Organic ChemistryZumdahl Chapter 22

Alkanes: Saturated Hydrocarbons• Hydrocarbons are molecules composed of carbon & hydrogen

– Each carbon atom forms 4 chemical bonds– A saturated hydrocarbon is one where all C - C bonds are “single”

bonds & the molecule contains the maximum number of H-atoms– Saturated hydrocarbons are called ALKANES

Normal vs Branched Alkanes• NORMAL alkanes consist of

continuous chains of carbon atoms

• Alkanes that are NOT continuous chains of carbon atoms contain branches

• The longest continuous chain of carbons is called the parent chain

CH3

CH2CH2

CH2CH3

CH3

CH2CH

CH3

CH3

Structural Isomerism• Structural isomers are

molecules with the same chemical formulas but different molecular structures - different “connectivity”.

• They arise because of the many ways to create branched hydrocarbons.

• a.k.a. “Constitutional Isomers”

CH3

CH2CH2

CH2CH3

CH3

CH2CH

CH3

CH3

n-pentane, C5H12

2-methlbutane, C5H12

The First 10 “Normal” AlkanesNameName FormulaFormula M.P.M.P. B.P.B.P. # Structural Isomers# Structural Isomers

• Methane CH4 -183 -162 1

• Ethane C2H6 -172 -89 1

• Propane C3H8 -187 -42 1

• Butane C4H10 -138 0 2

• Pentane C5H12 -130 36 3

• Hexane C6H14 -95 68 5

• Heptane C7H16 -91 98 9

• Octane C8H18 -57 126 18

• Nonane C9H20 -54 151 35

• Decane C10H22 -30 174 75

C1 - C4 are Gases C1 - C4 are Gases at Room Temperatureat Room Temperature

C5 - C16 are Liquids C5 - C16 are Liquids at Room Temperatureat Room Temperature

IUPAC Rules for Naming Branched Alkanes

– Find and name the parent chainFind and name the parent chain in the hydrocarbon - this forms the root of the hydrocarbon name

– Number the carbon atoms in the parent chainNumber the carbon atoms in the parent chain starting at the end closest to the branching

– Name alkane branchesName alkane branches by dropping the “ane” from the names and adding “yl”. A one-carbon branch is called “methyl”, a two-carbon branch is “ethyl”, etc…

– When there are more than one type of branch (ethyl and methyl, for example), they are named alphabeticallyalphabetically

– Finally, use prefixesuse prefixes to indicate multiple branches

Example 1: 2,2-dimethylpentane2,2-dimethylpentane• The parent chain is indicated by

the ROOT of the name - “pentanepentane”. This means there are 5 carbons in the parent chain.

CH3

CH2CH2

CH2CH3

• “dimethyldimethyl” tells us that there are TWO methyl branches on the parent chain. A methyl branch is made of a single carbon atom.

• “2,22,2-” tell us that BOTH methyl branches are on the second carbon atom in the parent chain.

CH31

CCH23

CH2

4

CH35

CH3

CH3

1

2

3

4

5

Example 2: 3-ethyl-2,4-dimethylheptane3-ethyl-2,4-dimethylheptane• The parent chain is indicated by

the ROOT of the name - “heptaneheptane”. This means there are 7 carbons in the parent chain.

CH3

CH2CH2

CH2CH2

CH2CH3

• “2,4-dimethyl2,4-dimethyl” tells us there are TWO methyl branches on the parent chain, at carbons #2 and #4.

• “3-ethyl3-ethyl-” tell us there is an ethyl branch (2-carbon branch) on carbon #3 of the parent chain.

1

2

3

4

5

76

CH3

CHCH

CHCH2

CH2CH3

CH2

CH3

CH3 CH3

Example 3: 2,3,3-trimethyl-4-propyloctane2,3,3-trimethyl-4-propyloctane• The parent chain is indicated by

the ROOT of the name - “octaneoctane”. This means there are 8 carbons in the parent chain.

• “2,3,3-trimethyl2,3,3-trimethyl” tells us there are THREE methyl branches - one on carbon #2 and two on carbon #3.

• “4-propyl4-propyl-” tell us there is a propyl branch (3-carbon branch) on carbon #4 of the parent chain.

1

2

34

5

7

6

8

1

23

45

7

6

8

CHC

CHCH2

CH2

CH2CH3

CH3

CH3

CH3

CH3CH2

CH2

CH3

Example 4: Name the molecules shown!

• parent chain has 5 carbons - “pentane”

• two methyl branches - start counting from the right - #2 and #3

• 2,3-dimethylpentane2,3-dimethylpentane

CH3

CH2

CHCH

CH3

CH3

CH3

• parent chain has 8 carbons - “octane”• two methyl branches - start counting

from the left - #3 and #4• one ethyl branch - #5• name branches alphabetically

3,4-dimethyl3,4-dimethyl

4433

octaneoctane

55

5-ethyl-5-ethyl-

HOMEWORK ASSIGNMENTHOMEWORK ASSIGNMENT• Read Zumdahl section 22-1Read Zumdahl section 22-1

– make notes on REACTIONS OF ALKANES and on CYCLIC ALKANES

– Copy table 22.2 on page 1040– Pay attention to sample exercises!

