1 Fall, 2009 Organic Chemistry I Alkanes Organic Chemistry I Alkanes Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

11Fall, 2009Fall, 2009

Organic Chemistry IOrganic Chemistry IAlkanesAlkanes

Dr. Ralph C. GatroneDr. Ralph C. GatroneDepartment of Chemistry and PhysicsDepartment of Chemistry and Physics

Virginia State UniversityVirginia State University

Fall, 2009Fall, 2009 22

ObjectivesObjectives

• NomenclatureNomenclature

• IsomerizationIsomerization

• ConformationConformation

• ReactionsReactions

Fall, 2009Fall, 2009 33

The AlkanesThe Alkanes

• Paraffin hydrocarbonsParaffin hydrocarbons

• Aliphatic hydrocarbonsAliphatic hydrocarbons

• Saturated hydrocarbonsSaturated hydrocarbons

• Class only contains C and HClass only contains C and H

• C has 4 bondsC has 4 bonds

• Every bond is a sigma bond to a C or Every bond is a sigma bond to a C or HH

Fall, 2009Fall, 2009 44

AlkanesAlkanes

General formula is CGeneral formula is CnnHH2n+22n+2

Know names of CHKnow names of CH44 to C to C1212HH2626 (see table on next (see table on next slide)slide)

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CarbonsCarbons NameName (C(CnnHH22nn+2+2))

11 MethaneMethane CHCH44

22 EthaneEthane CC22HH66

33 PropanePropane CC33HH88

44 ButaneButane CC44HH1010

55 PentanePentane CC55HH1212

66 HexaneHexane CC66HH1414

77 HeptaneHeptane CC77HH1616

88 OctaneOctane CC88HH1818

99 NonaneNonane CC99HH2020

1010 DecaneDecane CC1010HH2222

1111 UndecaneUndecane CC1111HH2424

1212 DodecaneDodecane CC1212HH2626

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Methane, Ethane, PropaneMethane, Ethane, Propane

H

H

H

H

H

H

H

H

H

H H

H

H

H

H

H

H

H H

H

H

H

H

HH

H

Only one compound regardless of point of Only one compound regardless of point of attachmentattachment

Consider butaneConsider butane

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ButaneButane

• CC44HH1010

H

H

H

H

H

H

H

H

H

H H

H H H

H

HHH HH

n-butaneisobutane

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PentanePentane

• CC55HH1212

CH3CH3

CH3

CH3CH2CH2CH2CH3 CH3CH2CHCH3 CH3CCH3

n-pentane isopentane neopentane

Isomerism: constitutional isomersIsomerism: constitutional isomers

Same chemical formula different atom connectionsSame chemical formula different atom connections

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Alkane IsomersAlkane Isomers

• Constitutional isomersConstitutional isomers

• Straight chain alkanesStraight chain alkanes

• Branched chain alkanesBranched chain alkanes

• Number of possible isomers increases Number of possible isomers increases with n (number of carbon atoms)with n (number of carbon atoms)

• CC66 has 5 isomers; C has 5 isomers; C88 has 18 isomers has 18 isomers

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Constitutional IsomersConstitutional Isomers

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PentanePentane

CH3CH3

CH3

CH3CH2CH2CH2CH3 CH3CH2CHCH3 CH3CCH3

n-pentane isopentane neopentane

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Alkyl GroupsAlkyl Groups

• portion of an alkane moleculeportion of an alkane molecule• remove one H from an alkaneremove one H from an alkane• general abbreviation “R” (for Radical)general abbreviation “R” (for Radical)• an incomplete species or the “rest” of the moleculean incomplete species or the “rest” of the molecule• Nomenclature of Alkyl GroupsNomenclature of Alkyl Groups: : • replace -replace -aneane ending of alkane with - ending of alkane with -ylyl ending ending• CHCH33 is “methyl” (from methane) is “methyl” (from methane)• CHCH22CHCH33 is “ethyl” from ethane is “ethyl” from ethane• must know name and structure ofmust know name and structure of• propyl, isopropyl, butyl, sec-butyl, isobutyl, and propyl, isopropyl, butyl, sec-butyl, isobutyl, and • tert-butyl (see next slide)tert-butyl (see next slide)

