Techniques in Organic Chemistry

Techniquesin

Organic Chemistry

Four Techniques:

1. Recrystallization 2. Distillation3. Sublimation4. Chromatography

TECHNIQUES IN ORGANIC CHEMISTRY

What is Recrystallization?- The process where a solution is created

by dissolving a solute in a solvent at or near its boiling point.

Principle: the amount of solute that can be dissolved by a solvent increases with temperature.

1. Recrystallization

Its success depends on the increasing solubility of the crystals in hot solvent and their decreasing solubility when the solution cools, thereby causing the compound to recrystallize.


Crystal Formation Crystal formation of a solute from a solution

is a selective process. In recrystallization, dissolution of the impure

solid in a suitable hot solvent destroys the impure crystal lattice, and recrystallization from the cold solvent selectively produces a new, more perfect (purer) crystal lattice.


Crystal Formation Slow cooling of the saturated solution

promotes formation of pure crystals because the molecules of the impurities, which do not fit as well into the newly forming crystal lattice, have time to return to the solution.

Therefore, crystals that form slowly are larger and purer than ones that form quickly.


Solvent Properties The appropriate choice of a recrystallization

solvent is a trial-and-error process, a relationship exists between the solvent’s structure and the solubility of the solute.

This relationship is simply described as like dissolves like.


Solvent Properties In general, a solvent with a structure

similar to that of the solute will dissolve more solute than will solvents with dissimilar structures.


Solvent Properties

1. High-polarity solventsAmong the more polar organic solvents,

both methanol and ethanol are commonly used for recrystallization because they dissolve a wide range of both polar and nonpolar compounds to the appropriate degree.


Solvent Properties

1. High-polarity solventsEthanol and methanol also evaporate easily and possess water solubility, which allows recrystallization from an alcohol/water mixture.


Solvent Properties

2. Low-polarity solventsAmong the low-polarity solvents,

diethyl ether and ethyl acetate appear to provide the best solvent properties, although the low boiling point of diethyl ethyl (35°C) is a disadvantage and its extreme flammability.


Solvent Properties3. Boiling point of the solvent.

The boiling point of the solvent is another important property because the solvent needs to be volatile enough to evaporate fairly quickly from the crystals after they are recovered from the recrystallization solution.


Solvent Properties3. Boiling point of the solvent.

Therefore, most commonly used recrystallization solvents have boiling points at or below 100°C.

Recrystallization

•1. Dissolve the solid sample in a minimum volume of hot solvent with a boiling stone or boiling stick present.•2. If insoluble impurities are present or charcoal treatment is used, gravity filter the hot solution through a fluted filter paper.

Steps in Recrystallization

•3. Cool the solution slowly to room temperature and then in an ice-water bath to induce crystallization.•4. Recover the crystals from the cooled recrystallization mixture by vacuum filtration.•5. Wash the crystals with a small amount of cold solvent.


•6. Allow the crystals to air-dry completely on a watch glass before weighing them and determining their melting point.


It is a procedure by which two liquids with different boiling points can be separated.

Distillation can be used effectively to separate liquids that have at least fifty degrees difference in their boiling points.

Distillation

Simple Set-up of Distillation

Procedures

•Check the calibration of the thermometer that is to be used.•Fill the distillation flask.•Heat the distillation flask slowly until the liquid begins to boil.

Procedures

•As the distillate begins to drop from the condenser, the temperature observed on the thermometer should be changing steadily. •Remove the heat source from the distillation flask before all of the liquid is vaporized.

It is the phase transition from the solid to the gaseous phase, without passing through an intermediate liquid phase.

Sublimation

Sublimation

Substances such as water and carbon dioxide (CO2) can be charted on a pressure versus temperature plot to reveal their state of matter (solid, liquid, or gas) at a given temperature and pressure. At a typical atmospheric pressure, we know that water is a solid at temperatures below 0 degrees Celsius, a liquid from 0 to 100 degrees Celsius, and a gas at higher temperatures.

How Sublimation Works

Atmospheric pressure, however, can change, particularly with altitude. Higher altitudes yield lower atmospheric pressures.

How Sublimation Works

Chromatography

It is a method used by scientists for separating organic and inorganic compounds so that they can be analyzed and studied.

The word chromatography means "color writing" which is a way that a chemist can test liquid mixtures.

Chromatography

1. Stationary Phase is the phase that doesn't move.2. Mobile Phase is the phase that does move.

How it Works?

The mobile phase moves through the stationary phase picking up the compounds to be tested. As the mobile phase continues to travel through the stationary phase it takes the compounds with it. At different points in the stationary phase the different components of the compound are going to be absorbed and are going to stop moving with the mobile phase.

How it Works?

This is how the results of any chromatography are gotten, from the point at which the different components of the compound stop moving and separate from the other components.

How it Works?

The different techniques in Organic Chemistry used by chemists are very important in separating liquids to liquids, solid to solid, solids to liquids and solid to gas.

TECHNIQUES IN ORGANIC CHEMISTRY

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Techniques in Organic Chemistry