Organic Chemistry - Some Basic Principles and Techniques

Earth is a unique planet in our solar system and the known universe which sustains life. It has been able to do so because of the presence on a wide variety of organic compounds here on earth. All life forms – plants, animals, microbes etc. – process and synthesize organic compounds. These can be as complex as the structure of DNA or as simple as methane gas. Organic compounds are the building blocks of life – literally and figuratively. It is literal, because our organs and body parts are made of organic compounds. It is figurative, because it was their presence on earth which helped life to develop and evolve. Long thought to be entirely different from inorganic compounds in a way that these could only be obtained from living organisms, this notion got debunked when chemists were able to make organic compounds like Urea, Acetic Acid and Methane in the laboratory by starting from inorganic compounds. The key characteristic of such compounds is the presence of carbon, which can combine with other elements like hydrogen, sulphur, nitrogen etc. to form covalent bonds, and hence organic compounds.

Tetravalence of Carbon: Shapes of Organic Compounds

Carbon is essentially a tetravalent element, i.e. it has a valency of 4. The electronic configuration of carbon atom is 1s22s22p2. There are 4 electrons in the valence shell, hence a valency of 4. Carbon has a strong tendency to form covalent bonds to complete the outer shell. Since the 2nd shell can have a maximum of 8 electrons, carbon can form 4 covalent bonds simultaneously. These bonds could be single, double or triple. In single bonds, such as in Methane (CH4), carbon has sp3 hybridization. In double bonds, such as those in Ethene (C2H4), carbon has sp2 hybridization. In triple bonds, like in Ethyne  (C2H2), carbon has sp hybridization. 

Higher the ‘s’ orbital character in hybridization, shorter the bond length and stronger the bond. Energy required to break such a bond is also high. Higher presence of ‘s’ character also increases the electronegativity of carbon atom. 

In double and triple bond molecules, carbon also forms ‘pi’ bonds. Pi bonds happen when 2 lobes of one orbital in one of the atoms overlap with corresponding lobes of a single orbital in another atom. In a double bond of carbon, one bond would be ‘sigma’ while the second would be pi. A triple bond will have 2 pi and 1 sigma bonds. Because the p orbitals, which form pi bonds in carbon compounds, are perpendicular to the plane of the molecule, they form 2 electron charge clouds above and below the plane of the molecule. Hence, an incoming reagent can attack these electron clouds, making pi bonds highly reactive.

Structural Representations of Organic Compounds

Primarily, there are four ways to represent organic compounds. These are Lewis structure (dot structure), dash structure, condensed structure and bond line structure. The most commonly used structures are the latter three. In a dash structure, each bond is represented as a dash. Hence, double or triples bonds will be represented as double or triple dashes. In condensed structure, a portion, or sometimes all, of the dashes are omitted from a typical dash structure while retaining the element symbols. In a bond line structure, all elements except carbon and hydrogen are specifically written in their elemental symbols. A vertice in a zig-zag line represents a carbon atom and hydrogen atoms required to satisfy carbon’s valency. Terminal ends of the zig-zag line are all methyl groups, unless specified. We can take an example of 2-bromobutane which can be represented in three structure types as below –

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Dash structure

CH3CHBrCH2CH3

Condensed Structure

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Bond line Structure

Cyclic compounds can also be similarly shown in different ways. Some examples are below –

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Representation of Cyclic Compound 

What we discussed above is a two dimensional or 2D representation of organic molecules. We can also depict a 3D representation using wedges. Typically, two types of wedges are used – solid and dashed. Solid wedge represents bond coming out of the paper towards the observer while dashed wedge depicts bond coming out of the paper away from the observer. The bonds which lie in the plane of paper are depicted by normal lines or dashes. 2-bromobutane can be represented as –

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3D Representation 

Depending on the orientation, some compounds can have multiple 3D structures. Such compounds are called chiral compounds.

