R

Recrystallization
t lli ti
Identification of an unknown

CHE-231
CHE 231 Organic Chemistry
University of Kentucky
 Introduction
• One of the most common p
purification techniques
q

• Solid dissolved in solvent then p

precipitated
p back
out as a purified material as impurities remain
soluble in the solvent

• Choose solvent such that impurities are more

soluble than the substance being isolated

• Used when amount of impurities isn’t

isn t too large
 Outline
• Select appropriate solvent
• Dissolve solid near b.p. of solvent
• Filter hot solution to remove impurities if
necessary
• Crystallize solid by cooling
• Separate crystals out
• Wash and dryy crystals
y
 Selecting Solvent
• Solute should be soluble in the solvent at high
t
temperature
t andd insoluble
i l bl att room ttemperature
t

• B.P. of the solvent should be low enough to

easily remove from solute during drying step

• B.P. of solvent < M.P. of solute

• Solvent should not react with the solute

For example…
solvent soluble cold soluble hot
H 2O water yes yes
CH 3 OH methanol
th l yes yes
O
yes yes
CH 3CCH 3 acetone
CH 3COOH acetic acid yes yes

CHCl3 chloroform yes yes

CH 3CH 2OCH 2CH 3 ether no yes

CCl4 carbon no no
tetrachloride
C 5H 12 pentane no no

C 6 H 14 hexane no no
C 7H 16 heptane no no
Solubility at Room Temperature (RT)

• Solute insoluble
in solvent at
Room temperature
• So heat the solvent
and test it for complete
solubility at high
t
temperature
t
Solubility at high temperature

• Upon heating the

p
compound dissolved
completely in the
selected solvent
• It is a good solvent for
recrystallization
lli i
 Mixing Solvent
• Use mixture of solvents only if
single solvents do not work

• Mix according to suggested lists in

handout
 Predicting solubility

• If you know the structure of the compound

to purify…
– Dielectric
Di l t i constant
t t Є
– Hydrogen bonding capabilities
 Like dissolves like
CH3
dissolves in CH3CH2CH2CH2CH2CH3
hexane
t l
toluene both have C C and C H bonds
O

H3C C OH dissolves in H OH
acetic acid water

both have OH
b th are highly
both hi hl polar
l
 Length of Carbon chain
• C5 rule
– Structures with less than 5 carbons and a polar
function are water soluble
– Branched chain compounds are more soluble than
corresponding
di straight
i h chain
h i compounds.
d (b(branched
h d
lowers intermolecular forces and intermolecular
attraction.))

C H 3C H 2 O H s o lu b le in H 2 O .

CH3

OH in s o lu b le in H 2 O .

CH3
 Solution
• Place solid (~100 mg) in Erlenmeyer flask w/
3-5 mL of solvent
• Heat up to B.P. (stir constantly)
• Add enough solvent to dissolve solute
• Perform hot filtration if needed
• Remove colored impurity w/ activated
charcoal [colored impurities get ADSORBED
(not absorbed) on the surface of charcoal]
 Hot Filtration
• Removes insoluble impurities and activated
charcoal
h l

• Use short stem glass funnel w/ filter paper

or use the Buchner funnel with vacuum line

• Pour near top of paper to maximize filtration

• Pour into beaker on hot plate containing

small amount of solvent heated near B.P.
 Crystallization
• Cool filtrate slowly at room temperature

• Agitate only if very large crystals form

• If nothing happens use seed crystal (leftover

starting material)

• If oil forms, use seed crystal or recrystallize

 Filtration
• Wet filter paper with solvent. Filter cooled
mixture by suction w/ Buchner funnel and
vacuum flask

• Use stirring rod to transfer crystals

• Release suction after solvent has passed

through filter

• Wash crystals with cold solvent then

reapply suction
 Drying
• Evaporate solvent by vacuum

• Transfer crystals to clean watch glass

• Allow crystals to air dry until next lab

• Do not p
put crystals
y in oven!
 Melting points
• Physical characteristic of pure compound

• If compound is pure, reproducible over range of

0 5 2 0°C
0.5-2.0°C

• IImpurity
it causes both
b th d
depression
i off M
M.P.
P and
d
melting to occur over wide range

• M.P. range can estimate purity

 Determining M.P range
• Use Mel-Temp apparatus, not larger boiling
point apparatus

• Crush portion of dry solid into powder

• Tapp some p
powder ((~2-3 mm)) into capillary
p y
tube

• Invert and tap powder down to sealed bottom

of tube
 Determining M.P range
• If powder shrivels, changes color or
sweats out solvent,
solvent recrystallize

• R
Record d ttemperature
t when
h fifirstt d
drop off
liquid appears and when the entire sample
is melted

• These values represent M

M.P.
P range
 Infrared Spectrum
• Molecules vibrate
andd rotate
t t
• The energies of these
vibrations and rotations
correspond to the
infrared region of the
electromagnetic spectrum
Source: boomeria.org/chemlectures/bonding/modes.jpg

• IR spectrum divided into 2 regions: functional

group region and fingerprint region
Infrared Spectrum continued…
• Functional group region: absorptions are
observed for particular groups ((-OH,
OH -
NH2, etc.)

