2 2-1 Organic Chemistry William H. Brown & Christopher S. Foote William H. Brown & Christopher S. Foote.

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2-1

Organic Organic ChemistryChemistry

William H. Brown &William H. Brown &

Christopher S. FooteChristopher S. Foote

William H. Brown &William H. Brown &

Christopher S. FooteChristopher S. Foote

22

2-2

Chapter TwoChapter Two

AlkanesAlkanes andand

CycloalkanesCycloalkanes

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2-3

StructureStructure Hydrocarbon:Hydrocarbon: a compound composed only of

carbon and hydrogen Saturated hydrocarbon:Saturated hydrocarbon: a hydrocarbon

containing only single bonds Alkane:Alkane: a saturated hydrocarbon whose carbons

are arranged in a open chain Aliphatic hydrocarbon:Aliphatic hydrocarbon: another name for an

alkane

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2-4

StructureStructure Shape

• tetrahedral about carbon• all bond angles are approximately 109.5°

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2-5

NomenclatureNomenclature Alkanes have the general formula CnH2n+2

CondensedStructural Formula

MolecularFormulaName

heptane

hexane

pentane

butane

propaneethanemethane CH4 CH4

C2H6 CH3CH3

C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3

C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3

C7H16 CH3(CH2)5CH3

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NomenclatureNomenclature Alkanes (contd.)

tetradecane

hexadecane

octadecaneeicosane

decane

nonane

octane C8H18 CH3(CH2)6CH3

C9H20 CH3(CH2)7CH3C10 H22 CH3(CH2)8CH3

C14 H30 CH3(CH2)12 CH3

C16 H34 CH3(CH2)14 CH3

C18 H38 CH3(CH2)16 CH3C20 H42 CH3(CH2)18 CH3

dodecane C12 H26 CH3(CH2)10 CH3

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Constitutional IsomerismConstitutional Isomerism Constitutional isomers:Constitutional isomers: compounds with the

same molecular formula but a different connectivity (order of attachment of their atoms)• example: C4H10

CH3CH2CH2CH3 CH3CHCH3

CH3

Butane 2-Methylpropane

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2-8

Constitutional IsomerismConstitutional Isomerism

World populationWorld populationis aboutis about6,000,000,0006,000,000,000

4,111,846,763

4,347

75

31

Constitutional Isomers

MolecularFormula

CH4

C5H12

C10H22

C15H32

C30H62

36,797,588C25H52

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2-9

Nomenclature - IUPACNomenclature - IUPAC Suffix -aneane specifies an alkane Prefix tells the number of carbon atoms

undec-dodec-

tetradec-pentadec-hexadec-heptadec-

nonadec-eicos-

tridec-

11121314151617

octadec- 181920

Prefixmeth-eth-prop-but-pent-hex-

oct-non-dec-

1234567hept-89

10

Carbons CarbonsPrefix

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Nomenclature - IUPACNomenclature - IUPAC Parent name: the longest carbon chain Substituent: a group attached to the parent chain

• alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-

Alkane Alkyl group

CH4

Name Name

Methane CH3- Methyl group

CH3CH3 Ethane CH3CH2- Ethyl group

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2-11

NomenclatureNomenclature1.The name of a saturated hydrocarbon with an

unbranched chain consists of a prefix and suffix

2. For branched alkanes, the parent chain is the longest chain of carbon atoms

3. Each substituent is given a name and a number

4. If there is one substituent, number the chain from the end that gives it the lower number

CH3CHCH3

CH3

2-Methylpropane1

23

2-Methylpentane

CH3

CH3CH2CH2CHCH3 14 3 2

5

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2-12

NomenclatureNomenclature6. If there are two or more different substituents,• list them in alphabetical order• number from the end of the chain that gives the

substituent encountered first the lower number

CH3CH2CHCH2CHCH2CH3

3-Ethyl-5-methylheptane

CH3

CH2CH3

65

43

21 7

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NomenclatureNomenclature7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization

8. Substituents are named by the same set of rules.

CH3CCH2CHCH2CH3

CH3 CH2CH3

CH3

4-Ethyl-2,2-dimethylhexane

1 2 34 5

6

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NomenclatureNomenclature Alkyl groups

1-methylethyl (isopropyl)

propyl

ethyl

methyl

CondensedStructural FormulaName

CH3

-CH2CH3

-CH3

-CH2CH2CH3

-CHCH3

1,1-dimethylethyl (tert-butyl)

1-methylpropyl (sec-butyl)

