INTRODUCTORY AND IMPORTANCE OF ORGANIC CHEMISTRY

INTRODUCTORY AND IMPORTANCE OF ORGANIC CHEMISTRY

Organic chemistry is the study of the compounds of carbon. Carbon compounds are far more numerous than those of other elements because carbon are able to bond together to form a wide range of chains and rings. The subject is named organic chemistry because living organisms are composed of carbon compounds.

Organic chemistry could be regarded as the chemistry of life.  Organic Chemistry is of great medical, economic and technological importance. It touches almost every areas of our daily life (food, medicinal drugs, paper, ink, paint, plastic, fuel, automobiles, textiles, pesticides, dyes, explosives, etc).

Billions of years ago most of the carbon atoms on the earth existed as CH₄:

CH₄, H₂O, NH₃, H₂ were the main components of the primordial atmosphere.

Electrical charges and other forms of highly energetic radiation caused these simple compounds to fragment into highly reactive pieces which combine into more complex compounds such as amino acids, formaldehyde, hydrogen cyanide, purines, and pyrimidines.

Amino acids reacted with each other to form the first protein.

Formaldehyde reacted with each other to become sugars, and some of these sugars, together with inorganic phosphates, combined with purines and pyrimides to become simple molecules of ribonucleic acids (RNAs) and DNA.

Living organisms are nature’s laboratory where many chemical transformations are taking place independently, simultaneously and continuously. Hence most organic samples are mixtures of compounds.

However, the need for identification of these numerous compounds of carbon had led to their division into groups/family based on some of their common features. One of the major questions requiring an answer in organic Chemistry and Chemistry in general is what is present in a given sample of material.    These will often involve determining the class of the compounds and the arrangement of atoms in the molecules i.e. the structure of the compound(s). The correct answer to these question and many more will help to come up with the identity of the compound(s) in the sample.

Peculiarities of carbon chemistry are:

Ability of carbon to form bonds not only with other elements but also with itself.

(a)    The existence of different types of isomeric compounds

 The basic structure of any organic compound be it plastic, protein, medicine or fuel  consist of a skeleton of carbon atoms joined together in chains and rings. This ability of an element to form chains of atoms bonded together is known as catenation.

The carbon atom has four (4) valence electrons on its outermost shell, this is the reason for having for having four elements establishing 4 covalent bonds around a carbon atom. 

  Qualitative Analysis of Organic Compounds

   The detection of various elements present in an organic compound is called qualitative analysis. Carbon and hydrogen are present in almost all the organic compounds. Other commonly present element in organic compounds are oxygen, nitrogen, sulphur and sometimes phosphorus.

The principle of various elements present in any organic compound can be detected as follows:

Detection of Carbon and Hydrogen

Principle: Carbon and hydrogen are detected by heating the organic compound with cupric oxide (CuO) strongly, where carbon is oxidized to carbon (IV) oxide and hydrogen to water. CO₂ is tested by lime water, whereas water is tested by anhydrous copper sulphate test.

The given organic compound is mixed with dry copper oxide (CuO) and heated in a hard glass tube. The products of the reaction are passed over (white) anhydrous Copper sulphate and then bubbled through lime water. If copper sulphate turns blue due to the formation of  CuSO₄.5H₂O (by water vapour) then the compound contains hydrogen. If the lime water is turned milk by CO2, then the compound contains carbon.

Detection of Nitrogen, Sulphur, and Halogen

Nitrogen, sulphur, and halogen in  any organic compound are detected by Lassaigne’s test.

Theory: Elements like nitrogen, sulphur and halogen are bonded covalently in the organic compouns. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution, and can be detected by simple chemical tests.

 Preparation of  Lassaigne’s extract: A small piece of sodium is heated gently in an ignition tube till the sodium melts. About 50 to 60 mg of the organic compound is added to this and the tube heated strongly for 2-3 minutes to fuse the material inside it. After cooling, the tube is carefully broken in a china dish containing about 20 to 30 mL of distilled water. The fused material along with the pieces of ignition tube is crushed with the help of a glass rod and the contents of the china dish are boiled for a few minutes. The sodium salts formed in the above reactions (i.e. NaCN, Na₂S, NaX or NaSCN) dissolve. Excess of sodium reacts with water to give sodium hydroxide. This alkaline solution is called Lassaigne's extract or sodium extract. The solution is then filtered to remove the insoluble materials and the filtrate is used for making the tests for nitrogen, sulphur and halogens.

Detection of nitrogen

A small quantity of the sodium extract is taken in a test tube. It is made alkaline by adding 2-3 drops of sodium hydroxide (NaOH) solution. 1 mL of freshly prepared solution of ferrous sulphate is added to this solution. The mixture of the two solutions is boiled and then acidified with dilute sulphuric acid. The appearance of prussian blue or green colouration of the precipitate confirms the presence of nitrogen in the given organic compound.

The carbon and nitrogen present in the organic compound on fusion with sodium metal give sodium cyanide (NaCN) soluble in water. So, the sodium extract contains sodium cyanide which, on reaction with ferrous sulphate, gives sodium ferrocyanide. Some of the ferrous salt is oxidised to the ferric salt on heating and this reacts with sodium ferrocyanide to form ferric ferrocyanide.

