Experiment 3: Distillation

Part A: Simple DistillationPart B: Fractional Distillation

Reading: Mohrig, Hammond & Schatz Ch. 13 pgs 141-164; 166-173 Ch. 6 pgs 49-53; 55-58

watch the technique video on the course website!

Distillation

• Common method for the purification of organic liquids

• Process involves: 1. vaporization of a compound to separate it from it's less volatile

contaminants 2. condensation & collection of the "pure" distillate

• Various types of distillation; method used will depend on the boiling point of the compound to be isolated & the specifics of the desired separation - simple distillation: useful for the separation of liquids with boiling points that differ by 70°C or more - fractional distillation: useful for the separation of compounds that boil at

less than 70°C of one another - vacuum distillation: run in a sealed apparatus under vacuum; used for

extremely high boiling liquids or those that decompose at high temp

DistillationSimple Distillation Apparatus

distillingflask

distillationhead

water condenser

adapter

reciever

Distillation

applyheat

coolreceiver

• heat liquid• vapor pressure increases until equal to applied pressure; liquid begins to boil • vapors rise through the apparatus (condense on thermometer; register temp)• vapors pass through side arm, condense and are collected

Boiling Point: temperature at whichthe vapor pressure of a liquid equalsthe applied pressure

so, if reduce the applied pressure(e.g. apply vacuum), we effectively lower boiling point!

notelocation

Temperature vs. Time Behavior

• Pure Liquid - vapor temperature increases to the boiling point - composition of vapor/liquid remain constant (thermal equilibrium) - distillation proceeds at a relatively constant temperature

• Mixture - more complicated! - liquid vapor equilibrium changes over the course of the distillation - temperature varies as the distillation proceeds

pure liquid

Temperature vs. Time Behavior

Mixture 1bp differ by < 70°C

Mixture 2bp differ by > 70°C

Temperature vs. Time Behavior• These temperature changes described by two principles:

Dalton's Law: vapor pressure of a liquid (P) is the same as the partial pressure of the individual components

Raoult's Law: the partial vapor pressure of a compound in a mixture is equal to the vapor pressure of the pure compound times it's mole fraction

P = PA + PB

PA = PApure • XA

When a mixture begins to boil, the vapor always contains a higher % of the more volatile component than does the liquid

As the distillation proceeds, the residual liquid is enriched in the higher boiling component, the temperature required to make residue boil increases, & the composition of the vapor changes

both the vapor pressure & the amount of each component is important

Phase Diagram (plot of vapor + liquid composition vs. temp)

• Mixture 2: 50:50 Mixture of A + B (BPs differ by > 70°C)

bp of A

bp of B

Phase Diagram• Mixture 2: 50:50 Mixture of A + B (BPs differ by > 70°C)

50:50 A:B - vapor contains almost exclusively A

bp of A

bp of B

Phase Diagram• Mixture 2: 50:50 Mixture of A + B (BPs differ by > 70°C)

10:90 A:B - vapor still contains almost exclusively A!

bp of A

bp of B

Temperature vs. Time Behavior

Mixture 2bp differ by > 70°C

expect clean separation of A and Bby simple distillation

Phase Diagram• Mixture 1: 50:50 Mixture of A + B (BPs differ by < 70°C)

bp of B

bp of A

Phase Diagram• Mixture 1: 50:50 Mixture of A + B (BPs differ by < 70°C)

50:50 A:B - vapor contains ~ 90% A

Phase Diagram• Mixture 1: 50:50 Mixture of A + B (BPs differ by < 70°C)

20:80 A:B - vapor still only about 80% A

Temperature vs. Time Behavior

Mixture 1bp differ by < 70°C

can't adequately separate A and B by simple distillation

How Can We Isolate A? BPs < 70° apart

• Successive distillations

- isolate partially purified material (say 70:30 A:B)

- distill again

- and again (etc.)

• Fractional distillation!!

- basic principles the same

- now provide surface for repeated vaporization & condensation - the fractionating column!

