Chapter 2 Alkanes and Cycloalkanes: Introduction to Hydrocarbons.

Chapter 2Chapter 2Alkanes and Cycloalkanes: Alkanes and Cycloalkanes:

Introduction to Introduction to HydrocarbonsHydrocarbons

Classes of HydrocarbonsClasses of Hydrocarbons

Hydrocarbons only contain carbon and hydrogen atoms.

Hydrocarbons are either classed as aliphatic or aromatic.

Aliphatic hydrocarbons contain three main groups: alkanes which only have carbon-carbon single bonds,

alkenes which have a carbon-carbon double bond, or alkynes which have a carbon-carbon triple bond.


Aromatic hydrocarbons are more complex but the simplest aromatic hydrocarbon is benzene. Aromatic hydrocarbons are called arenes.


Electron Waves andElectron Waves andChemical BondsChemical Bonds

The Lewis model of chemical bonding predates the idea

that electrons have wave properties.

Two widely used theories of bonding based on the wave

nature of an electron are:

Valence Bond Theory, and

Molecular Orbital Theory

Models for Chemical BondingModels for Chemical Bonding

Which electrostatic forces are involved as two hydrogen

atoms approach each other and form a H-H bond.

These electrostatic forces are:• attractions between the electrons and the nuclei• repulsions between the two nuclei • repulsions between the two electrons

+ e– + e–

Formation of HFormation of H22 from Two Hydrogen Atoms from Two Hydrogen Atoms

Potential

energyH• + H•

Internuclear distance

H H

weak net attraction at

long distances

Potential Energy vs Distance Potential Energy vs Distance Between Two Hydrogen Atoms Between Two Hydrogen Atoms

Potential

energyH• + H•


H H

H H

H H

attractive forces increase

faster than repulsive forces

as atoms approach each other


Potential

energyH• + H•

H2


74 pm

H H

H H

H H-436 kJ/mol

maximum net attraction

(minimum potential energy)

at 74 pm internuclear distance


Potential

energyH• + H•

H2


74 pm

H H

H H

H H-436 kJ/mol

repulsive forces increase

faster than attractive forces

at distances closer than 74 pm


Valence Bond Theory

constructive interference between two half-filled atomic

orbitals is basis of shared-electron bond

Molecular Orbital Theory

derive wave functions of molecules

by combining wave functions of atoms

Models for Chemical BondingModels for Chemical Bonding

Waves interactions include:

Constructive interference when the waves are in phase

and reinforce each other

Destructive interference when the waves are out of

phase and oppose each other

Behavior of WavesBehavior of Waves

Electron pair can be shared when half-filled orbital of

one atom overlaps in phase with half-filled orbital of

another. For example with overlap of two 1s orbitals

of two hydrogen atoms shown below:

Valence Bond Model for Valence Bond Model for Bonding in HydrogenBonding in Hydrogen

The approach of the two hydrogen atoms can be modeled

showing electrostatic potential maps. The high electron

density between the nuclei is apparent.

Valence Bond ModelValence Bond Model

Electrons feel the

attractive force of the

protons

Orbitals begin

to overlap

Optimal distance

between nucleiHigh electron density

between the nuclei

The Sigma (The Sigma () Bond) Bond

A bond in which the orbitals overlap along a line

connecting the atoms is called a sigma (s) bond.

Two perpendicular views are shown below.

Electrons in molecules occupy molecular orbitals (MOs)

just as electrons in an atom occupy atomic orbitals (AOs).

MOs are combinations of AOs.

Two electrons per MO.

The additive combination of two atomic orbitals

generates one bonding orbital.

The subtractive combination of the two atomic orbitals

generates an antibonding orbital.

Bonding in HBonding in H22::

The Molecular Orbital ModelThe Molecular Orbital Model

Addition of the AOs to form the bonding MO ()

Molecular Orbital Model for HMolecular Orbital Model for H22

Subtraction of the AOs to form the antibonding MO (*)

Format is AOs on the sides and MOs in the middle.

Combination of n AOs results in n MOs.

Bonding MOs lower in energy than antibonding MOs.

Fill electrons in MOs the same as for AOs – lowest first.

