SECTION II (CHAP. 4)
ALKANES & CYCLOALKANES
FUNDAMENTAL CONCEPTS
 Naming alkanes
 Naming cyclic alkanes
 Naming alkyl halides
 Naming bicyclic compounds
 Newman project
 Conformational analysis
NAMING ALKANES (CnH2n+2)
Number of
Carbons
Generic
Name
Example
Number of
Carbons
Generic
Name
Example
1C
Methane
CH4
11C
Undecane
CH3(CH2)9CH3
2C
Ethane
CH3CH3
12C
Dodecane
CH3(CH2)10CH3
3C
Propane
CH3CH2CH3
13C
Tridecane
CH3(CH2)11CH3
4C
Butane
CH3(CH2)2CH3
14C
Tetradecane
CH3(CH2)12CH3
5C
Pentane
CH3(CH2)3CH3
15C
Pentadecane
CH3(CH2)13CH3
6C
Hexane
CH3(CH2)4CH3
16C
Hexadecane
CH3(CH2)14CH3
7C
Heptane
CH3(CH2)5CH3
17C
Heptadecane
CH3(CH2)15CH3
8C
Octane
CH3(CH2)6CH3
18C
Octadecane
CH3(CH2)16CH3
9C
Nonane
CH3(CH2)7CH3
19C
Nonadecane
CH3(CH2)17CH3
10C
Decane
CH3(CH2)8CH3
20C
Icosane
CH3(CH2)18CH3
NAMING ALKANES (CnH2n+2)
In 1892, as the number of known molecules grew, chemists decided that a
SYSTEMATIC naming system was needed.
The IUPAC system (International Union of Pure and Applied Chemistry) was
born. UIPAC’s Rules
Rule 1: Find the longest continuous chain of carbon atoms, and use the name
of this chain as the base name of the compound.
Rule 2: Number the longest chain, beginning with the end of the chain
nearest a substituent.
Rule 3: Name the groups attached to the longest chain as alkyl groups. Give
the location of each alkyl group by the number of the main chain carbon
atom to which it is attached.
Rule 4: Write the alkyl groups in alphabetical order regardless of their position
on the chain.
Rule 5: When two or more of the same alkyl substituent are present, use the
prefixes di-, tri-, tetra-, penta-, and so on (ignored in alphabetizing) to avoid
having to name the alkyl group twice.
NAMING ALKANES (CnH2n+2)
When there are two longest chains of equal length, use the chain with the
greatest number of substituents.
NAMING ALKANES (CnH2n+2)
To name the substituents, count the number of carbons in each side group,
and use the terms from Table to name the substituents by replacing the
“ane” end “yl”.
CH3 –
CH3CH2 –
CH3CH2CH2 –
Methyl
Ethyl
n-Propyl
CH3CH2CH2CH2–
CH3CH2CH2 CH2CH2–
CH3CH2CH2CH2CH2CH2 –
n-Butyl
n-Pentyl
n-Hexyl
NAMING ALKANES (CnH2n+2)
Some branched substituents have common names that you may want to
memorize
NAMING ALKANES (CnH2n+2)
However, these complex substituents can be named differently:
 Number the longest carbon chain WITHIN
the substituent starting with the carbon
directly attached to the main chain
 Name the substituent (in this case butyl)
1
4
2
3
 Name and Number the substituent’s side
(2-methylbutyl)
group (in this case 2-methyl)
Problem 4.65 P. 184; 4.69 P. 185
NAMING ALKYL HALIDES
When using the IUPAC nomenclature, name the compound as a
haloalkane:
 Use prefixes such as “fluoro” for a fluorine atom, “chloro” for a chlorine
atom, “bromo” for a bromine or “iodo” for an iodine atom.
 If you have more than one of the same halogen atoms, use “di”, “tri”,
“tetra” etc… to indicate the number of these atoms.
