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Nature of Organic Compounds: Properties,Production and Purification

KEY POINTS
Nature of organic compounds
Features of organic compounds
Importance of Organic Chemistry
Methods of Production of Organic Compounds
Carbonization:
Coal Gasification:
Liquefaction:
Purification of organic compounds
Crystallization
Fractional Crystallization
Sublimation:
Distillation:
Solvent Extraction
Chromatography
There are different types of chromatography
Paper chromatography
Thin-layer chromatography (TLC)
Column chromatography
Gas liquid chromatography(GLC)
High performance liquid chromatography
Ion exchange chromatography  


                  
                   Nature of organic compounds 

Compounds into two major classes namely: Organic and inorganic compounds. Organic compounds are those compounds containing carbon While inorganic compounds are compounds not containing carbon. 

However, Some carbon compounds such as carbon monoxide. Carbon dioxide Cabinets Carbon disulfide etc. Conventionally Studied together with inorganic compounds. Organic chemistry is the chemistry of carbon compounds with the exception of those studied under in organic chemistry. 

                              Features of organic compounds 

  • Organic compounds are numerous. 
  • Any compound show isomerism? Both structural and sterile actual mechanism 
  • Organic reactions are slow. 
  • Organic compounds involve only a few elements, which include Carbon, hydrogen, oxygen, nitrogen halogens, phosphorus and some metals. Whereas inorganic compounds include many elements (over 100 elements).
  •           Organic compounds readily decompose on heating, unlike- inorganic compounds, which are fairly stable on heating.

    Carbon is able to form so many compounds because of the following reasons:

  1. The ability of carbon to catenate: Carbon atom can attach themselves to one another to an extent not possible for atoms of any other element.
  2. Carbon atoms can involve indifferent types of bonding such as (i) Covalent bonding as in single, double and triple bonding and (ii) Ionic bonding — mostly when organic compounds react with metals.

                                 Importance of Organic Chemistry

Organic Chemistry is of immense (great) importance it is the chemistry of dyes and pigments, drugs, paper, ink, paints, plastics, gasoline and rubber tires; it is the chemistry of food we eat and the clothing we wear.
Organic chemistry is fundamental to biology and medicine. Apart from water, living organisms are made up of mainly organic compounds; the molecules of “molecular biology”.


Sources of organic compounds

  1. Coal: This is the main source of aromatic hydrocarbons: Aliphatic compounds also obtained from coal.
  2. Petroleum (crude oil): this is the major source of aliphatic hydrocarbon.
  3. Biological materials: refers to plants and animals. Organic compounds can be obtained from plants and animals. Compounds from plants and animals are generally   called natural products. Many dyes and drugs are obtained from biological materials. 
  4. Synthesis: The term synthesis means building up a compound step by step from a   simpler substance of known structure. New organic compounds can be prepared by the modification or reaction of compounds obtained from any of the other sources.



Methods of Production of Organic Compounds

1. From Coal: There are various methods of obtaining organic compounds from coal. These include:

      a. Carbonization:

 This involves destructive distillation of coal or heating of coal in the absence of air at 700—-1200o_C, to give coke (pure carbon), coal gas and liquid by-products (benzol and coal tar). The coke is a major raw material in iron and steel industry (Metallurgical industry). The coal tar is a mixture of aromatic compounds, mainly naphthalene,

(C13 H10), anthrAcene, phenanthrene, phenol and pyridine. 
other aromatic hydrocarbons such as toluene, xylene extent olefins (cyclopentadienes) and aliphatic.

Coal+heat(700-1200)=coke + coal gas + Benzol coal

(b) Coal Gasification: 

This involves the conversion of coal into synthesis gas with steam. Synthesis gas is a mixture of carbon(ll) oxide and hydrogen. 

The synthesis gas is used for the preparation of aliphatic chemicals and ammonia.

c. Liquefaction: involves the addition of hydrogen to coal by either hydrogenation or means of solvent having hydrogen donor properties Example of solvent that has hydrogen donor properties is tetralin,
Liquefaction increases the hydrogen to carbon ratio of coal (by making the coal more saturated) and improves the fuel quality. Chemicals or compounds that are obtained from liquefaction of coal are mainly aromatic and naphtheic in nature and include benzene, xylene, toluene, cyclohexane, methylcyclohexane, naphthalene and anthracene. 