Molecular Orbital DigramsMolecular Orbital Digrams

Energy-Level Diagram for HEnergy-Level Diagram for H22 MOs MOs

Introduction to Alkanes:Introduction to Alkanes:Methane, Ethane, and PropaneMethane, Ethane, and Propane

Small AlkanesSmall Alkanes

General formula for alkanes is CnH2n+2.

Smallest alkane is methane CH4 - also the most abundant.

Ethane (C2H6) and propane (C3H8) are the next alkanes.

Natural gas is 75% methane 10% ethane and 5% propane.

These alkanes have the lowest boiling points.

Structures of AlkanesStructures of Alkanes

All carbons in methane, ethane and propane have four

bonds.

Bond angles (which are close to 109.5o) and bond lengths

are:

spsp33 Hybridization Hybridizationandand

Bonding in MethaneBonding in Methane

The dilemma:

Methane has tetrahedral geometry.

This is inconsistent with electron configuration of carbon of1s2, 2s2, 2px

1,2py1 with only two unfilled orbitals.

Structure and Bonding TheoryStructure and Bonding Theory

spsp33 Hybrid Orbitals Hybrid Orbitals

Linus Pauling proposed a mixing or hybridization of the s

and three p orbitals to create 4 equal unfilled orbitals called

sp3 orbitals.

Properties of Properties of spsp33 Hybrid Orbitals Hybrid Orbitals

All four sp3 orbitals are of equal energy.

The axes of the sp3 orbitals point toward the corners of a

tetrahedron.

σ Bonds involving sp3 hybrid orbitals of carbon are

stronger than those involving unhybridized

2s or 2p orbitals.

Bonding with Bonding with spsp33 Hybrid Orbitals Hybrid Orbitals

Bonding in methane involves orbital overlap between each

partially filled carbon sp3 orbital and a partially filled s

orbital of the hydrogen atom.

Bonding and Structure of EthaneBonding and Structure of Ethane

Ethane also has tetrahedral geometry about the

carbon

atoms.

Hybridization can be used to rationalize the bonding.

The C-H bonds are formed as described for methane.

The C-C bond is formed by overlap of sp3 orbitals on

each of the carbon atoms.

C-C Bond Formation in EthaneC-C Bond Formation in Ethane

Two half-filled sp3

orbitals on each C

Electrons with

opposite spin

Overlap of orbitals

to form a bonding

orbital.

Structure of Ethylene and sp2 HybridizationStructure of Ethylene and sp2 Hybridization

Ethylene is planar with bond angles close to 120o.

sp3 Hybridization cannot be used to explain this

bonding.

Three atoms are bonded to each carbon so three

hybrid

orbitals are formed. Called sp2 orbitals.

One p orbital is not hybridized.

spsp22 Hybrid Orbitals Hybrid Orbitals

The 2s and two of the 2p orbitals are mixed to form

three sp2 orbitals with a trigonal planar arrangement.The 2pz orbital remains half filled.

Sigma (Sigma () Bonding in Ethylene) Bonding in Ethylene

Form C-H bonds

by overlap of sp2

and s orbitals

These are all sigma () bonds. An unfilled p

orbital remains on each carbon atom.

Form C-C bond

by overlap of sp2

orbitals on each

carbon

Pi (Pi () Bonding in Ethylene) Bonding in Ethylene

This called a pi () bond and the electrons in the

bond are called electrons.

Form second C-C

bond by overlap

of p orbitals on

each carbon

Structure of Acetylene and sp HybridizationStructure of Acetylene and sp Hybridization

Acetylene is linear with bond angles of 180o.

sp3 and sp2 Hybridization cannot explain this bonding.

sp Hybridization explains this. There are two half filled

p orbitals no hybridized.

spsp Hybrid Orbitals Hybrid Orbitals

The 2s and one of the 2p orbitals are mixed to form

two sp orbitals with a linear arrangement. The 2py and 2pz orbitals remain half filled.

Sigma (Sigma () Bonding in Acetylene) Bonding in Acetylene

Form C-H bonds

by overlap of sp

and s orbitals

These are all sigma () bonds. Two unfilled p

orbitals remain on each carbon atom.

Form C-C bond

by overlap of sp

orbitals on each

carbon

Pi (Pi () Bonding in Acetylene) Bonding in Acetylene

There are two pi () bonds and a total of 4 electrons.

