Alkanes Bettelheim, Brown, Campbell and Farrell Chapter 11

Alkanes

Bettelheim, Brown, Campbell and Farrell

Chapter 11

Hybrid Orbitals

• Type Bond Angle Bonds to ? Atoms

• sp 180o 2• sp2 120o 3• sp3 109.5o 4

• Each hybrid orbital is identical

• Molecular Formula: – Gives number of each type of atom in compound

• Structural Formula– Gives number of each type of atom AND shows how

the atoms are connected

• Condensed Structural Formula– Gives number of each type of atom AND shows how

the atoms are connected using “shorthand” structures

Line-Angle Structural Formulas

– Line represents C-C single bond– Each vertex (point) of angle represents a C atom– Hydrogen atoms are not shown

CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3PentaneButanePropane PentaneButanePropane

Condensedstructural

formula

Line-angleformula

Ball-and-stick model

Hydrocarbons• Contain only carbon and hydrogen

H-C C-HH-C-C-HH

H

H

H HC C

H

H H

Hydrocarbons

Alkanes(Chapter 11)

Alkenes(Chapter 12)

Alkynes(Chapter 12)

Arenes(Chapter 13)

Only carbon-carbon single

bonds

One or more carbon-carbondouble bonds

One or morecarbon-carbontriple bonds

One or morebenzene-like

rings

Ethane Ethene(Ethylene)

Ethyne(Acetylene)

Benzene

Saturated--only single bonds Unsaturated—not just single bonds

Alkanes

• Saturated hydrocarbons – Contain only single bonds– May be straight-chains, branched or cyclic– First two alkanes are methane and ethane

H-C-HH

HH-C-C-HH

H

H

H

Methane Ethane

Alkanes– First 10 “straight-chain” alkanes

• No branches

• Generic Formula CnH2n+2 (n = number of C atoms)

CH4 CH4C2H6 CH3CH3C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3C7H16 CH3(CH2)5CH3C8H18 CH3(CH2)6CH3C9H20 CH3(CH2)7CH3C10H22 CH3(CH2)8CH3

CH4 CH4C2H6 CH3CH3C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3C7H16 CH3(CH2)5CH3C8H18C9H20C10H22

CondensedStructural Formula

MolecularFormulaName

decane

nonane

octane

heptane

hexane

pentane

butane

propane

ethanemethane





decane

nonane

octane

heptane

hexane

pentane

butane

propane

ethanemethane

CondensedStructural FormulaName

ALKANES

• Straight chain or “normal” alkanes have all of their carbons connected in sequence

• C1 connected to C2 connected to C3 connected to C4, etc.

• A straight chain has angles of 109.5o between adjacent carbons. Physically the straight chain appears to be “zig-zag” rather than a straight line

Constitutional Isomerism (Structural Isomers)

• Compounds that have the same molecular formula but different structural formulas– Up to 3 C—only one structure possible

– For 4 C (C4H10), two constitutional isomers are possible

CH3CH2CH2CH3 CH3CHCH3

CH3

Butane(bp -0.5°C)

2-Methylpropane(bp -11.6°C)

Constitutional Isomerism

– Do the structural formulas in each set represent the same compound or constitutional isomers?

CH3CH2CH2CH2CH2CH3 CH3CH2CH2CH2CH2CH3

CH3CHCH2CHCH3

CH3

CH3

CH3CH2CHCHCH3

CH3

CH3

(a) and (each is C6H14)

(b) and (each is C7H16)

• Free Rotation around C-C Single Bonds

• Conformations– Different shapes which molecule can have

when it rotates around single bonds


(a) The same compound—just different conformations

(b) Constitutional isomers—connected differently

CH3CHCH2CH

CH3

CH3

CH3CH3CH2CHCHCH3

CH3

CH35

1 12 23 3

4

455

2 5 4 3

2 1

431 and

CH3CH2CH2CH2CH2CH3 CH3CH2CH2

CH2CH2CH3

654321

654

321

and

21

12

3 3

4

45

5

6

6


Draw structural formulas for the five constitutional isomers of molecular formula C6H14


– Draw structural formulas for the five constitutional isomers of molecular formula C6H14

Six carbons in an unbranched chain

Five carbons in a chain; one carbon as a branch

1 32 4

56

1 12

23 3

4 4

5 5

Four carbons in a chain; two carbons as branches

1 12 2

3 34 4

IUPAC Names for Unbranched Alkanes• The IUPAC name has two parts:

– (1) a root name (prefix that shows the number of carbon atoms in the chain)

– (2) the suffix -ane-ane: shows that the compound is a saturated hydrocarbon

Prefixmeth-eth-prop-but-pent-

hex-

oct-non-dec-

12345

67hept-8910

Number ofCarbon Atoms

Number ofCarbon AtomsPrefix

Root names for 1 to 4 carbons new

Root names for 5 or more carbons same as molecular prefixes

Alkyl Group Names• Substituent group derived from alkane by removing a

hydrogen atom– commonly represented by the symbol R-– named by dropping the -aneane from the name of the

parent alkane and adding the suffix -ylyl

-CH2CH3

-CH3

-CH2CH2CH3

-CHCH3CH3

-CH2CH2CH2CH3

-CH2CHCH3CH3

-CHCH2CH3CH3

-CCH3

CH3

CH3

tert-butyl

sec-butyl

isobutyl

butyl

isopropyl

propyl

ethyl

methylName


CondensedStructural FormulaName

IUPAC Names

• The name of an alkane with a branched chain of carbon atom consists of:

