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Home , Addition reaction, Electrophilic addition, SN1 reaction, SN2 reaction, Substitution reaction

Group B IN (II)

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

1 Organic Reactions :

Organic reactions are chemical reactions which involve organic compounds.

Reaction Mechanism : The steps of an showing the breaking and formation of new bonds leading to the formation of product through transitory intermediates. In other words, In organic chemistry terms, a reaction mechanism is a formalized description of how a reaction takes place from reactants to products.

Intermediate Reactant or Product Transition State

Most of the attacking reagents carry either a positive or a negative charge.

2 Types of Organic Reaction

The reactions in organic chemistry are mainly classified into following classes:

1. Substitution

Reactions

2. Addition

Reactions Organic

Reaction Reactions

3. 3. Elimination

4. Rearrangement Reactions

3 1. Substitution Reactions

Substitution reactions are defined as reactions in which the of one is substituted by another group. or It is a reaction which involves the replacement of one or a molecule of a compound with another atom or molecule.

Examples: reacted with Cl2 will produce dichlorobenzene and HCl. This replaces the on the original molecule with the Cl atom.

4 Types of Substitution Reaction

Substitution reaction may be initiated by a , or free . Therefore, Substitution reactions are of three types:

1. Free- Reaction 2. Nucleophilic Substitution Reaction 3. Electrophilic Substitution Reaction

1. Free-Radical Substitution Reactions A free radical substitution reaction is initiated by free radical. A simple example of substitution is the reaction between and /bromine in the presence of UV light (or sunlight).

Free radicals : Free radicals are atoms or groups of atoms which have a single unpaired electron formed by homolytic fission ( studied in earlier lecture). 5 Mechanism: The mechanism for the chlorination of methane involves the following steps-

1. Initiation Step - A chlorine molecule undergoes homolytic fission in the presence of UV light to give chlorine free radicals.

Cl2 2Cl

2. Propagation Step – A chlorine free radical attacks the methane molecule to give methyl free radical and hydrogen chloride. Further, the methyl free radical attacks a chlorine molecule to yield methyl chloride and chlorine radical.

CH + Cl CH + HCl 4 3

CH3 + Cl2 CH3Cl + Cl

These propagation reactions are repeated again and again.

6 3. Termination Steps – These involve the formation of stable molecules by combination of free radicals.

Cl + Cl Cl2

CH3 + Cl CH3Cl

CH3 + CH3 CH3-CH3

2. Nucleophilic Substitution Reaction

When a substitution reaction involves the attack by a nucleophile, the reaction is referred to as SN (S stands for substitution and N for nucleophile) Nucleophilic Substitution Reaction.

_ _ Example: R-X + OH R - OH + X

The of halides by aqueos NaOH is an example of nucleophilic substitution reaction.

Remember the role of a nucleophile by its Greek roots: Nucleo-(nucleus)-phile-(lover) – it is attracted to the nucleus, which is positively charged! are therefore negatively charged or strongly δ-. 6 The nucleophilic substitution reactions are divided into two classes:

1. SN2 Reaction 2. SN1 Reaction

1. SN2 Reaction

The SN2 reaction is a nucleophilic substitution reaction where a bond is broken and another is formed simultaneously or we can say that where simultaneous attack of the nucleophile and displacement of the leaving grouptake place.

The term ‘SN2’ stands for – Substitution Nucleophilic Bimolecular. When the rate of a nucleophilic substitution reaction depends on the concentration of both the substrate and nucleophile,the reaction is second order reaction and termed as SN2 reaction.

Rate∞ [Substrate][Nucleophile]

Evidently, the rate determining step include the participation of both the substrate and the nucleophile.

7 SN2 Reaction Mechanism: Consider the hydrolysis of methyl chloride by aqueous NaOH. The reaction mechanism is represented here-

Fig. Nucleophilic substitution by SN2 Mechanism

This reaction proceeds through a backside attack by the nucleophile on the substrate. The nucleophile approaches the given substrate at an angle of 180o to the carbon- bond. The carbon-nucleophile bond forms and carbon-leaving group bond breaks simultaneously through a transition state. Notice that intermediate is not formed in an SN2 reaction, just a transition state is obtained. In the course of the reaction, the configuration of the carbon is inverted and designated as Walden Inversion. Factors Affecting Rate of SN2 Reaction :

1. Nucleophilicity : Since the nucleophile is involved in the rate-determining step of SN2 reactions, stronger nucleophiles react faster. Stronger nucleophiles are said to have increased nucleophilicity and thus rate of reaction will increase.

2. Effect : SN2 reactions are much faster in polar aprotic (e.g. acetonitrile, dimethylsulfoxide, dimethylformamide, etc.) compared with polar protic solvents (e.g. , ).

3. Steric Hindrance : SN2 reactions are particularly sensitive to steric factors, since they are greatly retarded by steric hindrance (crowding) at the site of reaction. In general, the order of reactivity of alkyl halides in SN2 reactions is:

methyl > 1° > 2°.

3° alkyl halides are so crowded that they do not generally react by an SN2 mechanism

In an SN2 reaction, the transition state has 5 groups around the central C atom. As a consequence of the steric requirements at this center, less highly substituted systems

(i.e. more smaller H groups) will favour an SN2 reaction by making it easier to achieve the transition state.

8 2. SN1 Reaction

The SN1 reaction is a unimolecular nucleophilic substitution reaction. When the rate of a nucleophilic substitution reaction depends only on the concentration of the alkyl halide, hence it is first order reaction. This reaction involves the formation of a intermediate.

