CHEM 237 #6 Extraction

Imagine we had an initial mixture consisting of benzoic acid, benzocaine, and acetanilide. You first wash your DCM layer with HCl, then NaOH (just as you did in lab a week ago). What will be left in the dichloromethane after this? How can you isolate it?

1. Once the initial mixture has been washed with HCl, then NaOH, the products we are left with are isolated Benzocaine, sodium benzoate, and acetanilide left in the dichloromethane. When separating the benzoic acid from the dichloromethane and acetanilide, two 15 mL portions of 3 M sodium hydroxide is used. During the two rounds of separation, the upper layer becomes the basic aqueous layer containing the sodium benzoate and the bottom organic layer contains the dichloromethane and acetanilide. Once you have separated the organic layer, you can then separate acetanilide from dichloromethane by heating the mixture until boiling occurs. This will boil off the dichloromethane because its boiling point is 40℃, compared to acetanilide's boiling point of 304℃. After boiling off all of the dichloromethane, only crude acetanilide will remain.

In extraction, soluble chemical compounds (solutes) are distributed between

2 immiscible solvents

benzoic acid is more soluble in

DCM

benzocaine is treated with

HCl

Extract benzoic acid from DCM with __ to form __

NaOH, sodium benzoate

Could you have extracted any benzoic acid when adding HCl to the mixture? Why or why not.

1. Benzoic acid is insoluble in hydrochloric acid. By adding sodium hydroxide into the organic compound mixture, benzoic acid is converted into a negatively charged sodium benzoate ion which is soluble in aqueous solutions. Then, to convert the sodium benzoate ion back into benzoic acid, we must add acid to turn it back into an acid by adding enough HCl to turn the pH highly acidic. When hydrochloric acid is added to the benzoate ion, the benzoate ion is turned back into benzoic acid. Therefore, we extract benzoic acid from the sodium benzoate when adding hydrochloric acid to the mixture.

You want to run a reaction, and then wash your reaction mixture with water after. Your choice of solvents are tetrahydrofuran and heptane. Which would work best for an extraction with water and why?

1. Heptane would be the best choice for this reaction because tetrahydrofuran is miscible in water. In a liquid-liquid extraction, when an impurity is removed from a liquid, the technique is known as "washing". The requirements for each of the two liquids to successfully separate the aqueous and organic layers is that they must be immiscible, they must have different densities, the two liquids must have different affinities for the material being isolated, and it must be possible to isolate your product from the liquid into which it is extracted. Therefore, this confirms that since tetrahydrofuran is miscible in water, its density is similar to water, and because you can't isolate a homogenous mixture using the separation funnel, heptane is the best solvent to use in this situation.

objective of this lab

To demonstrate the use of extraction as a method of separating mixtures of compounds

Recovery of benzoic acid from the aqueous basic layer

acidify the basic aqueous layer with HCl to protonate the carboxylate salt. Cool and crystal

What simple, non-destructive test could be used to determine which layer is which

add a little distilled water to see which layer the water adds to

To convert the benzocaine into a water-soluble salt

add acid to the 3-component mixture

to precipitate the benzocaine

add base to the aqueous acidic layer mixture

if emulsion/oil forms..

add small quantity of water, if that doesnt work add small amount of solvent

liquid-liquid extraction

based upon a difference in solubility of the compound between two immiscible liquids

Unknown of this lab is a mix of __ and found in what layer

benzoic acid, benzocaine, and benzamide, found in organic DCM layer

unequal distribution is related to

compound solubility

2nd type of extraction involves the separation of organic compounds that requires

compounds with different water solubilities, and compounds that can be converted to forms with greatly different water solubilities

easiest method to determine distribution coefficient is to

dissolve a carefully weighted sample of pure compound into distilled water and then extract it with an equal volume of pure solvent

How could eugenol be separated from a mixture of other non-acidic molecules

dissolve mixture in DCM, shake with NaOH, collect (aq) layer, and re-acidify to force precipitation of the eugenol

In an extraction, your success and recovery is governed by a value known as the

distribution coeffiecient (Kd)

advantages of extraction

efficient and simple, continuous extraction is used for large scale

Why is ethanol not a suitable solvent for the solvent extraction of benzoic acid and benzocaine from the aqueous solution

ethanol is miscible in water

Use sep funnel to

hasten the distribution to equilibrium by shaking

When you extract benzocaine with HCl, the ___ should dissolve and 2 layers would be produced

hydrochloride salt. Organic layer on bottom, DCM on top

Which layer is on top?

if organic layer is not chlorinated, it will be on top of aqueous phase

in-organic salts are very soluble in water, and very ___ in organic solvens

insoluble

acid-base reactions convert water insoluble neutral organic compounds into

ionic salts (possible only with compounds that are reasonable acidic or basic and react with dilute(aq) acid or base

now acidify the aqueous layer to

isolate the benzoic acid via protonation

If two compounds do not have such a large difference in water solubility

it limits the usefulness of extraction

To recover benzocaine from the aqueous acidic layer

make the acidic layer basic by adding NaOH so the the ester group hydrolyzes and allow to cool slowly to make crystals

The two liquids must possess several properties

must be immiscible, must have different densities, different affinities for the material being isolated, and must be possible for you to isolate your product from the liquid into which it is extracted

does benzamide react with weak acid or weak base

no

does density increase as molar mass increases?

no

What forms the aqueous layer

one compound is more soluble than the other

The organic products remain in the

organic phase, as well as the starting materials and organic by-products

Extraction is often used as early step of

purification and it increases the effectiveness of other purification technique

density

ratio fo a substances mass to its volume

extraction is an excellent technique for the

removal or isolation of a compound from a mixture

disadvantages of extraction

requires the use of an organic solvent that requires disposal or recycling by distillation- increases complexity and cost, solvent loss, pollution

washing

selectively removing impurities from a liquid

extraction and washing are typically performed in a

sep funnel

Liquid-liquid E works well for many situations, but mixtures of organic compounds may have similar solubility in the same solvents, making simple extraction impossible. Therefore,,,

take advantage of the acid-base properties of the mixture. It may be possible to convert an organic acid or base into its corresponding water soluble salt, which would allow it to be easily removed from an organic solution

If DCM layer is shaken with dilute HCl,

the amine group of benzocaine will be protonated to form an amine hydrochloride salt that will be water soluble (goes into aqueous layer)

To get effective mixing in a sep funnel, the combined levels of liquid should not exceed

the height of the widest part of the sep funnel

If HCl salt is treated with a dilute base,

the proton is removed to regenerate benzocaine. This will make benzocaine insoluble in the aqueous layer and it will precipitate out of solution to form crystals

Now add NaOH to the sep funnel with the benzoic acid

this de-protonates benzoic acid to from water soluble sodium benzoate. Now in aqueous layer

In a standard equilibrium situation, you would like the solvent you have chose to

totally remove the product you want from the other solution and leave everything else behind

in sep funnel you must have

two liquids in separate layers (or phases)

In todays lab, extraction makes us of ___ of a compound between 2 immiscible liquids (water and hexane)

unequal distribution

to avoid build up of pressure in sep funnel

vent through stop cock

NaCl is more soluble in

water

most often, the two immiscible solvents are

water (highly polar solvent) and much less polar organic solvent

What does different affinities for the material being isolated mean

you want the material you are trying to isolate to end up in one of the layers. This can be done by extracting your product into a specific layer (partitioning)