Note: When nitrogen and sulphur both are present in any organic compound, sodium thiocyanate is formed during fusion. When extracted with water sodium thiocyanate goes into the sodium extract and gives 'blood red coloration' with ferric ions due to the formation of ferric thiocyanate.

Detection of sulphur

The presence of sulphur in any organic compound is detected by using sodium extract as follows:

Lead acetate test

A small portion of sodium extract is acidified with acetic acid and lead acetate solution is added to it. A black precipitate of lead sulphide indicates the presence of sulphur.

Sodium nitroprusside test

To a small quantity of sodium extract taken in a test tube, 2 to 3 drops of sodium nitroprusside are added to the solution. A violet color indicates the presence of sulphur. This color fades away slowly on standing.

Note: The physical properties of qualitatively detected  substances can be identified by carrying out the following: melting point, colour, boiling point, texture, density, ductility, electrical conductivity, malleability, thermal conductivity, refractive index, and coefficient of linear expansion.

Isolation and Purification of Organic compounds

Isolation: This is basically the process of getting out, separating, or extracting organic compounds from its natural plant or synthesised products. Extraction is a physical process by which a compound (solute) is transferred from one phase to another, usually from a liquid or a solid to another liquid. The solute is removed from one phase by adding to it an immiscible solvent in which the solute is more soluble.

Liquid- liquid extraction: This involves the distribution, or partitioning, of a solute between two immiscible liquid phases. In organic chemistry laboratory, the most common process involves the extraction of an organic compound from one liquid phase to the other. The two liquid phases are usually, but not always, an aqueous solution and an organic solvent. Whenever extraction protocol is being used, most extraction operations in the organic laboratory are carried out in separation funnels.

                             

In synthetic organic experiment washes are introduced so as to remove unwanted chemicals associated the desired product. The following are examples of washes/when applicable:

(1)   Water, for removing salts and organics with reasonable good water solubility. Water washes are also used immediately following extractions of the mixture with either acid or base to ensure that all traces of acid or base have been removed;

(2)   Saturated aqueous salt (like NaCl, Na₂SO₄), for removing salt and organics when a salting-out effect is desirable (the effect of decreasing the solubility of molecular species by increasing salt concentration is known as “salting-out effect”) or to help prevent emulsions;

(3)   Aqueous acid (like HCl, CH₃COOH) for extracting basic compounds;

(4)   Aqueous base (like NaOH, Na₂CO₃, NaHCO₃) for extracting acid compounds.

  

 Choice of extraction solvent

Although water is almost always one of the liquids in the liquid-liquid extraction process, the choice of organic solvent is quite wide. A good extraction solvent needs four essential features namely:

      It has to be practically immiscible with water   

      It has to have a different density to water

      It needs good stability and volatility so that it can easily be removed from the organic compound by evaporation.

      The solute you want to extract has to dissolve easily in it

        

S0lid-liquid extraction: Solids can be extracted using organic solvents. One very simple way of doing this is to place the solid in an Erlenmeyer flask, cover the solid with the organic solvent and allow the flask to stand with occasional swirling. The organic compound that you are interested in will be slowly leached out of the solid. The unwanted solid can then be removed from the organic solution containing your compound by simple filtration. This is, however, a fairly inefficient technique in term of extracting all of the organic compound in question, although the efficiency of the extraction can be improved by using hot solvents.

A much more efficient way to extract solids is to use a soxhlet apparatus. In this technique, the solid to be extracted is placed into a special “thimble” made of thick filter paper. The thimble is placed in the apparatus as shown in Fig 3 below, and the whole extractor is placed on top of a well-supported round-bottomed flask containing the organic solvent. A reflux condenser is placed on top of the soxhlet extractor. The flask is heated is heated using a water, or steam bath (for flammable solvents) or some form of electrical heating, so that the solvent boils.

Quantitative analysis of organic compounds

Determining the empirical experimental formula:

The determination of the chemical formula of an unknown organic substance starts with a quantitative elemental analysis, which shows the percentage composition of different elements present in the substance. If each of these percentages is divided by the atomic weight of the particular element, this provides the ratios of the numbers of atoms of each of these elements.

The following calculations show the way in which this is done:

% composition                 C= 40.82%                 H=   8.63%                 N=  23.75%            Total= 73.20% Reminder= O= 26.80%                        100.00 %

Atomic weight C= 12 H=  1 N= 14 O= 16

% /Atomic weight 40.82/12 = 3.40 8.63/1     = 8.63 23.75/14 = 1.69 26.80/16 =1.67

This provides the atomic ratios of   C: H : N : O = 3.40:8.63:1.69:1.67. If each of these figures is divided by the lowest in numerical value, 1.67 the ratio of  C : H : N : O = 2:5:1:1 is obtained.  

 The empirical formula of a compound gives the ratio of the number of different atoms in the compound. For example, the empirical formula of ethanoic acid is CH₂O. This means that, for every atom of carbon in ethanoic acid, there are two atoms of hydrogen and one atom of oxygen. The empirical formula of a compound can be calculated from the percentage composition of the compound, the latter being determined by various forms of quantitative analysis.

The molecular formula of a compound gives the total number of atoms of the different elements present in a molecule of the compound. For example, the molecular formula of ethanoic acid is C₂H₄O₂. This means that in a molecule of ethanoic acid there are two atoms of carbon, four atoms of hydrogen, and two atoms of oxygen. In other to determine the molecular formula of a compound, its empirical formula must be known.