chapter 4 alkanes and cycloalkanes -...

Chapter 4 Alkanes and Cycloalkanes

Structure, Nomenclature and Conformations

Carbon BondingChapter 4

Hydrocarbons The Simplest Class of Organic Compounds

A. Contain only Carbon and Hydrogen B. Carbon can bond in different ways

C

H

HH

H

C

H

HH

H

CH4

C

H

H

C C

H

C

H

H

HH

H

CH

CH C CH H


B. Carbon can bond in different ways (multiple bonds)

C2H4

C2H2

C H

H

H

H

C C H

H

H

H

H

H

C C C

H

H

H

H

H

H

H

H

C C C C

H

H

H

H

H

H

H

H

H

H

Name Kekule structure Condensed structure Model

methane CH4

ethane CH3CH3

propane CH3CH2CH3

butane CH3CH2CH2CH3

Structural Representation of Simple Alkanes

Simple alkanes have the general formula CnH2n+2.


C. Hydrocarbons are further subclassified

C C C C C

UNSATURATED

ALIPHATIC AROMATIC

SATURATED

Nomenclature of Alkanes

n Molecular Condensed Name Melting Boiling

Formula Structural Formula point (oC) point (oC)

1 CH4 methane -182 -162

2 C2H6 ethane -183 -89

3 C3H8 propane -190 -42

4 C4H10 butane -138 -1

5 C5H12 pentane -130 36

6 C6H14 hexane -95 69

7 C7H16 heptane -91 98

8 C8H18 octane -57 126

9 C9H20 nonane -51 151

10 C10H22 decane -30 174

CH4

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH3

CH3CH2CH2CH2CH3

CH3CH2CH2CH3

CH3CH2CH3

CH3CH3

Formulas and Properties of “Normal” Alkanes

1º, 2º, 3º, and 4º Carbons

A carbon atom is classified as primary (1º), secondary (2º), tertiary (3º), or quaternary (4º) depending upon whether or not it is directly bonded to a total of one, two, three, or four other carbon atoms, respectively.

C

C

H

HH C

C

C

HH C

C

C

CH C

C

C

CC

1º 2º 3º 4º

1º, 2º, 3º, and 4º Carbons - Examples

Indicate the number of primary, secondary, tertiary, and quaternary carbons in the following molecules:

1º: 52º: 23º: 14º: 1

1º: 22º: 23º: 24º: 0

Hydrocarbon NomenclatureWhy is the IUPAC system Necessary ?

Consider the following “carbon hydrides”

CH4 C2H6 C2H4 C2H2

Each formula represents a unique compound.

When structural formulas are considered, however,

C3H7 has only 1 isomer. C4H10 has 2 isomers.

C5H12 has 3 isomers. C6H14 has 5 isomers.

C10H22 has 75 isomers. C30H62 has 4 x 109 isomers.

Hydrocarbon Nomenclature

Basis of the IUPAC System of naming-The Root Name

1. The root name of a compound is derived from the longest continuous chain of carbon atoms or the longest continuous chain of carbon atoms containing a given special structural feature.

a. Common stems are used to indicate the length of carbon chains.

meth-, eth-, pro-, but-, pent-, hex-, hept-, oct-, non-, dec-

b. Common suffixes indicate the class of compounds.

Alkane Alkene Alkyne

C CC CC C


2. If the carbon chain contains “unsaturation” (double or triple bonds), the carbon chain is numbered so as to give double or triple bond(s) the lowest number(s).

C C C C

H

H H H

H

HHH

1-Butene

1 2 3 4 C C C C

H

H

H

H

H

HHH

2-Butene

1 2 3 4

C C C C

H

H H H

H

HHH

3-Butene

1 2 3 4 C C C C

H

H H H

H

HHH

1-Butene

4 3 2 1


“Substituents” may be attached to a carbon chain

1. Substituents which are not pieces of alkanes:

F- Fluoro- -NO2 Nitro

Cl- Chloro- -NH2 Amino

Br- Bromo- -OH Hydroxy

I- Iodo-

Hydrocarbon Nomenclature2. Substituents which are pieces of alkanes - “Alkyl” groups

Names are derived from the parent alkanes.

