Chemistry Podcast

Monday, 28 March 2022

Organic Chemistry – Some Basic Principles and Techniques

 BASIC PRINCIPLES AND TECHNIQUES IN ORGANIC CHEMISTRY

Organic chemistry is the chemistry of carbon compounds except oxides of carbon and metal carbonates. The term organic literally means derived from living organism. All organic compounds contain carbon as their essential constituent. Carbon atom has unique property to form bonds with other carbon atoms. This property of forming bonds with atoms of the same element is called catenation

Carbon form strong bonds with many others elements and especially with other carbon atoms to form chains and rings that gives rise to millions of organic compounds.


CLASSIFICATION OF ORGANIC COMPOUNDS

Organic compounds are broadly classified in two ways.

1] Based on carbon skeleton

Aliphatic compounds

These are the compounds in which carbon atoms are joined to form an open chain. Their structure may consist of straight chain or branched chain.

e.g. CH3 – CH2 – CH2 – CH3

Cyclic compounds

These are the compounds in which carbon atoms are joined to form one or more rings. They are further classified into two types.

a] Homocyclic or carbocyclic

In these compounds, the ring is made up of carbon atoms only. They are further divided into two types.

i) Alicyclic compounds

These compounds show some of the properties similar to those of aliphatic compounds. Carbon atoms are linked by single bonds only.

Cyclopropane - Class 11th Organic chemistry Notes

ii) Aromatic compounds

These are the compounds which contains at least one aromatic ring which resembles benzene in their chemical behavior.

Phenol - Organic Chemistry Notes

b] Heterocyclic compounds

These compounds include one or more heteroatoms like O, N, S etc. in the ring of carbon atoms. They are also divided into two types.

i) Hetero – alicyclic compounds

Alicyclic compounds which contain at least one heteroatom in the ring are called hetero-alicyclic compounds.

Tetrahydrofuron

ii) Hetero – aromatic compounds

Aromatic compounds which contain at least one heteroatom in the ring are called hetero – aromatic compounds.

Pyridine Organic chemistry

2] Based on functional group

An atom or a group of atoms in the organic molecule which determines its characteristic chemical properties is called the functional group.

e.g.   – OH, - X, - CHO, - COOH, - NH2, etc. are functional group. The chemistry of every organic molecule is determined by the functional group it contains.

Homologous series

A series of organic compounds each containing a characteristic functional group, and the successive members differ from each other in molecular formula by a –CH2– (methylene) group is called homologous series.

 

 

PURIFICATION OF ORGANIC COMPOUNDS

The purification of organic compounds involves the following processes.

1. Crystallization

This is the most common method used to purify organic solids which dissolve in particular solvent. The purification is done on the basis of differences in solubility of a given organic compound and impurities.

 

2. Sublimation

It is a process which is used for solids which directly change into vapor state upon heating without passing the liquid state and vapors on cooling give back the solid Substance. Impure samples of naphthalene, anthracene, camphor, etc. are purified by this method.

 

3. Distillation

The process in which liquid is converted into its vapor phase at its boiling point and the vapors is then condensed back to liquid on cooling is known as distillation. This method is used if the organic liquid is stable at its boiling point and it contains non – volatile impurities.

 

4. Fractional distillation

The process of separating and purifying the components of a mixture of two or more miscible liquids having different boiling points is known as fractional distillation. The liquid which is more volatile distills out first leaving behind the less volatile liquid in the distillation flask.

 

5. Steam distillation

Liquids which are immiscible with water but are steam volatile are separated by this technique. Aniline, nitrobenzene, bromobenzene etc. can be steam distilled.

 

6. Fractional crystallization

The process of separating the components of a mixture of two or more solids, having different solubilities in the same solvent at the same temperature, by step – wise crystallization is known as fractional crystallization.

 

7. Differential extraction

The organic solvents like benzene, chloroform, petroleum ether etc. are immiscible with water such solvents are used to extract an organic compound present in aqueous solution by the method of differential extraction.

 

8. Chromatography

Chromatography is the technique used for the separation, isolation, purification and identification of constituents of a mixture. This technique depends on the distribution of the mixture between two phase, one stationary phase and other moving or mobile phase. Depending upon the principle involved, it is divided into two types.

 

a) Adsorption chromatography

It is based on the principle of differential adsorption. Different compound are adsorbed on adsorbent to different degrees. It also has two types i) column chromatography and ii) thin – layer chromatography (TLC).