NAMING ALKANES (CnH2n+2)
Name the following molecules:
Practice 4.1 to 4.3 P. 141; 4.4, 4.5
P.143; 4.14 P. 153; 7.1 P. 283
Cyclic Alkanes
5- and 6-membered rings most stable: Bond angle closest to 109.5
Some alkanes are cyclic, thus they are called “cycloalkanes”. Their empiric
formula is CnH2n with n ≥ 3
NAMING CYCLOALKANES (CnH2n)
Because of the restriction by their cyclic geometry, the angle compression
generates a strong ring strain. Thus, they have weak bonds, and the ring can
be easily broken to relieve the strain.
NAMING CYCLOALKANES
Name the following molecules
Practice 4.6 P. 145, 4.8 to 4.10 P. 148
NAMING BICYCLIC ALKANES
There are three ways that two rings may be joined: Fused rings, Bridged rings
or Spirocyclic compounds
The naming of bicyclic compounds is based on the name of the alkane
having the same number of carbons as there are in the ring system.
For Fused rings and Bridged rings, the prefix bicyclo and a set of brackets
enclosing the number of carbons on each bridge, arranged in a decreasing
order precedes the name.
To number the bicyclo parent chain, start at a bridgehead carbon and
number the longest carbon chain connecters first.
Fused rings: the two rings share two adjacent carbons (more common)
bicyclo[4.3.0]nonane
NAMING BICYCLIC ALKANES
Bridged rings: the two rings share two nonadjacent carbons
bicyclo[2.2.1]heptane
Practice 4.11, 4.12 P. 150, Problems 4.36, 4.37, 4.40 P.
182 - 183
Spirocyclic compounds: the two rings
share only one atom of carbon
The name follows the prefix spiro and
a set of brackets enclosing the
number of carbons on each bridge,
arranged in a increasing order
The numbering must start from the
carbon adjacent to the bridgehead
and number the small ring first
10 11
6
9
8
7
1
5
2
3
4
spiro[5.5]undecane
THE CHEMIST TREE
PHYSICAL PROPERTIES OF ALKANES
Alkanes are non polar compounds, so they are insoluble in water
(hydrophobic).
Density: less than 1 g/mL
Melting and boiling points increase with increasing carbons (little less for
branched chains)
C1-C2: gases (natural gas) (gas at
their natural state)
C3-C4: liquefied petroleum (LPG) (gas
at their natural state)
C5-C8: gasoline (volatile liquids)
C9-C16: diesel, kerosene, jet fuel
(high-boiling
liquids,
somewhat
viscous)
C17-up: lubricating oils, heating oil
(oils)
PHYSICAL PROPERTIES OF ALKANES
As the number of carbons in an alkane
increases, the boiling point will increase due
to the larger surface area and the increased
van der Waals attractions.
The boiling point will
decrease
with
the
increase of branching
3D REPRESENTATION OF ALKANES
Methane
Tetrahedral
sp3 hybrid
carbon with angles of 109.5º
Ethane
Two sp3 hybrid carbons.
Rotation about the C—C sigma bond.
The Newman Projection
THE NEWMAN PROJECTION
Draw the Newman projection of the following molecules or convert a
Newman projection into the corresponding 3D
Practice 4.19, 4.20 P.163, Problems 4.42 P. 183; 4.56 P 184; 4.68 P. 185; 4.74
P. 186
CONFORMATIONAL ANALYSIS
Conformations are different arrangements of atoms caused by rotation
about a single bond, and a specific conformation is called a conformer.
Pure conformers cannot be isolated in most cases, because the molecules
are constantly rotating through all the possible conformations.
CONFORMATIONAL ANALYSIS
Staggered conformer has lowest energy
The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in
energy. At room temperature, this barrier is easily overcome, and the
molecules rotate constantly
Graph
of the
relative
torsional energies of ethane
CONFORMATIONAL ANALYSIS
The staggered conformations of propane are also lower in energy than the
eclipsed conformations. Since the methyl group occupies more space than
hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for
ethane
Graph of the relative torsional energies of propane
CONFORMATIONAL ANALYSIS
Butane Conformations
Butane has two different staggered conformations: gauche (60° between
the methyl groups) and anti (180° between the methyl groups).