• Fingerprint region: complex series of

absorptions specific to a single
absorptions,
compound
Infrared Spectrum continued…
100 light
nce
% trransmittan

0 Functional group region Fingerprint

Fingerprint dark
4000 cm-1 region
region 300
wavenumber
 I. R. of Benzoin

SP3 C-H
CH

SP2 C-H

O-H C=O

SP3 C-H
H

C C

O O
C=O
H H
SP2 C-H O-H
 I. R. of p-bromoaniline

SP2 C-H

NH2 C-Br

Br N

SP2 C-H H
I. R. of 4-cyanopyridine

SP2 C-H C≡N

N C N

H SP2 C-H
 I. R. of p-anisic acid

O-H SP3 C-H

SP2 C-H
C=O

O C O

SP3 C-H H C H

H H H SP2 C-H
 Procedure
• Determine M.P. range of unknown provided
by TA

• Find best solvent for recrystallizing unknown

(list attached to handout)
– Do not use all of the sample
p on the first trial ((use
~30 mg/trial)
– You may not receive additional unknown if you run
out

• Recrystallize unknown as described earlier

Impure!
p dull!! Not crystallized!!!
y
 Procedure continued…
• Dry and weigh purified crystals

• Calculate percent recovery by the following

formula:
Weight of recovered sample
y=
% Recovery X 100
Initial weight of unknown sample

• Obtain M.P. range for recrystallized solid

• Obtain IR spectrum of solid by KBr pellet

Pure! Shiny!!
y Recrystallized!!!
y ☺
 KBr pellet : for solid samples…
• KBr in the oven, important to remain dry

• Using mortar and pestle, grind ~100 mg of KBr

with a small amount (~1 mg) of your sample;
{KBr : Sample :: 100mg :1mg}

• Tightly screw one bolt into the mini-press then

loosen by two turns

• Pour KBr matrix into mini-press, using just

enoughh tto cover the
th fface off the
th bolt
b lt inside
i id
How much KBr should I use?

~ 100 mg KBr
How much sample should I use?
 KBr pellet continued…
• Keeping press upright, tighten other bolt

• Use two wrenches to make both bolts snug

• Hold snug for 5 seconds, then remove the

b lt
bolts

• Ensure the KBr matrix is stuck in the center

and thin enough that light can pass through
Hold the mini-press upright and tighten
1stt bolt…
b l
Spread the mixture of KBr and sample…
Tighten 2nd bolt…
Show your muscles…
Hey…this is your 1st IR pellet ☺
 Procedure continued…

• Taking into consideration the melting point of

your unknown

• Compare IR spectrum of your compound to IR

spectra
p of compounds
p melting
g in similar range
g

• Paper
ape cop
copies
es o
of IR spect
spectra
aoof poss
possible
be
compounds are in stock room (CP 333);
electronic copies are available on Bb.
 Some important aspects of the experiment for
writing post-lab
post lab
• Purpose
• M P ranges (for both
M.P. both, impure and pure sample)
• Solubility data for all test ran
• Percent recovery value with calculation
• IR spectrum & its analysis
• scuss o o
Discussion of results
esu ts

Post-lab:
Worksheet for this experiment is available on Bb.
 Waste Disposal
•Make a table for waste disposal. For example,

Hazardous Aq. Acidic Hazardous Non-

Liquid Waste Solid hazardous
O
Organici W t
Waste Solid
Waste Waste

MeOH, Unknown Soiled

Acetic acid
Acetone
Acetone, sample
sample, gloves
Hexane, Filter paper
Chloroform,
Ether
 Some friendly reminders….
• Clean up the lab before you leave
• Return
R t allll common equipment
i t where
h it b
belongs
l
e.g. supplies from IR kit, solvents, tubings,
Alunimum block,
block hot plate
plate, etc
etc.
• Thermometers are common equipment and
must be returned at the end of each lab periodperiod.
You are responsible for one thermometer. If it
breaks yyou must payp y for it.