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Alkyl GroupsAlkyl Groups

CH3

CH3

CH3

CH3

CH3

CH3- CH3CH2- CH3CH2CH2- CH3CHCH3

methyl ethyl propyl isopropyl

CH3CH2CH2CH2- CH3CH2CHCH3 HCCH2

butylsec-butyl isobutyl tert-butyl

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Note on Alkyl GroupsNote on Alkyl Groups

• Type of C is based on number of C’s Type of C is based on number of C’s bonded to itbonded to it

• Type of H is based on type of CType of H is based on type of C

Fall, 2009Fall, 2009 1515

Drawing Organic StructuresDrawing Organic Structures

CH3CH2CH2CH2CH2CH2CH3

heptane or n-heptane heptane or n-heptane

Where a line ends: CHWhere a line ends: CH33

Where two lines meet: CHWhere two lines meet: CH22

Hydrogens are not written.Hydrogens are not written.

Every C has Every C has 44, repeat, , repeat, 4 bonds4 bonds

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Alkane NomenclatureAlkane Nomenclature

• Name is based uponName is based upon• Prefix-Parent-SuffixPrefix-Parent-Suffix• RulesRules• Name the longest possible chainName the longest possible chain• Number the carbons in the longest chainNumber the carbons in the longest chain• Numbers start at closest branching pointNumbers start at closest branching point• Substituents are numbered at their point of attachmentSubstituents are numbered at their point of attachment• Substituents are named as alkyl groupSubstituents are named as alkyl group• Molecule is named as a single wordMolecule is named as a single word• Substituents are placed alphabeticallySubstituents are placed alphabetically

• Complex substituents are named as compounds would beComplex substituents are named as compounds would be• We will do some specific examplesWe will do some specific examples

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ExampleExample

1: Find the longest chain1: Find the longest chain

2. Number the longest chain2. Number the longest chain

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ExampleExample

12

34

56

7

8

910

11

11 carbons = undecane

Number 1 is nearest closest branch (C2)Number 1 is nearest closest branch (C2)

Identify substituentsIdentify substituents

Name and number substituentsName and number substituents

Using position number on the longest Using position number on the longest chainchain

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ExampleExample

12

34

56

7

8

910

11


C-2 has methyl group 2-methyl

C-5 has isopropyl group 5-isopropyl

C-8 has ethyl group 8-ethyl

Assemble nameAssemble name

Substituents are placed in name Substituents are placed in name alphabeticallyalphabetically

Fall, 2009Fall, 2009 2020

ExampleExample

12

356

7

8

910

11



C-5 has isopropyl group 5-isopropyl


8-ethyl-5-isopropyl-2-methylundecane

Note: isopropyl is placed using the Note: isopropyl is placed using the letter iletter i

Also true for isobutylAlso true for isobutyl

sec-butyl and t-butyl are placed using sec-butyl and t-butyl are placed using the letter bthe letter b

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Using Complex SubstituentUsing Complex Substituent

• An exampleAn example

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ExampleExample

1: Find the longest chain1: Find the longest chain

2. Number the longest chain2. Number the longest chain

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ExampleExample

12

34

56

7

8

910

11


Identify substituentsName using position on longest chain

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ExampleExample

12

34

56

7

8

910

11



C-5 has a complex substituent


How do we name the complex How do we name the complex substituent?substituent?

