Chemistry

Session GENERAL ORGANIC CHEMISTRY - 5

Session Objectives

Session Objective 1.Fission of a covalent bond: homolysis and heterolysis 2.Reaction intermediates: carbocations, carbanions and free radicals 3.Different reagents: electrophiles, nucleophiles and carbene 4.Brief idea about type of organic reactions: substitution, addition, elimination,condensation, rearrangement, isomerisation

Reaction Mechanism Detailed description of sequence of steps involved in group from reactants to products. Reactant intermediate product

Bond Cleavage + vely charged ion – carbocation - vely charged ion – carbaanion Heterolytic Cleavage Homolytic Cleavage Free radicals.

Carbonium ion Planar – sp 2 hybridised bond angle 120 o Has six electrons Stabilized by resonance or inductive effort or hyperconjugation

Examples of Carbonium ion no resonance hence unstable.

Stability of Cabocation (i) By inductive effect The resonance effect is always more predominant than the inductive effect in stabilizing an ion.

Stability of Cabocation (ii) By hyperconjugation Thus, tertiary carbocation is more stable than secondary and so on.

Carbanion Pyramidal - sp 3 hybridised bond angle Has eight electrons Stabilized by resonance or by inductive effect.

Stability of Carbanion (i) By resonance Cyclopentadienyl carbanion

Stability of Carbanion (ii) By inductive 3° 2° 1°

Stability of Carbanion (iii) Electron-donating groups destabilize a carbanion while electron-withdrawing groups stabilize it.

Free Radical Planer or Pyramidal Has seven electrons Stabilized by resonance or by inductive effect. Order of stability of free radical 3 o >2 o > 1 o

Classification of Reagents Nucleophilic Reagents (Nucleophiles) Attacks the positive end of a polar bond or nucleus-loving is known as nucleophile. Generally, negatively charged or electron rich species are nucleophilic. All nucleophiles are in general Lewis bases.

Classification of Reagents Electrophilic Reagents (Electrophiles) Attacks a region of high electron density or electron-loving is known as electrophile. All positively charged or electron deficient species are electrophilic.

Classification of Reagents Neutral reagents which contain an electron-deficient atom are also electrophiles. AlCl 3, SO 3, BF 3, SOCl 2, POCl 3, FeCl 3, ZnCl 2 All electrophiles are in general Lewis acids.

Carbenes Divalent carbon compound. Carbon atom is linked to two adjacent groups by covalent bonding. A carbene is neutral and possesses two free electrons, i.e. a total of six electrons. Electron deficient.

Carbenes Carbene is of two types (i)Singlet carbene: (ii)Triplet carbene: Triplet carbene is more stable than single carbene.

Types of Organic Reaction SubstitutionAddition EliminationRearrangement CondensationIsomerisation

Types of Organic Reaction Substitution Reaction Replacement of an atom or group by other atom of group Nucleophilic substitution: S N 1 Reaction: Unimolecular nucleophilic substitution reaction.

Types of Organic Reaction - S N 1 Reaction (1) (2)

Types of Organic Reaction - S N 2 Reaction S N 2 Reaction: This is called bimolecular nucleophilic substitution and it is one-step process.

Addition The reagent often adds to bond and the bond is converted into bond. Can be electrophilic addition or nucleophilic addition. (Hydration)

Elimination Reaction Two groups on adjacent atoms are lost as a double bond is formed. We divide elimination reactions into three classes. (1) E 1 (2) E 1 CB (3) E 2

Rearrangement Migration of a group takes place within the same molecule. (Beckmann rearrangement) (Dehydration and rearrangement)

Condensation Two molecules of same or different reactants combine to give a new product with the elimination of simple byproducts like H 2 O, NH 3, etc,

Isomerisation

Class Test

Class Exercise – 1 Select the most stable carbocation among the following. (a) (b) (c)(d)

Solution This carbocation is highly stabilised through resonance with three benzene rings. Hence answer is (b).

Class Exercise - 2 Which of the following is an addition reaction? (a) (b) (c) (d)

Solution Hence answer is (d).

Class Exercise - 3 Which of the following is the most effective group in stabilizing a free radical inductively? (a) F(b) I (c) Br(d) Cl

Solution Since free radical is electron deficient, any substituent with more electron releasing and less electron withdrawing ability will stabilize the radical inductively. The decreasing order of electronegativity of halogens is: F > Cl > Br > I Hence answer is (b).

Class Exercise - 4 Which of the following is not a nucleophile? (a) CN – (b) BF 3 (c) RNH 2 (d) OH –

Solution Among the following, BF 3 is only electron deficient. Hence, it will not act as a nucleophile. Hence answer is (b).

Class Exercise - 5 Which of the following is the correct order regarding –I effect of the substituents? (a) –NR 2 > –OR > –F (b) –NR 2 > –OR < –F (c) –NR 2 < –OR < –F (d) –OR > –NR 2 > –F

Solution –I effect increases with electronegativity of atom. The decreasing order of electronegativity is F > O > N The correct order for –I effect is –NR 2 < –OR < –F Hence answer is (c).

Class Exercise - 6 The least stable carbonium ion is (a) (b) (c) (d)

Solution Among the following, (a) is stabilised through +I effect and (b) is destabilized through –I effect of phenyl ring. Other two are stabilised through resonance. Hence answer is (c).

Class Exercise - 7 Arrange the following ions in the decreasing order of stability.

Solution. It is a primary cation. Hence, minimum stability. are secondary cations. Hence, stabilised through +I effect of –CH 3 group which decreases with distance. (c) is more stable as compared to (b). (d) is most stable as it is tertiary cation and stabilised through +I effect of –CH 3 group and hyperconjugation.  The order is (d) > (c) > (b) > (a)

Class Exercise - 8 Arrange the following radicals in order of their decreasing stability

Solution Radicals are stabilised through electron releasing resonance and inductive effect. More resonating structure

Solution One resonating structure, although both are primary radicals. Among and, later is a tertiary radical. Hence, more stable. The decreasing order of stability is

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