Lecture 5 - Alkenes & Alkynes

General

Organic Chemistry Two credits Second Semester 2009

King Saud bin Abdulaziz University for Health Science

Reference Book: Organic Chemistry: A Brief Course, by Robert C. Atkins and Francis A. Carey Third Edition

Instructor: Rabih O. Al-Kaysi, PhD.

Lecture 5

Chapter 4

Alkenes & Alkynes

Alkene Nomenclature

Alkenes Alkenes

Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by molecular formula CnH2n said to be "unsaturated"

Alkene AlkeneNomenclature Nomenclature

H2C

CH2

Ethene or Ethylene (both are acceptable IUPAC names)

H2C

CHCH3

Propene (Propylene is sometimes used but is not an acceptable IUPAC name)

Alkene AlkeneNomenclature Nomenclature H2C CHCH2CH3

1-Butene

1) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon.

Alkene AlkeneNomenclature Nomenclature H2CCHCHCH2Br CH3 4) If a substituent is present, identify its position by number. The double bond is first then alkyl groups and halogens when the chain is numbered. The compound shown above is 4-bromo-3-methyl-1-butene.

Alkene AlkeneNomenclature Nomenclature H2CCHCHCH2OH CH3 4) If a substituent is present, identify its position by number. Hydroxyl groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol.

Alkenyl Alkenylgroups groups methylene

H2C

vinyl

H2C

CH

allyl

H2C

CHCH2

isopropenyl

H2C

CCH3

Remember

Cycloalkene CycloalkeneNomenclature Nomenclature Cyclohexene

1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene.

Structure and Bonding in Alkenes

Structure Structureof ofEthylene Ethylene bond angles:

H-C-H = 117°

H-C-C = 121° bond distances:

C—H = 110 pm

C=C = 134 pm

planar

Bonding in Ethylene σ σ

σ

σ

σ

• Framework of σ bonds • Each carbon is sp2 hybridized

Bonding in Ethylene

•

Each carbon has a half-filled p orbital

Bonding in Ethylene

•

Side-by-side overlap of halffilled p orbitals gives a π bond

Isomerism in Alkenes

Isomers Isomers

Isomers are different compounds that have the same molecular formula.

Isomers Isomers

Constitutional Constitutionalisomers isomers

Stereoisomers Stereoisomers

Isomers Isomers

Constitutional Constitutionalisomers isomers

different connectivity

Stereoisomers Stereoisomers same connectivity; different arrangement of atoms in space

Isomers Isomers

Constitutional Constitutionalisomers isomers

Stereoisomers Stereoisomers

consider the isomeric alkenes of molecular formula C4H8

CH2CH3

H C

H 1-Butene

H3C

H

H C

C

H

C

H3C

H

2-Methylpropene CH3

C

H3C

H3C

H C

C H

cis-2-Butene

H

C CH3

trans-2-Butene

CH2CH3

H C

H 1-Butene

H3C

H C

C

H

H3C

C H

2-Methylpropene CH3

C H

H3C

C

Constitutional isomers H

cis-2-Butene

CH2CH3

H C

H3C C

C

H

H

H 1-Butene

C

H3C

H

2-Methylpropene H3C

H C

Constitutional isomers H

C CH3

trans-2-Butene

Stereoisomers H3C

CH3 C

H

H3C

H C

C H

cis-2-Butene

H

C CH3

trans-2-Butene

Stereochemical StereochemicalNotation Notation cis (identical or analogous substituents on same side)

trans (identical or analogous substitutents on opposite sides)

Naming Stereoisomeric Alkenes by the E-Z Notational System

Stereochemical StereochemicalNotation Notation CH2(CH2)6CO2H

CH3(CH2)6CH2 C H

C

Oleic acid H

cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond) cis and trans are ambiguous when analogies are not obvious

Cl Example Example

Br C

H

C F

What is needed: 1) 2)

systematic body of rules for ranking substituents new set of stereochemical symbols other than cis and trans

The TheE-Z E-ZNotational NotationalSystem System E : higher ranked substituents on opposite sides Z : higher ranked substituents on same side

higher

C lower

C

The TheE-Z E-ZNotational NotationalSystem System E : higher ranked substituents on opposite sides Z : higher ranked substituents on same side

lower

C

C higher

The TheE-Z E-ZNotational NotationalSystem System E : higher ranked substituents on opposite sides Z : higher ranked substituents on same side

higher

C lower

lower

C higher

Entgegen

The TheE-Z E-ZNotational NotationalSystem System E : higher ranked substituents on opposite sides Z : higher ranked substituents on same side

higher

C lower

lower

C higher

Entgegen

higher

C lower

higher

C lower

Zusammen

The TheE-Z E-ZNotational NotationalSystem System Question: How are substituents ranked? Answer: They are ranked in order of decreasing atomic number.

higher

C lower

lower

C higher

Entgegen

higher

C lower

higher

C lower

Zusammen

The TheCahn-Ingold-Prelog Cahn-Ingold-Prelog(CIP) (CIP)System System

The system that we use was devised by R. S. Cahn Sir Christopher Ingold Vladimir Prelog

CIP CIPRules Rules (1) Higher atomic number outranks lower atomic number Br > F

Cl > H

higher Br C lower

F

Cl

higher

H

lower

C

CIP CIPRules Rules (1) Higher atomic number outranks lower atomic number Br > F

Cl > H

higher Br C lower

F

Cl

higher

H

lower

C

(Z )-1-Bromo-2-chloro-1-fluoroethene * Do not remember the name just if it is E or Z

Physical Properties of Alkenes

Dipole Dipolemoments moments

H

What is direction of dipole moment? Does a methyl group donate electrons to the double bond, or does it withdraw them?

H C

C

H

H

µ =0D

H3C

H C

H

C

H µ = 0.3 D

µ = 1.4 D H H C H

H C

C

H

Cl Chlorine is electronegative and attracts electrons.

C

H

H

µ =0D

Dipole Dipolemoments moments

H3C

H C

H

C

H µ = 0.3 D

Dipole moment of 1chloropropene is equal to the sum of the dipole moments of vinyl chloride and propene.

µ = 1.4 D H H C

Dipole Dipolemoments moments

C

H

Cl

H3C

H C

H3C

H C

H

C

H µ = 0.3 D

C

Cl H µ = 1.7 D

µ = 1.4 D H H C Therefore, a methyl group donates electrons to the double bond.

Dipole Dipolemoments moments

C

H

Cl

H3C

H C

H

H3C

H C

H

C

H µ = 0.3 D

C Cl

µ = 1.7 D