C HH

H

H

Parent alkane When attached to another chain

Methane

CH

H

H

Parent Chain

Methyl- BASE NAME

C CH

H

H

H

H

H

Ethane

C CH

H

H

H

H

Parent Chain

Ethyl- BASE NAME


2. Substituents which are pieces of alkanes - “Alkyl” groups

Parent alkane When attached to another chain

C CH

H

H

H

H

H

Parent Chain

Propane Propyl- BASE NAME

C

H

H C CH

H

H

H

H

H

C

H

C CH

H

H

H H

C

H

H

Parent Chain Isopropyl- BASE NAME

Also called a “methyl ethyl”

group

Hydrocarbon Nomenclature -Alkyl Groups

CH

H

H

C

H

H

C H

H

H

C

H

H

Parent Chain

butyl- BASE NAME

CH

H

H

C

H

H

C

H

H

C

H

H

CH

H

H

C

H

H

C H

H

C

H

HParent Chain

s-butyl- BASE NAME

butane

Also called a “1-methyl propyl”

group

Hydrocarbon Nomenclature -Alkyl Groups

CH

H

H

C

H

C

H

H

HCH

H

HParent Chain

isobutyl- BASE NAME

CH

H

H

C

H

C

H

H

HCH

H

Parent Chain

t-butyl- BASE NAME

CH

H

H

C C

H

H

HCH

H

H

2-methylpropane (“isobutane”)

Also called a “2-methyl propyl”

group

Also called a “dimethyl ethyl”

group

methyl- CH3-

ethyl- CH3CH2-

propyl- CH3CH2CH2-

isopropyl- (CH3)2CH-

n-butyl- CH3CH2CH2CH2-

s-butyl- CH3CH2CHCH3

isobutyl- (CH3)2CH2CH-

t-butyl- (CH3)3CH-

Hydrocarbon Nomenclature -Alkyl GroupsName Formula Line structure

C3H7-

C4H9-

C2H5-

CH3-

Chapter 4 Alkanes and Cycloalkanes

Nomenclature-Putting the Pieces Together

Hydrocarbon Nomenclature -Final Step

Position of Attachment (“Locant”) of Substituents is Indicated

1. For carbon chains with double or triple bonds (alkenes and alkynes), the numbering system for the base carbon chain has already been established and does not change.

2. For saturated carbon chains (alkanes) the numbering system for the base carbon chain is determined by the positions of the substituents.

Substituent names (in alphabetical order) and numbers are added (prefixed) to the base name of the compound.

Hydrocarbon Nomenclature Putting it Together for a Simple Alkane

CH3 CH2 CH2 CH

CH CH3

CH3

CH CH3

CH2

CH3

7 6 5 4 3

2

1

Family

heptane

Longest chain

Base name

Alkyl groups

methylisopropyl

Position of alkyl groups

4- -3-

CH3 CH2 CH2 CH

CH CH3

CH3

CH CH3

CH2

CH3

Hydrocarbon Nomenclature A more complicated example

CHCH2CH3

CH3

C

CH2

CH2Cl

CH2

C

CH3

Cl

CH2CH2 Cl

CH2CH

CH3

CH3

CH

CH2

CH3 CH3

CCH2 CH2Cl

CH2 C

CH3

Cl

CH2CH2 ClCH2CHCH3CH3

CHCH2CH3

CH3

C

CH2

CH2Cl

CH2

C

CH3

Cl

CH2CH2 Cl

CH2CH

CH3

CH3

1,3,5-trichloro-5-isobutyl-3,9-dimethyl decane1,3,5-trichloro-3,9-dimethyl-5-(2-methylpropyl) decane

Name the following compound.

1 5

3-isopropylpentane

1

5

3-ethyl-2-methylpentane

Draw the structure of 3-ethyl-6-isopropyl-3,7-dimethyldecane.

1

7

3-isopropyl-2,2,6-trimethyl-heptane

*

Name the following compound:

Hydrocarbon Nomenclature -Other Carbon Groups

Main chain

Main chain

Main chain

Main chain

Main chain

“cyclopropyl”-

“cyclobutyl”-

“cyclopentyl”-

“cyclohexyl”-

“phenyl”-

Hydrocarbon Nomenclature -Cyclic Alkanes 1. Structures are usually abbreviated

2. Planar representations do not represent 3-D shape.

C

C C

H H

H

H

H

H

CC

CC

C

C

H HH

H

H

HH H

HH

H

H

3. Cyclic structures are numbered and named to give the lowest numbers to substituents.

Hydrocarbon Nomenclature -Cyclic Alkanes

CH3H3C1,2-dimethylcyclopentane 1,2,4-trimethylcyclohexane

CH3

H3C

H3C1 2

1

2 34

Draw the structure of 1-sec-butyl-3-isopropylcyclopentane.