 

b) Partition chromatography

This technique is based on continuous differential partitioning of components of a mixture between stationary and mobile phases.

 

 

Determination of empirical formula

The formula of a compound which gives the simple whole number ratio of the atoms of various elements present in one molecule of the compound is called its empirical formula. It involves following steps.

Step – I

The percentage composition of elements present in a given compound is determined by quantitative analysis.

Step – II

The percentage of each element is divided by the respective atomic mass. This is gives the atomic ratio of constituents atoms present in a given compound.

Step – III

The simplest ration is found by dividing all the atomic ratios by the smallest atomic ratio.

 

Molecular formula

The formula which gives the actual number of atoms of various elements present in one molecule of the compound is called its molecular formula. Molecular formula is determined from empirical formula as –

Molecular formula = n x empirical formula

Where n is whole integer given by-

N    =          Molecular mass/ Empirical formula mass

 

 

 

ISOMERISM

The compounds having same molecular formula and different structural formulae are called isomers of each other. This phenomenon is known as isomerism. It has two main types.

 

1. Stereoisomerism

The stereoisomers have the same structural formula but differ in the arrangement of atoms or groups in space.

 

 

2. Structural isomerism

The structural isomers differ from each other in the arrangement of the atoms or groups within the molecules.

It can be further divided as :

 

a) Chain or nuclear isomerism

It is the type of isomerism in which the isomers differ in the chain of carbon atoms. For example, C4H10 has two isomers.

Chain or nuclear isomerism

b) Position isomerism

It is the type of isomerism in which the different isomers differ in the position of the functional group. For example, C3H7OH has two isomers.

Position isomerism

c) Functional isomerism

It is the type of the isomerism in which isomers have different functional group. For example, C2H6O has two isomers :

Functional isomerism

d) Metamerism

It is the type of isomerism in which the isomers differ in the nature of alkyl group attached to the same functional group. For example, C5H10O has two isomers.

C2H5 – CO – C2H5    (Diethyl Ketone) and       CH3 – CO – C3H7 (Methyl propyl Ketone)

 

FUNDAMENTAL CONCEPTS IN ORGANIC REACTION MECHNAISM

Electron displacements in covalent bonds

 

Inductive effect

When an organic molecule has a polar covalent bond in its structure polarity is induced in adjacent carbon – carbon single bond too. This is called inductive effect. The arrow head put in the center of the bond is used to represent the inductive effect. The direction of arrow head indicates the direction of permanent electron displacement. For example,

inductive effect

This effect decreases rapidly as the length of carbon chain increases. There are two types of inductive effect.

i) – I effect

Atoms or group of atoms which are highly electronegative or carry positive charge are electron – withdrawing groups and such groups are said to have – I effect.

e.g. – F, - Cl, -Br, - I , - NO2, - CN, - COOH, - COOR, - SO3H, etc.

 

ii) + I effect

Atoms or group of atoms which are electropositive or carry negative charge are donating groups and are said to have + I effect.

e.g. Metals like Na, K, Mg, Zn etc. and alkyl groups such as – CH3, - CH2CH3, - CH(CH3)2, - C(CH3)3, - C(C2H5)3, etc.

 

Electromeric effect

The complete transference of shared pair of electrons to one of the atom joined by a multiple bond in presence of an attacking reagent this is known as electromeric effect.

This is a temporary effect and takes place only in the presence of an attacking reagent. For example:

 

Electromeric effect

Resonance effect

The polarity developed within a molecule is due to the interaction of two ∏ - bonds or a ∏ - bonds with the lone pair of electrons present on the adjacent atom is called the resonance effect or mesomeric effect. It is a permanent effect and depending, upon the direction of transfer of electrons it is of two types.

 

i) +R effect

When the transfer of electron is away from an atom or substituent group attached to the conjugated system, it is termed as +R effect. For example +R effect in aniline.

 

 ii) – R effect

When the transfer of electron is towards the atom or substituent group attached to the conjugated system, it is termed as – R effect. For example – R  effect in nitrobenzene.

 

 

Hyper conjugation

The delocalization of electrons due to overlap between a P – orbital and sigma (σ) bond of C – H is called hyper conjugation or no bond resonance. Greater the number of alkyl groups attached to a positively charged carbon atom, greater is the hyper conjugation interaction and greater is the stability of cation. Thus the relative stability of cation decreases in the order.

