Chapter 12 : Organic Chemistry-Some Principle and Techniques
Organic compounds are vital for sustaining life on earth and include complex molecules like genetic information bearing deoxyribonucleic acid (DNA) and proteins
That constitutes essential compounds of our blood, muscles and skin.
Organic chemicals appear in materials like clothing, fuels, polymers, dyes and medicines. These are some of the important areas of application of these compounds.
Topics covered in this snack-sized chapter:
Formation and the shapes of molecules like methane (CH4
), ethene (C2
), ethyne (C2
) are explained in terms of the use of sp3
and sp hybrid orbitals by carbon atoms in the respective molecules.
12.2 Tetra Valence of Carbon: Shapes of Organic Compounds arrow_upward
Hybridisation influences the bond length and bond enthalpy (strength) in organic compounds.
The sp hybrid orbital contains more s character and hence it is closer to its nucleus and forms shorter and stronger bonds than the sp3
hybrid orbital is intermediate in s character between sp and sp3
And hence, the length and enthalpy of the bonds it forms, are also intermediate between them.
12.2.1 The Shapes of Carbon Compounds
12.2.2 Some Characteristic Features of Bonds
In a bond formation, parallel orientation of the two p orbitals on adjacent atoms is necessary for a proper sideways overlap
Bonds provide the most reactive centres in the molecules containing multiple bonds
12.3 Structural Representations of Organic Compounds arrow_upward
In bond-line structural representation, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zigzag fashion.
In a condensed structural formula, covalent bonds are not always shown and atoms of the same type bonded to one another are grouped together
The complete structural formula indicates all of the carbon and hydrogen atoms.
12.3.1 Complete, Condensed and Bond-line Structural Formulas
Molecular models are physical devices that are used for a better visualization and perception of three-dimensional shapes of organic molecules.
Commonly three types of molecular models are used:
The three-dimensional (3-D) structure of organic molecules can be represented on paper by using certain conventions.
For example, by using solid ( ) and dashed ( ) wedge formula, the 3-D image of a molecule from a two-dimensional picture can be perceived.
12.3.2 Three-Dimensional Representation of Organic Molecules
Organic compounds are broadly classified as follows:
- Acyclic or open chain compounds
- Alicyclic or closed chain or ring compounds
Common method of naming has been developed and is known as the IUPAC system of nomenclature.
12.5 Nomenclature of organic Compounds
A systematic name of an organic compound is generally derived by identifying the parent hydrocarbon and the functional group(s).
Using prefixes and suffixes, the parent name can be modified to obtain the actual name.
12.5.1 The IUPAC System of Nomenclature
Common or Trivial Names of Some Organic Compounds
12.5.2 IUPAC Nomenclature of Alkanes
Some Alkyl Groups
The longest chain of carbon atoms containing the functional group is numbered in such a way that the functional group is attached at the carbon atom possessing lowest possible number in the chain.
12.5.3 Nomenclature of Organic Compounds having Functional Group(s)
Benzene ring is substituted, the position of substituents is defined by numbering the carbon atoms of the ring such that the substituents are located at the lowest numbers possible.
For example, the compound (b) is named as 1, 3-dibromobenzene and not as 1, 5-dibromobenzene.
The phenomenon of existence of two or more compounds possessing the same molecular formula but different properties is known as isomerism
Such compounds are called as isomers.
12.5.4 Nomenclature of Substituted Benzene Compounds
Compounds having the same molecular formula but different structures are classified as structural isomers.
Some typical example of different types of structural isomerism is given below:
12.6.1 Structural Isomerism
The compounds that have the same constitution and sequence of covalent bonds but differ in relative positions of their atoms or groups in space are called stereoisomers
Organic molecule (also referred as a substrate) reacts with an appropriate attacking reagent and leads to the formation of one or more intermediate(s) and finally product(s).
The bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments.
12.7 Fundamental Concepts in Organic Reaction Mechanism arrow_upward
A covalent bond can get cleaved either by:
12.7.1 Fission of a Covalent Bond
- Heterolytic cleavage, or by
A reagent that brings an electron pair is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic.
