Group B REACTION MECHANISM IN ORGANIC CHEMISTRY(I)

Dr. Akanksha Upadhyay Assistant Professor Department of Chemistry Women’s College, Samastipur

Inductive Effect

Definition : The partial displacement of sigma electron pairs towards more electronegative atom is said to be Inductive effect. OR It can be defined as the polarization of one bond due to the polarization of another bond.

Characteristics of Inductive effect:

• It takes place as a result of differences in the electronegativity of two atoms which form a sigma bond. • The physical and chemical properties of compounds is controlled by Inductive Effect. • As it moves away from the groups producing it, its magnitude decreases. • I effect is a permanent effect. • It performs on σ bonded electron. Types of Inductive Effect (I effect)

There are two types of Inductive Effect:

1. Positive Inductive Effect (+I effect) : Electron donating substituents tend to donate shared electron pair towards the carbon to which that group is connected. Hence carbon bears partial negative charge, this is known as positive inductive effect. The groups which cause such effect are called as +I groups.

+ I effect in increasing order:

H < CH2CH3 < C(CH3)3

2. Negative Inductive Effect (-I effect) : Electron withdrawing substituents tend to withdraw electron pair from the carbon to which that group is connected Hence carbon bears partial positive charge, this is known as negative inductive effect. The groups which cause such effect are called as -I groups

-I effect in increasing order: + H < C6H5< OR < I < Cl < CONH2 < COOH < CHO < CN < NH3

Greater the number of alkyl groups, the greater is the +I effect and higher will be the acidity of the compound and vice versa.. Applications of Inductive Effect

Inductive Effect is useful in explaining the strength of some organic acids and bases. a) Effect on Acidity and Basicity of the Compound: We can predict the acidity and basicity of compounds. It can be said as a generalisation the electron withdrawing groups (EWG) increase the acidity of a compound and electron donating group decrease the acidity of a compound.

• This is because, if we take the conjugate base of the acid, that is, RCOO-, if R is electron withdrawing, then the conjugate base is stabilised via delocalisation of the formed negative charge. • If R had been electron donating, then the conjugate base would be destabilised because of inter-electronic repulsions.

- I effect will increase the Acidity of carboxylic acids.

Q. Among the below four acids the Acidity is of the order

1. CH3-COOH 2. Cl-CH2-COOH 3. Cl2-CH-COOH 4. Cl3C-COOH

Solution. 4 > 3 > 2 > 1

The - I effect of chlorine increases the Acidity of the substituted chloro- Acetic acids in the above order. Groups with +I effect will increase the basicity of amines.

Q. Why (CH3)2-NH2 Dimethyl amine is more basic than CH3-NH2– Methyl amine?,

Solution : Because the methyl groups are electron donating and makes the availability of the lone pair of electron on the N-Atom easier to make the secondary amine more basic than the primary amine. More the availability of the electron pair on the Nitrogen atom of the amino group, the more will be it's basicity.

 Suppose if the amino group is directly attached to a PHENYL group the corresponding amine viz. ANILINE will be less basic than methyl amine, because the phenyl group is electron withdrawing, ie., it has - I effect.

So C6H5-NH2 is less basic than Methyl amine. b) Stability of Carbocation and Carbanion:

Since, carbocation is an electron deficient species, Hence, EDG and +I groups increases the stability of carbocation.

 Stability of Carbocation is directly proportional to +I effect and inversely proportional to –I effect.

 Stability of Carbanion is directly proportional to -I effect and inversely proportional to +I effect.

 Thus, we can say that greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the +I effect and higher will be the stabilization of the carbocation. On the other hand, stability of the carbanion will decrease with increase in the number of alkyl group. Electromeric Effect

Electromeric Effect can be observed only in organic compounds which contain multiple bonds. It is a temporary effect that when the compound is subjected to an attacking reagent.

Definition: The instantaneous formation of a dipole in the molecule of an organic compound due to the complete transfer of shared pi electron pairs to one of the atoms under the influence of an attacking reagent is referred to as the Electromeric effect.

 This effect can be observed in organic compounds that contain at least one multiple bond. When the atoms participating in this multiple bond come under the influence of an attacking reagent, one pi bonding pair of electrons is completely transferred to one of the two atoms.

 The electromeric effect is a temporary that remains as long as the attacking reagent is present and exposed to the organic compound. Once this attacking reagent is removed from the system, the molecule that was polarized goes back to its original state.

Types of Electromeric Effects

The electromeric effect can be broken down into two types, namely the +E effect and the -E effect. This classification is done based on the direction in which the electron pair is transferred.

