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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 hydrogens2o (secondary) hydrogens also known as methylene hydrogens3o (tertiary) hydrogens also known as methine hydrogens
(Hydrogens on CH4 or NH4+ are called quaternary hydrogens)
Simplified IUPAC Nomenclature Rules
1. Locate longest alkyl chain. The length of this chain determines the parent name.
2. If there are chains of equal length, choose the one with larger number of substituents (avoids complex substituents)
3. 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.
4. Substituent names are placed in alphabetical order (ignore prefixes such as di, tri, tetra, sec & tert, etc. BUT NOT cyclo and iso).
5. The final name has no spaces between words. Commas and hyphens are used between “words”.
Alkyl Halides and Alcohols
Cycloalkanes
(lower set of numbersin 1,3 vs 1,5)
(lower set of numbersin 1,2,4 vs 1,3,4)
4-Ethyl-3-methylcyclohexanol
6123
4
OH5
1-Ethyl-2-methylcyclohexan-4-ol
5432
1
OH6
NOT
(Here, the alcohol takes precedence and numbering starts there)
Bicyclic Compounds
Start numbering at the bridgehead and go around the longest bridge first
1-isopropylbicyclo[2.2.2]octane
2
34
5
6
7
8
1
7-Methylbicyclo[4.3.0]nonane
12
3
45
67
8
9
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
Torsional strain: due to unfavourable “eclipsing” orbital interactions
Steric strain: due to “crowding” of atomic nuclei
Interaction Cause EnergyEclipsing H Torsional strain 4.0 kJ/molEclipsing H and CH3 Mostly torsional strain 6.0Eclipsing CH3 Torsional and steric 11Gauche CH3 Steric 4
The Zig-Zag Structure of Linear Alkanes
H
H
H
HH
H
H
Why do we draw alkanes in a zig-zag manner?
What’s in common in these compounds?
DiamondDodecahedrane
(Hint: JRR Tolkien's first book of his famous trilogy)
cyclohexane
Cyclohexane, the true Lord of the Rings (in organic chemistry; mastering it will give you great power indeed!)
Lord of the Rings in Middle Earth. The bearer has immense power, but it’s all fantasy.
Lord of the Rings
The Cycloalkanes
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.
cyclopropane cyclobutane cyclopentane cyclohexane
60o 90o 108o 120o
[109o is the perfect tetrahedral angle (sp3 hybrid)]
Rings have angle strain, in addition to torsion and steric strains
“flat” “bent” “envelope” “chair”
The True Shapes of Cycloalkanes
cyclopropane cyclobutane cyclopentane cyclohexane
60o 90o 108o 120o
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
1
4
space filling model
model from your molecular model set
Chair and Boat Conformations
chair boat
(lowest energy form) (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
CH3
CH3
CH3
CH3
1,2-Dimethyl Cyclohexanes
CH3
CH3
CH3
CH3
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.
“flexible” “rigid” (more stable)
The Decalins
Synthesis of Alkanes: Hydrogenation of Alkenes and Alkynes
Corticosteroids: metabolism & immune functionAnabolic Steroids: muscle & bone growthSex Steroids: human reproduction
Cholesterol is a waxy steroid found in the cell membranes. It is an essential component of mammalian cell membranes.
The Importance of Rings (The Steriods)
An Infinite Array of Cyclohexane Rings(minus the hydrogens)
Diamond
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