General Organic Chemistry
1. Electronic Displacements in Organic Molecules
(a) Inductive effect: Inductive effect may be defined as the permanent displacement of electrons forming a covalent bond towards the more electronegative element or group.
The inductive effect is represented by the symbol, the arrow pointing towards the more electronegative element or group of elements. Thus in case of n-butyl chloride inductive effect may be represented as below.
Any atom or group, if attracts electrons more strongly than hydrogen, it is said to have a -I effect (electron-attracting or electron-withdrawing), viz NO2, Cl, Br, I, F, COOH OCH3, etc. while if atom or group attracts electrons less stronlgy than hydrogen it is said to have +I effect (electron repelling or electron-releasing) viz, CH3, C2H5, Me2CH and Me3C groups. The important atoms or groups which cause negative or positive inductive effect are arranged below in the order of decreasing effect.
-I (Electron-attracting) groups:
+I (Electron-attracting) groups:
(b) Resonance and Resonance (Mesomeric) effect.
Resonance: The phenomenon in which two or more structures can be written for the true structure of a molecules, but none of them can be said to represent if uniquely, is referred to as resonacne or mesomerism. The true structure of the molecule is said to be a resonance hybrid of the various possible alternative structures which themselves are known as resonating structures or canonical structures. Every two adjacent resonating structures are represented by inserting a double headed arrow between them. Thus the actual structure of benzene may be represented in the following two ways.
Necessary conditions for resonance
1. All resonating structures must have the same arrangement of atomic nuclei.
2. The resonating structurs must have the same number of paird and unpaired electrons. However, they differ in the way of distribution of electrons.
3. The energies of the various limiting structures must be sane or nearly the same.
4. Resonating structures must be planar.
All the resonating structures do not contribute equal to the real molecule and hence only the major contributing forms are used while represeenting a resonance hybrid.
The mesomeric effect may be defined as the permanent effect in which p electrons are transfered from a multiple bond to an atom, or from a multiple bond to a single covalent bond or lone pair(s) of p-electrons from an atom to the adjacent single covalent bond.
Like inductive effect, the mesomeric effect (denoted by M) may be +M and -M. It is +M when the transference of electron pair is away from the atom and -M when transference of electron pair is toward the atom. In general.
Some common atoms or groups which cause +M and -M effect are given below
+M groups. -Cl, -Br, -I, -NH2, -NR2, -OH, -OCH3
- M groups. -NO2, -CN, >C=O
(c) Electromeric effect: This type of temporary displacement of electrons take place in compounds containing multiple covalent bonds (e.g. C=C, C=O, C°N, etc.) or an atom with a lone pair of electrons adjacent to a covalent bond. The effect involves complete transference of a pair of electrons from a multiple bond to an atom, or from a multiple bond to another bond, or from an atom with a free pair of electrons to a bond. it is the p-electrons of a multiple bond, or the p-electrons of an atom, which are transfered. Since the effect involes complete transference of electrons, it leads to the development of full + and - charges within the molecule. It is important to note that the electromeric effect is purely a temporary effect and is brought into play only the requirement of attacking reagent, it vanishes out as soon as the attacking reagent is removed from reaction mixture.
(d) Hyperconjugation: The alkyl groups with at least one hydrogen atom on the a-carbon atom, attached to an unsaturated carbon atom, are able to release electrons by a mechanism similar to that of the electromeric effect, e.g.
Note that the delocalization involves s and p bond orbitals (or p orbitals in case of free radicals); thus it is also known as s-p conjugation. This type of electron release due to the presence of the system H - C - C = C is known as hyperconjugation.
2. Breaking of a Covalend Bond
(a) Homolytic fission or homolysis: In homolytic bond fission one electron of the bonding pair goes with each of the departing atom or group resulting in two electrically neutral fragments or atoms generally known as free radicals. e.g.
Thus a free radical may be definedas the atom or group of atoms having a single, odd or unpaired electron. Now since the homolytic fission always results in the formation of free radicals, the reactions involving such (homolytic) fission are known as free radical reactions and are said to proceed via a free radical mechanism.
