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Chapter 4 Alkanes and Cycloalkanes
!Structure, Nomenclature and Conformations
Carbon Bonding
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 CH
ethane CH
propane CH
butane CH
Structural Representation of Simple Alkanes
Simple alkanes have the general formula CnH2n+2.
Hydrocarbons The Simplest Class of Organic Compounds
C. Hydrocarbons are further subclassified
C C C C C
UNSATURATED
ALIPHATIC AROMATIC
SATURATED
Nomenclature
n Molecular Condensed Name Melting Boiling
Formula Structural Formula point ( point (
1 CH methane -182 -1622 C ethane -183 -893 C propane -190 -424 C butane -138 -15 C pentane -130 366 C hexane -95 697 C heptane -91 988 C octane -57 1269 C nonane -51 151
10 C 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
Hydrocarbon Nomenclature
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
Hydrocarbon Nomenclature
“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
Hydrocarbon Nomenclature
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- CH
ethyl- CH
propyl- CH
isopropyl- (CH
n-butyl- CH
s-butyl- CH
isobutyl- (CH
t-butyl- (CH
Hydrocarbon Nomenclature -Alkyl GroupsName Formula Line structure
C3H7-
C4H9-
C2H5-
CH3-
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
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
12 3
4
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
34
5
6 7
8
910
C10H18
cubane basketane
housane churchane
Question: Are pentane and cyclopentane isomers ???
C5H12 C5H10
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
Isomerism What is the relationship between these two
molecules?
and
2,4-dimethylpentane 2,2-dimethylpentane
Isomerism What is the relationship between these two
molecules?
2,3-dimethylpentane 2,3-dimethylpentane
Combustion of Hydrocarbons
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
Combustion of Hydrocarbons
n Δ Δ
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
least stable
most stable
Combustion of Hydrocarbons
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 t h e f o o t o f t h e 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
