ALKANES - Alkanes Nomenclature, Conformational Analysis,...

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© E.V. Blackburn, 2005 Alkanes Nomenclature, Conformational Analysis, and an Introduction to Synthesis
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© E.V. Blackburn, 2005 Alkanes saturated aliphatic hydrocarbons paraffins general formula C n H 2n+2 acyclic hydrocarbons
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© E.V. Blackburn, 2005 Sources of methane major constituent of natural gas (97%) “firedamp” of coal mines “marsh gas” product of anaerobic plant decay
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© E.V. Blackburn, 2005 Cycloalkanes Single ring cycloalkanes have the general formula C n H 2n thus they have two fewer hydrogen atoms than alkanes.
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© E.V. Blackburn, 2005 Methane – its structure tetrahedral H H H H sp 3 109.5 o
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© E.V. Blackburn, 2005 Methane – its structure “Fischer Structure” “Lewis Structure” H H H H H H H H C : : .. ..
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© E.V. Blackburn, 2005 Space-filling models Space-filling models depict atoms as spheres and therefore show the volume occupied by atoms and molecules.
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© E.V. Blackburn, 2005 Ethane - C 2 H 6 sp 3 1.10Å 1.53Å A structural formula is a Lewis structure which shows the connectivity of its atoms - the order in which atoms are connected. C C H H H H H H
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© E.V. Blackburn, 2005 What is ethane’s structure? staggered eclipsed Or something in between?
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© E.V. Blackburn, 2005 Conformations Conformations are structures that are interconvertible by rotation about single bonds. This is the staggered conformation of ethane: staggered This is an example of a sawhorse formula.
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© E.V. Blackburn, 2005 Newman projections staggered eclipsed θ = 60 ο Look along the C-C bond. The nearest carbon masks the rear carbon but all six bonds to the two carbons are visible. The nearest carbon is represented by the point where the three bonds meet. The rear carbon is represented by the circle.
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© E.V. Blackburn, 2005 Space-filling model of ethane staggered eclipsed
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© E.V. Blackburn, 2005 Potential energy Stability of conformations rotation eclipsed staggered eclipsed 12 kJ/mol 60 o 120 o 180 o
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© E.V. Blackburn, 2005 Torsional strain Torsional energy is the energy required to rotate the molecule about the C-C bond. The relative instability of the eclipsed conformation is said to be due to torsional strain .
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© E.V. Blackburn, 2005 Propane - C 3 H 8 energy barrier = 14 kJ/mol H C C H H H H C H H H
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© E.V. Blackburn, 2005 Butane - C 4 H 10 compound A B bp -12 0 C mp -159 -138 solubility 1320 1813 mL/100mL C 2 H 5 OH CH 3 CH 2 CH 2 CH 3 (CH 3 ) 3 CH H 3 C CH 3 CH 3 H
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© E.V. Blackburn, 2005 Conformations H 3 C H H H H CH 3 anti All conformations are free of torsional strain. H H CH 3 H H CH 3 H CH 3 H H H CH 3 gauche
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The methyl groups in the gauche conformations are crowded together and steric repulsion results. These conformations are less stable due to steric strain. Stability of conformations
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This note was uploaded on 04/27/2011 for the course CHEM 261 taught by Professor Drclive during the Spring '09 term at University of Alberta.

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ALKANES - Alkanes Nomenclature, Conformational Analysis,...

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