Separation and Purification of organic compounds

Physical properties of chemicals & separation and purification methods

SHARDA PUBLIC SCHOOL, ALMORA


Presentation by:

Dr. Tanuja NautiyalDepartment of Chemistry

Sharda Public SchoolAlmora


Physical Properties

Each organic compound has certain physical and chemical properties.

some of the important physical properties of organic compounds are:

Melting point, Boiling point, Density, Solubility


Melting Point

Melting point for an organic substance is the temperature, at which it changes from solid state to liquid state.

This temperature is also referred to as freezing point at which a compound changes its state from liquid to solid.


Melting Point

Melting point determinations have importance in various applications. Such as:Making a comparison with the literature data to observe the

same temperature. Having a very rough idea on the purity of a substance (there

are much more available and accurate methods applied to decide it).


There are lots of factors that affect the melting point. The chemical structure is the main factor.

Melting points are higher for higher molecular weight compounds.

Impurities decrease the melting point. ice-salt mixtures , Salt and soil , Ethylene glycol.

Intramolecular and, in particular intermolecular forces, such as H-bonds, dipole-dipole interactions increase the melting point.

Factors affect the Melting Point


Loosing symmetry decreases the chance of intermolecular interactions. Solid state formation is deeply affected through the possibility of interaction among molecules.

Therefore, loosing symmetry, in particular through branching lowers the melting point.

Trans isomers are more stable than cis isomers, therefore they have higher melting points.

Factors affect the Melting Point


Measuring the Melting Point

Old Fashion Model


New fashion model

Measuring the Melting Point


Boiling Point

Boiling point for an organic substance is the temperature at which it changes from liquid state to gas (vapor) state. The vapor pressure of a liquid is also equal to the pressure of the current environment at its boiling point temperature.


Factors affecting the Boiling Point

The lower atmospheric pressure: the lower the boiling point

There are structural aspects having effect on boiling point:

Higher molecular weight ---higher boiling pointIntermolecular and intra-molecular interactionHydrogen bonding, Dipole-dipole interaction, Vander

wals forces, Impurities, Branching; increase boiling point


Density

A density of a substance is calculated by dividing its weight to its volume.

The unit generally employed for density is gm/mL or gm/cm3.


Pressure TemperaturePolar groups Molecular weightBranchingDouble and triple bonds

Factors affecting the Density


Measurement of Density

By using Pycnometers


In this equation:W1= the weight of empty pycnometerW2= the weight of pycnometer filled with the sample liquid W3= the weight of pycnometer filled with the inert liquid.

Measurement the density for liquid sample


Measuring the density for solid sample

In this equation;w1 = the weight of pycnometer filled with the inert liquid.w2 = the weight of empty pycnometer. w3 = the weight of pycnometer added a little amount of solid sample.w4 = the weight of inert liquid filled w3.


Solubility

Solubility is the degree of dissolution of a substance in a solvent.

The solubility of a liquid substance in another liquid matter ---miscibility and immiscibility,

See table for solubility page 68


There are lots of interaction types that might take place in dissolution of a substance in a liquid.

The organic molecules can not dissolve in water because; long carbon skeletons, absence of polar functional groups, absence of heteroatoms, absence of ionic structures, hydrophobic character

Solubility


like dissolves likeLike dissolves like, organic substances are soluble in

organic solvent, but not all, varying depending on the structure of both the organic substance and the organic solvent.

Acetone has the ability of dissolve most of the organic molecules. For instance, the solubility of naproxen in acetone is 0.726 mol/L, which is quite higher than its solubility in other solvents.

Solubility


The acidic or basic property of substances is measured in terms of pH.

It is a measurement of the hydrogen ion concentration.

pH is defined as the negative logarithm (base 10) of hydrogen ion concentration.

pH of a Solution


PH < 7 ACIDIC

PH = 7 NEUTRAL

PH > 7 BASIC

PH << 7 MORE ACIDIC

PH >> 7 MORE BASIC

pH of a Solution


pHHenderson-Hasselbach equation:

pH = pKa + log ([A-] / [HA])

•pH = pka; the concentration of ionized and non-ionized forms are equal•pH > pKa---the concentration of ionized >non-ionized forms •pH < pKa---the concentration of ionized <non-ionized forms

Carboxylic acid ---pKa=4-5

HA, PH=7 more than 99 % A-If an organic molecule is insoluble in a liquid, it precipitates. precipitation and dissolution might be evaluated as opposite actions


Miscibility

Miscibility of organic solvents is quite important.

For instance; Some reactions utilize more than one organic solvent to conduct a reaction as reagents or to increase the solubility.

Some work-up and purification studies also require the employment of more than one solvent.


Partition Coefficient

Organic compound has different solubility in different organic solvents have applications in various experiments.

Partition coefficient is the ratio of concentration of an organic molecule in two immiscible organic solvents. The organic compound must be at unionized (neutral). It is important to adjust the pH that guarantees the non-ionized state of a molecule.


Partition Coefficient


Distribution Coefficient


Application of Distribution Coefficient

Solubility in water and lipid systems are both required for a drug molecule to pass through different biological fluid systems to reach to its active site.