Form one bond by overlap of py orbitals

on each carbon

Form second

bond by overlap of pz orbitals on each

carbon

Hybridization of CarbonHybridization of Carbon

Carbons bonded to four atoms are sp3 hybridized with

bond angles of approximately 109.5o.

Carbons bonded to three atoms are sp2 hybridized with

bond angles of approximately 1200 and one C-C -bond.

Carbons bonded to two atoms are sp hybridized with

bond angles of approximately 1800 and two C-C -bonds.

Which Theory of Which Theory of Chemical Bonding Is Best?Chemical Bonding Is Best?

Theories of Chemical BondingTheories of Chemical Bonding

Approaches to chemical bonding:

1.Lewis model;

2.Orbital hybridization model;

3.Molecular orbital model.

Considerations of Chemical BondingConsiderations of Chemical Bonding

Lewis and Orbital hybridization models work together

and success in organic depends on writing correct Lewis

structures.

Molecular orbital theory provides insights into structure

and reactivity lacking in the other models. This model

requires higher level theory which will not be presented.

The results of MO theory will be used – for example

electrostatic potential maps.

Isomers of ButaneIsomers of Butane

There is only one isomer for each of the molecular formulas CH4, C2H6 and C3H8.

For C4H10 there are two distinct connectivities of the

carbon atoms. They are constitutional isomers.

H

H

C

H

C

H

C

H

C

HH HH

HH

H

CH

C

H

CH

C

H HH

H

H

Bondline

formulas

Isomers of ButaneIsomers of Butane

The isomers have different physical properties.

All carbon atoms are sp3 hybridized.

Higher n-AlkanesHigher n-Alkanes

n-Alkanes are straight-chain alkanes with general formula CH3(CH2)nCH3. n-Pentane is CH3CH2CH2CH2CH3 and

n-hexane is CH3CH2CH2CH2CH2CH3. These formulas can be

abbreviated as CH3(CH2)3CH3 or CH3(CH2)4CH3.

Isomers of CIsomers of C55HH1212

There are three isomers C5H12.

It is important to realize that these are all representations

of isopentane.

Isomers of higher n-alkanesIsomers of higher n-alkanes

For higher n-alkanes there are many isomers and it is not

possible to easily predict how many isomers can be

formed.

IUPAC Nomenclature ofIUPAC Nomenclature ofUnbranched AlkanesUnbranched Alkanes

IUPAC Naming IUPAC Naming

Alkane names are the basis of the IUPAC system of

nomenclature. The –ane suffix is specific to alkanes.

The IUPAC Rules for Branched AlkanesThe IUPAC Rules for Branched Alkanes

Rules for naming branched alkanes:

1.Find the longest continuous carbon chain and its IUPAC name. This is the parent alkane.

2.Identify the substituents on this chain.

substituent

longest chain

(5 carbons)



3. Number the longest continuous chain in the direction that gives the lowest number to the first substituent.

4. Write the name of the compound. The parent alkane is the last part of the name and is preceded by the names of the substituents and their numerical locations (locants). Hyphens separate the locants from the words.

2-methylpentane



5.When the same substituent appears more than once,

use the multiplying prefixes di-, tri-, tetra-, and so on. A separate locant for each substituent. Locants are separated from each other by commas and from the words by hyphens.

2,2-dimethylbutane 2,3-dimethylbutane

Alkyl GroupsAlkyl Groups

Alkyl groups are substituents derived from alkanes.

They lack one hydrogen at the point of attachment.

The alkyl group is named from the alkane by replacing the

-ane suffix with –yl.

For example a CH3CH2CH2CH2- substituent is a butyl group.

Classification of Carbon AtomsClassification of Carbon Atoms

Carbon atoms are defined as primary, secondary, tertiary

or quaternary.

A primary carbon is directly attached to one other carbon.

A secondary carbon is directly attached to two other

carbons. A tertiary carbon to 3 and a quaternary carbon

to 4.

Complex Alkyl Groups (Substituents)Complex Alkyl Groups (Substituents)

Secondary and tertiary groups may have common names

and IUPAC names.

The base name of these groups is the longest chain

including the attachment carbon form and the

substituents are located on this chain.

Naming Highly Branched AlkanesNaming Highly Branched Alkanes

When two or more different substituents are present

number from the end closest to the first point of difference.

