Chapter 2Chapter 2

Alkanes

Hydrocarbons

Alkanes Alkenes Alkynes Arenes

carbon-carbonsingle bond

carbon-carbondouble bond

carbon-carbontriple bond

one or morebenzene-likerings

C C C C C C

Classification

HydrocarbonsHydrocarbonsCompounds that contains only Carbons and

HydrogensTwo types

◦ Saturated hydrocarbons Carbon-Carbon single bond

◦ Unsaturated hydrocarbons One or more Carbon-Carbon double

bonds or triple bonds or benzene rings

2.1 Alkanes2.1 AlkanesSaturated hydrocarbonAlso referred as aliphatic hydrocarbondsGeneral formula: CnH2n+2

2.2 How do we write structural 2.2 How do we write structural formulas of Alkanesformulas of AlkanesLine-angle formulaLine-angle formula

• A line represents a carbon-carbon bond.• A vertex and a line terminus represent a

carbon atom.• Hydrogen atoms are not shown in line-angle

formulas.

Table 2.1 The first 10 alkanes Table 2.1 The first 10 alkanes with unbranched chainswith unbranched chains

2.3 What are Constitutional 2.3 What are Constitutional Isomers?Isomers?Constitutional isomers:Constitutional isomers: Compounds that have the

same molecular formula but different structural formulas (a different connectivity of their atoms).

• For the molecular formulas CH4, C2H6, and C3H8, only one structural formula is possible. There are no constitutional isomers for these molecular formulas.

• For the molecular formula C4H10, two constitutional isomers are possible.

2.3Constitutional Isomers2.3Constitutional IsomersProblem:Problem: Do the structural formulas in each set represent the same compound or constitutional isomers?

2.3 Constitutional Isomers2.3 Constitutional Isomers

Problem:Problem: Do the line-angle formulas in each set represent the same compound or constitutional isomers?

and

and

a)

b)

2.3 Constitutional 2.3 Constitutional IsomerismIsomerism

Problem:Problem: Draw line-angle for the three constitutional isomers of molecular formula C5H12.

2.4 How Do We Name 2.4 How Do We Name Alkanes?Alkanes?

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

atoms.• substituent names: the groups bonded to

the parent chain.

IUPAC NamesIUPAC NamesThe IUPAC name of an alkane with an unbranched chain of carbon atoms consists of two parts:

(1) A prefix shows the number of carbon atoms in the chain.

(2) The suffix -ane-ane: shows that the compound is a saturated hydrocarbon. 1

2

3

4

4 C's = but-and no branchesbutane

Alkyl GroupsAlkyl Groups

Alkyl group:Alkyl group: A substituent group derived from an alkane by removal of a hydrogen atom (Table 2.3).• Commonly represented by the symbol R-.• Named by dropping the -aneane from the name

of the parent alkane and adding the suffix -ylyl.

C

H

HH

H

- HCH

H

H

Ready to connect to another group

Methane Methyl

Alkyl groupsAlkyl groups

Table 2.3

Rules for naming alkanesRules for naming alkanes1. Determine the longest chain

of carbon-carbon bonding without disconnecting the chain (parent) then give its name that ends with –ane

2. Determine the number of substituents and identify them. (alkyl ends with –yl)

3. Determine the position(s) of the carbons that contains those subsituents.

4. If more than one of the same substituent occurred then indicate them with Latin prefixes (di-, tri-, tetra-, etc…)

3.@ C2 and C44.2,4-Dimethyl

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6

Hexane

1.

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5

6

2 CH3-Methyl

2.

Rules for naming alkanesRules for naming alkanes1. If more than two

different substituents, list them in alphabetical order

6. Indicate all position of carbons that has a substituent even if they’re the same subsituents

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5

6

methyl ethyl

4-ethyl-2-methyl

E before M

Rules for naming alkanesRules for naming alkanes7. Name the alkanes in

this order

1. Postition-substituent then parent

2. Give the substituent lowest position as possible

3. Use hyphen to separate each substituents

4. Use comma to separate position of carbons that has the same substituents

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5

6

2-ethyl-4-methylhexane

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5

6

2,4-dimethylhexane

ExamplesExamples Write the molecular formula and IUPAC name for

these alkanes

(a)(b)

ExamplesExamples Draw a line-angle formula for these alkanes

◦ 2,3-dimethylbutane

◦ 4-isobutyl-2,5-dimethyloctane

Common NamesCommon NamesCommon names; an older system

• The number of carbon atoms determines the name.• The first three alkanes are methane, ethane, and propane.

• All alkanes with the molecular formula C4H10 are named butanes, all those with the molecular formula C5H12 are named pentanes, etc.

• For alkanes beyond propane, isoiso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH-

For more complex alkanes, use the IUPAC system.

2.5 Sources of Alkanes2.5 Sources of AlkanesNatural gas• 90 to 95 percent methane. • 5 to 10 percent ethane, and • A mixture of other relatively low-

boiling alkanes, chiefly propane, butane, and 2-methylpropane .

Petroleum• A thick, viscous liquid mixture of

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

PetroleumPetroleumFigure 2.3 Fractional distillation of petroleum.

2.6 Cycloalkanes2.6 CycloalkanesCyclic 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 (have only carbon-carbon single bonds).

Cycloalkanes with ring sizes of from 3 to over 30 carbon atoms are found in nature.• Five-membered (cyclopentane) and six-membered

(cyclohexane) rings are especially abundant in nature.

2.6 Cycloalkanes2.6 Cycloalkanes

Nomenclature• To name a cycloalkane, prefix the name of

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

• If there is only one substituent on the ring, there is no need to give it a location number.

