Vicinal Dihalide: Two Halogen Atoms Are Bonded to Adjacent Carbons

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

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.

' 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

(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

• 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

• 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

(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

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.

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