Methods of Purification of Organic Compounds

Methods of Purification of Organic Compounds

Need for Purification:

In order to study the structure, physical properties, chemical properties and biological properties of organic compounds they must be in the pure state. There are several methods by which organic compounds can be purified. The methods employed for purification depend upon the nature of impurity and the nature of organic compound. The most widely used technique for the separation and purification of organic compounds are:

  • Crystallisation,
  • Sublimation
  • Distillation
  • Fractional Distillation
  • Steam Distillation
  • Azeotropic Distillation
  • Diffrential Extraction and
  • Chromatography

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Sublimation:

Few substances like benzoic acid, naphthalene and camphor when heated pass directly from solid to vapor without melting (ie liquid). On cooling the vapours will give back solids. Such phenomenon is called sublimation. It is a useful technique to separate volatile and non-volatile solid. It has limited application because only a few substance will sublime.

Substances to be purified is taken in a beaker. It is covered with a watch glass. The beaker is heated for a while and the resulting vapours condense on the bottom of the watch glass. Then the watch glass is removed and the crystals are collected. This method is applicable for organic substance which has high vapour pressure at temperature below their melting point.

Substances like naphthalene, benzoic acid can be sublimed quickly. Substance which has very small vapour pressure will decompose upon heating are purified by sublimation under reduced pressure. This apparatus consists of large heating and large cooling surface with small distance in between because the amount of the substance in the vapour phase is much too small in case of a substance with low vapour pressure.

Purification of Organic Compounds

Crystallization:

It is the most widely used method for the purification of solid organic compound. This process is carried out in by the following step

(i) Selection of Solvent:

Most of the organic substances being covalent do not dissolve in polar solvents like water, hence selection of solvent (suitable) becomes necessary. Hence the powdered organic substance is taken in a test tube and the solvent is added little by little with constant stirring and heating, till the amount added is just sufficient to dissolve the solute (ie) organic compound.

If the solid dissolves upon heating and throws out maximum crystals on cooling, then the solvent is suitable. This process is repeated with other solvents like benzene, ether, acetone and alcohol till the most suitably one is sorted out.

(ii) Preparation of Solution:

The organic substance is dissolved in a minimum quantity of suitable solvent. Small amount of animal charcoal can be added to decolorize any colored substance. The heating may be done over a wire gauze or water bath depending upon the nature of liquid (ie) whether the solvent is low boiling or high boiling.

Purification of Organic Compounds

(iii) Filtration of Hot Solution:

The hot solution so obtained is filtered through a fluted filter paper placed in a funnel.

(iv) Crystallization:

The hot filtrate is then allowed to cool. Most of the impurities are removed on the filter paper, the pure solid substance separate as crystal. When copious amount of crystal has been obtained, then the crystallization is complete. If the rate of crystallization is slow, it is induced either by scratching the walls of the beaker with a glass rod or by adding a few crystals of the pure compounds to the solution.

(v) Isolation and Drying of Crystals:

The crystals are separated from the mother liquor by fitration. Filtration is done under reduced pressure using a Bucher funnel. When the whole of the mother liquor has been drained into the filtration flask, the crystals are washed with small quantities of the pure cold solvent and then dried.

Distillation:

This method is to purify liquids from non-volatile impurities, and used for separating the constituents of a liquid mixture which differ in their boiling points. There are various methods of distillation depending upon the diffrence in the boiling points of the constituents. The methods are

  • Simple Distillation
  • Fractional Distillation and
  • Steam Distillation.

The process of distillation involves the impure liquid when boiled gives out vapour and the vapour so formed is collected and condensed to give back the pure liquid in the receiver. This method is called simple distillation. Liquids with large difference in boiling point (about 40k) and do not decompose under ordinary pressure can be purified by simply distillation eg. The mixture of C6H5NO2 (b.p 484k) & C6H6 (354k) and mixture of diethyl ether (b.p 308k) and ethyl alcohol (b.p 351k).

