Distillation
Experiment 3: 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
Part A: Simple Distillation
Part B: Fractional Distillation
• Various types of distillation; method used will depend on the boiling
point of the compound to be isolated & the specifics of the desired
separation
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
- 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
Distillation
Simple Distillation Apparatus
distillation
head
so, if reduce the applied pressure
(e.g. apply vacuum), we effectively
lower boiling point!
water condenser
distilling
flask
Boiling Point: temperature at which
the vapor pressure of a liquid equals
the applied pressure
note
location
adapter
apply
heat
reciever
cool
receiver
• 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
Temperature vs. Time Behavior
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
pure liquid
• Mixture
Mixture 1
bp differ by < 70°C
- more complicated!
Mixture 2
bp differ by > 70°C
- liquid vapor equilibrium changes over the course of the distillation
- temperature varies as the distillation proceeds
Temperature vs. Time Behavior
• These temperature changes described by two principles:
Phase Diagram
(plot of vapor + liquid composition vs. temp)
• Mixture 2: 50:50 Mixture of A + B (BPs differ by > 70°C)
Dalton's Law: vapor pressure of a liquid (P) is the same as the partial
pressure of the individual components
P = PA + PB
bp of B
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
PA = PApure • XA
both the vapor pressure & the amount of each component is important
! 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
bp of A
Phase Diagram
Phase Diagram
• Mixture 2: 50:50 Mixture of A + B (BPs differ by > 70°C)
• Mixture 2: 50:50 Mixture of A + B (BPs differ by > 70°C)
bp of B
bp of A
bp of B
bp of A
50:50 A:B - vapor contains almost exclusively A
Temperature vs. Time Behavior
10:90 A:B - vapor still contains almost exclusively A!
Phase Diagram
• Mixture 1: 50:50 Mixture of A + B (BPs differ by < 70°C)
bp of B
bp of A
Mixture 2
bp differ by > 70°C
expect clean separation of A and B
by simple distillation
Phase Diagram
Phase Diagram
• Mixture 1: 50:50 Mixture of A + B (BPs differ by < 70°C)
• Mixture 1: 50:50 Mixture of A + B (BPs differ by < 70°C)
50:50 A:B - vapor contains ~ 90% A
Temperature vs. Time Behavior
20:80 A:B - vapor still only about 80% A
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
Mixture 1
bp differ by < 70°C
- now provide surface for
repeated vaporization &
condensation - the
fractionating column!
fractionating
column
can't adequately separate A and B by simple distillation
Fractional Distillation Apparatus
Fractional Distillation
BPs < 70° apart
Temperature vs. Time Behavior
L3 = 50% A, bp 63°
Fractional Distillation
bp differ by < 70°C
5:95 mixture of A & B
Next Week
(September 26-30)
Experiment 3: Distillation
expect clean separation of A and B
as if bps differ by a greater amount
Experimental Details - Parts A & B
Work in pairs (each student does one distillation):
1. Obtain cyclohexane:toluene mixture - add to clamped distilling flask
A. Simple Distillation
B. Fractional Distillation
DUE:
2. Add a boiling chip
provides surface for nucleation - promotes vaporization
helps to disperse heat; prevents hot spots & "bumping"
separation of cyclohexane & toluene by these methods
3. Assemble microscale apparatus (simple or fractional - record which)
will work in pairs (one distillation each; exchange data)
lab reports are done individually
4. Before you begin
check to be sure all joints are sealed & apparatus is secure
check thermometer position!!
Recrystallization Lab Report (exp 2)
Lab Reports are due at the beginning of your regular lab session
5. Proceed with distillation
carefully control temperature; optimal rate - 2 drops per minute
record 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)
Simple Distillation: Microscale Apparatus
note thermometer position:
bulb is below the elbow
wrap loosely
with foil to
help maintain
temperature
thermometer
adapter
distillation
head
connector with
support rod
long-neck
roundbottom flask
boiling chip
collection
flask
Fractional Distillation: Microscale Apparatus
thermometer
adapter
bulb location
distillation
head
air condenser
packed with
stainless sponge
connector with
support rod
collection
flask
ice bath
ice bath
short-neck
roundbottom flask
wrap loosely
with foil to
help maintain
temperature
boiling chip
stir sand around to control
temp (HOT! use a spatula)
sand bath
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)
sand bath
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
See pgs 9-12 and pg 44 in your lab manual for more detailed instructions
# Purpose
- technique experiment: what will you learn?
- what conclusions will you reach?
Writing the Lab Report: Exp #2 Recrystallization
# Results & Discussion (continued)
- Part B: recrystallization of phenacetin
evaluate success of your purification
present your data
specifically identify solvent you would use to recrystallize compound
Is 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 address
clearly explain why you made this choice (refer to your observations)
(a clear decision!)
whether or not the purification was a success!
comment of the efficiency of your recrystallization
# Results & Discussion
- Part A: solubility experiments
how 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 Recovery
amount of compound you got back
versus "percent yield" # amount of compound that you made
% recovery =
amount pure product recovered (g)
amount of crude material used (g)
x 100