never heat a system the is totally closed from
  the outside
• Pressure builds up, can be very dangerous
• Never heat a system to dryness
• Explosion possible
• Do not overfill distillation flask (2/3 full max)
• Add boiling chips/ magnetic stir bar to reduce
  bumping.
• Do not add boiling chips/ stir bar while solution
  is boiling (can bump because the chips/ stir bar
  are cold )
• Place thermometer below neck of distilling head
• Water inlet is the lower condenser nipple, outlet
  is upper nipple
• Control rate of heating carefully (overheating
  can cause bumping)
• Hand hot plates carefully, for the tops get quite
  hot
• Distilling high-boiling solvent, wrap distilling
  head and flask with foil to provide insulation
• If BP gt150 C, you dont need to run water through
  the condenser

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Procedure

• Lay out glassware, check for cracks
• Pipet 10 ml each of cyclohexane and toluene into
  a 50 ml RB flask
• Add spin bar
• Assemble apparatus
• Connect water lines
• Have myself or your TA check apparatus
• Position the bulb of the thermometer below the
  neck of the distillation head
• Position the graduated cylinder below bent
  adaptor
• Turn on water slowly. You only need a slow
  flow
• Turn on heat to 6. Increase rate to 8 when you
  see vapors contact thermometer bulb
• Collect first few drops of distillate and discard
• Collect distillate at a rate 1-2 drops per second
• Record temperature vs. volume of distillate for
  entire distillation
• Take readings every 1-2 ml, or more frequently if
  the temperature is going up rapidly
• Increase heat as you proceed, because the
  distillation pot will enrichen with toluene as
  you proceed
• Distill until 1-2 ml remains in the distillation
  flask
• Break down your apparatus, clean with soap/water
  and put everything away

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Final Notes

• Do not correct boiling points to std pressure
• Estimate and record the temperature ranges at
  which fractions were collected,
• estimate volumes of distillate collected at each
  temperature
• Graph boiling temp vs. volume (ml) of distillate

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Questions to Answer

• 1) What is the relationship between volatility
  and the vapor pressure of a solvent?
• 2) Explain what happens to the vapor pressure of
  a liquid as temperature increases. Are
  temperature and vapor pressure directly
  proportional? If not, explain the relationship
  with a graph.
• 3) Why is it preferable to allow cold water to
  enter at the bottom of a condensor and exit at
  the top rather than vice-versa?
• 4) Can the vapor pressure of a liquid be greater
  than 1 atm? Explain.
• 5) What possible hazard might occur if a
  distillation vessel is heated all the way to
  dryness?