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