/
Purification of Purification of

Purification of - PowerPoint Presentation

olivia-moreira
olivia-moreira . @olivia-moreira
Follow
393 views
Uploaded On 2017-01-30

Purification of - PPT Presentation

Benzaldehyde Week 2 Benzaldehyde Most aldehydes are relatively easily oxidized by air into carboxylic acids Benzaldehyde is oxidized to benzoic acid Benzaldehyde is commonly contaminated with benzoic ID: 515587

hexanes benzaldehyde stilbene reaction benzaldehyde hexanes reaction stilbene part week catalyst thiamine pour chemistry reagent wittig stilbenes distill base

Share:

Link:

Embed:

Download Presentation from below link

Download Presentation The PPT/PDF document "Purification of" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.


Presentation Transcript

Slide1

Purification of Benzaldehyde

Week 2Slide2

Benzaldehyde

Most aldehydes are relatively easily oxidized by air into carboxylic

acids.

Benzaldehyde is oxidized to benzoic acid:Benzaldehyde is commonly contaminated with benzoic acid.Slide3

Section 6.4B – Purification of benzaldehyde

Poorly soluble in water

Very soluble in waterSlide4

Utility of hydroquinone during drying

Hydroquinone reacts more readily with O

2

than benzaldehyde, thus protecting the benzaldehyde from oxidation.Slide5

We will not distill

Benzaldehyde

must be distilled under low pressure and at low temperature, or it will oxidize

quickly.The present organic chemistry teaching lab is not equipped for low pressure distillations. Extracting out the benzoic acid with base, and protecting with hydroquinone is usually sufficient to render benzaldehyde pure for most chemistry. Slide6

Benzoin Condensation

Week 2Slide7

Benzoin Condensation

Traditional approach involves cyanide ion (

-

C≡N) as the catalyst. The cyanide mechanism is outlined in detail in you book.Cyanide is highly toxic and not used in our organic chemistry labs.

We will be using thiamine (vitamin B1) as an alternative catalyst. Slide8

Thiamine X

-

Thiamine exists as a salt. The X

-

anion could varyWe will use X = Cl-

Thiamine

hydrochloride

Reacts readily with hydroxide base to make the zwitterion catalystSlide9

R

1

, R

2 short hand to simplify structuresSlide10

Mechanism:

Benzoin

condensation using thiamine

HCl

catalyst

benzaldehydeSlide11

Mechanism – Part 2

Anion Stabilized by resonance

charge separated –

less stable resonance contribution

more stable resonance contribution –

makes

proton

more acidicSlide12

Mechanism – Part 3

benzoin

catalyst gets used againSlide13

Hazards

You will be working with a

NaOH

solution. This can cause chemical burns. Slide14

Stilbene by Wittig reaction

Week 2Slide15

Wittig reagent

The

reagent

will be provided to you, but, when necessary, it is usually made in the following manner:

Note that this reaction is usually quantitative.Triphenyl phospine (TPP) is used because phosphines

are toxic and TPP is not very

volatile.

Also the TPP Witting reagent is usually crystalline and easy to

isolate.Slide16

Wittig reaction – Part 1

The reagent is first activated by deprotonation by

base:

The chloride anion pairs with the sodium cation from the

NaOH.Slide17

Wittig reaction – Part 2

stilbeneSlide18

Reaction produces a mixture of cis

-

and

trans- stilbene, mostly trans.

t

rans

-

stilbene

cis

-stilbene

You do not need to separate these, as both will work for subsequent

reaction.Slide19

Cautions

Treat the phosphorus containing compounds as toxic.

You are using a strong base,

NaOH which can cause serious chemical burns.Dichloromethane (DCM) is a cancer suspect agent – use in the hood.Distill off the solvent cautiously.

Do not distill to drynessDistilling any solvent to dryness can cause an explosion hazard.Reduce the volume, then pour the last 5mL onto a watch glass, and allow to evaporate to dryness. Rinse out any remaining solid from the flask onto the watch glass with a few mL of DCM.

Use all other cautions provided to you about working with organic compounds and working in this lab.Slide20

Trituration with warm hexanes

Hexane is a nonpolar

solvent.

The polar triphenylphosphine oxide will not be extracted, while the nonpolar stilbenes will extracted.We do not have a steam bath. Gently warm the hexanes in a flask on your hotplate.

Caution: fire hazard! Hexanes vapor can ignite on your hotplate.This will not explode, but may cause a flame to appear at the top of your flask. If this happens, notify your TA for help. Slide21

Trituration – part 2

Swirl your solid mixture in the warm

hexanes.

Again, the salt (NaCl) and phosphine oxide will not dissolve, while your stilbenes will dissolve.Pour the suspension through a filter

paper.Pour additional aliquots of hexanes through to ensure you have washed through the stilbenes.Discard the solid on the filter paper.

Distill off (not to dryness – caution – explosion hazard!) the hexanes in the flask to obtain your pure

stilbenes

.

Pour off the remaining few mL

onto a watch glass as done earlier. Slide22

Convenient places to stop if you are running out of time

Carrying out reaction

Distilling of

dicholormethaneYou could stop after this step, if necessary, and complete the hexanes trituration the following week.Hexanes triturationYou could stop after this step, if necessary, and complete the TLC the following

week.TLC analysis of final productSlide23

Do not recrystallize

Your product comes out reasonably

pure.

Recrystallization is time consuming, and one often does not recover enough product for the subsequent steps in the synthesis. Also, the two stilbene isomers act as impurities to each other in recrystallization, so the process eliminates one of the isomers, though both isomers work in the bromination

chemistry.Slide24

END