1-(1-methylethyl)-3-(1-methylpropyl)cyclopentane

1

3 1

3

Nomenclature of Bicyclic Compounds

bicyclo [4.4.0] decanebicyclo [6.2.0] decane

bicyclo [3.3.2] decane

12

3

4

56

7

8

9

101

23

4

5

6 7

8 9

10

1

2

3

45

6

7

8

9

10

spiro [4,5] decane

12

3

4

5

6 7

8

910

C10H18

1-cyclobutyl-2-iodo-4-isobutyl cyclohexaneI

5-iodo-7-isobutyl spiro[3,5]nonaneI

2-iodo-4-isobutyl bicyclo[4.2.0]octaneI

1 23

456

12

34

56

789

12345

6

7 8

cubane basketane

housane churchane

Chapter 4 Isomerism Revisited

n-butane iso-butanebutane 2-methylpropane

Isomerism

Isomerism Two Compounds

The compounds are not isomers.

Are the chemical formulas the same ?NoYes

The compounds are constitutional isomers.

Are the compounds superimposable ?

Yes

No

The compounds are stereoisomers.

The compounds are identical.

The compounds are enantiomers.

The compounds are diastereomers.

Are the connectivities of the atoms the same ?

Yes No

Are the compounds mirror images?

NoYes

Isomerism What is the relationship between these two

molecules?

2,3-dimethylpentane 2,3-dimethylhexane


molecules?

and

2,4-dimethylpentane 2,2-dimethylpentane


molecules?

2,3-dimethylpentane 2,3-dimethylpentane

Question: Are pentane and cyclopentane isomers ???

C5H12 C5H10

Cyclic Alkanes - cis/trans Isomerism

Consider the formula C5H10 There appear to be five constitutional isomers

which do not contain a double bond:

Cyclic Alkanes - cis/trans Isomerism

1,2-Dimethylcyclopropane can actually exist as two geometric isomers:

cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane

stereoisomers

H H

H CH3

H

CH3

H CH3

H H

H

CH3

Combustion of Hydrocarbons


C H 4 + O 2 ➛ C O 2 + H 2 O

C 2 H 2 + O 2 ➛ C O 2 + H 2 O

C 3 H 8 + O 2 ➛ C O 2 + H 2 O

C 4 H 1 0 + O 2 ➛ C O 2 + H 2 O

C 8 H 1 8 + O 2 ➛ C O 2 + H 2 O

C + O2 ➛ CO2

2 2

2.5 2

5 3 4

6.5 4 5 12.5 8 9


n ΔHcomb (kJ/mol) ΔHcomb/C (kJ/mol)

2 -1559 -779

3 -2217 -739

4 -2874 -718

6 -4188 -698

8 -5505 -688

10 -6821 -682

12 -8138 -678

CnH2n+2 + O2 → nCO2 + (n+1) H2O + ΔHcomb (kJ/mol) (n-alkane)

(3n+1) 2

+ 12.5 O2 8 CO2 + 9 H2O

+ 12.5 O2 8 CO2 + 9 H2O

+ 12.5 O2 8 CO2 + 9 H2O

∆Hº = -5470 kJ/mol

∆Hº = -5460 kJ/mol

∆Hº = -5452 kJ/mol

leaststable

moststable


C8H18

Sources of Hydrocarbons

Boiling Points for Straight-Chain Alkanes

Physical Properties of Alkanes The Basis for Fractional Distillation

Hot crude oil is fed in at the foot of the fractionating column and mixed with steam. Various constituents, or fractions, of the crude oil separate at various points of the column.

# C’s 1-4

7

13

16

19

22

25+

Physical Properties of Alkanes The Basis for Fractional Distillation

In general, the larger an alkane molecule, the more strongly it is attracted to other molecules through London dispersion forces. Alkanes composed of larger molecules are harder to melt than alkanes composed of smaller molecules. Alkanes with even numbers of carbon atoms fall on a melting point curve that is slightly higher than the melting point curve for alkanes with odd numbers of carbon atoms.

Uses of Hydrocarbons

chapter 4 alkanes and cycloalkanes -...

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