Chapter 3 Structure and Stereochemistry of Alkanes

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Chapter 3Structure and Stereochemistryof Alkanes
Organic Chemistry, 5th EditionL. G. Wade, Jr.
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Alkane Formulas

• All C-C single bonds
• Saturated with hydrogens
• Ratio CnH2n2
• Alkane homologs CH3(CH2)nCH3
• Same ratio for branched alkanes

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Common Names

• Isobutane, isomer of butane
• Isopentane, isohexane, etc., methyl branch on
  next-to-last carbon in chain.
• Neopentane, most highly branched
• Five possible isomers of hexane,18 isomers of
  octane and 75 for decane!
  gt

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Pentanes
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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.
  gt

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Longest Chain

• The number of carbons in the longest chain
  determines the base name ethane, hexane.
• If there are two possible chains with the same
  number of carbons, use the chain with the most
  substituents.

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Number the Carbons

• Start at the end closest to the first attached
  group.
• If two substituents are equidistant, look for the
  next closest group.

gt
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Name Alkyl Groups

• CH3-, methyl
• CH3CH2-, ethyl
• CH3CH2CH2-, n-propyl
• CH3CH2CH2CH2-, n-butyl

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Propyl Groups
H
H
n-propyl
isopropyl
A secondary carbon
gt
A primary carbon
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Butyl Groups
H
H
n-butyl
sec-butyl
A secondary carbon
gt
A primary carbon
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Isobutyl Groups
H
H
isobutyl
tert-butyl
A tertiary carbon
gt
A primary carbon
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Alphabetize

• Alphabetize substituents by name.
• Ignore di-, tri-, etc. for alphabetizing.

3-ethyl-2,6-dimethylheptane

gt
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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-methyl-3-(1,2-dimethylpropyl)cyclohexane gt
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Physical Properties

• Solubility hydrophobic
• Density less than water 1 g/mL
• Boiling points increase with increasing carbons
  (little less for branched chains).

Melting points increase with
increasing carbons (less for odd- number of
carbons).

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Boiling Points of Alkanes
Branched alkanes have less surface area
contact, so weaker intermolecular forces.
gt
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Melting Points of Alkanes
Branched alkanes pack more efficiently into a
crystalline structure, so have higher m.p.
gt
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Branched Alkanes

• Lower b.p. with increased branching
• Higher m.p. with increased branching
• Examples

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Major Uses of Alkanes

• C1-C2 gases (natural gas)
• C3-C4 liquified petroleum (LPG)
• C5-C8 gasoline
• C9-C16 diesel, kerosene, jet fuel
• C17-up lubricating oils, heating oil
• Origin petroleum refining
  gt

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Reactions of Alkanes

• Combustion

• Cracking and hydrocracking (industrial)

• Halogenation

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Conformers of Alkanes

• Structures resulting from the free rotation of a
  C-C single bond
• May differ in energy. The lowest-energy
  conformer is most prevalent.
• Molecules constantly rotate through all the
  possible conformations.
  gt

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Ethane Conformers

• Staggered conformer has lowest energy.
• Dihedral angle 60 degrees

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Ethane Conformers (2)

• Eclipsed conformer has highest energy
• Dihedral angle 0 degrees

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Conformational Analysis

• Torsional strain resistance to rotation.
• For ethane, only 3.0 kcal/mol

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Propane Conformers

• Note slight increase in torsional strain
• due to the more bulky methyl group.

gt
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Butane Conformers C2-C3

• Highest energy has methyl groups eclipsed.
• Steric hindrance
• Dihedral angle 0 degrees

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Butane Conformers (2)

• Lowest energy has methyl groups anti.
• Dihedral angle 180 degrees

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Butane Conformers (3)

• Methyl groups eclipsed with hydrogens
• Higher energy than staggered conformer
• Dihedral angle 120 degrees

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Butane Conformers (4)

• Gauche, staggered conformer
• Methyls closer than in anti conformer
• Dihedral angle 60 degrees

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Conformational Analysis
gt
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Higher Alkanes

• Anti conformation is lowest in energy.
• Straight chain actually is zigzag.

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Cycloalkanes

• Rings of carbon atoms (CH2 groups)
• Formula CnH2n
• Nonpolar, insoluble in water
• Compact shape
• Melting and boiling points similar to branched
  alkanes with same number of carbons
  gt

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Naming Cycloalkanes

• Cycloalkane usually base compound
• Number carbons in ring if gt1 substituent.
• First in alphabet gets lowest number.
• May be cycloalkyl attachment to chain.

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Cis-Trans Isomerism

• Cis like groups on same side of ring
• Trans like groups on opposite sides of ring
  
  gt

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Cycloalkane Stability

• 5- and 6-membered rings most stable
• Bond angle closest to 109.5?
• Angle (Baeyer) strain
• Measured by heats of combustion per -CH2 -
  
  gt

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Heats of Combustion Alkane O2 ? CO2 H2O
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Cyclopropane

• Large ring strain due to angle compression
• Very reactive, weak bonds

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Cyclopropane (2)

• Torsional strain because of eclipsed ydrogens

gt
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Cyclobutane

• Angle strain due to compression
• Torsional strain partially relieved by
  ring-puckering

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Cyclopentane

• If planar, angles would be 108?, but all
  hydrogens would be eclipsed.
• Puckered conformer reduces torsional strain.

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Cyclohexane

• Combustion data shows its 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.
  
  gt

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Chair Conformer
gt
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Boat Conformer
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Conformational Energy
gt
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Axial and Equatorial Positions
gt
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Monosubstituted Cyclohexanes
gt
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1,3-Diaxial Interactions
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Disubstituted Cyclohexanes
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Cis-Trans Isomers

• Bonds that are cis, alternate axial-equatorial
  around the ring.

gt
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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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Bicyclic Alkanes

• Fused rings share two adjacent carbons.
• Bridged rings share two nonadjacent Cs.

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Cis- and Trans-Decalin

• Fused cyclohexane chair conformers
• Bridgehead Hs cis, structure more flexible
• Bridgehead Hs trans, no ring flip possible.

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End of Chapter 3