(b) Hetereolytic fission or heterolysis: In this type of fission the electrons pair forming the covalent bond goes to a single atom or group and thus electricity charged fragments (ions) are formed. Thus the reaction involving heterolytic fision are known as ionic reaction sand are said to proceed via ionic (polar) mechanism. The heterolytic fission of the covalent bond can occur in either of the following two ways.
(i) When the electrons pair between C and X leaves the organic group and remains with the departing substituent X and thus the latter attains a negative charge (due to gain of electrons) while the former attains a positive charge (due to loss of electrons)
Such organic species which has only six paired electrons (i.e. three pairs of electrons) and a positive charge at its carbon centre is known as carbonium ion. Carbonium ions are generally symbolized as . Reactions in whcih carbonium ions are formed as intermediate are said to proceed by carbonium ion mechanism.
(ii) When the electrons pair between C and X remains with the organic group and the substituent X is devoid of its bonding electron and thus gets positive charge while the organic group (which has gained electrons) is negatively charged.
Such organic species which has eight paired electrons (i.e. four pair of electrons) and a negative charge on one of its carbon centre is known as carbanion. Carbanions are generally symbolized as . Reactions in which carbanions are formed as intermediate are said to proceed by a carbanion mechanism.
3. Reaction Intermediates
(a) Free radicals: A free radical is a species which has as odd or unpaired electron. Free radicals themselves are electricially neutral, however, due to the presence of odd electron, these are paramagnetic in nature. Again because of the presence of odd electron, free radicals are in constant search for another electron to pair up and hence these are highly reactive species. Carbon free radicals are named after the parent alkyl group and adding the word free radical.
A free radical may have an sp2 hybridized carbon in which odd electron remains in the p orbital the shape of this type of free radical will be planar. Alternatively, free radical may have sp3 hybridized carbon atom.
Order of Stability:
Triphenylmethyl > benzyl > allyl > tertiary > secodnary > primary > methyl > vinyl
(b) Carbonium ion (Carbocations): A carbonium ion is an orgnaic species in which a carbon atom has only six (paired) electrons (i.e. three pairs of electrons) and a positive charge, i.e. it (carbon) lacks a pair of electrons in its valency shell.
Carbocations are generally formed in acid catalysed reactions. Carbocations are classified as primary, secondary or tertiary depending upon the nature of the carbon atom bearing the positive charge. For example,
Structurally, the carbon atom of a carbonium ion is sp2hybridized. The three sp2hybrid orbitals are utilized in forming bonds to the three substituents, the remaining unused p orbital remains order.
Order of Stability:
(c) Carbanions: A carbanion is negatively charged organic species in which negative charge resides on the carbon atom. Thus the carbon atom in carbanion although contains four pairs of electrons, its one pair is free. These are named after the parent alkyl group and adding the word carbanion. For example,
Carbanions are generally formed during base catalyzed carbanion (e.g. R3C-:) is similar to that of amines. The carbon atom of the carbanion is in sp3 hybridized state; three sp3 hybrid orbitals form covalend bonds with three atoms/groups while the fourth sp3hybrid orbital has a non-bonding pair of electrons. Thus a carbanion has a pyramidal shape similar to that of ammonia.
Order of stability:
primary (1°) > secondary (2°) > Tertiary (3°)
The presence of electron withdrawing group increases of stability of carbanions.
(d) Carbenes: Carbenes are neutral divalent carbon species in whcih carbon is bonded to two monovalent atoms or groups and is surrounded by a sextet of electrons, e.g. :CH2(methylene and :O (dichlorocarbene). These are of two types.
(i) Singlet; In singlet carbenes, the two electrons have opposite spins and are paired in one orbital (sp2hybridization). Thus a singlet carbene has a bent structure.
(ii) Triplet: In triplet carbenes, the central atoms is sp hybridized and two electrons have the same spin but are in different orbitals. Thus a triple carbene has a linear structure and behaves as a diradical or bivalent free radical.
Due to inter-electronic repulsions between the two electrons present in the same orbital, a singlet carbene is generally less stable than the triple carbene.
4. Classification of reagents
(a) Electrophilic reagents (Electrophilies): As the name implies electrophilic (electro-electron, phile-love) reagents are electrong-seeking or loving and thus attack the substances at the point of maximum electron density. Thus an electrophilic reagent or an electrophile is a species having electron-deficient atom or centre. The electrophilic reagent may be positively charged species, or neutral molecule with electron deficient centrre. Some of the important electrophiles are given below.
