ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
UNIT 12
Introductions
STRUCTURAL REPRESENTATION OF ORGANIC COMPOUNDS
Complete, condensed and bond line structural formulas
bond by a dash (–).
Thus, ethane (C2H6), ethene (C2H4), ethyne (C2H2)
Such structural representations are called
complete structural formula
H H
H C
C
H
H H
Ethane
H
H
C C
H
H
Ethene
H
C
C
H
Ethyne
These structures can also be represented by the following
ways
CH3 CH3 H2C H2C HC
Ethane Ethene
HC
Ethyne
organic chemists use another way of representing the structures, in which only lines are used. In this bond-line structural representation of organic compounds, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zig-zag fashion.
For example
3-Methyloctane can be represented in various forms as:
i) CH3CH2CHCH2CH2CH2CH2CH3
| CH3
Three-Dimensional Representation of Organic Molecules
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.
solid-wedge is used to indicate a bond projecting out of the plane of paper, towards the observer.
The dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer.
The bonds lying in plane of the paper are depicted by using a normal line (—).
Wedge-and-dash representation of CH4
CLASSIFICATION OF ORGANIC COMPOUNDS
Acyclic or open chain compounds
These compounds are also called as aliphatic compounds and consist of straight or branched chain compounds, for example:
Alicyclic or closed chain or ring compounds
Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring (homocyclic). Sometimes atoms other than carbon are also present in the ring (heterocylic).
Some examples are
Aromatic compounds
Aromatic compounds are special types of compounds.
These include benzene and other related ring compounds (benzenoid).
In order to clearly identify compound, a systematic method of naming has been developed and is known as the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature.
The names are correlated with the structure such that the
reader or listener can deduce the structure from the name.
The traditional names are considered a trivial or common
names.
Hydrocarbons :
Compounds containing carbon and hydrogen
only are called hydrocarbons.
Straight chain hydrocarbons: The names of such compounds are based on their chain structure, and end with suffix ‘-ane’ and carry a prefix indicating the number of carbon atoms present in the chain (except from CH4 to C4H10, where the prefixes are derived from trivial names).
Branched chain hydrocarbons: In a branched chain compound small chains of carbon atoms are attached at one or more carbon atoms of the parent chain. The small carbon chains (branches) are called alkyl groups.
In order to name such compounds, the names of alkyl groups are prefixed to the name of parent alkane. An alkyl group is derived from a saturated hydrocarbon by removing a hydrogen atom from carbon. Thus, CH4 becomes -CH3 and is called methyl group.
Nomenclature of branched chain alkanes
1. First of all, the longest carbon chain in the molecule is identified.
Example
2 . The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers.
3. If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: 6-ethyl-2- methylnonane.
[Note: the numbers are separated from the groups by hyphens and there is no break between methyl and nonane.]
4. If two or more identical substituent groups are present then the numbers are separated by commas. The names of identical substituents are not repeated, instead prefixes such as di (for 2), tri (for 3), tetra (for 4), penta (for 5), hexa (for 6) etc. are used.
While writing the trivial names of substituents’ in alphabetical order, the prefixes iso- and neo- are considered to be the part of the fundamental name of alkyl group. The prefixes sec- and tert- are not considered to be the part of the fundamental name.
For example
Cyclic Compounds: A saturated monocyclic : Compound is named by prefixing ‘cyclo’ to the corresponding straight chain alkane. If side chains are present, then the rules given above are applied. Names of some cyclic compounds
are given below.
-C≡C- .
For IUPAC nomenclature of substituted benzene compounds, the substituent is placed as prefix to the word benzene as shown in the following examples.
If benzene ring is disubstituted, 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.
In the trivial system of nomenclature the terms ortho (o), meta
(m) and para (p) are used as prefixes to indicate the relative
positions 1,2- ;1,3- and 1,4- respectively.
ISOMERISM
Structural Isomerism
(i) Chain isomerism: When two or more compounds have similar molecular formula but different carbon skeletons, these are referred to as chain isomers and the phenomenon is termed as chain isomerism.
(ii) Position isomerism: When two or more compounds differ in the position of substituent atom or functional group on the carbon skeleton, they are called position isomers and this phenomenon is termed as position isomerism.
(iii) Functional group isomerism: Two or more compounds having the same molecular formula but different functional groups are called functional isomers and this phenomenon is termed as functional group isomerism.
(iv) Metamerism: It arises due to different alkyl chains on
either side of the functional group in the molecule.
Stereoisomerism
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. This special type of isomerism is called as stereoisomerism and can be classified as geometrical and optical isomerism.
In an organic reaction, the 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 general reaction is depicted as follows :
bond and the other reactant is called reagent.
which attention is focused is called substrate.
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 (kinetics) is referred to as reaction mechanism.
Fission of a Covalent Bond
Cleavages of covalent bonds
(i) Heterolytic cleavage: In heterolytic cleavage, the bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments.
(ii) Homolytic cleavage: In homolytic cleavage, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms. Thus in homolytic cleavage, the movement of a single electron takes place instead of an electron pair.
Nucleophiles and Electrophiles
Nucleophile (Nu) : A reagent that brings an electron pair i.e., nucleus seeking and the reaction is then called nucleophilic.