The eclipsed conformation where the dihedral angle between the methyl
groups is 0° is referred to as totally eclipsed.
CONFORMATIONAL ANALYSIS
The stability of the different staggered conformations differs by 3.8 kJ/mol
The anti conformation has less steric hindrance
Problems 4.44, 4.53, 4.55 P.
183; 4.56, 4.57 P. 184, 4.67,
4.68 P.185. 1.75 P. 186
This kind of interference
between two bulky groups
is called steric strain or
steric hindrance
CONFORMATIONAL ANALYSIS
Graph of the relative torsional energies of ethyl-1,2-diamine (H2NCH2CH2NH2)
25
20
15
10
5
0
0
50
100
150
200
250
300
350
400
Cyclopropane
Torsional strain – eclipsing
C-H bonds all the way
around the ring – see the
Newman projection
Cyclopropane is 44 kJ/mol less stable than cyclohexane per CH2 group. It is
highly strained and very reactive.
CONFORMATIONAL ANALYSIS
To optimize the bond angles, most cycloalkanes are NOT flat in their most
stable conformation. Carbon atoms in cycloalkanes are sp3 hybridized.
Cyclobutane
The torsional strain is partially relieved by ring-puckering
Cyclopentane
If it was planar the angles
would be 108, and all
hydrogens
would
be
eclipsed.
The
puckered
conformer reduces torsional
strain.
CONFORMATIONAL ANALYSIS
Cyclohexane – chair conformation
If the molecule was planar the bond angles would be 120. The chair
conformer has 109.5 bond angles and all hydrogens are staggered. No
angle strain and no torsional strain
Practice 4.22 P. 168, 4.23 P. 169
Problems 4.46, P. 183; 4.59 P. 184
CONFORMATIONAL ANALYSIS
Cyclohexane – Boat conformation
Cyclohexane (Conformational Energy)
CONFORMATIONAL ANALYSIS
Cyclohexane (Axial and Equatorial Positions)
The most important result in chair conversion is that any substituent that is
axial in the original conformation becomes equatorial in the new
conformation.
Practice 4.25 P. 173; 4.28 P. 174
CONFORMATIONAL ANALYSIS
Cyclohexane (Axial and Equatorial Positions Chair 1 vs Chair 2 )
The equatorial methyl conformation (Chair 2) is favored over the axial
methyl conformation (Chair 1) by approximately 7.28 kJ/mol
ln Keq = ‒ (‒ 7280 J mol-1/ 8.314 J. K-1. Mol-1 x 298 K)
Keq = e2.938 = 18.9/1 = equatorial/axial
% (equatorial) = (18.9/18.9+1) x 100 = 95%
= 2.938
CONFORMATIONAL ANALYSIS
Trans-1,3-dimethylcyclohexane
Both conformations have one axial and one equatorial methyl group so they
have the same energy.
A. Draw both chair conformations of cis-1,2-diisopropylcyclohexane, and
determine which conformer is more stable.
B. Repeat for the trans isomer. Problems 4.47 to 4.52 P. 183, 4.61, 4.62 P. 184
4.66, 4.72 P. 185
C. Predict which isomer (cis or trans) is more stable
D. Draw
the
most
ethylcyclohexane
stable
conformation
of
trans-1-sec-butyl-3-
CIS/TRANS ISOMERISM
When naming a disubstituted cycloalkane, use the prefix cis when there are
two groups on the same side of the ring
Use the prefix trans when two substituents are on opposite sides of a ring
Polycyclic systems
POLYCYCLIC SYSTEMS
Diamonds are formed by fusing many chairs together three dimensionally in
all directions
There are many biologically important steroids, all of which involve fusing
cycloalkanes as part of their structure