2-methylpropyl (isobutyl)

butyl

CH3

CH3

CH3

CH3

-CH2CH2CH2 CH3

-CH2CHCH3

-CHCH2CH3

-CCH3

CondensedStructural FormulaName

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Classification of C & HClassification of C & H Primary (1°) C:Primary (1°) C: a carbon bonded to one other

carbon• 1° H: a hydrogen bonded to a 1° carbon

Secondary (2°) C:Secondary (2°) C: a carbon bonded to two other carbons• 2° H: a hydrogen bonded to a 2° carbon

Tertiary (3°) C:Tertiary (3°) C: a carbon bonded to three other carbons• 3° H: a hydrogen bonded to a 3° carbon

Quaternary (4°) CQuaternary (4°) C: a carbon bonded to four other carbons

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CycloalkanesCycloalkanes General formula CCnnHH2n2n

• five- and six-membered rings are the most common

Structure and nomenclature• to name, prefix the name of the corresponding open-

chain alkane with cyclo-cyclo-,, and name each substituent on the ring

• if only one substituent, no need to give it a number• if two substituents, number from the substituent of

lower alphabetical order• if three or more substituents, number to give them the

lowest set of numbers and then list substituents in alphabetical order

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CycloalkanesCycloalkanes Line-angle drawings

• each line represents a C-C bond• each angle represents a C

C

C CC

CC

C

C H2C

H2CCH2

CH

CH2

CH

CH3

CH3

C8H16

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CycloalkanesCycloalkanesExample:Example: name these cycloalkanes

(a) (b)

(c) (d)

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2-19

BicycloalkanesBicycloalkanes Bicycloalkane:Bicycloalkane: an alkane that contains two rings

that share two carbons

Bicyclo[4.4.0]decane(Decalin)

Bicyclo[4.3.0]nonane(Hydrindane)

Bicyclo[2.2.1]heptane(Norbornane)

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BicycloalkanesBicycloalkanes Nomenclature

• parent is the alkane of the same number of carbons as are in the rings

• number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc.

• show the lengths of bridges in brackets, from longest to shortest

bicyclo[2.2.1]heptane

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IUPAC - GeneralIUPAC - General prefix-infix-suffix

• prefix tells the number of carbon atoms in the parent • infix tells the nature of the carbon-carbon bonds• suffix tells the class of compound

one or more triple bonds

one or more double bonds

all single bonds

-yn-

-en--an-

Nature of Carbon-Carbon Bonds in the Parent ChainInfix

Suffix Class

-e

-ol

-al

-one

-oic acid

hydrocarbon

alcohol

aldehyde

ketonecarboxylic acid

-amine amine

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IUPAC - GeneralIUPAC - Generalprop-en-e = propene

eth-an-ol = ethanol

but-an-one = butanone

but-an-al = butanal

pent-an-oic acid = pentanoic acid

cyclohex-an-ol = cyclohexanol

eth-yn-e = ethyne

eth-an-amine = ethanamine

CH3CH=CH2

CH3CH2OHCH3CH2CH2CH2COH

O

HC CH

OH

CH3CH2CH2CH

O

CH3CCH2CH3

O

CH3CH2NH2

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ConformationsConformations Conformation:Conformation: any three-dimensional

arrangement of atoms in a molecule that results from rotation about a single bond

Newman projection:Newman projection: a way to view a molecule by looking along a carbon-carbon bond

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ConformationsConformations Staggered conformation:Staggered conformation: a conformation about a

carbon-carbon single bond in which the atoms on one carbon are as far apart as possible from the atoms on an adjacent carbon

H

H H

H H

H

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ConformationsConformations Eclipsed conformation:Eclipsed conformation: a conformation about a

carbon-carbon single bond in which the atoms on one carbon are as close as possible to the atoms on an adjacent carbon

H

H H

H

HH

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ConformationsConformations Torsional strain:Torsional strain: the force that opposes the

rotation of one part of a molecule about a bond while the other part of the molecule is held fixed• the torsional strain between eclipsed and staggered

ethane is approximately 12.6 kJ (3.0 kcal)/mol

+12.6 kJ/mol

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ConformationsConformations

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ConformationsConformations anti conformation

• a conformation about a single bond in which the groups lie at a dihedral angle of 180°

CH3

H H

H H

CH3

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ConformationsConformations Nonbonded interaction strain:

• the strain that arises when atoms not bonded to each other are forced abnormally close to one another

