Notes-Part-2-Class-11-Science-Chemistry-Chapter-14-Basic Principles of Organic Chemistry-Maharashtra Board

Isomerism :

The phenomenon of existence of two or more compounds possessing the same molecular formula is known as isomerism.

• Each individual compound is called an isomer.
• Isomerism is mainly of two types : (i) Structural isomerism (ii) Stereoisomerism

 Structural isomerism :

When two or more compounds have same molecular formula and different structural formula they are said to be structural isomers of each other. The phenomenon is known as structural isomerism.

Structural isomerism is further classified as

• chain isomerism,
• position isomerism,
• functional group isomerism,
• metamerism
• tautomerism

(i) Chain isomerism : When two or more compounds have the same molecular formula but different parent chain or different carbon skeleton, it is refered to as chain isomerism and such isomers are known as chain isomers.

Example :

• Butane : (CH₃ - CH₂ - CH₂ - CH₃)
• 2-methylpropane : [CH₃ - CH(CH)₃ -CH₃]

The above two compounds are chain isomers of each other as they have different skeletons and the same molecular formula C₄H₁₀.

(ii) Position isomerism : The phenomenon in which different compounds having the same functional groups at different positions on the parent chain is known as position isomerism.

Example :

• But-2-ene [CH₃ - CH = CH - CH₃]
• but-1-ene [CH₂ = CH - CH₂ - CH₃]

These are position isomers of each other as they have the same molecular formula C₄H₈ and the same carbon skeleton but the double bonds are located at different positions.

(iii) Functional group isomerism : The phenomenon in which different compounds with same molecular formula have different functional groups is known as functional group isomerism.

Example :

• Dimethylether (CH₃ − O − CH₃)
• ethyl alcohol (CH₃ − CH₂ − OH)

These have same molecular formula C₂H₆O but the former has ether (− O −) functional group and the latter has alcohol (− OH) functional group.

(iv) Metamerism : Different compounds with the same molecular formula and the

same functional group, but having unequal distribution of carbon atoms on either side of the functional group is refered to as metamerism and the isomers are known as metamers.

Example :

• Ethoxyethane (CH₃ − CH₂ − O − CH₂ − CH₃)
• methoxypropane (CH₃ − O − CH₂ − CH₂ − CH₃)

These have the same molecular formula C₄H₁₀O and the same functional group ether but have different distribution of carbon atoms attached to etheral oxygen. These are metamers of each other.

(v) Tautomerism : Same compounds exists as mixture of two or more stucturally distinct molecules which are in rapid equilibrium with each other. This phenomenon is referred to as tautomerism. Such interconverting isomers are called tautomers.

Example :

Keto-enol tautomerism is very common form of tautomerism. It is found in carbonyl compounds. Here a hydrogen atom shifts reversibly from the ∝-carbon of the keto form to oxygen atom of the enol.

Theoretical basis of organic reactions :

Types of cleavage of covalent bond :

A covalent bond can undergo cleavage or fission in two ways

• Homolytic cleavage
• Heterolytic cleavage

(i) Homolytic fission : The symmetrical breaking of covalent bond between two atoms such that each atoms retains one electron of the shared pair forming free radicals is known as homolytic fission.

• The movement of a single headed arrow or fish hook
• In this fission each departing species are electrically neutral and contain are unpaired electron is called free radical.
• A free radical is unstable and a reactive species having a tendency to accept an electron for pairing.
• The fission takes place at high temperature or in the presence of UV light or suitable peroxide and in the non polar solvent.

Free radical : A uncharged species with incomplete octet which is electrically neutral and contains an unpaired electron is called free radical. Free radical is highly reactive and short lived.

(a) A carbon free radical is sp² hybridized and reveals planar trigonal geometry.

Alkyl free radicals are classified as primary (1⁰), secondary (2⁰) and tertiary (3⁰).

The observed stability order of alkyl free radicals is :

(ii) Heterolytic fission :

• The unsymmetrical breaking of a covalent bond between two atoms in such a way that one of the departing atoms (more electronegative atom) retains the shared electron pair of the bond, forming charged ions is known as heterolytic fission. This fission can be shown as,
• This fission takes place when two bonding atoms differ largely in electronegativity.
• The more electronegative atom retains electron pair and acquires negative charge (anion) and another atom gets positive charge (cation).

