R t lli tiRecrystallizationIdentification of an unknownIdentification of an unknown

CHE 231 Organic ChemistryCHE-231 Organic ChemistryUniversity of Kentucky

Introduction

• One of the most common purification techniquesp q

• Solid dissolved in solvent then precipitated back p pout as a purified material as impurities remain soluble in the solvent

• Choose solvent such that impurities are more soluble than the substance being isolatedsoluble than the substance being isolated

• Used when amount of impurities isn’t too largeUsed when amount of impurities 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 dry crystalsy y

Selecting Solvent• Solute should be soluble in the solvent at high

t t d i l bl t t ttemperature and insoluble at room temperature

• 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 hotyesyeswaterH2O

CH OH th l yes

yes

yes

yes

CH3OH

CH3CCH3

methanolO

acetoneyesyes

yesyes

acetic acid

chloroform

CH3COOH

CHCl3yesno

nono

ethercarbon

tetrachloride

CH3CH2OCH2CH3

CCl4

no

no

no

no

pentane

hexane

C5H12

C6H14

nonoheptaneC7H16

Solubility at Room Temperature (RT)

• Solute insoluble• Solute insoluble in solvent at Room temperature

• So heat the solvent• So heat the solvent and test it for completesolubility at hight ttemperature

Solubility at high temperature

• Upon heating the compound dissolved pcompletely in the selected solvent

• It is a good solvent for lli irecrystallization

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• If you know the structure of the compound to purify…

Di l t i t t Є– Dielectric constant Є – Hydrogen bonding capabilities

Like dissolves likeCH3

dissolves in CH3CH2CH2CH2CH2CH3 hexane

t l C and C H bondsO

toluene both have C

H3C C OH H OHacetic acid

dissolves inwater

OHboth have

b th hi hl lboth are highly polar

Length of Carbon chain• C5 rule

– Structures with less than 5 carbons and a polarStructures with less than 5 carbons and a polar function are water soluble

– Branched chain compounds are more soluble than di i h h i d (b h dcorresponding straight chain compounds. (branched

lowers intermolecular forces and intermolecular attraction.))

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

in s o lu b le in H 2 O .

C H 3

O H

C H 3

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 h lcharcoal

Use short stem glass funnel w/ filter paper• 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 andmixture 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 suctionreapply suction

Drying• Evaporate solvent by vacuum

• Transfer crystals to clean watch glass

• Allow crystals to air dry until next lab

• Do not put crystals in oven!p y

Melting points• Physical characteristic of pure compound

• If compound is pure, reproducible over range of 0 5 2 0°C0.5-2.0°C

I it b th d i f M P d• Impurity causes both depression of M.P. and melting to occur over wide range

• M.P. range can estimate purity

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

point apparatuspoint apparatus

• Crush portion of dry solid into powderCrush portion of dry solid into powder

• Tap some powder (~2-3 mm) into capillary p p ( ) p ytube

• Invert and tap powder down to sealed bottom of tube

Determining M.P range• If powder shrivels, changes color or

sweats out solvent recrystallizesweats out solvent, recrystallize

R d t t h fi t d f• Record temperature when first drop of liquid appears and when the entire sample is meltedis melted

• These values represent M P range• These values represent M.P. range

Infrared Spectrum• Molecules vibrate

d t tand rotate• 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 ( OHobserved for particular groups (-OH, -NH2, etc.)

• Fingerprint region: complex series of absorptions specific to a singleabsorptions, specific to a single compound

Infrared Spectrum continued…

light100 light

nce

ran s

mitt

an

0

%tr

Functional group region FingerprintFingerprintcm-1

wavenumber4000 300

dark0 Functional group region FingerprintregionFingerprintregion

I. R. of Benzoin

SP3 C H

SP2 C-H

SP3 C-H

O-H C=O

C

H

C

SP3 C-H

C

H

O

C

O

HSP2 C-H O-H

C=O

I. R. of p-bromoaniline

SP2 C-H

NH2 C-Br

N

H

Br N

H

Br

HSP2 C-H

I. R. of 4-cyanopyridine

SP2 C-H C≡N

N C NN C N

H SP2 C-H

I. R. of p-anisic acid

SP3 C-HO-H

SP2 C-HC=O

O C O

O

O

CH

HH

C O

H

H SP2 C-HSP3 C-H

Procedure• Determine M.P. range of unknown provided

by TAby TA

• Find best solvent for recrystallizing unknownFind best solvent for recrystallizing unknown (list attached to handout)– Do not use all of the sample on the first trial (use p (

~30 mg/trial)– You may not receive additional unknown if you run

outout

• Recrystallize unknown as described earlier• Recrystallize unknown as described earlier

Impure! dull!! Not crystallized!!! p y

Procedure continued…• Dry and weigh purified crystals

• Calculate percent recovery by the following formula:formula:

% Recovery = Weight of recovered sample

X 100yInitial weight of unknown sample

X 100

• Obtain M.P. range for recrystallized solid

• Obtain IR spectrum of solid by KBr pellet

Pure! Shiny!! Recrystallized!!! ☺y 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;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 h t th f f th b lt i idenough to cover the face of the bolt inside

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 ltbolts

• Ensure the KBr matrix is stuck in the center and thin enough that light can pass through

Hold the mini-press upright and tighten 1 t b l1st bolt…

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 ofTaking into consideration the melting point of your unknown

• Compare IR spectrum of your compound to IR spectra of compounds melting in similar range p p g g

• Paper copies of IR spectra of possible ape cop es o spect a o poss b ecompounds are in stock room (CP 333); electronic copies are available on Bb.

Some important aspects of the experiment for writing post-labwriting post lab

• Purpose• M P ranges (for both impure and pure sample)M.P. ranges (for both, impure and pure sample)• Solubility data for all test ran• Percent recovery value with calculationPercent recovery value with calculation• IR spectrum & its analysis• Discussion of resultsscuss o o esu ts

Post-lab: Worksheet for this experiment is available on Bb.

Waste Disposal•Make a table for waste disposal. For example,

Hazardous Liquid O i

Aq. Acidic Waste

Hazardous Solid W t

Non-hazardous

Organic Waste

Waste Solid Waste

MeOH,Acetone

Acetic acid Unknown sample

Soiled glovesAcetone,

Hexane,Chloroform,

sample,Filter paper

gloves

Ether

Some friendly reminders….• Clean up the lab before you leave

R t ll i t h it b l• Return all common equipment where it belongs e.g. supplies from IR kit, solvents, tubings, Alunimum block hot plate etcAlunimum block, hot plate, etc.

• Thermometers are common equipment and must be returned at the end of each lab periodmust be returned at the end of each lab period. You are responsible for one thermometer. If it breaks you must pay for it.y p y