Purification of Organic Compounds

Fractional Distillation:

This is one method to purify and separate liquids present in the mixture having their boiling point close to each other. In the fractional distillation, a fractionating column is fitted with distillation flask and a condenser. A thermometer is fited in the fractionating column near the mouth of the condenser.

The process of separation of the components in a liquid mixture at their respective boiling points in the form of vapours and the subsequent condensation of those vapours is called fractional distillation. The process of fractional distillation is repeated, if necessary. This method finds a remarkable application in distillation of petroleum, coal-tar and crude oil.

Steam Distillation:

This method is applicable for solids and liquids. If the compound to be steam distilled the it should not decompose at the steam temperature, should have a fairly high vapour pressure at 373k, it should be insoluble in water and the impurities present should be non-volatile.

The impure liquid along with little water is taken in a round-bottom flask which is connected to a boiler on one side and water condenser on the other side, the flask is kept in a slanting position so that no droplets of the mixture will enter into the condenser on the brisk boiling and bubbling of steam.

The mixture in the flask is heated and then a current of steam passed in to it. The vapours of the compound mix up with steam and escape into the condenser. The condensate obtained is a mixture of water and organic compound which can be separated. This method is used to recover essential oils from plants and flowers, also in the manufacture of aniline and turpentine oil. (see Fig. 11.4)

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Azeotropic Distillation

These are the mixture of liquids that cannot be separated by fractional distillation. The mixtures that can be purified only by azeotropic distillation are called as azeotropes. These azeotropes are constant boiling mixtures, which distilled as a single component at a fixed temperature. For example ethanol and water in the ratio of 95.87:4.13.

In this method the presence of a third component like C6H6, CCl4, ether, glycerol, glycol which act as a dehydrating agent depress the partial pressure of one component of azeotropic mixture and raises the boiling point of that component and thus other component will distill over.

Subtances like C6H6, CCl4 have low boiling points and reduce the partial vapour pressure of alcohol more than that of water while subtances like glycerol & glycol etc have high boiling point and reduce the partial vapour pressure of water more than that of alcohol.

Purification of Organic Compounds

Differential Extraction:

The process of removing a substance from its aqueous solution by shaking with a suitable organic solvent is termed extraction. When an organic substance present as solution in water can be recovered from the solution by means of a separating funnel.

The aqueous solution is taken in a separating funnel with little quantity of ether or chloroform (CHCl3). The organic solvent immiscible with water will form a separate layer and the contents are shaken gently. The solute being more soluble in the organic solvent is transfered to it. The solvent layer is then separated by opening the tap of the separating funnel, and the substance is recovered.

Chromatography:

The most valuable method for the separation and purification of small quantity of mixtures. As name implies chroma-colour and graphed writing it was first applied to separation of different colored constituents of chlorophyll in 1906 by M.S Tswett, a Russian botanist.

He achieved it by passing a petroleum ether solution of chlorophyll present in leaves through a column of CaCO3 firmly packed into a narrow glass tube. Different components of the pigments got separated into land or zones of different colors and now this technique is equally well applied to separation of colorless substances.

The principle behind chromatography is selective distribution of the mixture of organic substances between two phases – a stationary phase and a moving phase. The stationary phase can be a solid or liquid, while the moving phase is a liquid or a gas.

When the stationary phase is a solid, the moving phase is a liquid or a gas. If the stationary phase is solid, the basis is adsorption, and when it is a liquid, the basis is partition. So the Chromatography is defined as a technique for the separation of a mixture brought about by differential movement of the individual compound through porous medium under the influence of moving solvent. The various methods of chromatography are

  • Column chromatography (CC)
  • Then layer chromatography (TLC)
  • Paper chromatography (PC)
  • Gas-liquid chromatography (GLC)
  • Ion-exchange chromatography

Purification of Organic Compounds

Adsorption Chromatography:

The principle involved is different compounds are adsorbed on an adsorbent to different degree. Silica gel and alumina are the commonly used adsorbent. The components of the mixture move by varying distances over the stationary phase Column chromatography and thin layer chromatography are the techniques based on the principle of differential adsorption.

