Alkanes
Saturated hydrocarbons
Structure of alkanes
Contain only C & H
Single bonds only – all carbons are sp3
and tetrahedral
All bond angles close to 109.5º
General formula – CnH2n+2
Constitutional (structural) isomerism
Compounds with the same empirical formula
but different structural formulas
Structure of alkanes
Straight chain (n-)
CH3CH2CH2CH3
n-butane
Branched
CH3
|
CH3CH2CH3
i-butane (isobutane)
Nomenclature of alkanes
 IUPAC system
 Prefixes
1=meth
5=pent
2=eth
6=hex
3=prop
7=hept
4=but
CH3CH2CH2CH2CH3
pentane
8=oct
9=non
10=dec
Nomenclature of alkanes
Common names
isobutane
isopentane
Nomenclature of alkanes
Endings
-ane – all single bonds
-ene – at least one double bond
-yne – at least one triple bond
H2C=CHCH3
propene
HC≡CCH3
propyne
Nomenclature of alkanes
Branches are named after the number of
carbons in the branch, + -yl
-CH3 = methyl
-CH2CH3 = ethyl
-CH2CH2CH3 = propyl
Common names for substituents
|
CH3CHCH3
Isopropyl (methylethyl)
Nomenclature of alkanes
|
CH3CHCH2CH3
sec-butyl (1-methylpropyl)
-CH2CHCH3
|
CH3
Isobutyl (2-methylpropyl)
Nomenclature of alkanes
|
CH3CCH3
|
CH3
tert-butyl
(2,2-dimethylpropyl)
Nomenclature of alkanes
Numbering – the longest chain is
numbered so that the first branch has the
lowest possible number.
2-methylpentane
Nomenclature of alkanes
3-methylpentane
Numbering is applied to the longest
possible chain
incorrect
Nomenclature of alkanes
3-methylhexane
Multiple identical substituents use prefixes
2 = di
3 = tri
4 = tetra
5 = penta
6 = hexa, etc.
Nomenclature of alkanes
Each substituent is designated by number
2, 4-dimethylpentane
Nomenclature of alkanes
Different substituents are named in
alphabetical order
4-ethyl-2-methylhexane
Cycloalkanes
Add “cyclo-“ in front of the chain name
Nomenclature of alkanes
cyclopropane
Can also be represented by a line-angle
drawing
Nomenclature of alkanes
cyclopropane
cyclobutane
Substituents are named as in other
alkanes
Draw a structure for methylcyclohexane
Nomenclature of alkanes
methylcyclohexane
1,4-dimethylcyclohexane
1-ethyl-4-methylcyclohexane
Nomenclature of alkanes
When the ring has fewer carbons than the
longest straight chain, the ring is named
as a substituent.
2-cyclopropylbutane
Physical properties
 Most alkanes are liquids or gases (C4 or below).
 Alkanes are nonpolar due to nonpolar nature or
C-H bond.
 Only intermolecular forces are weak London
dispersion forces (a type of van der Waals
interaction) – dispersion forces increase with
increasing molecular surface area
 Boiling point increases with molar mass
 Boiling point decreases with increased
branching
Physical properties
Solubility – insoluble in water or smaller
alcohols, soluble in each other or any
nonpolar solvent (carbon tetrachloride,
benzene, ether)
Density – less than water
Strong odor
Insulator
Conformations
Straight chain alkanes
Ethane
Newman projections
Conformations
Energy barrier to rotation is 2.8-3.0
kcal/mol.
At RT there is enough thermal energy to
allow rapid rotation about the C-C bond.
1 kcal/mol for each H-H interaction
Conformations
Conformations
Butane conformations along the central CH bond
Conformations
methyl-methyl interactions are 3kcal/mol
methyl-H interactions are 1.25 kcal/mol
Conformations of butane
Conformations of cycloalkanes
Cyclopentane – envelope conformation –
all bond angles  109.5º
Cyclohexanes
Chair conformation – most stable
Conformations of cycloalkanes
Boat and twist conformations – less stable
twist
boat
Conformations of cycloalkanes
Ring positions – equatorial and axial
Conformations of cycloalkanes
Equatorial positions are less sterically
hindered.
If large groups are attached to the ring
they will most always be in the equatorial
position.
Large groups in the axial position interfere
with groups in the other axial positions.
Conformations of cycloalkanes
Cis-trans isomerism in cycloalkanes
Cyclobutanes and cyclopentanes
trans-1,2-dimethylcyclobutane
cis-1,2-dimethylcyclobutane
Cis-trans isomerism in cycloalkanes
Cyclohexanes
cis-1,2-dimethylcyclohexane
Cis-trans isomerism in cycloalkanes
Adjacent (1,2) cis substituents are
equatorial-axial
cis-1,3 substituents are
equatorial/equatorial
cis-1,4 substituents are equatorial-axial
Chemical reactions
Mostly unreactive
Combustion
2C6H14+19O2  12CO2+14H2O
Halogenation
2C3H8 + Cl2  2CH3CHClCH3
F2>Cl2>Br2>I2
3º>2º>1º
Sources of Alkanes
Petroleum – products of refining process
Natural gas