PPT - Gmu

advertisement

Isolation of Caffeine

References:

Slayden: pp. 47- 49

Pavia: pp. 24, 73 – 84

Note: These pages are not in the GMU version of the Pavia text sold in the book, only in the primary text that we do not use anymore .

1/17/2015 1

Isolation of Caffeine

Overview

An aqueous Vivarin/Sodium Carbonate mixture is extracted with Dichloromethane (Methylene

Chloride)

The organic fraction is separated from the aqueous solution and evaporated to dryness

The product is recrystallized from Acetone

The mass and % recovery of the product are determined

Product identity is verified by Melting Point

1/17/2015 2

Isolation of Caffeine

Background

Caffeine (a stimulant) is a bitter, white crystalline alkaloid derivative of Xanthine (note methyl substitution on primary amino groups)

1/17/2015

Caffeine is synthesized from the purine nucleotides AMP, GMP, and IMP in plants

These in turn are transformed into Xanthosine and then Theobromine, the latter being the penultimate precursor of Caffeine

3

1/17/2015

Isolation of Caffeine

The two Amide groups exist as zwitterionic resonance structures where the Nitrogen and

Carbon atoms are double bonded to each other so that both of the Nitrogen atoms are essentially planer (in sp 2 orbital hybridization)

A zwitterion is a neutral molecule with a positive and a negative electrical charge (not dipoles) at different locations within that molecule

Zwitterions are sometimes also called inner salts

The fused ring system therefore contains a total of ten pi (  ) electrons, thus, according to

Huckel’s rule, is aromatic

4

Isolation of Caffeine

Zwitterionic Resonance

1/17/2015 5

Isolation of Caffeine

Chemical Properties

Caffeine acts as a general nervous system stimulant, warding off drowsiness

Caffeine is considered a “psycho active” substance, but is generally legal and unregulated

Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, acting as a natural pesticide by paralyzing and killing insects

Sources for human use include coffee plants, tea leaves, and kola nuts

1/17/2015 6

Isolation of Caffeine

Procedure

Obtain 2 Vivarin tablets noting the actual mass of

Caffeine in each tablet

Determine the mass of the tablets

Obtain and determine mass of approximately 8 g of Sodium Carbonate

Crush the tablets with a pestal & mortor

Add the crushed sample to a 150 mL beaker

Add 60 mL of Distilled Water

Add the Sodium Carbonate (Na solution

2

CO

3

) to the

Add a Teflon boiling chip

Heat solution to boiling on a hot plate – maintain boiling point for 2 minutes

1/17/2015 7

Isolation of Caffeine

Procedure (Cont’d)

Allow the hot mixture to settle slightly

- some solid particles will remain undissolved

Attach an iron ring to a ring stand

Insert a 125 ml Separatory Funnel into the iron ring using a clay triangle if necessary to support funnel

Adjust height of iron ring so that stem of separatory funnel fits just inside a

150 ml beaker

Place a glass funnel into the neck of the separatory funnel

Make a boat of glass wool and place in the bottom of glass funnel

1/17/2015 8

Isolation of Caffeine

Procedure (con’t)

Decant the sample solution into the bottom of the glass wool boat in the glass funnel allowing the liquid to drain into the 125 mL Separatory Funnel

Rinse the beaker with a small amount of distilled water and pass the water through the glass funnel into the separatory funnel

Allow the filtrate in the Separatory Funnel to cool to room temperature

1/17/2015 9

Isolation of Caffeine

Procedure (con’t)

Liquid–liquid Extraction, also known as solvent extraction and partitioning, is a method to separate two immiscible liquids, usually an aqueous component and an organic solvent component

The process involves the extraction of a substance from one liquid phase into the other liquid phase utilizing a separatory funnel

Use the following steps to extract the Caffeine from the aqueous phase into the organic phase consisting of Methylene Chloride solvent

(density – 1.33 g/ml)

1/17/2015

Three (3) extractions will be performed

10

Isolation of Caffeine

Procedure (Cont’d)

Add 8.0 mL of Dichloroethane (Methylene

Chloride) to the mixture in the separatory funnel

Place the stopper in the top opening and make sure it is secure, i.e. won’t leak

Invert the funnel pointing the stopcock end away from your face

Use “gentle”, (not vigorous swirling to avoid formation of an emulsion) for 15-20 seconds

Slowly open the stopcock to relieve any pressure

1/17/2015 11

Isolation of Caffeine

Close the stopcock and continuing swirling with venting every 15 seconds until no more gas is released (two or three times should be sufficient)

Continue the swirling for a total of 5 minutes

Replace the separatory funnel in the ring stand and allow the mixture to settle for a few minutes

Remove the stopper from the Separatory Funnel

Open the stopcock slowly and drain the clear bottom (organic) layer into a clean 150 mL beaker

1/17/2015

Repeat the extraction process two more times with new 8.0 mL aliquots of Methylene Chloride, but reduce the total swirling time to 2 minutes

Each time, add the clear bottom layer to the 150 mL beaker

12

Isolation of Caffeine

1/17/2015

Procedure (Cont’d)

With the help of the instructor, add Anhydrous

Sodium Sulfate as a drying agent

Leaving the particles settled on the bottom of the beaker, slowly pour (decant) the liquid into a clean, dry, 50 mL beaker

Rinse the beaker containing the drying agent particles on the bottom with one additional small portion of Dichloromethane and add to the sample beaker

Evaporate the mixture to dryness in the hood

Note: Set hot plate setting to about 3

Note: Crude yield at this point is roughly 90%

13

Isolation of Caffeine

Procedure (Cont’d)

Recrystallization using a mixed solvent of Acetone and

Petroleum Ether

Dissolve the dried product in 12-15 mL cold Acetone

(from ice/water bath)

Heat solution gently on hot plate

If solution is incomplete, remove beaker from hot plate and add more Acetone in 2 mL increments

Up to 20 mL of additional Acetone may be required until a clear pale-yellow solution is obtained

Remove solution from hot plate and allow to cool slightly

1/17/2015 14

Isolation of Caffeine

Procedure (Cont’d)

Add Petroleum Ether, drop by drop, with swirling from a full plastic pipet until the first appearance of pervasive cloudiness throughout the solution

Continue cooling until crystallization is complete

Note: Maximum yield is obtained when the addition of

Petroleum Ether is stopped at the first appearance of a pervasive cloudiness throughout the solution

 It may be necessary to evaporate some of the solution to effect recrystallization

Separate the crystals from the solution by Vacuum

Filtration rinsing the beaker and crystals in the funnel with small amounts of Petroleum Ether

1/17/2015 15

1/17/2015

Isolation of Caffeine

Immerse beaker in an ice-water bath for about a minute

Air-dry the sample from the evaporation procedure or the vacuum filtration procedure on a pre-weighed weighing tray for one week

Compute the mass of the purified sample

Compute the percent recovery of the product relative to the amount of caffeine indicated in the tablets

Determine melting point  236 o C

Note: Decomposition of this crude material occurs beginning at 236 o C over a range of 5 o C or so

16

Download