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Level 185

Nomenclature & Conformations of Alkanes & Cycloalk


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Sources of Alkanes
Natural gas and petroleum. Formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure. Gas contains C1-C5 alkanes. Petroleum contains a very complex mixture.
Alkanes and Alkane Isomers
The formula for an alkane with no rings in it must be CnH2n+n where the number of C's is n.
Alkanes
- Compounds with C-C single bonds and C-H bonds only (no functional groups).
Alkane Isomers
- Alkanes with C's connected to no more than 2 other C's are straight- chain or normal alkanes.
Straight-Chain Alkyl Groups
An alkane group that is part of the larger molecule
General Name Formula
Prefix - Parent - Suffix
Cycloalkanes
Alkanes that have carbon atoms that form a ring (called alicyclic compounds).
Complex Cycloalkanes
Naturally occurring materials contain cycloalkane structures.
Physical Properties
Almost totally insoluble in water due to low polarity and lack of hydrogen bonding.
Properties of Cycloalkanes
Melting points are affected by the shapes and the way that crystals back so they do not change uniformly.
Conformations of Ethane
Conformers interconvert rapidly and actual structure is an average of conformers.
Representing Conformations
Sawhorse Representation and Newman Projections
Sawhorse Representation
Show molecules at an angle, showing a molecular model.
Newman projections
A way of drawing the conformations of a molecule by looking straight down the bond connecting two carbon atoms.
Ethane's Conformations
The barrier to rotation between conformations is small. The most stable conformations of ethane has all six C-H bonds away from each other (staggered). The least stable conformation has all six C-H bonds as c…
Conformations of Other Alkanes
- Propane (C3H8) torsional barrier around the carbon-carbon bonds.
Angle Strain
Energetically unfavorable due to bond angles being non-optimal
torsional strain
Decreased stability of a molecule associated with eclipsed bonds
Steric strain
An increase in energy resulting when atoms are forced too close to one another
Stability of Cycloalkanes: The Baeyer Strain Theory
Rings from 3 to 30 C's do exist but are stained due to bond bending distortions and steric interactions.
Cyclopropane: An Orbital View
3-membered ring must have planar structure.
Conformations of Cyclobutane
Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens.
cyclopentane
bond angle is 108 but wants 109.5; really stable; envelope confirmation
Conformations of Cyclohexane
- The cyclohexane ring is free of angle strain and torsional strain (all neighboring C-H bonds are perfectly staggered).
Axial and Equatorial Bonds
The chair conformation has two kinds of positions for substituents on the ring: axial positions and equatorial positions.
Axial and Equatorial Bonds in Cyclohexane
Chair cyclohexane has six axial hydrogens perpendicular to the ring and six equatorial hydrogens near the plane of the ring.