Chemistry note for CTEVT diploma level : Organic chemistry (II)

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Contents

Chapter – 3 : Isomerism :
Chapter- 4 : Organic reaction :
References :

Chapter – 3 : Isomerism :

Isomers and isomerism in organic compounds :

Different compounds having same molecular formula but different properties are called isomers. The phenomenon of existence of such compounds is called isomerism.Eg.

CH₃– CH₂– OH CH₃– O – CH₃

Ethanol methoxymethane

(liquid at room temp.) ( gas at room temp.)

Isomerism can be divided into two broad classes :

Structural isomerism.
Stereoisomerism.

Structural isomerism : Compounds having same molecular formula but different structural formula are called structural isomers. Phenomenon of existence of such compounds is called structural isomerism.

Structural isomerism can be categorized into following types:

Chain isomerism
Position isomerism
Functional isomerism
Metamerism
Tautomerism

Chain isomerism : Compounds having same molecular formula and same functional group but different in their carbon chain are called chain isomers. The phenomenon of existence of such compounds is called chain isomerism. Examples:

chain isomerism

Position isomerism : The isomers having same carbon chains but different positions of functional group, substituent or multiple bonds( i.e. double or triple bond) are called position isomers . Examples :

position isomerism

Functional isomerism : Compounds having same molecular formula but different functional groups are called functional isomers. Phenomenon of existence of such compounds is called functional isomerism. Examples :

Note : Functional isomers:

Alcohol ↔ Ether

Aldehyde ↔ Ketone

Carboxylic acid ↔ Ester

Metamerism : Compounds having same molecular formula but different in alkyl groups present in the either side of the same functional group are called metamers. The phenomenon is called metamerism. Example:

CH₃ – CH₂ – O – CH₂ – CH₃ (ethoxyethane) and

CH₃ – CH₂ – CH₂– O – CH₃ (methoxypropane)

Chapter- 4 : Organic reaction :

Inductive effect :

Development of polarity in between the two covalently bonded atoms due to the difference in electronegativity is called inductive effect. This effect is also called field effect or space phenomenon. The inductive effect decreases on increasing the distance

Inductive effect may be represented by arrow in the middle of the bond towards electron withdrawing atom or group.

Negative inductive effect (-I effect) :

If an atom or group of atoms withdraws electrons, the effect shown by such group is called negative inductive effect (- I effect). Such atoms or groups are more electronegative than carbon atom and also called electron withdrawing species. Some common atoms or groups which cause – I effect are: NO₂, CN, F, Cl, OH, etc.

Positive inductive effect (+I effect) :

If an atom or group of atoms releases electrons, the effect shown by such group is called positive inductive effect (+ I effect). Such atoms or groups are more electropositive than carbon atom and also called electron releasing species. Some common atoms or groups which cause + I effect are: CH₃ – , CH₃CH₂ – , – O – , etc.

Applications(significance) of Inductive effect:

In deciding acidic strength of carboxylic acids.
It’s effect on dipole moment and bond length.
To decide the stability of carbocation, carboanions and carbon free radicals .

Bond fission( cleavage) :

1. Homolytic bond fission(homolysis) : The cleavage (breaking) of covalent bond in between two atoms in such a way that each atom retains(keeps) one electron of the shared pair of electrons is known as hemolytic fission or homolysis.

Homolytic fission leads to the formation of free radicals. This types of fission takes place when bonding elements have equal or nearly equal electronegativity.

Eg. homolytic bond fission

2. Heterolytic bond fission(heterolysis) : The cleavage (breaking) of covalent bond in between two atoms in which one of the bonded atoms acquires both of the electrons from shared pair is known as heterolytic fission or heterolysis.

Heterolytic fission leads to the formation of cations and anions. This types of fission takes place when bonding elements have different electronegativity. Eg.

Types of reagents:

1. Electrophiles: Electron deficient atoms or group of atoms which attack on electron rich centre during the reaction are called electrophiles. They are positively charged species or neutral species having electron deficient centre. They are also called electron loving species.

Eg. H⁺, R⁺, AlCl₃, BF₃, etc.

2. Nucleophiles : Electron rich atoms or group of atoms which attack on electron deficient centre during the reaction are called nucleophiles. They are negatively charged species or neutral species having electron rich centre. They are also called nucleus loving species.

Eg. OH ^– , CN ^–, Cl ^– ,:NH₃, H₂O, etc.

Carbocation: The fragments of organic molecules in which carbon atom bears positive charge are called carbocations. Eg.

CH₃ ⁺ ( methyl carbocation or methylium cation)

CH₃ – CH₂⁺ ( ethylium cation), etc.

Carbanion: The fragments of organic molecules in which carbon atom bears negative charge are called carbanions. Eg.

CH₃ ^– (methylium anion)

CH₃ – CH₂^– ( ethylium anion), etc.

Types of organic reactions :

1. Substitution reactions : A reaction in which an atom or group of atoms is replaced by another atom or group of atoms is called substitution reaction. Types:

i. Nucleophilic substitution reaction

ii. Electrophilic substitution reaction

i. Nucleophilic substitution reaction : A nucleophilic substitution reaction is a chemical reaction which involves the displacement of leaving group by a nucleophile.

nucleophilic substitution reaction

It is of two types :

SN₁ reaction :

→ SN₁ indicates the unimolecular nucleophilic substitution reaction.

→ The rate of SN₁ reaction depends only upon the concentration of the substrate.

i.e. Rate = k[Substrate]

→ The reaction occurs in two steps. In first step carbocation is formed and in second step nucleophile attacks the carbocation to give substituted product. Eg.

SN₂ reaction :

→ SN₂ indicates the bimolecular nucleophilic substitution reaction.

→ The rate of SN₂ reaction depends upon the concentration of the both substrate and nucleophile.

i.e. Rate = k[Substrate][:Nu ^–]

→ The reaction occurs in single step. . SN₂ reaction occurs through a transition state as shown below: Eg.

ii. Electrophilic substitition reaction :

Electrophilic substitution is a chemical reaction that involves the displacement of a functional group/leaving group by an electrophile.

2. Elimination reaction:

The reaction in which two atoms or groups of atoms are removed from a molecule and a double or triple bond is formed is called elimination reaction. Eg. dehydrohalogenation reaction of haloalkane.