Column Chromatography:

This is the simplest chromatographic method carried out in long glass column having a stop cock near the lower end. This method involves separation of a mixture over a column of adsorbent (Stationery phase) packed in a column. In the column a plug of cotton or glass wool is placed at the lower end of the column to support the adsorbent powder. The tube is uniformely packed with suitable absorbent constitute the stationary phase. (Activated aluminum oxides (alumina), Magnesium oxide, starch are also used as absorbents).

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The mixture to be separated is placed on the top of the adsorbent column. Eluent which is a liquid or a mixture of liquids is allowed to flow down the column slowly. Different components are eluted depending upon the degree to which the components are adsorbed and complete separation takes place. The most readily adsorbed substances are retained near the top and others come down to various distances in the column.

Purification of Organic Compounds

Thin layer Chromatography:

This method is an another type of adsorption chromatography with this method it is possible to separate even minute quantities of mixtures. A sheet of a glass is coated with a thin layer of adsorbent (cellulose, silica gel or alumina). This sheet of glass is called chromoplate or thin layer chromatography plate.

After drying the plate, a drop of the mixture is placed just above one edge and the plate is then placed in a closed jar containing eluent (solvent). The eluent is drawn up the adsorbent layer by capillary action. The components of the mixture move up along with the eluent to different distances depending upon their degree of adsorption of each component of the mixture. It is expressed in terms of its retention factor (ie) Rf value

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The spots of colored compounds are visible on TLC plate due to their original color. The colorless compounds are viewed under uv light or in another method using iodine crystals or by using appropriate reagent.

Partition Chromatography:

Paper chromatography (PC) is an example of partition chromatography. The same procedure is followed as in thin layer chromatography except that a strip of paper acts as an adsorbent. This method involves continues differential portioning of components of a mixture between stationary and mobile phase. In paper chromatography, a special quality paper known as chromatography paper is used. This paper act as a stationary phase.

A strip of chromatographic paper spotted at the base with the solution of the mixture is suspended in a suitable solvent which act as the mobile phase. The solvent rises up and flows over the spot. The paper selectively retains different components according to their different partition in the two phases where a chromatogram is developed.

The spots of the separated colored compounds are visible at different heights from the position of initial spots on the chromatogram. The spots of the separated colorless compounds may be observed either under ultraviolent light or by the use of an appropriate spray reagent.

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IUPAC Nomenclature Of Organic Compounds

IUPAC Nomenclature Of Organic Compounds

The International Union of Pure and Applied Chemistry (IUPAC) is the world authority on chemical nomenclature and terminology, naming of new elements in the periodic table standardized methods for measurement; atomic weights, and many other critically-evaluated data. According to IUPAC recommendations to name any organic compound, it is considered as a derivative of its parent saturated hydrocarbon. The IUPAC name of an organic compound consists of three parts.

prefix + root word + suffix

Root word denotes the number of carbon atoms in the longest continiuous chain in molecules. Prefix denotes the group(s) attached to the main chain which is placed before the root. Suffix denotes the funtional group and is placed after the root word.

Number of carbons in parent chain and the corresponding root words

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Nomenclature of Organic Compounds:

Suffix:

There are two types of suffix. They are primary suffix and secondary suffix

Primary Suffix:

It denotes the saturation/unsaturation of organic compounds. It is added immediately after the root word. Primary suffix for various saturated and unsaturated carbon chains are as follows:

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Secondary Suffix:

It is used to denote the nature of functional group present in the organic compound. It is added to the primary suffix by removig its terminal ‘e’. Secondary suffix names for some functional groups is listed below in table 11.4

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Nomenclature of Organic Compounds:

Prefix:

Substituents that are attached to the parent carbon chain are denoted by adding prefix names before the root word. The prefix names for some common substituents are listed below. If the functional groups are not part of the parent chain, they are considered as substituents. In such cases its prefix name is added before the root word. Prefix names for some functional groups mentioned along with their secondary prefix are listed in table 11.4

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IUPAC rules for nomenclature of organic compounds

The following steps should be followed for naming an organic compound as per IUPAC nomenclature.