2. From Petroleum: Organic compounds can be obtained from distillation, which separates the components into fractions such as petrol (gasoline); kerosene, diesel and gas oil. This method is used in petroleum refineries.

3. Biological materials: Organic substances are extracted from plant materials such as leaves, stems or roots by the use of suitable solvents or other processes. The biological materials can be dried before milling to reduce moisture. Some of the substances that can be obtained from biological materials include drugs, oils, colorants (dyes and pigments) and perfumes. 

4.   from synthesis: this involves the reaction of one, two or more compounds to produce desired compounds.



                              Purification of organic compounds 

Methods used in each case depends on the physical characteristics or properties of the individual compounds present in the mixture. It should be noted that the greater the difference in any given physical property between two substances the easier it is to separate them.
Some common physical methods used in the separation of organic mixtures include the following:

 l.  (a) Crystallization: This method is used for the purification of impure crystalline or solid
compounds. The purification of solids by crystallization is based upon differences in their solubility in a given solvent or mixture of solvents. The process involves dissolving the impure substance in a minimum amount of solvent at or near the boiling point to get a saturated solution. The hot solvent is then filtered to remove insoluble, impurities, The filtrate is allowed to cool gradually during which the crystals the pure substance  separate and are filtered. If after this process the solid is still impure, the process is   repeated until a pure solid is obtained the repetition process is called re-crystallization,
Examples of mixtures that can be separated by crystallization are:
 (i) a mixture of sugar and charcoal (ii) a mixture of sugar and sand


 (b): Fractional Crystallization: This method is used when different solubility in a given solvent. It  solvent as in crystallization method. As the hot filtrate cools, the solid with lower solubility   crystallizes out first and can be filtered off. The filtrate is allowed to cool further during which the  second component crystallizes out. 


C.  Sublimation: This is used for the purification of substances that can change directly from solid state to gaseous state without changing to liquid. Examples of such substances include naphthalene, camphor, iodine and ammonium chloride. Note that the impurity must be a substance that does not sublime. Examples of mixtures that can be separated by sublimation are a nurture of naphthalene and common salt or a mixture of camphor and calcium chloride.  Sublimation method involves heating the mixture of impure substance and allowing the vapor  formed to come in contact with a cold surface where it then solidifies as a pure compound and is collected.


D. Distillation: This method is used when the components of the mixture have different boiling —points. There are different types of distillation which include the following: 
 I. Simple distillation: This method is used to purify an impure volatile (easy to evaporate)  liquid contaminated with a non-Volatile impurities, which could be solid or liquids with high  boiling points. The process involves boiling the mixture in a flask and allowing the vapor to  pass through a condenser where the vapor is condensed into a liquid. The pure liquid, which is called distillate is collected in a receiver (beaker or flask), while the impurities remain in the distillation flask. 

 II. Fractional distillation: This method is used to separate a mixture of two or more volatile miscible liquids with different boiling points. The apparatus for fractional distillation is similar to that of a simple distillation except that a fraction column is incorporated in the set up.
Fractional distillation is used in separating and purifying petroleum fractions.

 III.   Distillation under reduced pressure (vacuum distillation): This method is used to purify a liquid, which decomposes at a temperature below its boiling point. It is also used to concentrate a
solution in which the solute is capable of decomposing at the boiling point of the solvent. In distillation under reduced pressure, a device for evacuation of air from the set-up (usually a pump), is incorporated into the distillation set-up. This reduces the pressure within the flask and the liquid can boil at a lower temperature. The flask is heated by means of a water or oil bath.

IV.  Steam distillation: This method is used for separating high boiling liquid that is immiscible with water, from non-volatile impurities. The process involves volatilizing a liquid by passing steam into a mixture of the compound and water. The liquid boils and its liquid, vapor together with water vapor are passed over the condenser and are collected as the distillate. In this way the high boiling steam volatile liquid is distilled at the temperature of steam, which is lower than its normal boiling point, Since the liquid is immiscible with water the distillate can easily be separated from water using a separation funnel. For example, a mixture of oil and water.

