Ch 24 From Petroleum to Pharmaceuticals Classes of Hydrocarbons

HydrocarbonsHydrocarbons AromaticAromaticAliphaticAliphatic

HydrocarbonsHydrocarbons AromaticAromaticAliphaticAliphatic AlkanesAlkanes AlkynesAlkynes AlkenesAlkenes

HydrocarbonsHydrocarbons AliphaticAliphatic AlkanesAlkanes Alkanes are hydrocarbons in which all of the bonds are single bonds. C C H H H HHH

HydrocarbonsHydrocarbons AliphaticAliphatic AlkenesAlkenes Alkenes are hydrocarbons that contain a carbon- carbon double bond. C C H H HH

HydrocarbonsHydrocarbons AliphaticAliphatic AlkynesAlkynes Alkynes are hydrocarbons that contain a carbon- carbon triple bond. HCCH

HydrocarbonsHydrocarbons AromaticAromatic The most common aromatic hydrocarbons are those that contain a benzene ring. H H H HHH

Reactive Sites in Hydrocarbons The Functional Group Concept

Functional Group a structural unit in a molecule responsible for its characteristic chemical behavior and its spectroscopic characteristics

AlkanesAlkanes functional group is a hydrogen atom the reaction that takes place is termed a substitution one of the hydrogens is substituted by some other atom or group, X R—H R—X

AlkanesAlkanes functional group is a hydrogen the reaction that takes place is substitution one of the hydrogens is substituted by some other atom or group R—H R—X + Cl 2 C C H H H HHH + HCl C C H Cl H HHH

Functional Groups in Hydrocarbons alkanesRH alkenes RH, double bond alkynes RH, triple bond aromaticsArH, double bond

Some Key Functional Groups

Families of organic compounds and their functional groups AlcoholsROH Alkyl halidesRX (X = F, Cl, Br, I) Aminesprimary amine: RNH 2 secondary amine: R 2 NH secondary amine: R 2 NH tertiary amine: R 3 N tertiary amine: R 3 N EthersROR

Many classes of organic compounds contain a carbonyl group O C Carbonyl group O C Acyl group R

Many classes of organic compounds contain a carbonyl group O C Carbonyl group O C Aldehyde R H

Many classes of organic compounds contain a carbonyl group O C Carbonyl group O C Ketone R R'

Many classes of organic compounds contain a carbonyl group O C Carbonyl group O C Carboxylic acid R OH

Many classes of organic compounds contain a carbonyl group O C Carbonyl group O C Ester R OR'

Many classes of organic compounds contain a carbonyl group O C Carbonyl group O C Amide R NH 2

Introduction to Alkanes: Methane, CH 4 Ethane, C 2 H 6 Ethane, C 2 H 6 Propane, C 3 H 8 C n H 2n+2 General formula for an alkane

The simplest alkanes Methane (CH 4 ) CH 4 Ethane (C 2 H 6 )CH 3 CH 3 Propane (C 3 H 8 )CH 3 CH 2 CH 3 bp -160°C bp -89°C bp -42°C No isomers possible for C1, C2, C 3 hydrocarbons

Isomeric Alkanes: The Butanes C 4 H 10 General formula for any butane

n-Butane Isobutane CH 3 CH 2 CH 2 CH 3 (CH 3 ) 3 CH bp -0.4°C bp -10.2°C C 4 H 10

Higher n-Alkanes Pentane (C 5 H 12 ) and Beyond C n H 2n+2 n > 4

CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 n-Pentane n-Hexane CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 n-Heptane C n H 2n+2 n > 4

The C 5 H 12 Isomers

n-Pentane CH 3 CH 2 CH 2 CH 2 CH 3 Isopentane (CH 3 ) 2 CHCH 2 CH 3 Neopentane (CH 3 ) 4 C C 5 H 12

The number of isomeric alkanes increases as the number of carbons increase. There is no simple way to predict how many isomers there are for a particular molecular formula. How many isomers?

Table 2.3 Number of Constitutionally Isomeric Alkanes CH 4 1 C 2 H 6 1 C 3 H 8 1 C 4 H 10 2 C 5 H 12 3 C 6 H 14 5 C 7 H 16 9

Table 2.3 Number of Constitutionally Isomeric Alkanes CH 4 1 C 8 H C 2 H 6 1 C 9 H C 3 H 8 1 C 10 H C 4 H 10 2 C 15 H 32 4,347 C 5 H 12 3 C 20 H ,319 C 6 H 14 5 C 40 H 82 62,491,178,805,831 C 7 H 16 9

C 6 H 6 Isomers How many isomers with the composition C 6 H 6 can you draw?