Fractional Distillation Apparatus

fractionatingcolumn

Fractional Distillation BPs < 70° apart

5:95 mixture of A & B

L3 = 50% A, bp 63°

Temperature vs. Time Behavior

Fractional Distillationbp differ by < 70°C

expect clean separation of A and Bas if bps differ by a greater amount

Next Week (September 26-30)

A. Simple Distillation

B. Fractional Distillation

separation of cyclohexane & toluene by these methodswill work in pairs (one distillation each; exchange data)lab reports are done individually

Experiment 3: Distillation

DUE: Recrystallization Lab Report (exp 2)

Lab Reports are due at the beginning of your regular lab session

Experimental Details - Parts A & B

Work in pairs (each student does one distillation):

1. Obtain cyclohexane:toluene mixture - add to clamped distilling flask

2. Add a boiling chipprovides surface for nucleation - promotes vaporizationhelps to disperse heat; prevents hot spots & "bumping"

3. Assemble microscale apparatus (simple or fractional - record which)

4. Before you begincheck to be sure all joints are sealed & apparatus is securecheck thermometer position!!

5. Proceed with distillationcarefully control temperature; optimal rate - 2 drops per minuterecord temperature with increasing distillate volume (every 2 drops)

e.g. at 2 drops, at 4 drops, at 6 drops. etc.record total volume collected and "hold-up"

6. Exchange data with your lab partner (record their name)

long-neckroundbottom flask

distillationhead

collectionflask

ice bathboiling chip

sand bath

thermometeradapter

connector withsupport rod

Simple Distillation: Microscale Apparatus

note thermometer position:bulb is below the elbow

stir sand around to controltemp (HOT! use a spatula)

wrap looselywith foil to

help maintain temperature

thermometeradapter

connector withsupport rod

distillationhead

collectionflask

ice bath

sand bath

boiling chip

short-neckroundbottom flask

air condenserpacked with

stainless sponge

Fractional Distillation: Microscale Apparatus

bulb location

wrap looselywith foil to

help maintain temperature

Some Pointers:• Control the temperature carefully - don't overheat - collect distillate at a rate no faster than 2 drops per minute

(be sure collection flask is arranged so can see individual drops!) - if faster, separation (results) will be poor

• Watch for leaks - check connectors before you begin - be sure joints are tight (avoid evaporation of distillate along the way)

• Do not stop the distillation too soon - continue until only about 0.4mL solution remains in the roundbottom - temperatures may fluctuate a bit

a sudden drop in temperature may signify your distillation is over alternatively, may mean you have exhausted the first component, with the second yet to distill commonly seen in fractional distillation (sometimes the simple) check the volume in the "still pot" (= roundbottom flask)

Some Pointers:

• Don't distill to dryness - flask may overheat and break

• Data Collection - make a table in your notebook before you begin to collect data

volume of distillate (# drops, e.g. 2, 4, 6, 8 …) versus temperature (°C)

- measure the total volume of distillate collected (recovery) transfer to graduated test tube to get an accurate measurement

- measure the volume of distillate that remains in the roundbottom again, transfer to graduated test tube this is your "hold-up" - the amount of liquid not recovered the amount of liquid retained by the apparatus

Writing the Lab Report: Exp #2 Recrystallization

Purpose - technique experiment: what will you learn? - what conclusions will you reach?

Results & Discussion - Part A: solubility experiments

present your dataspecifically identify solvent you would use to recrystallize compoundclearly explain why you made this choice (refer to your observations)

(a clear decision!)

See pgs 9-12 and pg 44 in your lab manual for more detailed instructions

Writing the Lab Report: Exp #2 Recrystallization

Results & Discussion (continued) - Part B: recrystallization of phenacetin

evaluate success of your purificationIs the compound pure?

what data do you have at your disposal?physical appearance?TLC and melting point

what do you expect to see if the compound is pure?what do you expect to see if the compound is impure?

only after you answer these questions can you addresswhether or not the purification was a success!

comment of the efficiency of your recrystallizationhow much did you get back?% recovery

Writing the Lab Report: Exp #2 Recrystallization

Appendix A: Calculations - Rf values (just one!)

yes, even though you've done it before

- Percent Recoveryamount of compound you got backversus "percent yield" amount of compound that you made

% recovery =amount pure product recovered (g)amount of crude material used (g) x 100