A reagent that takes away an electron pair is called electrophile (E+) i.e., electron seeking and the reaction is called electrophilic.
12.7.2 Nucleophiles and Electrophiles
The movement of electrons in organic reactions can be shown by curved-arrow notation.
It shows how changes in bonding occur due to electronic redistribution during the reaction.
12.7.3 Electron Movement in Organic Reactions
The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or asubstituent group or in the presence of an appropriate attacking reagent.
12.7.4 Electron Displacement Effects in Covalent Bonds
The inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule, resulting in a permanent dipole in a bond.
The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule.
12.7.6 Resonance Structure
The resonance effect is defined as ‘the polarity produced in the molecule by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom’.
There are two types of resonance or mesomeric effect designated as R or M.
- Positive Resonance Effect (+R effects)
- Negative Resonance Effect (-R effects)
It is a temporary effect. The organic compounds having a multiple bond (a double or triple bond) show this effect in the presence of an attacking reagent only.
12.7.8 Electromeric Effect (E effect)
It is defined as the complete transfer of a shared pair of electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent.
Hyperconjugation is a general stabilizing interaction.
It involves delocalisation of electrons of C—H bond of an alkyl group directly attached to an atom of unsaturated system or to an atom with an unshared p
Organic reactions can be classified into the following categories:
12.7.10 Types of Organic Reactions and Mechanisms
An organic compound is extracted from a natural source or synthesized in the laboratory.
The common techniques used for purification are follow:
12.8 Methods of Purification of Organic Compounds arrow_upward
- Differential extraction and
Some solid substances change from solid to vapour state without passing through liquid state, is known as sublimation
Used to separate sublimable compounds from non-sublimable impurities
It is carried out in 4 steps:
- Preparation of the solution in a suitable solvent
- Filtration of the hot solution
- Crystallisation by cooling the hot filtrate
- Isolation and drying of the purified substance
Simple distillation involves conversion of a liquid into its vapour by heating in a distilling flask and then condensation of the vapour into a liquid in the receiver
The vapours of lower boiling fraction reach the top of the column first followed by vapours of higher boiling fractions.
12.8.4 Differential Extraction
Column Chromatography: Column chromatography involves separation of a mixture over a column of adsorbent (stationary phase) packed in a glass tube.
The elements present in organic compounds are carbon and hydrogen.
Carbon and hydrogen are detected by heating the compound with copper (II) oxide.
Both are detected by heating the compound with copper (II) oxide.
12.9.1 Detection of Carbon and Hydrogen
Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by “Lassaigne’s test”.
Nitrogen test- The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid.
12.9.2 Detection of other Elements
Sulphur test-The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulphide indicates the presence of sulphur
Phosphorus test- The compound is heated with an oxidizing agent (sodium peroxide). The phosphorus present in the compound is oxidized to phosphate.
The percentage composition of elements present in an organic compound is determined by the methods based on the following principles:
Both carbon and hydrogen are estimated in one experiment. A known mass of an organic compound is burnt in the presence of excess of oxygen and copper (II) oxide.
The mass of organic compound be mg, mass of water and carbon dioxide produced be m
1 and m
12.10.1 Carbon and Hydrogen
Dumas method: The nitrogen containing organic compound, when heated with copper oxide in an atmosphere of carbon dioxide, yields free nitrogen in addition to carbon dioxide and water
Carius method: a known mass of an organic compound is heated with fuming nitric acid in the presence of silver nitrate contained in a hard glass tube known as Carius tube.
It is filtered, washed, dried and weighed.
A known mass of an organic compound is heated in a carius tube with sodium peroxide or fuming nitric acid
Sulphur present in the compound is oxidised to sulphuric acid
A Known mass of an organic compound is heated with fuming nitric acid whereupon phosphorus present in the compound is oxidised to phosphoric acid.
The percentage of oxygen in an organic compound is usually found by difference between the total percentage composition (100) and the sum of the percentages of all other elements.