• Answer end-of-chapter problemsAnswer end-of-chapter problems:• 15, 16, 17, 18

Alkanes Review - Alkanes Review - CycloalkanesCycloalkanes• A cycloalkane is made of a hydrocarbon chain

that has been joined to make a “ring”.

CH3

CH2

CH3 CH2

CH2

CH2

n-propaneC3H8

cyclopropaneC3H6

60° bond angleunstable!!

109.5° bond angle

•Note that two hydrogen atoms were lost in forming the ring!

•What is the general formula for a cycloalkane?

Other Cycloalkanes

cyclobutaneC4H8 - ~90° bond angles

cylcopentaneC5H10 ~109.5° bond angles

cyclohexaneC6H12 = 109.5° bond angles

Angle (ring) StrainAngle (ring) Strain - results from compression of the internal bond angles. Cyclopropane has the greatest angle strain (60° bond angles) while the strain is eliminated in cyclohexane.

Torsional StrainTorsional Strain - a barrier to free rotation around single bonds, due to the eclipsing of atoms in a molecule. This results when atoms are brought too close together in a particular conformationconformation of a molecule.

Cycloalkanes: Cis-Trans Isomerism• Consider 1,2-dimethylcycloalkane1,2-dimethylcycloalkane - a molecule that illustrates

GEOMETRIC ISOMERISM GEOMETRIC ISOMERISM - compounds with the same molecular formula and connectivity but differ in their geometries.

• The molecule on the left shows the two methyl branches on OPPOSITE SIDES of the ring. The molecule on the right shows the two methyl branches on the SAME SIDE of the ring.

Trans-1,2-dimethylcyclopentaneTrans-1,2-dimethylcyclopentane Cis-1,2-dimethylcyclopentaneCis-1,2-dimethylcyclopentane

• The Trans-isomer is the molecule with branches on OPPOSITE sides of the ring

• The Cis-isomer is the molecule with branches on the SAME SIDE of the ring.

• Cis-Trans isomerism is one type of geometric isomerism

Cis-Trans Isomers - Examples

Cl Cl

Cl

CH1

CH2

2

CH3

CH34CH25

CH3

6

cis-1,3-dimethylcyclobutanecis-1,3-dimethylcyclobutane

1-chloro-1-methylcyclohexane1-chloro-1-methylcyclohexane

cis-1,2-dichlorocyclohexanecis-1,2-dichlorocyclohexane

trans-1-ethyl-2-methylcyclopropanetrans-1-ethyl-2-methylcyclopropane

Cyclohexane - Boat & Chair Conformations• Cyclohexane is NOT a planar molecule. To achieve its

109.5° bond angles and reduce angle strain, it adopts several different conformations.

• The BOATBOAT and CHAIRCHAIR (99%) are two conformations

Alkenes & AlkynesAlkenes & Alkynes• Alkenes are

hydrocarbons that contain at least one carbon-carbon double double bondbond

• Alkynes are hydrocarbons that contain at least one carbon-carbon triple triple bondbond

• The suffix for the parent chains are changed from “ane” to “ene” and “yne”– e.g. ethene, propyne

• Where it is ambiguous, the BONDS are numbered like branches so that the location of the multiple bond may be indicated

Alkenes & Alkynes: Examples

CH2 CH2 CHCH CH2 C

H

CH3

C16

CH

CH3 CH2 CH

CH2

CH3C C CH2CH3

CH3

ethene ethyne propene

propyne 1-butene 2-pentyne

Cis-Trans Isomerism…Again!• Like rings, alkenes and alkynes show

restricted rotation - this time about the multiple bonds

• Because of the 120° bond angles in alkenes, cis-trans isomerism is possible– If one of the carbons in the double bond

have two of the same substituents, there is NO cis-trans isomerism!

– Remember that “trans” means opposite sides of the double bond and “cis” means the same side of the bond!

– Use molar mass to prioritize the substituents to decide cis-trans isomerism

CH

CH

CH3 CH3

CH

CH

CH3

CH3

cis-2-butene

trans-2-butene

CH

CH

CH3 CH3

CH

CH

CH3

CH3

cis-2-butene

trans-2-butene

Name those alkenes...

CH

CH

CH3 CH CH3

CH3

CH

CH

CH2

CH2

CH2

Br

cis-4-methyl-2-pentene 2-methyl-2-hexene

cyclopentenecis-3-heptene

trans-2-bromo-3-methyl-2-pentene

Homework Assignment• READ sections 22.2 (Alkenes & Alkynes), 22.3

(Aromatic Hydrocarbons) and 22.4 (The Petrochemical industry)– Don’t get hung up on “ and bonds”– Copy figure 22.11 and 22.12– Summarize the use of ortho, meta, para prefixes when

naming benzene derivatives– Make notes on section 22.4 - the Petrochemical Industry

• Complete Questions 19 - 25, 27, 29, 31, 32, 54 - 56