(CH3)3C+ > (CH3)2CH+ > CH3CH2+ > CH3+

 

 

Bond fission

In any chemical reaction, when a reactant is converted into products one or more bonds in the reactant are broken and new bonds are formed. The process of breaking or cleavage of covalent bond is known as bond fission. The bond fission takes place in two ways.

 

i) Hemolytic fission

The symmetrical breaking of a covalent bond between two atoms such that each atom acquires one electron of the shared pair is called hemolytic fission or homolysis.

Such fission takes place in the presence of ultraviolet light or high temperature. The cleavage of a bond results in the formation of free radicals. A free radical is a neutral species (atom or group) which contain an unpaired electron. A hemolytic fission is represented as:

hemolytic fission or homolysis.

ii) Heterolytic fission

The unsymmetrical breaking of a covalent bond between two atoms such that the more electronegative atom acquires both the electrons of the shared pairs is called heterolytic fission or heterolysis .

Such a fission takes place in the presence of polar solvent. The cleavage of a bond results in the formation of ions. This ions formed are unstable and reactive. One of the ion has a positive charge called as cation and the other ion has at least one lone pair and a negative charge called as an anion. Organic reactions which proceed by heterolysis are called ionic or  heteropolar simply polar reaction. A heterolytic fission is represented as:

heterolytic fission

The species in which carbon atom passes a positive charge is called carbanion and the species in which carbon atom possesses negative charge is called carbanion.

Example of heterolysis is,

Carbocation - Organic Chemistry - Some basic principle and techniques - class 11th chemistry notes

Reaction intermediates

 

i) Free radicals

An uncharged species which is electrically neutral and contains a single electron is called free radical.

Free radical is highly reactive and therefore has a transitory existence i.e. it is short – lived. The stability of free radicals decreases in the order triphenyl methyl > benzyl > allyl > 3ᵒ > 2ᵒ > 1ᵒ > methyl > vinyl.

 

ii) Carbocation

A species in which carbon atom bears a positive charge is called a carbocation. Positively charged carbon is sp2 hybridized. It is electron – deficient. It is Lewis acid and act as an electrophile. It has planer geometry.

e.g. tetra – butyl carbocation (CH3)3C+.

 

iii) Carbanion

A species in which carbon atom bears a negative charge is called a carbanion. Negatively charged carbon is Sp3 / Sp2 hybridized. It is electron – rich. It is Lewis base and act as a nucleophile. It has pyramidal geometry.

e.g. methyl carbanion      CH3.

 

iv) Carbenes

The highly reactive intermediate containing neutral and divalent carbon atoms is called carbene.

For example, CH2

 

 

Types of attacking reagent.

1) Electrophiles

Electrophiles are electron – deficient species. They are either positively charge species like H+, NO+2 etc. or molecules having incomplete octet of electrons like BF3, AlCl3, ZnCl2 etc.

Since electrophiles are electron deficient they accept a pair of electrons from donor atoms and thus they are electron loving reagents. All electrophiles are basically lewis acids.

 

2) Nucleophiles

Nucleophiles are electron rich species. They are either negatively charged species like OH-, CN-, Cl-, Br-etc. or molecules containing at least one lone pair of electrons on central atom like H2O, NH3, H2S, R-OH, R-NH2, etc. since nucleophiles are electron rich, they donates pair of electrons to acceptor atoms and thus they are nucleus loving reagents. All nucleophiles are Lewis base.

 

 

 

Types of organic reactions

1) Substitution reaction

A reaction in which an attacking species replaces another atom or group in substrate is called substitution reaction.

e.g.

Substitution reaction - organic reaction

2) Addition reaction

When two molecules combine to form one product molecule, it is called an addition reaction.

e.g.

CH2 = CH2 (Ethylene)   +     HCl    →    CH3 – CH2 – Cl (Ethyl chloride)

  

3) Elimination reaction

When one molecule is split into two fragment molecules, it is called an elimination reaction.

e.g.

Elimination reaction - Organic chemistry reaction

4) Rearrangement reaction

A reaction in which either the carbon skeleton or the functional group or both are modified is known as rearrangement reaction.

e.g.

Rearrangement reaction - Chemistry Notes Info

3 comments:

Anonymous said...

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Anonymous said...

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Anonymous said...

NOTE:

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