To better understand the structure of molecules, chemists and chemistry students also rely on physical models, which are also known as molecular models. There are three types of molecular models – framework model, ball & stick model and space filling model. Framework models represent bonds as sticks and there is no representation of atoms. Ball and stick models also represent atoms as balls along with sticks. Sometimes, springs can be used to represent double bonds, hence such models are also sometimes called as ball and spring models. In space filling models, relative sizes of each atom are represented. Continuing our example of 2-bromobutane, it can be represented as various molecular models below –

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Framework or stick model 

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Ball and Stick Model

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Space Filling Model

Classification of Organic Compounds

A large number of organic compounds are found in nature and has been synthesized for various purposes. Therefore, soon chemists felt the need of classification of the organic compounds. 

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Acyclic Compounds – Those organic compounds which are either branched or chain compounds are called acyclic compounds. These are also known as open chain compounds or aliphatic compounds. Examples – 

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Propane

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Isopentane 

Cyclic Compounds – Those organic compounds in which carbon atoms are joined together in the form of a ring are called cyclic compounds. These are also known as closed chain or ring compounds or alicyclic compounds. Examples – 

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Cyclobutane 

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Cyclopentane 

Homocyclic Compounds – Those alicyclic compounds which consists of only carbon atoms are joined together to form a ring or closed chain are called homocyclic compounds. Examples – cyclobutane, cyclopentane etc. 

Heterocyclic Compounds – Those alicyclic compounds which consists of the atoms other than carbon in the ring are called heterocyclic compounds. Examples – Piperidine, dioxane etc. 

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Piperidine 

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1,4 – dioxane 

Aromatic Compounds – The chemical compounds that contain conjugated planar ring systems with delocalized pi electron clouds instead of discrete alternating single and double bonds are called aromatic compounds. They are also known as aromatics or arenes. The most common example of aromatic compounds is benzene. These are unsaturated compounds which are stable in nature. Examples – Naphthalene, pyridine etc. 

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Naphthalene 

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Pyridine 

Benzenoid Compounds – In benzenoid aromatic compounds, the aromatic ring consists of only carbon atoms. Examples – Naphthalene, benzene etc. 

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Benzene 

Non – Benzenoid Compounds – Those aromatic compounds which do not contain benzene ring are called non – benzenoid compounds. Examples – Azulene, tropolone etc. 

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Azulene 

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Tropolone 

Heterocyclic Aromatic Compounds - In non - benzenoid aromatic compounds, the aromatic ring consists of the atoms other than carbon as well. Examples – Pyridine, Furan etc. 

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Furan 

Functional Groups – Functional groups are an atom or group of atoms joined in a specific manner which is responsible for the characteristic chemical properties of the organic compounds. Examples – -CHO (aldehyde), -OH (alcohol) etc. 

Homologous Series – Homologous series is a series of organic compounds containing a characteristic functional group. The members of the series are called homologues. They can be27 represented by the general formula. The successive members differ from each other by -CH2 unit. Examples – alkanes, alkenes, alkynes etc. 

Nomenclature of Organic Compounds 

In the older times, organic compounds were given names based on their sources or physical characteristics. That system used to work then because of a relatively low number of compounds that chemists had to deal with. These days, there are innumerable number of compounds that have or can be developed within the confines of organic chemistry. Hence, it is necessary to have a system of nomenclature to name and identify such compounds. That’s why we use IUPAC or International Union of Pure and Applied Chemistry’s nomenclature rules. 

In the IUPAC system of nomenclature, first you have to identify the parent hydrocarbon in the given organic compound. Parent hydrocarbon shows the number of carbon atoms in the parent chain. Then we add a suffix in the name of parent hydrocarbon which reflects the type of functional group. Other groups such as halogens which are attached to the parent chain are called substituents. For substituents, prefixes are used. For example, in 2-Chloropropane, 2 denotes the position of halogen group (Cl-chloro), prop denotes the parent hydrocarbon chain which contains three carbon atoms and ‘ane’ is suffix which represents that the compounds belongs from alkanes functional group and all carbon atoms have single bond between them.