4. Solvent Extraction: This method is based on the solubility of organic ‘substances in organic solvents. Organic compounds are usually more soluble in organic solvents that that in water or other inorganic solvents. Hence organic solvents are used to extract organic solids: liquids from an aqueous solution ‘ or suspension containing Other contaminants. The involves the separation of an organic compound from a solution or suspension (as either a solid or liquid) in an aqueous medium, by shaking with an organic solvent in which the compound is soluble and which is immiscible with water. There will be two phases at the end of the extraction; the aqueous phase and the organic phase. The two phases are separated using a separatory funnel. The organic solvent is removed from the organic extract by evaporation to get the desired organic compound, while the aqueous phase may be discarded. Solvent extraction is used in the extraction of oils from seeds, dyes and pigments from plant materials, and drugs (active agents) from plant materials. Some of the solvents used for extraction are diethyl ether, diisopropyl ether, n-hexane, toluene, dichloromethane and light petroleum. The solvent selected will depend upon the solubility of the substance to be extracted in that solvent and upon the ease with which the solvent can be separated from the solute.

5. Chromatography: This is a separation technique that depends on the differential distribution of the components of a mixture between two phases, namely a stationary phase and a mobile phase. The stationary phase may be a solid or a liquid in which a solid is coated with a nonvolatile liquid. The mobile phase could be a liquid or a -gas. When the stationary phase is a solid, the process is called adsorption chromatography. But when the stationary phase is a liquid adsorbed on a solid support, it is known as partition chromatography. In partition chromatography the mobile phase may be either a liquid (i.e. liquid —liquid partition chromatography or a gas (i.e. gas-liquid partition chromatography gas chromatography)
In adsorption chromatography the mobile phase is usually a liquid hence it is also known as liquid-solid chromatography.
Usually, the mixture is dissolved in the moving phase and passed over the stationary phase.

There are different types of chromatography

  1. Paper chromatography
  2. Thin-layer chromatography (TLC)
  3. Column chromatography
  4. Gas liquid chromatography(GLC)
  5. High performance liquid chromatography
  6. Ion exchange chromatography





1. Paper chromatography: this involves the use of paper (e.g. filter paper) as a solid support. The stationary liquid is trapped in the pores of the paper, while the mobile phase is any other suitable solvent. The mixture to be separated is spotted near the base of the strip of the filter paper and the paper is placed in such a way that the spot does not touch the solvent. The solvent will travel up the paper by capillary action carrying the individual components of   the mixture with it. After sometime, the components of the mixture will be Separated at different  spots on the paper. This technique can be used for the separation of black ink or mixture of dyes. The ratio of the distance travelled by the solvent to the distance travelled by the -substance  is known as the retention- factor or rate of flow (Rf). The retention factor (Rf) for each substance  is constant under the same condition of paper type, solvent, temperature and pressure. Thus Rf is a characteristic of a compound and Can be used for identification of such compound. 


Rf value of substance A =0.75
Rf value of substance B= 0.58

Note that if the components are colorless substances it will be difficult to identify the positions of the separated components. This problem is solved by using developing agent such as iodine and ninhydrin (for amino acids). The developing agent is. spread on the paper after bringing it out of the solvent. The positions of the components are indicated by deep brown patches of iodine. Alternatively, the paper can be viewed under ultraviolet light to detect the positions of the separated components. Paper chromatography is an example of partition Chromatography
i.e. liquid — liquid partition chromatography.

2. Thin layer chromatography: This is similar to paper chromatography except that the stationary phase is a solid adsorbent coated on a glass plate. The most commonly used Solid adsorbents are silica gel, alumina, keselguhr and cellulose powder. The plate is prepared by first preparing the slurry of the adsorbent with dichloromethane and coating thin film of the slurry on the glass plate using spreader. The plate is then dried in an oven at 100 oc to activate it. The activated plate is called a chromatoplate.
After the activation the mixture to be separated is spotted on the glass plate and developed in a suitable solvent as in paper chromatography. The plate is viewed under ultraviolet light or in iodine tank to detect the separation of the components of the mixture. The Rf value can be 
calculated in addition, the spots can be scrapped of separately and washed with a suitable solvent and filtered. The pure substance is obtained by evaporation of the solvent. thin-layer chromatography is both qualitative and quantitative analysis. It is an example of adsorption chromatography. it is a liquid-solid chromatography.



