Cycloalkanes

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• Cycloalkanes
• Alkanes that form rings are called cycloalkanes.
• Cyclopropane and cyclobutanes are strained
  because the C-C-C bond angles in the ring are
  less than 109.5? required for the tetrahedral
  geometry.
• Because of the strain in the ring, cyclopropane
  is very reactive.

2

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• Reactions of Alkanes
• The C-C and C-H bonds are very strong and
  relatively nonpolar. Therefore, alkanes are very
  unreactive.
• At room temperature alkanes do not react with
  acids, bases, or strong oxidizing agents.
• Alkanes do combust in air (making them good
  fuels)
• 2C2H6(g) 7O2(g) ? 4CO2(g) 6H2O(l) ?H -2855
  kJ
• C2H6 O2 ? CO H2O (incomplete combustion)

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• Substitution halogen atoms replace hydrogen
  atoms (UV light is used to break Bonds)
• CH4 X-X CH3X H-X
• step 1 initiation (formation of free radical
  by homolytic fission) Cl2 ? 2 Cl. (by UV light)
• step 2 - propagation
• Cl. CH4 ? CH3. HCl
• CH3. Cl2 ? CH3Cl Cl.
• step 3 termination
• Cl. Cl. ? Cl2
• Cl. CH3. ? CH3Cl
• CH3. CH3. ? C2H6
• Dehydrogenation H2 is removed, causing a CC
  bond
• CH3-CH3 CH2CH2 H2

5
Unsaturated Hydrocarbons

• Alkenes
• Alkenes contain C, H atoms and single and double
  bonds.
• The simplest alkenes are H2CCH2 (ethene) and
  CH3CHCH2 (propene)
• their trivial names are ethylene and propylene.
• Alkenes are named in the same way as alkanes with
  the suffix -ene replacing the -ane in alkanes.
• The location of the double bond is indicated by a
  number.

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• Geometrical isomers are possible since there is
  no rotation about a CC ? bond. Geometrical
  isomers have different physical properties.
• Cis the same side
• Trans across

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• Examples 1,2-dichloroethane

Cis mp 60.3ºC trans mp 47.5ºC
But-2-ene
cis mp -139ºC trans mp -106ºC
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• Examples name the following
• 1.
• transpent-2-ene
• 2.
• 5-methyl-cis-hex-2-ene

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• Alkynes
• Alkynes are hydrocarbons with one or more C?C
  bond. Therefore, alkynes have one ? and two ?
  bonds between two C atoms.
• Ethyne (acetylene) is a reactive alkyne HC?CH.
• When acetylene is burned in the presence of
  oxygen (oxyacetylene torch) the temperature is
  about 3200 K.
• Alkynes are named in the same way as alkenes with
  the suffix -yne replacing the -ene for alkenes.

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Unsaturated Hydrocarbons

• Addition Reactions of Alkenes and Alkynes
• The most dominant reaction for alkenes and
  alkynes involves the addition of something
  (hydrogen, halogen, water) to the two atoms which
  form the double bond
• Note that the C-C ? bond has been replaced by two
  C-Br ? bonds.
• Alkenes will react with bromine water by
  addition, while alkanes will show no reaction
  with bromine water. This is one way to
  distinguish between alkanes and alkenes.

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• A common addition reaction is hydrogenation
• CH3CHCHCH3 H2 ? CH3CH2CH2CH3
• Hydrogenation requires high temperatures and
  pressures as well as the presence of a catalyst.
• It is possible to cause hydrogen halides and
  water to add across ? bonds
• CH2CH2 HBr ? CH3CH2Br
• CH2CH2 H2O ? CH3CH2OH

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• Mechanism of Addition Reactions
• Consider the reaction between 2-butene and HBr
• Careful kinetics experiments show the rate law to
  be
• Therefore, both 2-butene and HBr must be involved
  in the rate determining step.

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• From the kinetics data, we can propose the
  following mechanism
• The p-electrons in the alkene attack the d H
  atom of the HBr to leave a positive charge on one
  carbon (slow step)

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• Then the pair of electrons on bromide attacks the
  carbon with a positive charge to give the product.

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• Polymers
• Polymers are large chains built from smaller
  molecules (monomers)
• Addition Polymerization (ex. polyethene)
• Initiation free radical is formed (species with
  an odd electron)
• Propagation happens over and over again chain
  becomes very long

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• Termination 2 radicals combine
• Economic Importance of alkene reactions
• Hydrogenation of vegetable oil used in the
  manufacture of margarine.
• Hydration of ethene used in the manufacture of
  ethanol
• Polymerization used in the manufacture of most
  plastics.