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ExampleExample

12





5-(1-methylethyl)-


Place substituents alphabeticallyPlace substituents alphabetically

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ExampleExample1

2





5-(1-methylethyl)-

8-ethyl-2-methyl-5-(1-methylethyl)-undecane

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Example 2Example 2

Find longest chainFind longest chain

Number from closest branchNumber from closest branch

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Example 2Example 2

123

45

67

8

8 carbons = octane

2-methyl

3-isopropyl

4-isopropyl

6-methyl

6-methyl


3,4-diisopropyl-2,6,6-trimethyloctane3,4-diisopropyl-2,6,6-trimethyloctane

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Combining SubstituentsCombining Substituents

• Substituents are combinedSubstituents are combined

• di = two, tri = three, tetra = fourdi = two, tri = three, tetra = four

• Alphabetized by substituentAlphabetized by substituent

• Not by the prefix di, tri, tetra, etc.Not by the prefix di, tri, tetra, etc.

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NomenclatureNomenclature

• Every exam will have two sections of Every exam will have two sections of nomenclaturenomenclature

• First: you name the structure givenFirst: you name the structure given• Second: you draw the structure Second: you draw the structure

based upon the name givenbased upon the name given• Usually these sections represent 40% Usually these sections represent 40%

of test valueof test value• Learn the rules. Follow the rules.Learn the rules. Follow the rules.

Fall, 2009Fall, 2009 3131

Chemical Properties of Chemical Properties of AlkanesAlkanes

• Paraffinic Hydrocarbon (little affinity)Paraffinic Hydrocarbon (little affinity)

• Alkanes are unreactiveAlkanes are unreactive

• Only sigma bonds C-C and C-HOnly sigma bonds C-C and C-H

• Electrons are not available for Electrons are not available for reactionsreactions

Fall, 2009Fall, 2009 3232

Chemical PropertiesChemical PropertiesAlkanes Burn

CH4 + O2 CO2 + H2O + heat + lightignition source

Alkanes react with Halogen (Cl2 and Br2)

CH4 + X2

lightCH3X

Reaction isn't clean. Gives multiple productsCH3X + CH2X2 + CHX3 + CX4

Reaction requires light. No light = No reaction (NR)

Fall, 2009Fall, 2009 3333

The Shape of AlkanesThe Shape of Alkanes

• The three-dimensional shape of molecules The three-dimensional shape of molecules result from many forces result from many forces

• A molecule may assume different shapes, A molecule may assume different shapes, called called conformationsconformations, that are in , that are in equilibrium at room temperature equilibrium at room temperature

• The conformational isomers are called The conformational isomers are called conformersconformers

• The systematic study of the shapes The systematic study of the shapes molecules is molecules is stereochemistrystereochemistry

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Consider EthaneConsider Ethane

CH3CH3

a C-C sigma bondpipe-like symmetryhas free rotation

How do we represent three dimensional How do we represent three dimensional shape of the ethane molecule?shape of the ethane molecule?

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Three-Dimensional Shape: Three-Dimensional Shape: EthaneEthane

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Conformations of EthaneConformations of Ethane

• Conformers interconvert rapidlyConformers interconvert rapidly

• Molecular models are three Molecular models are three dimensional objects that enable us to dimensional objects that enable us to visualize conformers visualize conformers

• Representing three dimensional Representing three dimensional conformers in two dimensions is conformers in two dimensions is done with standard types of drawings done with standard types of drawings

Fall, 2009Fall, 2009 3737

Drawing ConformationsDrawing Conformations

• Sawhorse Sawhorse representationsrepresentations – C-C bonds are at an C-C bonds are at an

angle to the edge of angle to the edge of the page and all C-H the page and all C-H bonds are shownbonds are shown

• NewmanNewman projectionsprojections– Bonds to front carbon Bonds to front carbon

are lines going to the are lines going to the centercenter

– Bonds to rear carbon Bonds to rear carbon are lines going to the are lines going to the edge of the circleedge of the circle

Fall, 2009Fall, 2009 3838

Dihedral AngleDihedral Angle

• Four Bond Angle – the angle between C-H Four Bond Angle – the angle between C-H bonds on the front and back carbons as bonds on the front and back carbons as viewed in a Newman Projectionviewed in a Newman Projection

H

H

H

H

H

H

Fall, 2009Fall, 2009 3939


• Free rotation is not freeFree rotation is not free

• Energy barrier of 12kJ/mole is observed.Energy barrier of 12kJ/mole is observed.

