4. NOMENCLATURE AND CONFORMATIONS OF
ALKANES AND CYCLOALKANES.
(approx. lecture time: 4 hr)
Topics covered: 4.1-4.17.
Hydrocarbons from Petroleum
The Unbranched Alkanes
Nomenclature
1o (primary) hydrogens also known as methyl hydrogens
2o (secondary) hydrogens also known as methylene hydrogens
3o (tertiary) hydrogens also known as methine hydrogens
(Hydrogens on CH4 or NH4+ are called quaternary hydrogens)
Simplified IUPAC Nomenclature Rules
1.
2.
3.
4.
5.
Locate longest alkyl chain. The length of this chain
determines the parent name.
If there are chains of equal length, choose the one with
larger number of substituents (avoids complex substituents)
Number the chain beginning at end closer to the first
substituent. If there is a tie, begin numbering at the end
closer to the second substituent.
Substituent names are placed in alphabetical order (ignore
prefixes such as di, tri, tetra, sec & tert, etc. BUT NOT
cyclo and iso).
The final name has no spaces between words. Commas and
hyphens are used between “words”.
Alkyl Halides and Alcohols
Cycloalkanes
(lower set of numbers
in 1,3 vs 1,5)
(lower set of numbers
in 1,2,4 vs 1,3,4)
(Here, the alcohol takes precedence
and numbering starts there)
Bicyclic Compounds
Start numbering at the
bridgehead and go around
the longest bridge first
Alkenes
For simple cyclic alkenes (no alcohol
function), the alkene must be
contained in the numbers “1,2”
The presence of the
alcohol takes
precedence. Start
numbering here.
Alkynes and Enynes
Numbering enynes (ene and yne): start
from the end closer to the first multiple
bond. If there is a tie, give the alkene the
lower number
Conformational Analysis of Ethane
Energy Diagram for C-C Bond Rotation
Conformational Analysis of Butane
Energy Diagram for C-C Bond Rotation
Strain Energies
Interaction
Eclipsing H
Eclipsing H and CH3
Eclipsing CH3
Gauche CH3
Cause
Torsional strain
Mostly torsional strain
Torsional and steric
Steric
Energy
4.0 kJ/mol
6.0
11
4
Torsional strain: due to unfavourable “eclipsing”
orbital interactions
Steric strain: due to “crowding” of atomic nuclei
The Zig-Zag Structure of Linear Alkanes
Why do we draw alkanes in a zig-zag manner?
What’s in common in these compounds?
cyclohexane
Diamond
Dodecahedrane
(Hint: JRR Tolkien's first book of his famous trilogy)
Lord of the Rings
Lord of the Rings in
Middle Earth. The
bearer has immense
power, but it’s all
fantasy.
Cyclohexane, the true Lord
of the Rings (in organic
chemistry; mastering it will
give you great power
indeed!)
The Cycloalkanes
[109o is the perfect tetrahedral angle (sp3 hybrid)]
Rings have angle strain, in addition to
torsion and steric strains
Angle Strain: due to expansion or compression of required
bond angles.
Torsional strain: due to unfavourable eclipsing
interactions of bonding electrons.
Steric strain: due to repulsive interactions of atoms in
close proximity.
The True Shapes of Cycloalkanes
“flat”
“bent”
“envelope”
“chair”
Total Strain Energy vs Ring Size
Cycloalkane
Total Strain
Cyclopropane
115 kJ/mol
Cyclobutane
110
Cyclopentane
25
Cyclohexane
0
Cycloheptane
24
Cyclooctane
40
Cyclononane
53
Conformational Analysis of Cyclohexane
model from your
molecular model set
space filling model
4
1
Chair and Boat Conformations
chair
(lowest energy form)
boat
(A high energy form)
The Cyclohexane Ring Flip
Energy Level Diagram for Cyclohexane Ring Flip
(For information only. Do not memorize!)
Conformational Analysis for Methylcyclohexane
1,3-diaxial
interactions
1,3-Dixial Interactions are Gauche Interactions
1,4-Dimethyl Cyclohexanes
1,2-Dimethyl Cyclohexanes
Importance of 3-D Structures
Controlling and understanding the 3-D shapes of
organic molecules are essential for designing
molecules that will function in desired ways.
Chemical information is “stored” and “transferred”
by the 3-D shapes of organic molecules.
The Decalins
“flexible”
“rigid” (more stable)
Synthesis of Alkanes: Hydrogenation of
Alkenes and Alkynes
The Importance of Rings
(The Steriods)
Cholesterol is a waxy steroid
found in the cell membranes.
It is an essential component of
mammalian cell membranes.
Corticosteroids: metabolism
& immune function
Anabolic Steroids: muscle &
bone growth
Sex Steroids: human
reproduction
An Infinite Array of Cyclohexane Rings
(minus the hydrogens)
Diamond