  1. Choose the longest carbon chain. (Root word). Consider all the other groups attached to this chain as substitutents.
  2. Numbering of the longest carbon chain
  3. Naming of the substituents (prefies or suffixs)
  4. Arrange the substitutents in the alphabetical order
  5. Write the name of the compound as below

“prefix + root word + primary suffix + secondary suffix

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The following are guide lines for writing IUPAC of the organic compound.

1. The IUPAC names are always written as single word, with notable exception of organic salts, acids and acid derivatives.

2. Commas are used between two adjacent number or letter symbols, and hypens are used to separate numbers and letter symbol in names Eg: 2, 2-Dimethyl-3-hexene N, N-Dimethyl methanamide

3. Structural prefix such as, meso-, cis-, trans-, are italicised and joined to the name by a hypen. These prefixes are omitted in alphabetising compound names or in capitalising names at the beginning of a sentence.Eg: trans-2-Butene

4 .Structural prefixes such as di, tri, tetra are treated as a part of the basic name and therefore are neither italicised nor separated by a hypen. These prefixes are not taken into account in alphabetising compound names eg: 4-Ethyl -2,2-dimethyl hexane.

5. To name alicyclic compounds , the additional rules should be followed as illustrated in the table 11.6

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Nomenclature of Organic Compounds:

NOMENCLATURE OF AROMATIC COMPOUNDS:

An aromatic compound consists of two parts nucleus and side chain

(A) Nucleus:
The benzene ring present in aromatic compound is called nucleus. It is represented as follows

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(B) Side chain:
Alkyl or any other aliphatic group attached to the benzene nucleus by replacing one or more hydrogen atom is called the side chain.

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If one hydrogen atom, (or) two hydrogen atoms or three hydrogen atoms are replaced in the benzene ring by some other groups, they are termed as mono substituted, di substituted or tri substituted derivative respectively.

Example

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If more than one hydrogen atom of benzene ring is replaced by some other atom or group, then their position is mentioned by Arabic numerals 1,2,3 ….. In case of disubstitution, respective position of two groups can also be mentioned as follows.

ortho – adjacent; represented as – o
meta – alternate; represented as – m
Para – opposite; represented as – p

Aromatic compounds are basically of two types:

Nomenclature of Organic Compounds:

1. Nuclear substituted aromatic compounds:

These are the compounds in which the functional group is directly attached to the benzene ring. They are named as derivatives of benzene.

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Nuclear Substituaed Aramatic Halogen Derivatives Compounds.

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2. Side chain substituted aromatic compounds:

These are the compounds in which the functional group is present in the side chain of the benzene ring. These are named as phenyl derivatives of the corresponding aliphatic compounds.

Nomenclature of Organic Compounds:

Side Chain Substituted

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Aryl Groups

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Selection of parent hydrocarbon out of side chain and benzene ring is based on (more or less) some rule as for the alicyclic compounds.

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Detection of Elements in Organic Compounds

Detection of Elements in Organic Compounds

Introduction

The first step in the analysis of an organic compound is the detection of elements present in it. The principal elements are carbon, hydrogen and oxygen In addition to these they may contain nitrogen, sulphur and halogens. Phosphorous. Metals like Li, Mg, Zn are present in certain organometalic compounds.

Detection of Carbon and Hydrogen

If the compound under investigation is organic, there is no need to test for carbon. This test is performed only to establish whether a given compound is organic or not. With the exception of few compounds like CCl4, CS2 all organic compounds also contain hydrogen. The presence of both these elements is confirmed by the following common test.