1. Parent name: the longest chain of carbon atoms

2. Substituent names and locations: the groups bonded to the parent chain

CH3

CH3CH2CH2CHCH2CH2CH2CH38

substituent

4-Methyloctane

1 2 3 4 5 6 7 8

parent chain

12

34

5

6

7

IUPAC Alkane Names1.An unbranched alkane: Prefix showing the

number of carbon atoms plus -aneane

2.Branched-chain alkanes: Parent name is name of longest carbon chain

3. Name and number each substituent on the parent chain: Use a hyphen between number and name

CH3CHCH3

CH3

2-Methylpropane

12

3

IUPAC Names

4.Number the parent chain from the end that gives the substituents the lower numbers

CH3

CH3CH2CH2CHCH35

2-Methylpentane(not 4-methylpentane)

123

4

IUPAC Names

5. If the same substituent occurs more than once,– Number parent chain from the end that gives the

lower number to the substituent encountered first– Use prefix (di-, tri-, tetra-,penta-, hexa- etc.) to show

how many times the substituent – Use a comma to separate numbers

CH3CH2CHCH2CHCH3

CH3 CH3

2,4-Dimethylhexane(not 3,5-dimethylhexane)

12

34

56

IUPAC Names6. For two or more different substituents

– List them in alphabetical order– Number chain to give the lower number to the

substituent encountered first– For different substituents in same positions on

opposite ends of the parent chain, the substituent that is first in the alphabet gets the lower number

CH3CH2CHCH2CHCH2CH3

CH3

CH2CH3

12

34

56

7

3-Ethyl-5-methylheptane(not 3-methyl-5-ethylheptane)

IUPAC Names

7. Do NOT alphabetize the prefixes di-, tri-, tetra, etc., or the prefixes sec- and tert- in alphabetizing; – Alphabetize the names of substituents first,

and then insert these prefixes

CH2CH3

CH3CCH2CHCH2CH3

CH3

CH34-Ethyl-2,2-dimethylhexane

(not 2,2-dimethyl-4-ethylhexane)

23

45

61

Common Names• Common names still in use

– Number of carbon atoms determines the name– First three alkanes are methane, ethane, and

propane

– All alkanes of formula C4H10 are butanes, all alkanes of formula C5H12 are called pentanes, etc.

– The prefix isoiso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH-

– For more complex alkanes, use the IUPAC system

CH3CHCH3

CH3

CH3CH2CHCH3

CH3

IsopentaneIsobutane

Name these compounds

Cycloalkanes• Cyclic hydrocarbon: Cyclic hydrocarbon: a hydrocarbon that

contains carbon atoms joined to form a ring

• Cycloalkane:Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated – Cycloalkanes of ring sizes ranging from 3 to over

30 carbon atoms are found in nature– Five-membered (cyclopentane) and six-membered

(cyclohexane) rings are especially abundant in nature

Cyclopentane Cyclohexane

Generic Formula

• Alkane: CnH2n+2 n = # of C

• Cycloalkane: CnH2n

Note that making a ring results in 2 fewer hydrogens than in straight chain

Cycloalkanes• Nomenclature

– Add cyclo-cyclo- to the name of the corresponding open-chain alkane and name each substituent on the ring

– If only one substituent, it does not need to be numbered

– If there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

Isopropylcyclopentane 1-tert-butyl-4-methylcyclohexane

1 4

Conformations - Alkanes

• Conformation:Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a C-C single bond– Three conformations for a butane molecule

Most crowdedconformation

rotate by 120°

rotate by 60°

Least crowdedconformation

Intermediatecrowding

Conformations - Alkanes

• Ring formation limits free rotation

• Can only rotate partly without breaking bonds

Cyclopentane

• The most stable conformation of a cyclopentane ring is an envelope

conformation

Cyclohexane

• The most stable conformation of a cyclohexane ring is the chair conformation– all bond angles are approximately 109.5°

Cyclohexane

Cyclohexane• In a chair conformation,

– six C-H bonds are equatorial equatorial (red) – six C-H bonds are axial axial (blue)

HH

H

HHH

(a) Ball-and-stick modelshowing all 12 hydrogens

axis through thecenter of the ring

H H

H

H

H

H

(b) The six equatorialC-H bonds

(c) The six axial C-H bonds

Cyclohexane

– the more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial

CH3

Equatorial methylcyclohexane

CH3

Axial methylcyclohexane

Cis-Trans Isomers• Cis:Cis: on the same side of ring• Trans:Trans: on opposite sides of ring

– Look at molecule edge-on…

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

H3C

H

HH

HH

H

Same side Opposite side

Cis-Trans Isomers….or view it from above (use wedge notation)