• This fission takes place in a polar solvent since ions are stabilised by solvation.

• Organic reactions proceeding by heterolysis are called ionic or heteropolar or polar reactions.

Carbocation :

A carbon atom having sextet of electrons and a positive charge is known as carbocation,

Structure of methyl carbocation :

• In methyl carbocation, positively charged carbon atom has six electrons.
• The central carbon atom of carbocation is sp²-hybrid state and has trigonal planar geometry.
• It is bonded to three hydrogen atoms and H-C-H bond angle is 120°.
• The P, atomic orbital is empty and lies perpendicular to the plane containing the three C-H sigma bonds at the carbon.

Classification of Carbocations :

Carbocations are classified as primary (1⁰), secondary (2⁰) and tertiary (3⁰). The observed stability of alkyl carbocations follows the order :

Carbanion :

Carbon atom having octet of electrons and a negative charge is known as carbanion.

Heterolytic bond fission generates a species called carbanion in which carbon gets the shared pair of electrons. This happens when a carbon atom is bonded to the electropositive atom.

Structure of methyl carbanion :

• In methyl carbanion, negatively charged carbon atom has eight electrons.
• The central carbon atom of carbanion is sp³-hybridised and has distorted tetrahedral geometry.
• It is bonded to three hydrogen atoms.
• Carbanion is unstable and reactive species.
• It is favoured in the polar solvents.

Types of reagent :

There are two types of reagents as follows :

• Electrophiles or Electrophilic centre (H⁺, NO₂⁺)
• Nucleophiles or Nucleophilic centre (OH^—, Cl^—)

Electrophiles or Electrophilic centre :

• The species which are electron seeking species i.e. electron deficient species are called electrophiles or electrophilic centre.
• A polyatomic electrophile has an electron deficient atom in it called the electrophilic centre.
• They are positively charged species like H⁺, NO₂⁺or neutral molecules with incomplete octet of electrons like BF₃, AlCl₃
• The electrophilic centre of the electrophile AlCl₃ is Al which has only 6 valence electrons.

Nucleophiles or Nucleophilic centre :

• The species which are nucleous seeking species i.e. electron rich species are called nucleophiles or nucleophilic centre.
• A polyatomic nucleophile has an electron rich atom in it is called the nucleophilic centre.
• They are negatively charged species OH^—, Cl^— or neutral species like H₂O in which ‘O’ has two lone pairs of electrons.

Example :

The structural formulae of two reagents  NH₃ and CH₃⁺ .showing all the valence electrons are :

• Thus NH₃ contains N with a lone pair of electrons which can be given away to another species. Therefore NH₃ is a nucleophile and ‘N’ in it is the nucleophilic centre.
• The CH₃⁺ is a positively charged electron deficient species having a vacant orbital on the carbon. It is an electrophile. The ‘C’ in it is the electrophilic centre.

Inductive effect :

• When an organic molecule has a polar covalent bond in its structure, polarity is induced in adjacent carbon-carbon single bonds too. This is called inductive effect.
• The more electronegative atom or the group which pulls the bonding electrons is said to have negative inductive effect (—I) and the one which is less electronegative repels the bonding electrons is said to have positive inductive effect (+1). The effect is transmitted along a chain of covalent bonds, due to the mobility of electrons through single bonds.
• Example : In methyl chloride, chlorine is more electronegative than carbon atom. Hence, chlorine pulls the bonding pair of electrons towards itself. Thus, the chlorine atom acquires a fractional negative charge, while the carbon atom acquires a fractional positive charge.

• This mobility or effect is passed on to the subsequent bonds and decreases rapidly with the ‘increase in the number of single bonds, (+δ₁ > +δ₂ ...).
• The effect becomes insignificant after four C-C bonds.
• The electron withdrawing atoms or groups of atoms having, —I effect are highly electronegative. For example, F, Cl, Br, L.
• The electron repelling atoms or groups of atoms with + I effect are electropositive. For example, metals, alkyl groups, CH₃—CH₂—CH₃
• Inductive effect helps to understand reaction mechanisms.

Resonance structures :

Structure of benzene : The cyclic structure of benzene consists of three alternating C—C single bonds and three C=C double bonds states two distinct bond length of 154 pm and 133 pm respectively.

