13'7 How an Alkene Addition Reaction Occurs

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13.7 How an Alkene Addition Reaction Occurs

• Reaction mechanism A description of the
  individual steps by which old bonds are broken
  and new bonds are formed.

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• In the 1st step, the alkene reacts with H from
  the acid HBr and produces a carbocation. In the
  2nd step, this reactive carbocation immediately
  reacts with Br- ion to form the product.

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• Cation An atom or group of atoms with a positive
  charge.
• Carbocation A carbon atom with a positive
  charge. This is just a special name given to the
  cation formed by carbon atoms.

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13.8 Alkene Polymers

• Polymer A large molecule formed by the
  repetitive bonding together of many smaller
  molecules called monomers. Many simple alkenes
  undergo polymerization reaction when treated with
  the proper catalyst.

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Ethene or Ethylene
Polyethylene
Propene or Propylene
Polypropylene
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the process continues in this fashion forming a
long chain that we call polymer
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In this course I will only make you responsible
for writing structures of polymers that are
derived from ethene (ethylene) with or without
substituents.
We can think of this molecule as a substituted
ethylene.
To see this, lets write the molecule in the
following way
The substituent part
The ethylene part
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déjà vu Oh no! It is orbitals and electrons
again!

• Electrons are neither particle nor waves. They
  share properties of both at the same time.
• One of the properties of electrons that make them
  similar to waves is that we cannot pinpoint at
  any time their exact position.
• The best we can do is to define a region of space
  in which the probability of finding the electron
  has a certain value. This value is normally taken
  as 90. These regions of space where the
  probability of finding an electron has a certain
  value is what we call an orbital.

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• In atoms, orbitals are given the names s (a in
  the figure below), p (b in the figure below), d,
  f.

• Notice that some of these orbitals have a
  particular direction in space.

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• Bonds are formed when two atoms get close and
  their orbitals overlap sharing their electrons

• The figure above shows how two hydrogen atoms
  form a hydrogen molecule by approaching one
  another until their orbitals overlap. Notice that
  a new orbital, called a molecular orbital, is
  formed in the process.

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• Sometimes it is energetically favorable to
  rearrange the electrons and orbitals of a
  particular energy level when an atom forms a
  compound.
• The reason for this is that the energy spent in
  the rearrangement is less than the energy gained
  in the new compound formed. This process is
  called hybridization.

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When C forms compounds containing double bonds,
these 3 orbitals from energy level 2 rearrange.
The resulting orbitals are called sp2 (in green).
Notice that there is one p orbital (orange)
unchanged. Each of these orbitals contain 1
electron and is ready for bonding.
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• Hydrocarbons that contain double or triple bonds
  contain two different types of molecular
  orbitals. The first set of molecular orbitals
  form the backbone of the hydrocarbon

• Here we see the backbone of the ethene (ethylene)
  molecule. These molecular orbitals and the bonds
  they form are called sigma (s).

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• Notice that the s bonds are formed by the carbon
  orbitals (green) that lie in the plane.
• Notice also that another orbital (in orange) is
  not involved in forming the backbone.

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• The unused p orbitals (depicted above in green
  before bonding) are used in forming the second
  bond between the carbons. Notice that for clarity
  I have omitted the s bonds.

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• When these two orbitals overlap they form a new
  molecular orbital called p orbital and the bond
  is called a p bond. Notice that the electronic
  density is on top and below the plane of the
  molecule. This is characteristic of p bonds.

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The chemical properties of systems containing
double or triple bonds is related to the p bonds
and electrons.