Organic Chemistry (BT 304) PDF

ORGANIC CHEMISTRY (BT 304) Course Co-ordinator Dr Mahera Moin Assistant Professor Dow College of Biotechnology Dow University of Health Sciences 1 chapter 6 chemistry of haloalkanes 2 haloalkanes...

ORGANIC CHEMISTRY (BT 304) Course Co-ordinator Dr Mahera Moin Assistant Professor Dow College of Biotechnology Dow University of Health Sciences 1 chapter 6 chemistry of haloalkanes 2 haloalkanes 3  Introduction Classification of Halogenated Organic Compounds Halogenated Organic Compounds Haloalkanes Vinyl Halides Aryl Halides 4  Introduction Haloalkanes: Organic molecules containing a halogen atom bonded to an sp3 hybridised carbon of the alkyl group 5 Introduction- Some Important Haloalkanes Cab Ca 6  Synthesis of Haloalkanes Haloalkanes are synthesised by the following methods Synthesis of Haloalkanes 1. From Alkenes 2. From Alkynes 3. From Alcohols Addition of Halogen a. Addition of Acids ( Refer to the Addition of Halogen Halogens Mode of Action of Acids Alkynes) b. Addition of Halogen Acids( Refer to the Mode of Action of Alkenes) 7  Synthesis of Haloalkanes 1. Preparation Of Haloalkanes From Alkenes a. Addition of Halogens Halogens add to alkenes to form vicinal dihalides. 8  Synthesis of Haloalkanes 1. Preparation Of Haloalkanes From Alkenes Mechanism of Halogen Addition s enem e Neither bromine nor ethylene is a polar molecule, but both are polarizable, and an induced-dipole/induced-dipole force causes them to be mutually attracted to each other. This induced-dipole/induced-dipole attraction sets the stage for Br2 to act as an electrophile. Electrons flow from the system of ethylene = to Br2, causing the weak bromine–bromine bond to break. 9  Synthesis of Haloalkanes 1. Preparation Of Haloalkanes From Alkenes Mechanism of Halogen Addition Step 1: Reaction of ethylene and bromine to form a bromonium ion intermediate: Step 2: Nucleophilic attack of bromide anion on the bromonium ion: 10  Synthesis of Haloalkanes 1. Preparation Of Haloalkanes From Alkenes Mechanism of Halogen Addition- Important Features Stereochemistry Of Halogen Addition The reaction of chlorine and bromine with cycloalkenes illustrates an important stereochemical feature of halogen addition. Anti addition is observed; the two bromine atoms of Br2 or the two chlorines of Cl2 add to opposite faces of the double bond 11  Synthesis of Haloalkanes 1. Preparation Of Haloalkanes From Alkenes Mechanism of Halogen Addition Practice Problems 12  Synthesis of Haloalkanes 3. Preparation Of Alkyl Halides From Alcohols And Hydrogen Halides The reaction of an alcohol with a hydrogen halide is a substitution. A halogen, usually chlorine or bromine, replaces a hydroxyl group as a substituent on carbon. Protonation converts the hydroxyl group from a poor leaving group (OH-) to a good leaving group (H2O). The order of reactivity of the hydrogen halides is: Among the various classes of alcohols, tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive 13  Synthesis of Haloalkanes Practice Problems 14  Mode of Action of Haloalkanes Basic Terminology Reaction Mechanism Nucleophile and Electrophile Nucleophilicity Substrate and Leaving Group Inversion of Configuration 15  Mode of Action of Haloalkanes Organic Reaction Mechanism In organic chemistry the arrows tells us how and where the electrons are moving during the course of a reaction. This process is called the reaction’s reaction mechanism Types of Organic Reaction Mechanisms 1. Ionic or Heterolytic Reaction Mechanisms ( 2 Electrons) Double barbed arrow shows the movement of 2 electrons 2. Homolytic Reaction Mechanisms ( 1 Electron) Single barbed arrow shows the movement of 1 electron 16  Mode of Action of Haloalkanes Nucleophile and Electrophile & Nucleopilicity Nucleophilic strength, or nucleophilicity, is a measure of how fast a Lewis base displaces a leaving group from a suitable substrate 17  Mode of Action of Haloalkanes Nucleophile and Electrophile Nucleophiles possess a negative charge or have a lone pair of electrons or have a pi bond Electrophiles possess a positive charge or have an empty p orbital 18  Mode of Action of Haloalkanes Practice Question: Identify each of the circled atom as electrophile, nucleophile or neither ri ni E Ni e T z N ~ 1. Neither 2. Electrophile 3. Nucleophile 4. Nucleophile - ~ 5. Nucleophile ~ 6. Nucleophile 7. Electrophile - 8. Electrophile - ~ 19  Mode of Action of Haloalkanes Substrate and Leaving Groups Substrate: A substrate is a chemical entity that contains a leaving group Leaving Group: Groups that best stabilize a negative charge are the best leaving groups. The weakest bases are most stable as anions and are the best leaving groups leave to wit -Eart 20  Mode of Action of Haloalkanes The Inversion Of Configuration: It resembles the way an umbrella turns inside out in the wind. It happens because the nucleophile attacks from the back side ( the side opposite to the leaving group) and turns the tetrahedron of the carbon inside out 21 Mode of Action of Haloalkanes 22  Mode of Action of Haloalkanes In the Nucleophilic substitution reactions of alkyl halides the carbon–halogen bond of the alkyl halide is broken heterolytically and the pair of electrons in that bond are lost with the leaving group 23 Mode of Action of Haloalkanes 24  Mode of Action of Haloalkanes The term “kinetics” refers to how the rate of a reaction varies with changes in concentration. Consider the nucleophilic substitution in which sodium hydroxide reacts with methyl bromide to form methyl alcohol and sodium bromide: 25  Mode of Action of Haloalkanes Hughes and Ingold observed that the hydrolysis of tert-butyl bromide, which occurs readily, is characterized by a first-order rate law: They found that the rate of hydrolysis depends only on the concentration of tert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. 26  Mode of Action of Haloalkanes The SN1 mechanism for hydrolysis of tertbutylbromide 27  Mode of Action of Haloalkanes The SN1 mechanism for hydrolysis of tertbutylbromide 28  Mode of Action of Haloalkanes FUNCTIONAL GROUP TRANSFORMATION BY NUCLEOPHILIC SUBSTITUTION The anionic portion of the salt substitutes for the halogen of an alkyl halide. The metal cation portion becomes a lithium, sodium, or potassium halide 29  Mode of Action of Haloalkanes FUNCTIONAL GROUP TRANSFORMATION BY NUCLEOPHILIC SUBSTITUTION Practice Question: Complete the following functional group transformations using the nucleophilic substitution of alkyl halides 30  Mode of Action of Haloalkanes FUNCTIONAL GROUP TRANSFORMATION BY NUCLEOPHILIC SUBSTITUTION Practice Question: Complete the following functional group transformations using the nucleophilic substitution of alkyl halides 31  Reference Francis A. Carey 32 Thank You 33