+E Effect :

This effect occurs when the electron pair of the pi bond is moved towards the attacking reagent.

The +E effect can be observed in the addition of acid to alkenes. The attacking reagent attaches itself to the atom which obtained an electron pair in the transfer. The +E effect is generally observed when the attacking reagent is an electrophile and the pi electrons are transferred towards the positively charged atom. An example where the +E effect occurs is the protonation of ethene which is illustrated above.

-E Effect :

This effect occurs when the electron pair of the pi bond is moved away from the attacking reagent. The attacking reagent attaches itself to the positively charged atom in the molecule, i.e. the atom which lost the electron pair in the transfer.

The -E effect is generally observed when the attacking reagent is a nucleophile and the pi electrons are transferred to the atom which the attacking reagent will not bond with. An example where the -E effect occurs would be the addition of nucleophiles to carbonyl compounds as illustrated above.

Mesomeric Effect

• The polarity developed between atoms of a conjugated system by the electron transfer or pi–bond electron transfer is known as the Mesomeric effect. In simple terms, we can describe mesomeric effect occurs when π electrons move away from or towards a substituent group in a conjugated orbital system.

The mesomeric effect can be subdivided into two types:

• +M effect • -M effect 1. +M effect (Positive mesomeric effect)

When the electrons or the pi electrons are transferred from a particular group towards a conjugate system, thus increasing the electron density of the conjugated system then such a phenomenon is known as (+M) effect or positive mesomeric effect. Eg;

• Group showing +M effect are;

• –NH, –NH2,–NHR, –NR2, – O, – OH, –OR, – F, – Cl, –O–COR, – NHCOR, –SH, – SR etc. 2. -M Effect (Negative mesomeric effect)

When the pi-bond electrons are transferred from the conjugate system to a particular group thus the electron density of the conjugate system is decreased, then this phenomenon is known as negative mesomeric (–M) effect. Eg;

• The group which shows –M effect include;

• –NO2, –CN, –COX, –SO3H, – CHO, –CONH2, –COR, –COOH, –COOR etc. Hyperconjugation

Hyperconjugation effect is a permanent effect in which localization of σ electrons of C-H bond of an alkyl group directly attached to an atom of the unsaturated system or to an atom with an unshared p orbital takes place.

From the above figure, we observe that one of the three C-H bonds of the methyl group can align in the plane of the empty p orbital and the electrons constituting the C-H bond in a plane with this p orbital can then be delocalized into the empty p orbital. stabilizes

• .The relative stability on the basis of hyperconjugation is given as,

• We also observe that the hyperconjugation stabilizes the carbocation as it helps in the dispersal of positive charge. Thus, we can say that greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilization of the carbocation. Fission in Chemistry

There is a release in energy when an atom splits into two parts. This process is known as fission. Types of Fission

1. Homolytic Fission:

Homolytic fission is that fission in which each atom in the bond has an electron which results in species called free radicals. Homolytic fission is symmetrical in nature and leads to the formation of atoms or groups of atoms that has unpaired electrons. During the homolytic fission of a neutral molecule, two free radicals are generated if the number of electrons is even in the molecule. 2. Heterolytic Fission:

• In heterolytic fission, when a covalent bond is broken, the shared pair of the electron is taken by one of the atoms. This is the fission in which the electrons are not divided equally. • In this type of cleavage, we also get carbon atoms which are negatively charged and are also called as carbanions and in the same way the carbon atoms which are positively charged, known as carbocations.

• An example of heterolytic fission is when methyl chloride is cleaved then both the bonded electrons are taken by chlorine and formation of carbocation takes place.

Types Of Organic reagents

Organic reagents are categorized into three sections according to their charge as electrophile ,nucleophile and free radicals.

1. Electrophilic Reagents ( Electrophile) : Groups which have less density of electrons or positive charges, are called electrophilic reagents. Electrophilic reagents can attract electron from lewis alkaline compounds (NH3) or high electrons density compounds (alkenes, benzene ring in the phenol or aniline).

Examples of electrophilic reagents: 2. Nucleophilic Reagents (Nucleophile): Groups which have high density of electron or negative charges or lone electron pair are defined as nucleophilic reagents. Nucleophilic reagents have the ability to accord electron to less electron groups. Examples of nucleophilic reagents;

3. Free radicals: Groups which have unpaired valence electron are called free radicals . Free radicals are very reactive than electrophilic reagents and nucleophilic reagents. Examples of Free radical; THANK YOU, Reaction Mechanism Part II will be updated soon.

Happy Learning Dear Students