It is interesting to note that since electrophiles are capabale of accepting electrons pair, they are Lewis acids.
The reactions involving the attack of electrophilies are known as electrophilic reactions, viz electrophilic addition and substitution reactions.
(b) Nucleophilic reagents (Nucleophiles): The reagents possessing at least one lone pair of electrons are known as nucleophilic reagents or nucleophiles (nucleo-nucleus, phile-love). Since they possess higher electron density, they attack the substrate at the point of minimum electron density. The nucleophilic reagent may be negatively charged species or neutral molecule with free electron pair(s). Some of the important nucleophiles are given below.
The star indicates the atom that denoates electrons to the substrate
5. Types of Organic Reactions
(a) substitution (displacment) reactions: The displacment of an atom or group from a molecule by a different atom or group is known as substitution reaction, e.g.
Substitution reactions are further classified into two types depending upon the mechanism:
(i) free-radical substitution reactions: those in which free radicals are formed as intermediates and
(ii) ionic substitution reactions: those in which ions are formed as intermediates.
(b) Addition reactions: Reactions in which atoms or group of atoms are added to a molecule (i.e. there is simply a net gain of the reagent atoms in the product molecule) are known as addition reactions. This type of reaction occurs only when there is a centre of unsaturation in the molecule which is generally due to the presence of a multiple bond between atoms, e.g.
Like substitution reactions, addition reactions may be initiated by electrophilies, nucleophiles or free radicals. Hence addition reactions may also be of three types depending upon the mechansim; electrophilic addition reactions, nucleophilic addition reactions and free radical addition reactions.
(c) Elimination reactions: As mentioned earlier, elimination reactions are reverse of addition reactions and involve the lose of atom(s) or group(s) from a molecule to form a multiple linkage.Most commonly, loss of atoms or group occurs from adjacent carbon atoms to yield an olefin, e.g.
(d) Rearrangements: Rearrangment reactions involve either the migration of a functional group to another position in the molecule containing a double bond or the reshuffling of the sequence of atoms forming the basic carbon skeleton of the molecule to form a product with new structure. For example,
6. Free-Radical Mechanism
The reaction between methane and chlorien in the presence of light is a typical example of the substitution reaction involving free-radical. The mechanism of this reaction involves the follwoign three steps.
- Initiation (Formation of free radicals)
- Propagation of free radicals
- Terminatin step (Termination of free radicals)
7. Electrophilic Substitution Reactions
These are generally written as SE(S for substitution and E for electrophilic) and are more common in aromatic compounds. Nitration, halogenation and sulphonation of benzene are few examples of this type of reactions. The mechanism involves the following three steps.
- Formation of the real electrophile, e.g. nitronium ion in nitration.
- Attack of electrophile on benzene
- Elimination of the proton to give substitution product
8. Nucleophilic substitution reactions
Nucleophilic substitution reactions are those substitution reactions which involve the attack of a nucleophile. These are usually written as SN(S stands for substitution and N for nucleophilic) and are more common in aliphatic compounds. Hydrolysis of alkyl halides by aqueous KOH is an example of nucleophilic substitution.
The nucleophilic substitution reactions may follow two mechanism, via unimolecualr and bimolecular.
(a) Unimolecular mechanism:
Nucleophilic substitution reactions proceeding via unimolecular mechanism are usually abbreviated as S
1 reactions (S stands for substitution, N for nucleophilic and 1 for unimolecular)
The rate of such reactions depends only on the concentration of the alkyl halide (substrate) and independent of base (nucleophile).
Such reactions (S
1 reactions) are believed to be completed in two stagse. In the first stage whcih is also the rate determining step (slow step), the alkyl halide ionises to give carbonium ion. In the second fast step, the carbonium ion combines with the nucleophilic reagent to form the final substituted product.
(b) Bimolecular mechanism:
Nucleophilic substitution reactions proceeding via bimolecular mechanism are usually abbreviated as S
2 reactions (2 stands for bimolecular).The rate of such reactions depends on the concentration of the alkyl halide (substrate) as well as the base (nucleophile).
Such reactions are believed to be completed in one step