C 6 H 6 Isomers: How many isomers with the composition C 6 H 6 can you draw?

Structure and Bonding in Alkenes

Structure of 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 Framework of  bonds Each carbon is sp 2 hybridized

Bonding in Ethylene Each carbon has a half-filled p orbital

Bonding in Ethylene Side-by-side overlap of half-filled p orbitals gives a  bond

Isomerism in Alkenes

IsomersIsomers Isomers are different compounds that have the same molecular formula (composition).

IsomersIsomers StereoisomersStereoisomers Constitutional isomers

IsomersIsomers StereoisomersStereoisomers different connectivity same connectivity; different arrangement of atoms in space

IsomersIsomers StereoisomersStereoisomers Constitutional isomers consider the isomeric alkenes of molecular formula C 4 H 8

2-Methylpropene 1-Butene cis-2-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C

2-Methylpropene 1-Butene cis-2-Butene C CHH H CH 2 CH 3 H CH 3 C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C Constitutional isomers

2-Methylpropene 1-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH C C H H H3CH3CH3CH3C H3CH3CH3CH3C Constitutional isomers

cis-2-Butene trans-2-Butene H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH Stereoisomers

Molecular Chirality: Enantiomers

A molecule is chiral if its two mirror image forms are not superposable upon one another. A molecule is achiral if its two mirror image forms are superposable. ChiralityChirality

Br Cl H F It cannot be superposed point for point on its mirror image. Bromochlorofluoromethane is chiral

Br Cl H F H Cl Br F To show nonsuperposability, rotate this model 180° around a vertical axis. Bromochlorofluoromethane is chiral

Br Cl H F H Cl Br F

Another look

are enantiomers with respect to each other and nonsuperposable mirror images are called enantiomers EnantiomersEnantiomers

stereoisomers constitutionalisomers Isomers

stereoisomers constitutionalisomers geometric isomers (cis/trans)enantiomers(optical) Isomers

Chlorodifluoromethane is achiral

The two structures are mirror images, but are not enantiomers, because they can be superposed on each other. Chlorodifluoromethane is achiral

Symmetry in Achiral Structures

Symmetry tests for achiral structures Any molecule with a plane of symmetry must be achiral.

A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry. Plane of symmetry

A plane of symmetry bisects a molecule into two mirror image halves. Chlorodifluoromethane has a plane of symmetry. Plane of symmetry

A plane of symmetry bisects a molecule into two mirror image halves. 1-Bromo-1-chloro-2-fluoroethene has a plane of symmetry. Plane of symmetry

A plane of symmetry bisects a molecule into two mirror image halves. 1-Bromo-1-chloro-2-fluoroethene has a plane of symmetry. Plane of symmetry

Physical Properties of Alkanes and Cycloalkanes

increase with increasing number of carbons more atoms, more electrons, more opportunities for induced dipole-induced dipole forces decrease with chain branching branched molecules are more compact with smaller surface area—fewer points of contact with other molecules Boiling Points

increase with increasing number of carbons more atoms, more electrons, more opportunities for induced dipole-induced dipole forces Boiling Points Heptane bp 98°C Octane bp 125°C Nonane bp 150°C

decrease with chain branching branched molecules are more compact with smaller surface area—fewer points of contact with other molecules Boiling Points Octane: bp 125°C 2-Methylheptane: bp 118°C 2,2,3,3-Tetramethylbutane: bp 107°C

Boiling Points of Alkanes governed by strength of intermolecular attractive forces alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent only forces of intermolecular attraction are induced dipole-induced dipole forces

Induced dipole-Induced dipole attractive forces + – + – two nonpolar molecules center of positive charge and center of negative charge coincide in each

Induced dipole-Induced dipole attractive forces + – + – movement of electrons creates an instantaneous dipole in one molecule (left)

Induced dipole-Induced dipole attractive forces + – + – temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)

Induced dipole-Induced dipole attractive forces + – + – temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)

Induced dipole-Induced dipole attractive forces + – + – the result is a small attractive force between the two molecules

Induced dipole-Induced dipole attractive forces + – + – the result is a small attractive force between the two molecules

Straight chain hydrocarbonBranched hydrocarbon Lots of intermolecular contacts Fewer intermolecular contacts