H

H

H

H

H

H HH

H H

HH

H

H

H

H

H

H

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H

H

H

H

H

H HH

H H

HH

H

H

H

H

H

H

staggerestaggeredd

staggerestaggereddeclipseeclipse

dd

Fall, 2009Fall, 2009 4141

Energy Differences of Energy Differences of ConfomersConfomers

• Staggered conformation: lower in energyStaggered conformation: lower in energy• Lower in energy = more stableLower in energy = more stable• Eclipsed conformation: higher in energyEclipsed conformation: higher in energy

Fall, 2009Fall, 2009 4242

Energy DifferencesEnergy Differences

• Hydrogens are close together in eclipsed Hydrogens are close together in eclipsed conformationsconformations

H

H

H

H

H

H HH

H H

HH

H

H

H

H

H

H

Fall, 2009Fall, 2009 4343

Eclipsing HydrogensEclipsing Hydrogens

• Eclipsing H’s interact with each otherEclipsing H’s interact with each other

• Torsional strainTorsional strain

• 12kJ/mole12kJ/mole

• There are 3 eclipsing hydrogensThere are 3 eclipsing hydrogens

• 4kJ/mole each4kJ/mole each

Fall, 2009Fall, 2009 4444

PropanePropane• CHCH33CHCH22-CH-CH3 3 – consider C-C bond shown– consider C-C bond shown

H

H

H

H

H

HH

HH

HCH3 CH3

H

H

H

H

H

CH3

Energy barrier is 14kJ/moleEnergy barrier is 14kJ/mole

Two eclipsing H’s = 8kJ/moleTwo eclipsing H’s = 8kJ/mole

Eclipsing H-CHEclipsing H-CH33 = 6kJ/mole = 6kJ/mole

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PropanePropane

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ButaneButane

• CHCH33CHCH22-CH-CH22CHCH3 3 – consider C2-C3 bond– consider C2-C3 bond

H

H

H

H

HH

HCH3 CH3

H

HH

H

CH3

CH3 CH3H

CH3

HH

H

CH3

CH3

HH

HH

H

CH3

CH3

H H

HCH3

CH3H

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Energy of Butane Energy of Butane InteractionsInteractions

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Conformations of ButaneConformations of Butane

• Anti – large groups are far apart (180Anti – large groups are far apart (180oo))• Eclipsed – large groups are close (0Eclipsed – large groups are close (0oo))• Gauche – large groups are close (60Gauche – large groups are close (60oo))

Fall, 2009Fall, 2009 4949

ButaneButane

• Anti conformationAnti conformation

• Large groups are far apartLarge groups are far apart

• All groups are staggeredAll groups are staggered

• Lowest energy conformationLowest energy conformation

H

H

H

H

CH3

CH3

Fall, 2009Fall, 2009 5050

ButaneButaneH H

HCH3

CH3H

Two methyl – H interactions (2 X Two methyl – H interactions (2 X 6kJ/mole)6kJ/mole)

One H-H interaction (1 X 4kJ/mole)One H-H interaction (1 X 4kJ/mole)

Total Energy = 16kJ/moleTotal Energy = 16kJ/mole

Fall, 2009Fall, 2009 5151

ButaneButane

• 2 H-H interactions – (2 X 4kJ/mole)2 H-H interactions – (2 X 4kJ/mole)