SN1 Reaction Mechanism: Consider the hydrolysis of tertiary butyl bromide as an example, the mechanism of the

SN1 reaction consists of two steps:

Step 1. Formation of Carbocation:

tert-butyl bromide Carbocation This is the rate determining step. The carbon-bromine bond is a polar . The cleavage of this bond allows the removal of the leaving group (bromide ion). When the bromide ion leaves the tertiary butyl bromide, a carbocation intermediate is formed.

9 Step 2. Attack of Nucleophile : The second step is a bond making process where the electron rich nucleophile attack over an electron poor electrophile (carbocation).

tert-butyl

Summary Of Nucleophilic Substitution Reaction Factors SN1 Reaction SN2 Reaction Unimolecular Bimolecular Kinetics First Order Second Order Steps Two steps One step Intermediates Carbocation No intermediate o o o o o o Alkyl halide 3 > 2 , No 1 or CH3 CH3 > 1 > 2 , No 3 Solvent Polar protic solvent Polar aprotic solvent Nucleophile Weak nucleophile Strong nucleophile

10 Addition Reactions

Addition Reactions are those in which atoms or group of atoms are added to a double or triple bond without the elimination of any atom or molecules.

Addition reactions are typical of unsaturated organic compounds— i.e., , which contain a C-C double bond, and , which have a C-C triple bond—and and , which have a C=O double bond. Types of Addition Reactions

These reactions may be initiated by or nucleophiles:

1. reactions 2. reactions

Electrophilic Addition reactions An electrophilic is a reaction in which a substrate is initially attacked by an electrophile, and the overall result is the addition of one or more relatively simple molecules across a multiple bond. The addition of HBr to is an example of electrophilic addition-

Mechanism:

+ - Br2 gives a Br (electrophile) and Br (nucleophile).

Nucleophilic Addition reactions

When an addition reaction involves the initial attack by a nucleophile, the reaction is referred to as nucleophilic addition reaction. Aldehydes and ketones which contain carbon-oxygen double bonds undergo such reactions.

Reactivity of aldehydes and ketones: and ketones demonstrate polar nature: Since, oxygen is more electronegative than carbon, so electron density is higher on the oxygen side of the bond and lower on the carbon side. Recall that bond polarity can be depicted with a dipole arrow, or by showing the oxygen as holding a partial negative charge and the carbonyl carbon a partial positive charge.

Carbon becomes more electrophilic

Fig. Structure of Therefore, C-centre behaves as an electrophilic target for attack by an electron-rich nucleophilic group.

Fig. Nucleophilic Addition Reaction

Relative Reactivity of Carbonyl Compounds to Nucleophilic Addition

Aldehydes are more reactive and readily undergo nucleophilic addition reactions in comparison to ketones. In the case of ketones, two large are present in the structure of ketones which causes steric hindrance when the nucleophile approaches the carbonyl carbon.However, aldehydes contain one and thus the steric hindrance to the approaching nucleophile is less. Moreover, electronically aldehydes demonstrate better reactivity than . This is because ketones contain two alkyl groups (+I effect) which decrease the electrophilicity of carbonyl carbon atom more than aldehydes.

Elimination Reaction

Elimination reaction is a type of reaction which is mainly used to convert saturated compounds (organic compounds which contain single carbon-carbon bonds) to unsaturated compounds (compounds containing double or triple carbon-carbon bonds). Besides, it is an important method for the synthesis of alkenes.

The elimination reaction consists of three fundamental steps:

1. Proton removal. 2. C-C 휋 bond is formed. 3. There is a breakage in the bond of the leaving group. Elimination reactions can occur mostly by two mechanisms:

1. E1 Elimination reaction 2. E2 Elimination reaction where E is referred to as elimination and the number represent the molecularity.

1. E1 Reaction

. E1 mechanism is also known as unimolecular elimination. . There are usually two steps involved – ionization and deprotonation. . During ionization, there is a formation of carbocation as an intermediate. In deprotonation, a proton is lost by the carbocation. . This happens in the presence of a base which further leads to the formation of a pi-bond in the molecule. . In E , the is also proportional to the concentration of the substance . 1 to be transformed. . It exhibits first-order kinetics. . The initial step is the formation of a carbocation intermediate through the loss of the leaving group. This slow step becomes the rate-determining step

. The rate of the E1 reaction is; Rate = k[RX]. Saytzeff’s rule: Major and minor product is decided on the basis of Saytzeff’s rule. Saytzeff or Zaitsev Rule states that the more substituted will be the major product. So by looking at the number of alkyl groups attached to the alkene, the degree of substitution and hence major and minor products can be determined. 2. E2 Reaction

. E2 reaction is bimolecular one-step elimination mechanism. . Here, the carbon-hydrogen and carbon- bonds mostly break off in the presence of base to form a new double bond. It exhibits second-order kinetics. . Example: The elimination products of 2-chloropentane

Major product

Minor product

In general, more substituted alkenes are more stable, and as a result,1- is minor product and 2-butene is major product (this is the regiochemical aspect of the outcome, and is often referred to as Zaitsev’s rule). In addition, trans–alkenes are generally more stable than cis-alkenes, so we can predict that more of the trans product will form compared to the cis product (stereochemical aspect). However, certain other eliminations favor the least substituted alkene as the predominant product, due to steric factors. Such a product is known as the Hoffmann product, and it is usually the opposite of the product predicted by Zaitsev’s Rule.

Example:

Here, less substituted alkene is major product rather than more substituted alkene in both the cases. This is because of the bulky nature of quaternary salt (steric hindrance).Hence, hoffmann product dominates over saytzeff product.