In general, a Log(Partition coefficient) value in 3-5 range is one of those drug-likely parameters that shows a good criteria for a drug candidate molecule. Chloroform is sometimes used as an alternative to n-octanol.


Polar groups, groups tend to make Hydrogen bonds, ionic bonds definitely trigger solubility in aqueous systems, therefore resulting in low Log(Partition coefficient) values relatively.

In contrast, lacking of those structural features and abundance of hydrophobic groups creating london -london forces positively affect lipophilicity and high Log(Partition coefficient) values.

Application of Distribution Coefficient


Methods of Purification

Extraction

Filtration

Distillation

Crystallization


Extraction Extraction is a separation technique.

Types of Extraction:• Solid-liquid Extractions: Solid liquid extractions simple aims to extract the

material from a solid mixture into a liquid.

• Liquid-liquid Extractions: Liquid-liquid extractions employ the solubility

difference of an organic compound in two immiscible liquids.


Extraction depends on several factors: • Solubility in two immiscible solvents• Temperature• The amount of each solvent used

Factors affect the Extraction


Separation funnels are utilized for liquid-liquid extraction.

Immiscible solvents separate in funnel such that heavier (higher density) stays at bottom.

Liquid-liquid Extraction


Liquid-liquid Extraction


After the extraction process, in general, the organic phase is evaporated to gain the organic compound.

In practice, most of the organic solvents have the ability to dissolve some amount of water as well. The amount of water transferred into organic phase changes depending on the organic solvent used. Anhydrous sodium sulfate, and anhydrous magnesium sulfate, anhydrous calcium chloride, anhydrous cupper sulfate, and anhydrous calcium sulfate

Drying of liquid


Therefore, some drying phase changes depending on the agents are used to have the organic solvent water free.

Some features required form drying agents:• No solubility in the organic solvent• High efficiency to dry • Inert material

Drying of liquid


Filtration

Filtration is one of the most applied processes in organic and pharmaceutical chemistry practices.

In general, filtration is categorized into two groups;

1. Gravity filtration 2. Vacuum filtration


Gravity Filtration Method

Gravity filtration is the simple form of filtration.

Requirement: Funnel, Filtration paper and a Erlenmeyer..


Vacuum FiltrationVacuum filtration is much faster than gravity filtration. The only difference is the employing of vacuum as a deriving force for filtration.


Distillation

Distillation is an old-known separation and purification technique, particularly applied for liquids.

It is based on a system consisting of both heating and cooling units.


Distillation


Fractional Distillation

Liquids that have different boiling points and do not make azeotrope mixtures can be separated employing the fractional distillation technique. The difference of fractional distillation from the simple distillation is the employment of fractional condenser.


Fractional Distillation


During the separation, some liquids can form azeotropes. The boiling point of the azeotropes might be lower or higher than the boiling point of each components of the azeotrope mixture.

For example: For instance, 96% ethanol boils at around 78.3 ºC which is lower than the boiling points of water and ethanol, 100 ºC, and 78.8 ºC, respectively. Due to the formation of azeotrope, technically it is not possible to separate ethanol-water mixture employing fractional distillation. In such a case, another solvent is added to break down the azeotrope, such as benzene addition to ethanol-water mixture.

Azeotropes


Steam Distillation

This type of distillation is especially for those liquids, that have high boiling points and decompose at high temperatures less than their boiling point.

The steam distillation system applies the in-situ generation of steam and applying it directly onto the material to activate it for distillation.


Steam Distillation


Vacuum Distillation

It is also referred to as low-pressure distillation. boiling point of an organic substance depends on several factors including the pressure.

The vacuum application to a distillation unit would be enough to decrease the pressure inside the system, resulting in the boiling of a liquid compound at temperatures lower than its regular boiling temperature.

For instance, dimethylsulfoxide has a relatively high boiling point, around 190 ºC. Vacuum application to the system physically takes down the boiling point of dimethylsulfoxide. An effective vacuum can even take it to less than 100 ºC.


Low-pressure Distillation


Crystallization

Crystallization is one of the oldest purification techniques applied for solid organic compounds. It has these stages of a continuous process summarized below:

• Dissolution• Hot filtration• Crystallization

• Filtration of crystal• Drying of crystal


Steps of Crystallization


Crystallization•The overall technique simply relies on the dissolution of an organic compound in the presence of impurities in a hot organic solvent concomitant with its crystallization in the same solvent during the cooling down of the organic solvent. •A crystal lattice is perfectly ordered for the same type of molecules. Therefore, crystals physically chemically tends to form between the same type of molecules. This also means that impurities are excluded from the crystal lattice.


CrystallizationSalt Chemical formula Shapes of crystalsPotash alum K2SO4.Al2(SO4)3.24H2O Octahedral

Blue vitriol CuSO4.5H2O Triclinic

Green vitriol FeSO4. 7H2O Monoclinic

Sodium chloride NaCl Cubic

Potassium nitrate KNO3 Rhombic

Washing soda Na2CO3.10H2O Monoclinic


Dr. Tanuja NautiyalDepartment of Chemistry

Sharda Public School

Education

Separation and Purification of organic compounds