When two or more different substituents are present, they

are listed in alphabetical order in the name. Prefixes such

as di-, tri-, and tetra- are used but ignored when alphabetizing.

tert-Butyl precedes isobutyl. sec-Butyl precedes tert-butyl.

4-ethyl-3,5-dimethyloctane

Naming Highly Branched AlkanesNaming Highly Branched Alkanes

When two or more different substituents are present

number from the end closest to the first point of difference.

If the first substituent is located an equal distance from each

end then the second substituent becomes the first potential

point of difference and so on.

Naming CycloalkanesNaming Cycloalkanes

Cycloalkanes contain a ring of carbons and have general formula CnH2n.

Add the prefix cyclo- to the name of the corresponding

alkane.


Identify and name substituents as before.

For one substituent no numbers are used.


For multiple substituents the locations must be specified.

Number the carbon atoms of the ring in the direction that

gives the lowest number to the substituents at the first

point of difference.

First substituent is on C1 by default.


If the ring has fewer carbons than the alkyl group attached

to it then the ring is the substituent.

Sources of Alkanes and CycloalkanesSources of Alkanes and Cycloalkanes

Natural is mainly methane with ethane and propane.

Petroleum is a liquid mixture containing approximately 150

hydrocarbons. Half of these are alkanes or cycloalkanes.

Distillation of crude oil gives fractions based on boiling point.

Petroleum RefiningPetroleum Refining

The yield of the more useful petroleum fraction used as

automotive fuel is increased by two processes:

Cracking. Cracking is the cleavage of carbon–carbon bonds

in high molecular weight alkanes induced by heat (thermal

cracking) or with catalysts (catalytic cracking).

Reforming. Reforming converts the hydrocarbons in

petroleum to aromatic hydrocarbons and highly branched

alkanes, both of which are better automotive fuels than

unbranched alkanes and cycloalkanes.

Other Natural Sources of AlkanesOther Natural Sources of Alkanes

Solid n-alkanes are waxy and coat the outer surface of many

living things to prevent loss of water. Examples include:Pentacosane (CH3(CH2)23CH3 is found in the waxy outer

layer of many insects.

Hentriacontane is a component of beeswax and the outer

layer of leaves of tobacco, peach trees and others.

Hopanes are found in petroleum

and geologic sediments.

Physical Properties of AlkanesPhysical Properties of Alkanesand Cycloalkanesand Cycloalkanes

Boiling PointBoiling Point

Boiling points of n-alkanes increase with increasing

molecular weight (number of carbons).

Branched alkanes generally have lower boiling points than

unbranched alkanes with the same number of carbons.

Intermolecular Forces and Boiling PointIntermolecular Forces and Boiling Point

Attractive forces between molecules in the liquid phase

affect the boiling point of the liquid.

These Intermolecular forces are van der Waals forces and

may be divided into three types:

Dipole-dipole (including hydrogen bonding);

Induced dipole-dipole; or

Induced dipole-induced dipole.

Intermolecular Forces and AlkanesIntermolecular Forces and Alkanes

Alkanes have no dipole so the van der Waals forces are the

temporary induced dipole-induced dipole.

This interaction is dynamic and fluctuates.


Long chain alkanes have more induced dipole-induced

dipole interactions so the boiling point increases with

increasing chain length.


Branched alkanes have lower surface area than isomeric n-

alkanes and therefore have lower boiling points.

Melting PointMelting Point

Solid n-alkanes are soft low melting solids. The same

intermolecular forces hold the molecules together in the

solid state.

Solubility of Alkanes in WaterSolubility of Alkanes in Water

Alkanes (and all hydrocarbons) are virtually insoluble in

water and are said to be hydrophobic.

The densities of most alkanes are in the range 0.6-0.8

g/mL therefore alkanes float on the surface of water.

Chemical Properties of Chemical Properties of AlkanesAlkanes

and Cycloalkanesand Cycloalkanes

Acidity of HydrocarbonsAcidity of Hydrocarbons

Hydrocarbons are very weak acids. Alkynes have the Hydrocarbons are very weak acids. Alkynes have the

lowest pKa.lowest pKa.