Electrophile (E+): A reagent that takes away an electron pair i.e., electron seeking and the reaction is called electrophilic.
Electron Movement in Organic Reactions
The movement of electrons in organic reactions can be shown by curved-arrow notation.
Presentation of shifting of electron pair is given below :
Electron Displacement Effects in Covalent Bonds
The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent.
examples of this type of electron displacements are:
Resonance Structure: A single Lewis structure cannot explain all the property of the molecule hence many Lewis structure are proposed for a some molecule to explain the properties.
Different Lewis structure proposed are called as resonance Lewis structure or canonical form of contributing structure the phenomenon is called resonance and the structures proposed are called resonance structure.
The following rules are applied while writing resonance
structures:
The resonance structures have
Resonance Effect: 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’.
(i) Positive Resonance Effect (+R effect) : In this effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system.
(ii) Negative Resonance Effect (- R effect): This effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugated system.
Inductive Effect: The process of electron displacement of electrons along the chain of carbon atoms due o the presence of a polar covalent bond at one end of the chain.
This is a permanent effect and is generally represented by an arrow as shown on the next slide.
There are two types of resonance effect
It may be noted that this effect decreases sharply as we move away from the atom involved in the initial polar bond and becomes negligible from the fourth atom onwards.
It arises whenever an electron withdrawing group is attached to end of a carbon chain.
Electromeric Effect (E effect)
It is defined as the complete transfer of a shared pair of π- electrons to one of the atoms joined by a multiple bond on the demand of an attacking reagent.
In this effect the π−electrons of the multiple bond are transferred to that atom to which the reagent gets attached. For example :
(ii) Negative Electromeric Effect (–E effect)
In this effect the π - electrons of the multiple bond are transferred
to that atom to which the attacking reagent does not get attached.
Hyperconjugation
Hyperconjugation is a general stabilising interaction. It involves delocalisation of σ 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 orbital. Hyperconjugation is a permanent effect.
Example:
METHODS OF PURIFICATION OF ORGANIC COMPOUNDS
Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it. Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it.
The common techniques used for purification are as follows :
impurities in a suitable solvent.
their boiling points.
Distillation is further divided in to :
the aqueous solution should be immiscible with each other so that they form two distinct layers which can be separated by separatory funnel.
or a gas is allowed to move slowly over the stationary phase.
Adsorption chromatography : Is based on the fact that different compounds are adsorbed on an adsorbent to different degrees.
Column chromatography: It involves separation of a mixture over a column of adsorbent (stationary phase) packed in a glass tube.
Thin layer chromatography (TLC): Is another type of adsorption chromatography, which involves separation of substances of a mixture over a thin layer of an adsorbent coated on glass plate.
Partition chromatography
Continuous differential partitioning of component of a mixture between stationary and mobile phase.
The relative adsorption of each component of the mixture is expressed in terms of its retardation factor.
QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
The elements present in organic compounds are carbon and hydrogen. In addition to these, they may also contain oxygen, nitrogen, sulphur, halogens and phosphorus.
Detection of Carbon and Hydrogen
Carbon and hydrogen are detected by heating the compound
with copper(II) oxide.
Carbon present in the compound is oxidised to carbon dioxide (tested with lime-water, which develops turbidity)
Hydrogen to water (tested with anhydrous copper sulphate,
which turns blue).
Detection of Other Elements: Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by “Lassaigne’s test”.
(A) Test for Nitrogen: The sodium fusion extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. The formation of Prussian blue colour confirms the presence of nitrogen.
(B) Test for Sulphur:
(a) 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.
(b) On treating sodium fusion extract with sodium nitroprusside, appearance of a violet colour further indicates the presence of sulphur.
(C) Test for Halogens: The sodium fusion extract is acidified with nitric acid and then treated with silver nitrate. Results:
the presence of chlorine.
(D) Test for Phosphorus: The compound is heated with an oxidising agent (sodium peroxide). The phosphorus present in the compound is oxidised to phosphate. The solution is boiled with nitric acid and then treated with ammonium molybdate.
Result: A yellow colouration or precipitate indicates the
presence of phosphorus.
QUANTITATIVE ANALYSIS
Carbon and Hydrogen: A known mass of an organic compound is burnt in the presence of excess of oxygen and copper(II) oxide.
Observation
The increase in masses of calcium chloride and potassium hydroxide gives the amounts of water and carbon dioxide from which the percentages of carbon and hydrogen are calculated.
(ii)Kjeldahl’s method.
(i) 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.
(ii) Kjeldahl’s method: The compound containing nitrogen is heated with concentrated sulphuric acid. Nitrogen in the compound gets converted to ammonium sulphate .
Drawbacks of this method: It is not applicable to compounds containing nitrogen in nitro and azo groups and nitrogen present in the ring (e.g. pyridine) as nitrogen of these compounds does not change to ammonium sulphate under these conditions.
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, in a furnace.
(AgX). It is filtered, washed, dried and weighed.
Sulphur: 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.
Phosphorus: A known mass of an organic compound is heated with fuming nitric acid where upon phosphorus present in the compound is oxidised to phosphoric acid.
Oxygen: A definite mass of an organic compound is decomposed by heating in a stream of nitrogen gas.