1. CHE-240 Unit 1 Structure and Stereochemistry of Alkanes CHAPTER THREE
Terrence P. Sherlock
Burlington County College
2004

2. IUPAC Names Find the longest continuous carbon chain.
Number the carbons, starting closest to the first branch.
Name the groups attached to the chain, using the carbon number as the locator.
Alphabetize substituents.
Use di-, tri-, etc., for multiples of same substituent =>
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3. Longest Chain
The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.)
If there are two possible chains with the same number of carbons, use the chain with the most substituents.
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4. Number the Carbons
Start at the end closest to the first attached group.
If two substituents are equidistant, look for the next closest group. 1 2 3 4 5 6 7
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5. Name Alkyl Groups CH3-, methyl CH3CH2-, ethyl CH3CH2CH2-, n-propyl
CH3CH2CH2CH2-, n-butyl
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6. Alphabetize Alphabetize substituents by name.
Ignore di-, tri-, etc. for alphabetizing.
3-ethyl-2,6-dimethylheptane
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7. Complex Substituents
If the branch has a branch, number the carbons from the point of attachment.
Name the branch off the branch using a locator number.
Parentheses are used around the complex branch name. 1 2 3
1-methyl-3-(1,2-dimethylpropyl)cyclohexane =>
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8. Boiling Points of Alkanes
Branched alkanes have less surface area contact,
so weaker intermolecular forces.
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9. Branched Alkanes Lower b.p. with increased branching
Higher m.p. with increased branching
Examples: H C 3 2
bp 60°C
mp -154°C
bp 58°C
mp -135°C
=>
bp 50°C
mp -98°C
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10. Ethane Conformers Staggered conformer has lowest energy.
Dihedral angle = 60 degrees
model H
Newman
projection
sawhorse
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11. Ethane Conformers (2) Eclipsed conformer has highest energy
Dihedral angle = 0 degrees
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12. Conformational Analysis
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13. Cis-Trans Isomerism Cis: like groups on same side of ring
Trans: like groups on opposite sides of ring =>
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14. Cycloalkane Stability
5- and 6-membered rings most stable
Bond angle closest to 109.5
Angle (Baeyer) strain
Measured by heats of combustion per -CH =>
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15. Heats of Combustion Alkane + O2  CO2 + H2O
166.6
164.0
158.7
158.6
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158.3
Long-chain
157.4
157.4
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16. Cyclopropane Large ring strain due to angle compression
Very reactive, weak bonds
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17. Cyclopropane (2) Torsional strain because of eclipsed hydrogens =>
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18. Cyclobutane Angle strain due to compression
Torsional strain partially relieved by ring-puckering
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19. Cyclopentane
If planar, angles would be 108, but all hydrogens would be eclipsed.
Puckered conformer reduces torsional strain.
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20. Cyclohexane Combustion data shows it’s unstrained.
Angles would be 120, if planar.
The chair conformer has 109.5 bond angles and all hydrogens are staggered.
No angle strain and no torsional strain =>
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21. Chair Conformer
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22. Boat Conformer
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23. Conformational Energy
Chapter 3
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24. Axial and Equatorial Positions
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25. Monosubstituted Cyclohexanes
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26. 1,3-Diaxial Interactions
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27. Disubstituted Cyclohexanes
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28. Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial around the ring.
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29. Bulky Groups
Groups like t-butyl cause a large energy difference between the axial and equatorial conformer.
Most stable conformer puts t-butyl equatorial regardless of other substituents.
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30. POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION” L. G
POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5TH EDITION” L.G. WADE ALL MATERIALS USED WITH PERMISSION OF AUTHOR PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE ORGANIC CHEMISTRY COURSE BY: ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN
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