Hydrocarbons are very weak acids. Alkynes have the Hydrocarbons are very weak acids. Alkynes have the

lowest pKa.lowest pKa.

Combustion of HydrocarbonsCombustion of Hydrocarbons

Combustion of hydrocarbons is exothermic generating Combustion of hydrocarbons is exothermic generating COCO22 and water. and water.Combustion of hydrocarbons is exothermic generating Combustion of hydrocarbons is exothermic generating COCO22 and water. and water.

Combustion of Relative StabilityCombustion of Relative Stability

All isomers of CAll isomers of C88HH1818 generate 8 molecules of CO generate 8 molecules of CO2 2 and 9 and 9

of Hof H22O yet different amounts of energy. This energy O yet different amounts of energy. This energy

difference must be directly related to the relative energies difference must be directly related to the relative energies

of the isomers.of the isomers.

All isomers of CAll isomers of C88HH1818 generate 8 molecules of CO generate 8 molecules of CO2 2 and 9 and 9

of Hof H22O yet different amounts of energy. This energy O yet different amounts of energy. This energy

difference must be directly related to the relative energies difference must be directly related to the relative energies

of the isomers.of the isomers.

Most stable

isomer

Most stable

isomer

Least stable

isomer

Least stable

isomer

Least energy

released

Oxidation and Reduction in Organic ChemistryOxidation and Reduction in Organic Chemistry

Assuming the oxidation state of H is +1 and O is -2 it is Assuming the oxidation state of H is +1 and O is -2 it is

possible to calculate the oxidation state of C in possible to calculate the oxidation state of C in

compounds containing C, H and O.compounds containing C, H and O.

Assuming the oxidation state of H is +1 and O is -2 it is Assuming the oxidation state of H is +1 and O is -2 it is

possible to calculate the oxidation state of C in possible to calculate the oxidation state of C in

compounds containing C, H and O.compounds containing C, H and O.


Oxidation of carbon corresponds to an increase in the Oxidation of carbon corresponds to an increase in the

number of bonds between carbon and oxygen or to a number of bonds between carbon and oxygen or to a

decrease in the number of carbon–hydrogen bonds. decrease in the number of carbon–hydrogen bonds.

Reduction corresponds to an increase in the number of Reduction corresponds to an increase in the number of

carbon–hydrogen bonds or to a decrease in the number carbon–hydrogen bonds or to a decrease in the number

of carbon–oxygen bonds.of carbon–oxygen bonds.

Oxidation of carbon corresponds to an increase in the Oxidation of carbon corresponds to an increase in the

number of bonds between carbon and oxygen or to a number of bonds between carbon and oxygen or to a

decrease in the number of carbon–hydrogen bonds. decrease in the number of carbon–hydrogen bonds.

Reduction corresponds to an increase in the number of Reduction corresponds to an increase in the number of

carbon–hydrogen bonds or to a decrease in the number carbon–hydrogen bonds or to a decrease in the number

of carbon–oxygen bonds.of carbon–oxygen bonds.


Any element more electronegative than C has the same Any element more electronegative than C has the same

effect as O on the oxidation state of C. effect as O on the oxidation state of C.

Oxidation state of C is +2 in CHOxidation state of C is +2 in CH33Cl and CHCl and CH33OH.OH.

Any element less electronegative than C has the same Any element less electronegative than C has the same

effect as H on the oxidation state of C.effect as H on the oxidation state of C.

Oxidation state of C is -4 in CHOxidation state of C is -4 in CH44 and CH and CH33Li.Li.

Any element more electronegative than C has the same Any element more electronegative than C has the same

effect as O on the oxidation state of C. effect as O on the oxidation state of C.

Oxidation state of C is +2 in CHOxidation state of C is +2 in CH33Cl and CHCl and CH33OH.OH.

Any element less electronegative than C has the same Any element less electronegative than C has the same

effect as H on the oxidation state of C.effect as H on the oxidation state of C.

Oxidation state of C is -4 in CHOxidation state of C is -4 in CH44 and CH and CH33Li.Li.

Chapter 2 Alkanes and Cycloalkanes: Introduction to Hydrocarbons.

Documents

pm h h h h h h

hh bond

hydrogen slide

hydrogen atoms

chemical bonding slide

behavior of waves slide

carboncarbon double

carboncarbon triple