Branched hydrocarbons have some complex rules which need to be learnt in order to name them in the right IUPAC manner. For branched chain alkanes, the rules are as follows:

• First of all, identify the longest chain of carbon atoms in a branched alkane. We refer to this as a parent chain.

• After identifying the longest chain, numbering is done on carbon atoms such that the carbon atoms with branches attached to them have lowest numbers. E.g. In the below branched alkane, the numbering can be done either left to right or right to left on the horizontal chain.

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If we do left to right numbering, the 2 carbon atoms with branches will get numbers 3 and 5. But if we do the same right to left, the 2 carbon atoms will get numbers 2 and 4. Clearly, right to left numbering gives lowest numbers and that’s what we should follow for this compound.

• The branched alkyl groups and their corresponding positions (numbering of the parent chain’s carbon atom) is prefixed to the parent chain. In case of multiple positions of the same groups, their positions can be clubbed together by using commas. The count of the same alkyl groups can be referred to as ‘di’, ‘tri’, ‘tetra’, ‘penta’ etc. Also, when prefixing the alkyl groups, they are written in alphabetical order. The prefixes such as ‘di’, ‘tri’, ‘tetra’, ‘penta’ etc. are not considered a part of alkyl group’s name when the groups are ordered in an alphabetical order.  E.g. the compound in previous point’s example will be named as :

4-Ethyl-2,2-dimethylhexane

• In case of branches being further branched between themselves, they also need to follow the nomenclature methodology discussed above, albeit they are written in brackets and the carbon atom which is attached to the parent chain is numbered 1. If there are 2 or more chains of equal length, we need to select the one with the highest number of branches. Further, while names of alkyl groups are written in alphabetical order as discussed earlier, prefixes like ‘iso’ and ‘neo’ are to be considered as constituent of alkyl group name while ‘sec’ and ‘tert’ are not to be considered in the same. E.g. let’s look at the following compound:

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The IUPAC name of this compound is 5-(2-Ethylbutyl)-3,3-dimethyldecane.

For cyclic compounds, the word ‘cyclo’ is prefixed to the name of alkane which has the same number of carbon atoms as in the cyclic alkane. E.g. 

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If there are any branches attached to cyclic alkanes, pretty much the same rules, as given for straight chain alkanes earlier, apply. E.g. 

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The name of this compound is 1-Ethyl-3-methyl cyclohexan.

Nomenclature of Organic Compounds with Functional Groups

Functional groups are associated with the site of highest reactivity in an organic compound. Many other elements beyond carbon and hydrogen can form functional groups in hydrocarbons. Identification of functional groups among different compounds helps us to group them together according to their similarity. Many functional groups have been identified such as -OH, -COOH, -CHO etc. These functional groups are generally written as suffixes to parent chain. When numbering the carbon atoms in parent chain, the numbering is done in such a manner that carbon having the functional group attached to it has the lowest number possible. 

If more than one functional groups are present, then they are looked at in terms of priority. Only one functional group is considered as the principal functional group, while the ones lying below it in terms of priority are chosen as subordinate functional groups. The principal group is named as a suffix while the subordinate groups are named as prefixes. Numbering is also done on the basis of the principal functional group getting the lowest number. As was the case with alkyl groups, multiple functional groups are named by attaching ‘di’, ‘tri’, ‘tetra’ etc. as prefix to their names. Some of the commonly used functional groups in descending order of their priority and their corresponding prefixes and suffixes are as below :

In addition to these functional groups, some groups such as -R, C6H5-, halogens, nitro and alkoxy groups are always prefixed. 

Nomenclature of Substituted Benzene Compounds

According to IUPAC system of nomenclature, in nomenclature of substituted benzene compounds if any substituent is attached to the benzene ring, prefix before the word benzene is used to indicate which substituent is attached to benzene ring. Although common names are also widely used for many benzene derivatives such as toluene (common name) for methylbenzene (IUPAC name). 

