Vicinal Dihalide: Two Halogen Atoms Are Bonded to Adjacent Carbons
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DAMIETTA UNIVERSITY CHEM-103: BASIC ORGANIC CHEMISTRY LECTURE 4 Dr Ali El-Agamey Types of reactions 1- Addition reaction They normally involves unsaturated compounds capable of accepting additional atoms. 2- Substitution reaction Atom or group replaces atom or group. Types of reactions 3- Elimination reaction Atoms are removed to produce unsaturated compounds or ring. Organic Chemistry, 7th Edition L. G. Wade, Jr. Alkanes © 2010, Prentice Hall Hydrocarbons Hydrocarbons are molecules that are made of carbon and hydrogen ONLY. Alkanes • General formula: CnH2n+2 • Found in everything from natural gas to petroleum. • The smaller alkanes have very low boiling points (b.p.) therefore they are gases. • CH4 C2H6 C3H8 b.p. -160oC -89oC -42oC Preparation of Alkanes • 1- Hydrogenation of alkenes • 2- Alkyl halides (a) Via hydrolysis of Grignard reagent (b) Reduction by metal and acid (c) Coupling with organo-copper compounds. Preparation of Alkanes • (a) Hydrolysis of Grignard reagent Preparation of Alkanes • (b) Reduction by metal and acid Preparation of Alkanes • (c) Coupling with organo-copper compounds. i ' Preparation of Alkanes • (c) Coupling with organo-copper compounds. i Homework (1) Show how can you prepare n-butane from:1 (a) n-butyl bromide (b) sec-butyl bromide and (c) 2-butene. Complete the following equations:1 12 Chapter 8 Reactions of Alkanes • 1- Combustion • 2- Cracking (Pyrolysis) • 3- Halogenation – General reaction; examples – General mechanism; mechanism of specific example (CH4; propane) – Calculation of relative reactivities; Product ratios Reactions of Alkanes • 1- Combustion 2-Cracking (Pyrolysis) (3) Halogenation Reactivity X2: Cl2 > Br2 o o o H: 3 > 2 > 1 > CH3-H Examples Homework • 4.40 Among the isomeric alkanes of molecular formula C5H12 , identify the one that on photochemical chlorination yields (a) A single monochloride (b) Four isomeric monochlorides (c) Three isomeric monochlorides (d) Two isomeric dichlorides Examples • Bromination of alkanes is much more selective than chlorination.1 • What is the relative reacitivity of primary and secondary hydrogens in each reaction? 18 Chapter 4 Alkyl Halides ©2010, Prentice Hall Alkyl Halides Classification • Methyl halides: halide is attached to a methyl group. • Primary alkyl halide: carbon to which halogen is bonded is attached to only one other carbon. • Secondary alkyl halide : carbon to which halogen is bonded is attached to two other carbons. • Tertiary alkyl halide : carbon to which halogen is bonded is attached to three other carbon. Chapter 6 20 Primary, Secondary, Tertiary Alkyl Halides * * primary alkyl halide secondary alkyl halide * tertiary alkyl halide Chapter 6 21 Types of Dihalides • Geminal dihalide: two halogen atoms are bonded to the same carbon. geminal dihalide • Vicinal dihalide: two halogen atoms are bonded to adjacent carbons. vicinal dihalide Chapter 6 22 Write the IUPAC names of the following compounds Fluoroethane 1-Chlorobutane 2-Bromopropane Iodocyclohexane1-Chloro-3-methylcyclopentane 4-(2-Fluoroethyl)heptane Homework: Write the IUPAC name of the following compounds Homework: Write the structures for the following compounds (ii) 3-Bromo-2-methylpentane (iv) 2-Bromo-3-ethyl-2-methylhexane (v) 1,1-Dibromopropane Preparation of Alkyl halides Preparation of Alkyl halides Polarity and Reactivity • Halogens are more electronegative than C. • Carbon—halogen bond is polar, so carbon has partial positive charge. • Carbon can be attacked by a nucleophile. • Halogen can leave with the electron pair. Chapter 6 27 Reactions of Alkyl halides (A) Substitution Reactions • The halogen atom on the alkyl halide is replaced with a nucleophile (Nuc-). • Since the halogen is more electronegative than carbon, the C—X bond breaks heterolytically and