3. Column chromatography: this is a liquid solid chromatography. In this technique the solid absorbent  (e.g. alumina or silica gel) is packed in a glass column and this serves as the  stationary phase. The mobile phase is a liquid. The packing of the column could be dry or wet packing. In dry packing a fine powder of the adsorbent is neatly introduced in long-glass. tube, the end of the glass near the tap is blocked with. cotton wool or glass wool to prevent the adsorbent powder from entering into the separated sample. The mixture to be t separated is poured at the top of the column and allowed to drain down. Pure solvent or a- mixture of solvents, which is the mobile phase, is then poured on top of the column. As the mixture moves down the column it separates as bands. This separation is known as chromatogram. As more solvent is added the individual components can emerge from the bottom of the column and are collected. The sample is obtained by evaporating the solvent.
 Separation is possible in column chromatography because some of the components of the  mixture have more affinity for the adsorbent and thus adsorb more tightly onto the adsorbent  while the others have more affinity for the mobile phase and therefore move. faster and further down the column with the solvent.
If the components are colored, their location on the column or in the elution fraction possess no problem. But if the components are colorless, the elute are collected in fraction and each
fraction can be examined by subjected. to ultraviolet spectroscopic, analysis whereby the absorbance at a specific wavelength can be measured. Alternatively, if the Components are colored, the adsorbent can be extruded from the glass tube and the various band cut with knife. Each band is then extracted with minimum amounts of solvents. The solvents are removed by evaporation and the pure sample collected  

4. Ion exchange chromatography: this operates with the simple principle of column chromatography but here the column is packed with cation or anion exchange resin instead of ordinary absorbent. the type of resin packed in the column depends on the desired separated ion. If it is for the preparation of positive ions, the column is packed with anion (negative) exchange resin. But if it is for the separation of negative ions, the column is packed with cation resins. The components of the mixture are separated because different ions are attracted to different degrees by The ions. Hence those that are attracted more strongly to the resin move behind those •that have less attraction for the resin. Thus the ion that •has least
attraction for the resin is eluted out of the column first. This technique is used in the separation of metal ions and for deionization of Water.


5.  Gas-liquid Chromatography (GLC): This is a type of partition chromatography. The mobile phase is a gas known as carrier gas. It can be N2, H2, helium, argon or CO2). The stationary phase is a non-volatile liquid adsorbed on an inert solid support. The stationary phase  is packed in along coiled glass or stainless steel Column. The mixture to be separated is injected into the column where it is evaporated and carried down the Column by the carrier gas. As the mixture passes through the column, the individual components are separated by partition between the mobile gas phase and the stationary liquid phase. The component that has the least affinity for the liquid stationary phase comes out first from the column to the detector, which Sends the signal to the recorder. The recorder presents the signal from each component of the graph known as chromatogram. The chromatogram is recorded in terms of retention time. the retention time           
Is the time required for a component to emerge from the column after injection of the mixture each peach in the graph represents a component and the area under the peak is the relative concentration of the components in the mixture. GLC gives both qualitative and quantitative analysis of the components.  



6. High performance liquid chromatography: this is a very important analytical method for these compounds which are non-volatile or thermally unstable that cannot be separated by GCL. Examples of such compounds are carbohydrates, steroids alkaloids, peptides and amino acids, antibiotics and nucleotides. HIPLC is similar to GLC. The only difference is that the mobile phase is a liquid. Thus higher pressure is required to move the liquid down the column. The apparatus has a pump, which helps in forcing the sample and the liquid mobile phase through the column to the detector and recorder at a faster rate. The components are detected by ultraviolet spectrometer and the signals come out, as peaks similar to those of gas-liquid 

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