Thus, for monosubstituted benzene compounds, we add prefix to the word benzene for the substituent attached to benzene ring as shown in the example below –

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Chlorobenzene

Disubstituted benzene compounds – If benzene ring is disubstituted, then to show their position on the ring we have to number each carbon atom in such a way that the substituents get the lowest possible number. For example, 1, 2- dichlorobenzene. 

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1,2-dichlorobenzene 

Position of substituents is also represented by using the words ortho, meta and para. Poly substituted benzene derivatives are also named in the same way. 

Isomerism 

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              Ethyl alcohol                                       Dimethyl ether 

As we can see above, ethyl alcohol and dimethyl ether have different structural formula. But their molecular formula is the same which is C2H6O. These types of compounds have not only different structural formula but show distinct properties as well. Thus, we can say a molecular formula can represent more than one compound. This phenomenon is known as isomerism. The word isomerism is made up of two words iso & meros, iso means same and meros means parts. So, word isomerism means the same parts. Ethyl alcohol and dimethyl ether are called isomers of each other. 

Definition of isomerism and isomer - “Compounds that are represented by the same molecular formulae, but different structural formulae are called isomers and this phenomenon is known as isomerism.”

Isomers not only show different structural formulae but different physical and chemical properties as well. 

Examples of Isomers

1. Ethyl alcohol and dimethyl ether are isomers of each other as both the compounds have the same molecular formula – C2H6O while different structural formulae. 

2. Compounds such as pentane, iso-pentane and neopentane are isomers of each other. These all three compounds have the same molecular formula- C5H12. But different structural formulae.                  

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 Pentane

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 Isopentane 

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Neopentane 

Types of Isomers or Isomerism – 

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Isomerism can be divided into mainly two types- Structural isomerism & Stereoisomerism. These can be further subdivided into different subtypes, which is illustrated in the diagram above. 

Structural Isomerism - Compounds which show isomerism due to difference in their structures are known as structural isomers. This phenomenon is known as structural isomerism. 

Examples of Structural Isomers – n-Butane and Isobutane. 

n-butane and isobutane have the same molecular formula- C4H10 but different structural formulae. 

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                     Butane                                            Isobutane 

Structural Isomerism Can Be Further Divided Into Following Subtypes

1. Chain isomerism 

2. Functional isomerism 

3. positional isomerism 

4. Metamerism isomerism 

5. Tautomerism isomerism 

6. Ring-chain isomerism 

1. Chain Isomerism – Those structural isomers which differ in chains of carbon atoms are known as chain isomers and the phenomenon is termed as chain isomerism. Thus, chain isomers differ in the arrangement of C-atoms in straight or branched chains of compounds. 

Examples of Chain Isomers – Hexane – C6H14

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Hexane 

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2-methyl pentane 

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3-methyl pentane 

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2,2-dimethyl butane 

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2,3- dimethyl butane 

Important Note – To show chain isomerism, organic compounds or carbon compounds should have minimum ‘4’ carbon atoms. 

2. Functional Isomerism – Those compounds which have same molecular formula, but different functional groups are called functional isomers and this phenomenon is known as functional isomerism or functional group isomerism. 

Example – Alcohol and Ethers 

Ethanol (functional group-OH) and dimethyl ether(functional group- R-O-R’)- Molecular formula – C2H6O

Structural formula of ethanol –                   Structural formula of dimethyl ether- 

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3. Positional isomerism – Those structural isomers which differ in position of substituents or functional group or multiple bonds, are known as positional isomers and this phenomenon is known as positional isomerism. 