X- leaves. Chapter 6 28 Reactions of Alkyl halides 29 Chapter 6 Structure and Synthesis of Alkenes ©2010, Prentice Hall Introduction • Alkenes are hydrocarbon with carbon-carbon double bonds. • Alkenes are also called olefins, meaning “oil- forming gas”. • The functional group of alkenes is the carbon-carbon double bond, which is reactive. Chapter 7 31 Sigma Bonds of Ethylene 32 Chapter 7 Bond Lengths and Angles • sp2 hybrid orbitals have more s character than the sp3 hybrid orbitals. • Pi overlap brings carbon atoms closer shortening the C—C bond from 1.54 Å in alkanes down to 1.33 Å in alkenes. 33 Chapter 7 Pi Bonding in Ethylene • The pi bond in ethylene is formed by overlap of the unhybridized p orbitals of the sp2 hybrid carbon atoms. • Each carbon has one unpaired electron in the p orbital. • This overlap requires the two ends of the molecule to be coplanar. Chapter 7 34 Physical Properties of Alkenes • Low boiling points, increasing with mass. • Branched alkenes have lower boiling points. • Less dense than water. • Slightly polar: – Pi bond is polarizable, so instantaneous dipole– dipole interactions occur. – Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment. Chapter 7 35 Disubstituted Isomers • Stability: cis < geminal < trans isomer • The less stable isomer has a higher exothermic heat of hydrogenation. cis-2-butene -120 kJ (CH ) C=CH iso-butene 3 2 2 -117 kJ trans-2-butene -116 kJ Chapter 7 36 Organic Chemistry, 7th Edition L. G. Wade, Jr. Syntheses and Reactions of Alkenes © 2010, Prentice Hall Preparation of Alkenes (1) Dehydrohalogenation of alkyl halide; (2) Dehalogenation of vicinal dihalides; (3) Dehydration of alcohols; (4) Reduction of Alkynes Preparation of Alkenes Main product Minor product Main product Minor product cis-But-2-ene trans-But-2-ene (1) Reactions of Alkenes Hydrogenation of Alkenes • Hydrogen (H2) can be added across the double bond in a process known as catalytic hydrogenation. • The reaction only takes place if a catalyst is used. The most commonly used catalysts are palladium (Pd), platinum (Pt), and nickel (Ni), but there are other metals that work just as well. • Syn addition of hydrogen. 41 Chapter 8 Mechanism of Catalytic Hydrogenation • The hydrogen and the alkene are adsorbed on the metal surface. • Once adsorbed, the hydrogens insert across the same face of the double bond and the reduced product is released from the metal. • The reaction has a syn stereochemistry since both hydrogens will add to the same side of the double bond. 42 Chapter 8 Influence of peroxides Markovnikov addition anti-Markovnikov addition Chapter 8 43 Problems Show how you would accomplish the following synthetic conversions: (a) Convert 1-methylcyclohexene to 1-bromo-1- methylcyclohexane. (b) Convert 1-methylcyclohexene to 1-bromo-2- methylcyclohexane. (c) Convert 1-methylcyclohexanol to 1-bromo-2- methylcyclohexane. Chapter 8 44 (2) Reactions of Alkenes anti-Markovnikov addition Halohydrin (BH3)2 is called diborane (1) Halohydrin formation; (2) Hydration (addition of water) (3) Reactions of Alkenes Epoxide 1,2-Diol 1,2-Diol (1) Ozonolysis; (2) Hydroxylation; (3) Epoxidation HCO2OH is called peroxyacid Solved Problem Ozonolysis of an unknown alkene gives an equimolar mixture of cyclohexanecarbaldehyde and 2-butanone. Determine the structure of the original alkene. 47 Chapter 8 Problem Ozonolysis of an unknown alkene gives compound A. Determine the structure of the original alkene. A 48 Chapter 8 Homework (1) What are the products of the ozonolysis of (a) 2-hexene (b) 3-hexene. (2) Ozonolysis of an unknown alkene gives acetone and formaldehyde. Determine the structure of the original alkene. 49 Chapter 8 Problems (1) Show how you would convert 1- methylcyclopentanol to 2-methylcyclopentanol. (2) Show how you would prepare 1-bromopropane from 2-bromopropane. (3) Show how you would prepare 1,2-epoxypropane from 2-propanol. Chapter 8 50 .