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Organic Chemistry

Introduction to Organic Chemistry

A Swedish chemist named Berzilius proposed the vital force theory, according to which a vital force existing in living organisms is responsible for the formation of oranic compounds. However, this was proved to be incorrect when F. Wohler, a German chemist synthesised urea- an organic compound from ammonium cyanate, an inorganic compound.

Shape of carbon compounds

sp orbital has 50% ‘s’ character. Thus, an sp hybridised carbon is more electronegative than an sp2 (33% s character) or sp3 (25% s-character) hybridised carbon.

Hybridisation influences the bond length and bond enthalpy.

Some Characteristic Features of π-Bonds

Parallel orientation of two p-orbitals on adjacent atoms is necessary for a proper sideways overlap to form π-bond.

Rotation about C = C double bond is restricted.

Electron charge cloud of π-bond is located above and below the plane of bonding atoms.

Electrons are easily available to attacking reagents.

Structural Representations of Organic Compounds

Complete Structural Formula

Examples:

Condensed Structural Formula

(Structural formula obtained by omitting some or all of the dashes representing covalent bonds and then using a subscript to indicate the number of identical groups attached to an atom)

Bond-Line Structural Formula

Only lines are used to represent the structure of organic compound.

Carbon and hydrogen atoms are not shown.

Lines representing the C − C bonds are drawn in a zig-zag fashion.

For example − bone-line formula of 2,3-dimethylhexane can be represented as

In cyclic compounds,

To test your knowledge of this concept, solve the following puzzle.

The general formula is.

Contain at least one triple bond between two carbon atoms

In IUPAC nomenclature:

They are named as derivatives of the corresponding alkanes replacing ‘ane’ by the suffix ‘yne’.

The position of the triple bond is indicated by the first the triply bonded carbon.

But-2-yne and but-1-yne differ in their structures due to the position of the triple bond. Hence, they are known as position isomers.

Structure of Triple Bond (Ethyne)

Each carbon atom of ethyne has two sp hybridised orbitals.

C − C sigma bond is formed by head-on overlapping of two sp hybridised orbitals of the two C atoms.

2p orbitals of one carbon atom undergo lateral or sideways overlapping with 2p orbitals of other carbon atoms to form two pi (π) bonds.

H − C − C bond angle is 180°.

C ≡ C bond enthalpy = 823 kJ mol−1

C ≡ C bond length is 120 pm, which is shorter than those of C = C (133 pm) and C − C (154 pm) bonds.

Preparation of Ethynes

From calcium carbide (CaC2) − Ethyne is prepared by treating calcium carbide (which is obtained by heating quick lime with coke) with water.

From vicinal dihalides

Physical Properties of Alkynes

First three members are gases; the next eight are liquids; and the higher ones are solids.

Colourless, odourless (except ethyne)

Weakly polar in nature

Lighter than water; immiscible with water, but soluble in organic solvents such as ethers, CCl4, benzene, etc.

Melting point, boiling point, and density increase with the increase in molar mass.

Chemical Properties of Alkynes

Hydrogen atoms of ethyne (or any other alkyne), attached to the triply bonded carbon atom, are acidic in nature.

Addition Reactions of Alkynes

In unsymmetrical alkynes, addition takes place according to

Markovnikov’s rule.

Addition of dihydrogen

Addition of halogens

It is used as a test for unsaturation.

Addition of hydrogen halides (HX; X = Cl, Br, I)

HX adds to alkynes to form gem halides (in which two halogens are attached to the same carbon atom).

Addition of water

Generally, these do not react with water. However, on warming alkynes with mercuric sulphate and dilute sulphuric acid at 333 K, one molecule of water adds to them to form carbonyl compounds.

Polymerisation

Linear polymerisation − Polyacetytene or polyethyne is produced by linear polymerisation of ethene.

Repeating units − CH = CH − CH = CH

Represented as −

Cyclic polymerisation

Oxidation 2 HC≡ CH + 5 O2 →        4 CO2 + 2 H2O + Heat Ozonolysis HC≡CH + O3 →        OH-C-C-H||O-O Uses: Some of the uses of ethyne are as follows: Oxy-acetylene welding at very high temperatures Illuminant in oxy-acetylene lamp Ripening and preservation of fruits Manufacture of several products like polymers. artificial rubber, oxalic acid, acetaldehyde, acetic acid, etc.

Test to Distinguish between Alkanes, Alkenes and Alkynes

Hydrocarbon Reagents Br2 in CCl4 Alk. KMnO4 Ammoniacal cuprous chloride Ammoniacal silver nitrate Alkanes No change No change No change No change Alkenes Decolourisation of Br2 solution Decolourisation of KMnO4 solution No change No change Alkynes Red precipitate is formed White precipitate is formed

 

The general formula is.

Contain at least one triple bond between two carbon atoms

In IUPAC nomenclature:

They are named as derivatives of the corresponding alkanes replacing ‘ane’ by the suffix ‘yne’.

The position of the triple bond is indicated by the first the triply bonded carbon.

But-2-yne and but-1-yne differ in their structures due to the position of the triple bond. Hence, they are known as position isomers.

Structure of Triple Bond (Ethyne)

Each carbon atom of ethyne has two sp hybridised orbitals.

C − C sigma bond is formed by head-on overlapping of two sp hybridised orbitals of the two C atoms.

2p orbitals of one carbon atom undergo lateral or sideways overlapping with 2p orbitals of other carbon atoms to form two pi (π) bonds.

H − C − C bond angle is 180°.

C ≡ C bond enthalpy = 823 kJ mol−1

C ≡ C bond length is 120 pm, which is shorter than those of C = C (133 pm) and C − C (154 pm) bonds.

Preparation of Ethynes

From calcium carbide (CaC2) − Ethyne is prepared by treating calcium carbide (which is obtained by heating quick lime with coke) with water.

From vicinal dihalides

Physical Properties of Alkynes

First three members are gases; the next eight are liquids; and the higher ones are solids.

Colourless, odourless (except ethyne)

Weakly polar in nature

Lighter than water; immiscible with water, but soluble in organic solvents such as ethers, CCl4, benzene, etc.

Melting point, boiling point, and density increase with the increase in molar mass.

Chemical Properties of Alkynes

Hydrogen atoms of ethyne (or any other alkyne), attached to the triply bonded carbon atom, are acidic in nature.

Addition Reactions of Alkynes

In unsymmetrical alkynes, addition takes place according to

Markovnikov’s rule.

Addition of dihydrogen

Addition of halogens

It is used as a test for unsaturation.

Addition of hydrogen halides (HX; X = Cl, Br, I)

HX adds to alkynes to form gem halides (in which two halogens are attached to the same carbon atom).

Addition of water

Generally, these do not react with water. However, on warming alkynes with mercuric sulphate and dilute sulphuric acid at 333 K, one molecule of water adds to them to form carbonyl compounds.

Polymerisation

Linear polymerisation − Polyacetytene or polyethyne is produced by linear polymerisation of ethene.

Repeating units − CH = CH − CH = CH

Represented as −

Cyclic polymerisation

Oxidation 2 HC≡ CH + 5 O2 →        4 CO2 + 2 H2O + Heat Ozonolysis HC≡CH + O3 →        OH-C-C-H||O-O Uses: Some of the uses of ethyne are as follows: Oxy-acetylene welding at very high temperatures Illuminant in oxy-acetylene lamp Ripening and preservation of fruits Manufacture of several products like polymers. artificial rubber, oxalic acid, acetaldehyde, acetic acid, etc.

Test to Distinguish between Alkanes, Alkenes and Alkynes

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