Experimental measurements show that benzene has a regular hexagonal shape and all the six carbon bonds, have the same bond length of 138 pm which is intermediate between C—C single bond and C=C double bond. This means that all the six carbon carbon bonds in benzene are equivalent.

Conjugated system of π bonds : When Lewis structure of a compound has two or more multiple bonds alternating with single bonds, it is called a conjugated system of π bonds.

Rules to be followed for writing resonance structure :

• Resonance structures can be written only when all the atoms involved in the π conjugated system lie in the same place.
• All the resonance structures must have the same number of unpaired electrons.
• Resonance structures in accordance to their energy or stability contribute to the resonance hybrid. Resonance structure (having low energy) are stable and contribute largely to resonance hybrid and are therefore, important.

Resonance Effect :

The polarity produced in the molecule by the interaction between conjugated π bonds (or that between π bond and p-orbital on attached atom) is called the resonance effect or mesomeric effect. The effect is transmitted through a chain of conjugated π bonds.

(i) Positive resonance effect or electron donating/releasing resonance effect (+ R effect) :

• If the substituent group has a lone pair of electrons to donate to the attached π bond or conjugated system of π bonds, the effect is called +R effect.
• The groups having lone pair of electrons show +R effect are —OH, —OR, —O^— —NHR, —halogen, etc.
• The +R effect increases electron density at certain positions in a molecule. The +R effect in aniline increases the electron density at ortho and para positions.

The following structures are resonance forms.

(ii) Negative resonance effect or electron withdrawing resonance effect (—R effect) :

• If the substituent group has a tendency to withdraw electrons from the attached π bond or conjugated system of π bonds towards itself, the effect is called —R effect. For example, groups such as —COOH, —CHO, —CO—, —CN, ~NO,, — COOR, etc. show —R effect.
• The —R effect results in developing a positive polarity at certain positions in a molecule.
• The —R effect in nitrobenzene develops positive polarity at the ortho and para positions.

The following structures are resonance forms.

Electromeric effect :

This is a temporary effect exhibited by multiple bonded groups in the excited state in

the presence of an attacking reagent. When a reagent approaches a multiple bond, the complete transfer of a shared pair of π electrons to one of the atoms joined by a multiple bond, giving a charged separated structure.

This effect is temporary and disappears when the reagent is removed from the reacting system.

Hyperconjugation : Hyperconjugation is a permanent electronic effect explains stability of a carbocation, free radical or alkene. It is delocalization of σ (sigma) electrons of a C—H bond of an alkyl group directly attached to a carbon atom which is part of an unsaturated system or has an empty p-orbital or a p-orbital with an unpaired electron.

Species stabilized by hyperconjugation

Hyperconjugation in ethyl cation :

• In ethyl cation, CH₃—C⁺H₂ positively charged carbon atom is attached to a methyl group.
• Positively charged carbon atom has six electrons due to three bond pairs, namely two C—H bonds and one C—C bond.
• C⁺ atom in ethyl cation is sp²-hybridised and it has one empty p-orbital available for hyperconjugation.
• The methyl group attatched to C⁺ rotates around C-C, hence one of its σ C—H bond is always aligned with the plane empty orbital of C⁺.
• The electrons of σ C-H bond are delocalised into empty p-orbital of C⁺ and decreases the positive charge and stabilises the ethyl cation.

Hyperconjugation in ethyl carbocation

Thus, hyperconguation in ethyl cation is a σ — π conjugation

The contributing structures II, III and IV involving delocalization of σ electrons of C-H bond shows no covalent bond between carbon and one of the ∝-hydrogens. Hyperconjugation is, therefore, called ‘no bond resonance’.

Hyperconjugation in propene OR No bond resonance in propene :

• Delocalization of electrons by hyperconjugation is also possible in alkenes. For example, consider hyperconjugation in propene.
• In propene, CH₃—CH=CH₂, sp³-hybridised carbon atom of methyl group is attached to sp²-hybridisecl carbon atom of the double bond.
• The hyperconjugation involves delocalization of σ C—H bond electrons into p-orbital of a carbon atom of a double bond.

 

Hyperconjugation structures :

• In the hyperconjugation structures, there is no bond between one of the H atoms and carbon atom attached to a double bond. Hence, this phenomenon is called ‘no bond resonance’. In this, ∝ C—H bond possesses ionic character.
• The effect of hyperconjugation is usually stronger than inductive effect.