• butane - gauche conformation; nonbonded interaction strain is approx. 3.8 kJ (0.9 kcal)/mol

CH3

H H

H CH3

H

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ConformationsConformations

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CyclopropaneCyclopropane• angle strain: the C-C-C bond angles are compressed

from 109.5° to 60°• torsional strain: there are 6 sets of eclipsed hydrogen

interactions• strain energy is about 116 kJ (27.7 kcal)/mol

H

H

H

H

H

H

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CyclobutaneCyclobutane• puckering from planar cyclobutane reduces

torsional strain but increases angle strain• the conformation of minimum energy is a puckered

“butterfly” conformation• strain energy is about 110 kJ (26.3 kcal)/mol

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CyclopentaneCyclopentane• puckering from planar cyclopentane reduces torsional

strain, but increases angle stain• the conformation of minimum energy is a puckered

“envelope” conformation• strain energy is about 42 kJ (6.5 kcal)/mol

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CyclohexaneCyclohexane Chair conformation:Chair conformation: the most stable puckered

conformation of a cyclohexane ring• all bond angles are approx. 109.5° • all bonds on adjacent carbons are staggered

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CyclohexaneCyclohexane In a chair conformation, six H are equatorial and

six are axial

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CyclohexaneCyclohexane For cyclohexane, there are two equivalent chair

conformations• all C-H bonds equatorial in one chair are axial in the

alternative chair, and vice versa

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CyclohexaneCyclohexane Boat conformation:Boat conformation: a puckered conformation of a

cyclohexane ring in which carbons 1 and 4 are bent toward each other• less stable than a chair conformation by 27 kJ (6.5

kcal)/mol

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CyclohexaneCyclohexane Twist-boat conformationTwist-boat conformation

• approx. 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation

• approx. 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

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CyclohexaneCyclohexane

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MethylcyclohexaneMethylcyclohexane Equatorial and axial methyl conformations

CH3

CH3

+7.28 kJ/mol

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G° axial ---> equatorialG° axial ---> equatorial

F

Cl

Br

I

OH

CH3

CH2CH3

CH(CH3)2

C(CH3)3

2.2

2.4

Group− °G

1.9

3.9

7.287.3

9.021.0

1.0

( / )kJ mol

C

CCOOHNH2

CH=CH2

0.85.95.9

7.11.7CH

N

Group ( / )kJ mol− °G

--> axial equatorial

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Cis,trans IsomerismCis,trans Isomerism Cis,trans isomers have:

• the same molecular formula• the same connectivity• a different arrangement of their atoms in space due to

the presence of either a ring or a carbon-carbon double bond (see Chapter 5)

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Cis,trans IsomerismCis,trans Isomerism 1,2-dimethylcyclopentane

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

CH3

H

HH

HH

H

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Cis,trans IsomerismCis,trans Isomerism 1,4-dimethylcyclohexane

• planar hexagon representations

trans-1,4-Dimethyl-cyclohexane

cis-1,4-Dimethyl-cyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

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Cis,trans IsomerismCis,trans Isomerism trans-1,4-dimethylcyclohexane

• the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = 14.56 kJ/mol

CH3

H

H

CH3

HH3C

CH3

H(more stable)(less stable)

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Cis,trans IsomerismCis,trans Isomerism cis-1,4-dimethylcyclohexane

conformations are of equal stability

H

CH3

H

CH3

H

H3C

H

CH3

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Cis,trans IsomerismCis,trans IsomerismH

H

trans-decalin

H

H

cis-decalin

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Physical PropertiesPhysical Properties Intermolecular forces of attraction

• ion-ion (Na+ and Cl- in NaCl)• ion-dipole (Na+ and Cl- solvated in aqueous solution)• dipole-dipole and hydrogen bonding• dispersion forces (very weak electrostatic attraction

between temporary dipoles)

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Physical PropertiesPhysical Properties Low-molecular-weight alkanes

(methane....butane) are gases at room temperature

Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature

High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature

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Physical PropertiesPhysical Properties Constitutional isomers have different physical

properties

NameDensity (g/mL)

hexane2-methylpentane

3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.3

63.358.0

49.7

-95

-154

-118-129

-98

0.659

0.653

0.6640.661

0.649

bp(°C)

mp(°C)

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Oxidation of AlkanesOxidation of Alkanes Oxidation is the basis for their use as energy

sources for heat and power• heat of combustion:heat of combustion: heat released when one mole of a

substance in its standard state is oxidized to carbon dioxide and water

Methane

++CH4 2O2 CO2 2H2O -890 (-212)

