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Reaction Mechanism in Organic Chemistry(Ii)

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Reaction Mechanism in Organic Chemistry(Ii) Group B REACTION MECHANISM IN ORGANIC CHEMISTRY(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 organic reaction 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 Addition 2. Reactions Organic Reaction Reactions 3. 3. Elimination 4. Rearrangement Reactions 3 1. Substitution Reactions Substitution reactions are defined as reactions in which the functional group of one chemical compound is substituted by another group. or It is a reaction which involves the replacement of one atom or a molecule of a compound with another atom or molecule. Examples: Benzene reacted with Cl2 will produce dichlorobenzene and HCl. This substitution reaction replaces the hydrogen atoms on the original molecule with the Cl atom. 4 Types of Substitution Reaction Substitution reaction may be initiated by a nucleophile, electrophile or free radical. Therefore, Substitution reactions are of three types: 1. Free-Radical Substitution 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 alkane and chlorine/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. CH4 + Cl CH3 + HCl 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 hydrolysis of alkyl 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! Nucleophiles 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-leaving group 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. Solvent Effect : SN2 reactions are much faster in polar aprotic solvents (e.g. acetonitrile, dimethylsulfoxide, dimethylformamide, etc.) compared with polar protic solvents (e.g. alcohols, water). 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 carbocation 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 covalent bond. 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 alcohol Summary Of Nucleophilic Substitution Reaction Factors SN1 Reaction SN2 Reaction Molecularity 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., alkenes, which contain a C-C double bond, and alkynes, which have a C-C triple bond—and aldehydes and ketones, which have a C=O double bond. Types of Addition Reactions These reactions may be initiated by electrophiles or nucleophiles: 1. Electrophilic Addition reactions 2. Nucleophilic Addition reactions Electrophilic Addition reactions An electrophilic addition reaction 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 ethylene 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: Aldehyde 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 Carbonyl group 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 substituents are present in the structure of ketones which causes steric hindrance when the nucleophile approaches the carbonyl carbon.However, aldehydes contain one substituent and thus the steric hindrance to the approaching nucleophile is less. Moreover, electronically aldehydes demonstrate better reactivity than ketone. This is because ketones contain two
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