Detection of Elements in Organic Compounds

Copper (II) Oxide Test:

The organic substance is mixed with about three times its weight of dry copper oxide by grinding. The mixture is then placed in a hard glass test tube fitted with a bent delivery tube. The other end of which is dipping into lime water in an another test tube. The mixture is heated strongly and the following reaction take place.

C + 2CuO → CO2 + 2Cu
2H + CuO → H2O + Cu

Thus if carbon is present, it is oxidized to CO2 which turns lime water milky. If hydrogen is also present, it will be oxidized to water which condenses in small droplets on the cooler wall of the test tube and inside the bulb. Water is collected on anhydrous CuSO4 which turns anhydrous CuSO4 blue. This confirms the presence of C and H in the compound.

Detection of nitrogen by lassaigne sodium fusion test:

This is a good test for the detection of nitrogen in all classes of nitrogenous compound and it involves the preparation of sodium fusion extract.

This method involves the conversion of covalently bonded N, S or halogen present in the organic compounds to corresponding water soluble ions in the form of sodium salts. For this purpose a small piece of Na dried by pressing between the folds of a filter paper is taken in a fusion tube and it is gently heated.

When it melts to a shining globule, put a pinch of the organic compound on it. Heat the tube till reaction ceases and becomes red hot. Plunge it in about 50 mL of distilled water taken in a china dish and break the bottom of the tube by striking against the dish. Boil the contents of the dish for about 10 mts and filter. This filtrate is known as lassaignes extract or sodium fusion extract and it used for detection of nitrogen, sulphur and halogens present in organic compounds.

Detection of Elements in Organic Compounds

Test for Nitrogen:

If nitrogen is present it gets converted to sodium cyanide which reacts with freshly prepared ferrous sulphate and ferric ion followed by conc. HCl and gives a Prussian blue color or green colour or precipitate. It confirms the presence of nitrogen. HCl is added to dissolve the greenish precipitate of ferrous hydroxide produced by the excess of NaOH on FeSO4 which would otherwise mark the Prussian blue precipitate. The following reaction takes part in the formation of Prussian blue.

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Incase if both N & S are present, a blood red colour is obtained due to the following reactions.

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Test for sulphur:

To a portion of the lassaigne’s extract, add freshly prepared sodium nitro prusside solution. A deep violet or purple colouration is obtained. This test is also used to detect S2- in inorganic salt analysis

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Acidify another portion of lassaigne’s extract with acetic acid and add lead acetate solution. A black precipitate is obtained.

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Oxidation Test:

The organic substances are fused with a mixture of KNO3 and Na2CO3. The sulphur, if present is oxidized to sulphate.

Na2CO3 + S + 3O → Na2SO4 + CO2

The fused mass is extracted with water, acidified with HCl and then BaCl2 solution is added to it. A white precipitate indicates the presence of sulphur.

BaCl2 + Na2SO4 → BaSO4 + 2NaCl

Detection of Elements in Organic Compounds

Test for Halogens:

To another portion of the lassaigne’s filtrate add dil HNO3 warm gently and add AgNO3 solution.

(a) Appearance of curdy white precipitate soluble in ammonia solution indicates the presence of chlorine.
(b) Appearance of pale yellow precipitate sparingly soluble in ammonia solution indicates the presence of bromine.
(c) Appearance of a yellow precipitate insoluble in ammonia solution indicates the presence of iodine.

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If N or S is present in the compound along with the halogen, we might obtain NaCN and Na2S in the solution, which interfere with the detection of the halogen in the AgNO3 test. Therefore we boil the lassaignes extract with HNO3 which decomposes NaCN and Na2S as

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Detection of Elements in Organic Compounds

Test for Phosphorous:

A solid compound is strongly heated with a mixture of Na2CO3 & KNO3 phosphorous present in the compound is oxidized to sodium phosphate. The residue is extracted with water and boiled with Conc. HNO3. A solution of ammonium molybdate is added to the above solution. A canary yellow coloration or precipitate shows the presence of phosphorous.