Positional Isomerism Example – 1. But-2-ene & But-1-ene (differ in position of double bond)

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But-2-ene                                                              But-1-ene 

2. 1-butanol & 2-butanol (differ in position of functional group -OH)

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                       1-butanol                                 2-butanol 

3. 1-chlorobutane & 2-chlorobutane (differ in position of substituent group -Cl)

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1-chlorobutane                                      2-chlorobutane 

4.Metamerism Isomerism – Those structural isomers which differ in their alkyl groups which are attached to their functional groups, are known as metamerism isomers and the phenomenon is known as metamerism isomerism. 

Example - Diethyl ether & methyl propyl ether 

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Diethyl ether [on both sides alkyl group -ethyl(C2H5) is attached]

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Methyl propyl ether [one side alkyl group-methyl(CH3) is attached while on another side alkyl group- propyl(C3H7) is attached]

5. Tautomerism Isomerism – Those structural isomers which differ in the position of hydrogen atoms or protons or electrons, are known as tautomers and this phenomenon is known as tautomerism. It is also known as desmotropism (desmos- bond & tropism-turn). Tautomers occur in equilibrium state with each other as they are easily inter-convertible. 

To show tautomerism, compounds must contain highly electronegative elements with multiple bonds. 

Example – Keto-enol tautomerism in acetone 

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Acetone (in keto form)                             Acetone(in enol form)

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Ketone                                                                      Enol 

6. Ring-chain Isomerism – Those structural isomers in which one isomer has open chain structure while another one has closed chain or ring structure, are known as ring-chain isomers and this phenomenon is called ring-chain isomerism. 

Examples - Butene & cyclobutane 

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Butene 

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Cyclobutane        

Stereoisomerism – Those compounds which have the same molecular formula but show different spatial arrangement of atoms in them are known as stereoisomers and the phenomenon is known as stereoisomerism. 

Example – 1,2-dichloroethene (C2H2Cl2) 

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Trans-1,2-dichloroethene                      Cis-1,2-dichloroetene 

Stereoisomerism Can Be Subdivided Into Following Subtypes

1. Geometrical Isomerism – It is also known as Cis-Trans isomerism. Those stereoisomers in which isomerism arises due to restricted rotation of carbon-carbon double bonds are known as geometrical isomers and this phenomenon is known as geometrical isomerism. In cis-trans isomerism it should be noted that the groups attached to the double bonded carbon atom should be different. 

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       Cis                                                                   Trans 

Geometric Isomers Example – cis-2-butene & trans-2-butene 

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           Cis-2-butene                                    Trans-2-butene 

2. Optical Isomerism – Those stereoisomers which are mirror images of each other or differ in optical activity are known as optical isomers and this phenomenon is known as optical isomerism. 

Conditions for Optical Isomerism

1. In optical isomers the carbon atom is attached to four different atoms or groups. 

2. Chiral center – Optical isomers have chiral centers, it means a carbon atom which is attached to the 4 different groups or atoms. 

3. Optical isomer which rotates plane polarized light towards right side is called dextro (d) while the optical isomer which rotates towards left side is called laevo(l). A mixture containing equal amounts of dextro & laevo isomers is called racemic mixture. Racemic mixtures show no effect on plane polarized light as rotations of d & l cancel out each other. 

Optical Isomers example or Example of d & l isomers 

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      D-mannose                                              L-mannose 

Those optical isomers which are mirror images of each other are called enantiomers. While non-mirror images of each other are called diastereomers. 

Examples – Enantiomers –

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Diastereomers – 

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                Cis                                                              Trans                  

Fundamental Concepts in Organic Reaction Mechanism 

Reaction Mechanism – A sequential account of each step, describing details of electron movement, energetics during bond cleavage and bond formation, and the rates of transformation of reactants into products is called reaction mechanism. 

Heterolytic Cleavage of The Covalent Bond – In this type of cleavage the covalent bond breaks in such a way that shared electron pairs remain with one of the atoms involved in the bond formation. 

Homolytic Cleavage of The Covalent Bond – In this type of cleavage the covalent bond breaks in such a way that the electrons of the shared electron pair go with each of the bonded atoms. 