CH3trans-1,2-Dimethyl-

cyclopentanecis-1,2-Dimethyl-

cyclopentane

H3C CH3 H3C

Cis-Trans Isomers– Often helpful to picture cyclohexane ring as hexagon

to determine which cis-trans isomer is present– StereoisomersStereoisomers differ in the orientation of atoms in

space (connected to same atoms)– Cis-trans isomers are one type of stereoisomers

trans-1,4-Dimethylcyclohexane cis-1,4-Dimethylcyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

or or

CH3

CH3

CH3

CH3

Physical Properties of Alkanes

• Most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity– Electronegativity difference between carbon and

hydrogen is 2.5 - 2.1 = 0.4– Thus C-H bond is nonpolar covalent– Alkanes are nonpolar compounds– Display only weak London dispersion forces

between molecules


• Melting and boiling points

– Low melting and boiling points compared to other types of compounds with same size

– Both boiling and melting points of alkanes increase with increasing molecular weight

• Greater number of London dispersion forces


CH4CH3CH3CH3CH2CH3CH3(CH2)2CH3

CH3(CH2)3CH3CH3(CH2)4CH3CH3(CH2)5CH3CH3(CH2)6CH3CH3(CH2)7CH3CH3(CH2)8CH3

methane

ethanepropane

butane

pentane

hexane

heptane

octanenonane

decane

Name

CondensedStructrualFormula

mp(°C)

bp(°C)

-182

-183

-190-138

-130

-95

-90

-57-51

-30

-164

-88

-420

36

69

98

126151

174

(a gas)

(a gas)

(a gas)(a gas)

0.626

0.659

0.684

0.7030.718

0.730

*For comparison, the density of H2O is 1 g/mL at 4°C.

Mol wt(amu)

16.0

30.144.1

58.1

72.2

86.2

100.2

114.2128.3142.3

Density of Liquid

(g/mL at 0° C)*

Physical Properties of Alkanes– Constitutional isomers are different compounds

with different physical and chemical properties

– C6H14 compounds:

bp (°C)Namehexane

2-methylpentane3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.363.3

58.0

49.7

Hexane

2,2-Dimethylbutane


• Solubility: “like dissolves like”– Nonpolar alkanes are NOT soluble in water– Do not form hydrogen bonds with water– Soluble in each other and in other nonpolar

organic compounds

• Density– Densities of 0.7-0.8 g/mL– Less dense than water (1.0 g/mL) so they

float on water

Chemical Properties of Alkanes

• Oxidation (Combustion)– Reaction with oxygen

– Products are CO2, H2O and heat

– Used as energy sources for heat and power

CH3CH2CH3 5O2 3CO2 4H2O++Propane

530 kcal/mol+

CH4 2O2 CO2 2H2OMethane

++ 212 kcal/mol+

Alkyl Halides

• Alkanes with halide substituents

F Cl Br I

CH2Cl2 CH3Cl CHCl3

CCl3F (Freon-11)

Reactions• Reaction with halogens (halogenation)

– Substitution reaction– Requires heat or light as catalyst

CH4 Cl2 CH3Cl HCl+ heator light

+Methane Cloromethane

(Methyl chloride)

CH3Cl Cl2 CH2Cl2 HCl+heat

+Dichloromethane

(Methylene chloride)

CH2Cl2Cl2

CHCl3Cl2

CCl4heat heatTrichloromethane

(Chloroform)Tetrachloromethane

(Carbon tetrachloride)

The Chlorofluorocarbons• Chlorofluorocarbons (CFCs)

– Freons

– CCl3F (Freon-11) and CCl2F2 (Freon-12)

– nontoxic, nonflammable, odorless, noncorrosive

• CFCs were used as – refrigerants– industrial cleaning solvents – propellants for aerosol sprays

CFC Replacements• Chlorofluorocarbons (CFCs) cause destruction

of the Earth’s stratospheric ozone layer

• Replacements are hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs)– More reactive than CFCs– Destroyed before they reach the stratosphere

H-C–C-FHCl

H ClF-C-C-HF

F H

F

HFC-134a HCFC-141b

Sources of Alkanes• Natural gas

– 90 to 95 percent methane, – 5 to 10 percent ethane, and – Mixture of other relatively low-boiling alkanes,

chiefly propane, butane, and 2-methylpropane

• Petroleum– Thick, viscous liquid mixture of thousands of

compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals

Refining of Crude Oil

Petroleum Distillation

Fraction Size Distills at oC

Gasoline C4 to C12 20 - 200

Kerosene C10 to C14 200 - 275

Fuel Oil/Diesel C14 to C18 275 - 350

Lubricating Oil C16 to C20 > 350

Residue >C20

(Asphalt/Greases)

CH3CCH2CHCHCH2CHCH2CH3

CH2CH3

CH3

Br

BrBr

CH3CH2CCH2CHCH3

Cl CH3

CH3

CH3

CH3CH2CH2CH3

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Alkanes Bettelheim, Brown, Campbell and Farrell Chapter 11