++

Propane

CH3CH2CH3 3CO25O2 4H2O -2220 (-530)

°H ( )/kJ kcal mol

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Heat of CombustionHeat of Combustion Heat of combustion for constitutional isomers

Hydrocarbon

Octane

2-Methylheptane

2,2-Dimethylhexane

2,2,3,3-Tetramethylbutane

H0

[ ( )/ ]kJ kcal mol Structural formula

-5470.6 (-1307.5)

-5465.6 (-1306.3)

-5458.4 (1304.6)

-5451.8 (1303.0)

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Heats of CombustionHeats of Combustion For constitutional isomers [kJ (kcal)/mol]

-5470.6 (-1307.5)-5465.6 (-1306.3)-5458.4 (1304.6)-5451.8 (1303.0)

8CO2 +9H2O

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Heat of CombustionHeat of Combustion• ring strain as determined by heats of combustion

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Sources of AlkanesSources of Alkanes Natural gas

• 90-95% methane

Petroleum• gases (bp below 20°C)• naphthas, including gasoline (bp 20 - 200°C)• kerosene (bp 175 - 275°C)• fuel oil (bp 250 - 400°C)• lubricating oils (bp above 350°C)• asphalt (residue after distillation)

Coal

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GasolineGasoline Octane rating:Octane rating: the percent 2,2,4-trimethylpentane

(isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties

CH3(CH2)5CH3 CH3CCH2CHCH3

CH3

CH3 CH3

Heptane(octane rating 0)

2,2,4-Trimethylpentane(octane rating 100)

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Synthesis GasSynthesis Gas A mixture of carbon monoxide and hydrogen in

varying proportions which depend on the means by which it is produced

C + H2O heat CO + H2Coal

1+catalyst +

2CH4 2H2COO2

Methane

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Synthesis GasSynthesis Gas Synthesis gas is a feedstock for the industrial

production of methanol and acetic acid

• it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed

CH3OH + COcatalyst

CH3COHO

Methanol Acetic acid

+CO 2H2 CH3OHcatalyst

Methanol

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Prob 2.19Prob 2.19 Which sets represent pairs of constitutional isomers?

(a) (b)CH3CH2OH andCH3OCH3 CH3CCH3 andCH3CH2CHO O

(c) (d)CH3COCH3 andCH3CH2COH CH3CHCH2CH3 andCH3CCH2CH3

O O OH O

(e) (f)andCH3CH2CH2CH2CH3 and CH2=CHCH2CH2CH3

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Prob 2.22Prob 2.22 Write the IUPAC name of each compound.

(a) (b) (c)

(d) (e)

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Prob 2.24Prob 2.24 Explain why each is an incorrect IUPAC name, and write

the correct IUPAC name.

(a) 1,3-dimethylbutane

(b) 4-methylpentane

(c) 2,2-dimethylbutane

(d) 2-ethyl-3-methylpentane

(e) 2-propylpentane

(f) 2,2-diethylheptane

(g) 2,2-dimethylcyclopropane

(h) 1-ethyl-5-methylcyclohexane

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Prob 2.26Prob 2.26 Write the IUPAC name of each compound.

(a) (b)

(c)

CH3CH2CCH3 CH3CH2CH

CH3CH2CH2CH2CH2COH

O O

O(d)

(e) (f)

CH3CHCH3

O OH

OH

(g) (h)CH3CH=CH2

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Prob 2.40Prob 2.40 Complete the table for cis,trans and equatorial,axial

substituted cyclohexanes.

Position ofSubstitution cis trans

1,4-1,3-

1,2-

a,e or e,a e,e or a,a____ or ____ ____ or ____

____ or ____ ____ or ____

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Prob 2.43Prob 2.43 Draw alternative chair conformations for each.

(a)

(d)(c)

(b)HO

H

OH

HOH

OH

H

OH

H

H

HO

CH3H

CH2OH

OHHHO

OH

H

H

H

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Prob 2.44Prob 2.44 Draw the more stable chair conformation of glucose.

OHO

HO

OH

OH

OH

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Prob 2.51Prob 2.51 Given these heats of combustion, which constitutional

isomer is the more stable?

CH3-CH H2C CH2

O

Acetaldehyde-1164 kJ (-278.8 kcal)/mol

Ethylene oxide-1264 kJ (-302.1 kcal)/mol

O

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Alkanes Alkanes

and and CycloalkanesCycloalkanes

End Chapter 2End Chapter 2