• Total Energy is 19kJ/moleTotal Energy is 19kJ/mole

• Methyl – methyl interaction = 11kJ/moleMethyl – methyl interaction = 11kJ/mole

• Highest energy conformationHighest energy conformation

HH

H

CH3

CH3

H

Fall, 2009Fall, 2009 5252

ButaneButane

• Staggered structureStaggered structure• Energy is not equal to 0kJ/moleEnergy is not equal to 0kJ/mole• Methyl groups are close togetherMethyl groups are close together• Interaction is 3.8kJ/moleInteraction is 3.8kJ/mole• gauchegauche

H

HH

H

CH3

CH3

Fall, 2009Fall, 2009 5353

ButaneButane

• Eclipsing methyl groupsEclipsing methyl groups

• Two large groups trying to be in the same spaceTwo large groups trying to be in the same space

• Steric StrainSteric Strain

• GaucheGauche

• Two methyl groups 60Two methyl groups 60o o apartapart

• Two large groups trying to be in the same spaceTwo large groups trying to be in the same space


Fall, 2009Fall, 2009 5454

Energy of Butane Energy of Butane InteractionsInteractions

Fall, 2009Fall, 2009 5555

Conformational InteractionsConformational Interactions

• Torsional StrainTorsional Strain

• Eclipsing groupsEclipsing groups


• Large groups occupying the same Large groups occupying the same spacespace

Fall, 2009Fall, 2009 5656

Preferred ConformationsPreferred Conformations• Molecule adopts the most stable Molecule adopts the most stable

(lowest energy) conformation where (lowest energy) conformation where all bondsall bonds– Are staggeredAre staggered– Large groups are anti to each otherLarge groups are anti to each other

• At Room Temperature there is At Room Temperature there is enough kinetic energy for all enough kinetic energy for all conformations to be in equilibriumconformations to be in equilibrium

LG

LG

Fall, 2009Fall, 2009 5757

Energy and EquilibriumEnergy and Equilibrium

• The relative The relative amounts of the amounts of the two conformers two conformers depend on their depend on their difference in difference in energy energy

EE = = RTRT ln ln KK• RR is the gas is the gas

constant [8.315 constant [8.315 J/(K•mol)], J/(K•mol)],

• TT is the is the temperature temperature (Kelvin) (Kelvin)

• KK is the is the equilibrium equilibrium constant between constant between isomersisomers

Fall, 2009Fall, 2009 5858

Determining the Value of KDetermining the Value of K

ΔE = -RTlnK ΔE = lnK -RT K = e-ΔE/RT

where ΔE = difference in energy of the conformers in J/mole; R = gas constant (8.315 J/Kmole) T = temperature in K

Fall, 2009Fall, 2009 5959

The Calculation of KThe Calculation of K

CH3

CH3

CH3

CH3

H

H H

H

HH

HH

no interactionsE = 0kJ/mole

2 eclipsed H's: 2 X 4.0 = 8kJ/mole

1 methyl - methyl eclipsed = 11 kJ/mole

E = 19 kJ/mole

E = most stable - least stable = 0 - 19 = -19kJ/mole

K = e_ E/RT

Fall, 2009Fall, 2009 6060

How much of each conformer How much of each conformer is present in solution?is present in solution?

• Calculate KCalculate K

K = e- E/RT

K = e-(-19,000J/mol)/(8.314J/K mol)(298 K)

K = e7.67

K = 2143 = 2143/1

Fall, 2009Fall, 2009 6161

How much of each conformer How much of each conformer is present in solution?is present in solution?

• Calculate KCalculate K

• K = 2143K = 2143

• K = A/B = 2143/1K = A/B = 2143/1

• Calculate PercentagesCalculate Percentages

• %A = (A/A+B ) X 100%A = (A/A+B ) X 100

• %A = (2143/2143 + 1) X 100%A = (2143/2143 + 1) X 100

• %A = 99.9%%A = 99.9%

Documents

1 Fall, 2009 Organic Chemistry I Alkanes Organic Chemistry I Alkanes Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University