Electrophiles - Electrophiles are electron deficient atoms or molecules which carry a positive charge. Electrophiles are electron lovers or electron pair acceptors. They are represented as E+. 

Nucleophiles – Nucleophiles are electron rich atoms or molecules which carry a negative charge. They are electron donors. They are represented as Nu:

Inductive Effect – The polarization of the bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is called inductive effect. When the bond gets polarized more electronegative element gets partially negative charge as it pulls the bonded electron pair towards itself while the less electronegative element gets partially positive charge. Inductive effect is shown by those molecules in which the covalent bond is formed between those atoms which possess large differences in their electronegativities. 

Resonance Structures – Some compounds cannot be represented by only one structure. They are hybrid of their many structures such as the structure of benzene is hybrid of its various structures. These structures are called resonance structures. Here, you need to note that the resonance structures are hypothetical and individually do not represent any real molecule.  

Methods of Purification of Organic Compounds 

Sublimation 

Sublimation is the process in which substance directly changes from solid state to liquid state. It is used as a separation technique for those mixtures which contain sublimable volatile substance and non-sublimable volatile components. Some substances such as ammonium chloride, camphor, naphthalene and anthracene are sublime substances. The mixture of ammonium chloride and salt can be separated by sublimation method of separation of mixtures. 

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Crystallization

In crystallization, pure crystals of the desired substance are produced. It is used for the purification of solid organic compounds. It is based on the difference between solubilities of the compound and impurities in a suitable solvent. Impure compound is dissolved in suitable solvent and a concentrated solution is prepared which is on cooling, gives pure crystals of the substance while the impurities remain in the filtrate. 

Distillation 

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It is used for the separation of components of a mixture containing two miscible liquids that boil without decomposition and have sufficient difference in their boiling point. In this technique liquid mixtures are boiled, vaporized, condensed and isolated. The mixture of acetone and water is separated by distillation. Boiling point of acetone is 56°∁ and water is 100°∁.

Fractional Distillation 

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It is the same technique as distillation, but it's apparatus has a fractionating column also. So that it can separate mixture of miscible liquids which has difference in their boiling point less than 25K. Separation of different gases from air can be done by fractional distillation. 

Separating Funnel

It is used to separate two immiscible liquids such as oil and water. This method is used in the extraction of iron also. The principle is that immiscible liquids separate out in layers depending on their densities. 

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Magnetic Separation

It is used in the separation of components of those mixtures in which one component shows magnetic properties and another one doesn’t. It is used in the extraction of metals to separate the metal from its impurity. 

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Chromatography

A laboratory method or technique of separation of a mixture in which components are distributed in two phases – mobile phase and stationary phase, is called chromatography. For example, different components of ink can be separated by chromatography. The term chromatography was coined by Mikhail Tswett in 1906. While the first use of chromatography for analysis was done in 1952 for fatty acids by James and Martin.

Principle of chromatography - Chromatography is based on the fact that different components of a mixture have different affinity towards stationary phase. Components get physically separated by distributing between stationary phase and mobile phase. The forces by which components get adsorbed by stationary phase are weak forces and nonionic forces such as hydrogen bond or Van der Waals forces. 

The various components of a mixture travel at different speeds and get separated. The separation is based on differential partitioning between the mobile and stationary phases. Component’s different partitioning coefficients result in differential retention on the stationary phase which results in their separation on stationary phase. 

Chromatography is of various types such as adsorption chromatography, column chromatography, thin layer chromatography, partition chromatography, paper chromatography etc. 

This ends our coverage on the summary of the unit “Organic Chemistry – Some Basic Principles and Techniques”. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various subtopics of this unit such as nomenclature of organic compounds, homologous series etc. on Vedantu website. We hope after reading this article you will be able to solve problems based on the topic. We have already provided detailed study notes or revision notes for this unit, which you can easily download by registering yourself on Vedantu website. Here in this article we have discussed the unit in a summarized way with the emphasis on important topics of the unit.  If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.