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Structural Representation of Organic Compounds

Structural Representation of Organic Compounds

Molecular formula of a compound is the simplest, least informative representation, showing the ratio of atoms present. The structure of an organic compound can be represented using any one of the below mentioned methods.

  1. Lewis structure or dot structure,
  2. Dash structure or line bond structure,
  3. Condensed structure
  4. Bond line structure

We know how to draw the Lewis structure for a molecule. The line bond structure is obtained by representing the two electron covalent bond by a dash or line (-) in a Lewis structure. A single line or dash represents single σ covalent bond, double line represents double bond (1σ bond, 1π bond) and a triple line represents triple bond (1σ bond, 2π bond). Lone pair of electrons on heteroatoms may or may not be shown. This represents the complete structural formula.

Structural Representation of Organic Compounds

This structural formula can be further abbreviated by omitting some or all of the dashes representing covalent bonds and by indicating the number of identical groups attached to an atom by a subscript.

The resulting expression of the compound is called a condensed structural formula. For further simplification, organic chemists use another way of representing the structures in which only lines are used.

In this type of representation of organic compounds, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are shown in a zigzag fashion. The only atoms specifically written are oxygen, chlorine, nitrogen etc. These representations can be easily understood by the following illustration.

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Molecular Models

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Molecular models are physical devices that are used for a better visualisation and perception of three dimensional shapes of organic molecules. These are made of wood, plastic or metal and are commercially available.

(i) Frame work model

(ii) Ball and stick model &

(iii) Space filling model. In the frame work model only the bonds connecting the atoms themselves are shown. This model emphasizes the pattern of bonds of a molecule while ignoring the size of the atom. In the ball and stick model, both the atoms and the bonds are shown.

Ball represent atoms and the stick a bond. Compounds containing C=C can be best represented by using springs in place of sticks and this model is termed as ball and spring model. The space filling model emphasizes the relative size of each atom based on its vander-waals radius.

Structural Representation of Organic Compounds

Three dimensional representation of organic molecules:

The simplest convention is solid and dashed wedge formula in which 3-D image of a molecule can be perceived from two dimensional picture. In this representation a tetrahedral molecule with four atoms or group a, b, c and d bonded to it can be represented by a wedge formula as follows.

A solid wedge Structural Representation of Organic Compounds img 3 (or a heavy line) is used to indicate a bond projecting above the plane of the paper and the dashed wedge Structural Representation of Organic Compounds img 4 (or a dashed line) is used to depict the bond below the plane. The bonds lying in the plane of the paper are shown by normal lines.

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Fisher Projection Formula:

This is a method of representing three dimensional structures in two dimension. In this method, the chiral atom(s) lies in the plane of paper. The horizontal substituents are pointing towards the observer and the vertical substituents are away from the observer. Fisher projection formula for tartaric acid is given below.

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Sawhorse Projection Formula:

Here the bond between two carbon atoms is drawn diagonally and slightly elongated. The lower left hand carbon is considered lying towards the front and the upper right hand carbon towards the back. The Fischer projection inadequately portrays the spatial relationship between ligands attached to adjacent atoms. The sawhorse projection attempts to clarify the relative location of the groups.

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Structural Representation of Organic Compounds

Newman Projection Formula:

In this method the molecules are viewed from the front along the carboncarbon bond axis. The two carbon atom forming the σ bond is represented by two circles. One behind the other so that only the front carbon is seen.

The front carbon atom is shown by a point where as the carbon lying further from the eye is represented by the origin of the circle. Therefore, the C-H bonds of the front carbon are depicted from the circle while C-H bonds of the back carbon are drawn from the circumference of the circle with an angle of 120° to each other.

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