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

Cyclopropane Methylcyclopropane

1

2

1-isopropyl-2-methylcyclopropane

ExamplesExamplesGive an IUPAC name for this molecule

Draw its structure using line-angle formulaethylcyclohexane

2.7 Conformations of 2.7 Conformations of AlkanesAlkanes

Conformation:Conformation: Any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond.

• Figure 2.5 Three conformations for a butane molecule.

2.7 Cyclopentane2.7 CyclopentaneFigure 2.5 The most stable conformation of a cyclopentane ring is an envelope conformation.

2.7 Cyclohexane2.7 Cyclohexane

The most stable conformation of a cyclohexane ring is the chair conformation.◦ All bond angles are approximately 109.5°.

CyclohexaneCyclohexane In a chair conformation,• six C-H bonds are equatorialequatorial..• six C-H bonds are axial.axial.

Figure 11.6 Chair conformation of Figure 11.6 Chair conformation of cyclohexanecyclohexane

CyclohexaneCyclohexane

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

Figure 11.7 Methylcyclohexane

2.8 Cis/Trans Isomers2.8 Cis/Trans Isomers

CisCis:: on the same side of the ring.

TransTrans:: on the opposite side of the ring,• In drawing cis-trans isomers of disubstituted

cyclopentanes, we can view a cyclopentane ring edge-on.

2.8 Cis-Trans2.8 Cis-Trans Isomers Isomers• Alternatively, we can view the

cyclopentane ring from above. Substituents are shown by solid wedges (above) or dashed wedges (below).

2.8 Cis-Trans Isomerism2.8 Cis-Trans Isomerism• To determine cis-trans isomers in disubstituted

cyclohexanes, we can view a cyclohexane ring either as a planar hexagon or viewed from above.

• Because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomersstereoisomers..

• Cis-trans isomers are one type of stereoisomer.

ExamplesExamplesFollowing is a chair conformation of

cyclohexane with carbon atoms numbered 1 through 6. ◦ Draw methyl groups that are equatorial on carbon

1,2 and 4◦ Draw methyl group that is axial on C2, C3 and

equatorial on C6

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6

ExamplesExamplesDoes the following cycloalkanes show cis-

trans isomerism? Why? For each that does, draw both isomers◦ 1,3-dimethylcyclopentane◦ Ethylcyclopentane◦ 1,3-dimethylcyclobutane

2.9 Physical Properties2.9 Physical Properties

The most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity.• The electronegativity difference between

carbon and hydrogen is 2.5 - 2.1 = 0.4 on the Pauling scale.

• Given this small difference, we classify a C-H bond as nonpolar covalent.

• Alkanes are nonpolar compounds and the only interaction between their molecules are the very weak London dispersion forces.

Melting PointsMelting PointsMelting and boiling points:• Boiling points of alkanes are lower than

those of almost any other type of compound of the same molecular weight.

• In general, both boiling and melting points of alkanes increase with increasing molecular weight.

• More branching, the lower the boiling point

Physical PropertiesPhysical Properties

Physical PropertiesPhysical Properties• Alkanes that are constitutional isomers

are different compounds and have different physical and chemical properties.

Physical PropertiesPhysical PropertiesSolubility: a case of “like dissolves like”.◦ Alkanes are not soluble in water because they

are unable to form hydrogen bonds with water.◦ Liquid alkanes are soluble in each other.◦ Alkanes are also soluble in other nonpolar

organic compounds, such as toluene and diethyl ether.

Density◦ The average density of the liquid alkanes listed

in Table 11.4 is about 0.7 g/mL; that of higher-molecular-weight alkanes is about 0.8 g/mL.

◦ All liquid and solid alkanes are less dense than water (1.0 g/mL) and, because they are both less dense and insoluble, they float on water.

ReactionsReactions

Oxidation (combustion)• Oxidation of hydrocarbons, including

alkanes and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel).

2.10Reactions of Alkanes2.10Reactions of Alkanes

Reaction with halogens (halogenation)• Halogenation of an alkane is a substitution

reaction.• Hydrogen from alkane carbon is replaced

with a halogen

Reactions of AlkanesReactions of Alkanes

IUPAC names of IUPAC names of haloalkaneshaloalkanesHalogen atoms act as subsituent

◦ Fluoro-◦ Chloro-◦ Bromo-◦ Iodo-Apply the same rules as for alkyl

substituents Cl

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4-chloro-2-methylhexane

ExamplesExamplesPredict all possible products

Reaction of propane with bromine gives two products, each with molecular formula C3H7Br. Draw structural formulas for these two compounds and give each an IUPAC name

+ Cl2 excess

light

2.10 The 2.10 The ChlorofluorocarbonsChlorofluorocarbons

Chlorofluorocarbons (CFCs) • Manufactured under the trade name Freon.• CFCs are nontoxic, nonflammable, odorless,

and noncorrosive.• Among the CFCs most widely used were

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

CFCs were used as;• Heat-transfer agents in refrigeration systems.• Industrial cleaning solvents to prepare

surfaces for coatings and to remove cutting oils from millings.

• Propellants for aerosol sprays.

CFC ReplacementsCFC ReplacementsChlorofluorocarbons (CFCs) cause destruction of the Earth ’s stratospheric ozone layer.

The most prominent replacements are the hydrofluorocarbons (HFCs) and the hydrochlorofluorocarbons (HCFCs).◦ These compounds are chemically more reactive than

CFCs and are destroyed before they reach the stratosphere.