Download Alkenes and Alkynes

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Alkenes, Alkynes and
Aromatic Structures
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Unsaturated Hydrocarbons
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- consist of carbon based molecules with multiple bonds
between the carbons
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Alkenes- functional group that is characterized by C-C
double bonds
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Alkynes- functional group characterized by C-C triple bonds
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Aromatics- structures that contain a benzene ring
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Characteristics of Alkenes
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Alkenes have a hybridization of sp2, a molecular geometry of
trigonal planar and a bond angle of roughly 120°
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General formula:
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They hybridization allows for 3 sp2 orbitals and 1 p orbital,
which is why the carbon is still able to have 4 bonds.
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They are hydrocarbons, so they are nonpolar in nature.
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Structural isomers exist (same chemical formula but different
arrangement of atoms).
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Geometric isomers also exist (more on this later.)
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Naming Alkenes
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Pretty similar to alkanes…but,
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The name of the longest carbon
chain ends in –ene.
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You start numbering the carbons
in the longest chain closest to
the double bond.
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You must indicate where the
double bond is in the
compound, so you use the lowest
numbered carbon that is
attached to the double bond.
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Example: 2-hexene or hex-2-ene
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Branches off of alkene chains
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Most are the same as alkanes
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Chloro
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Bromo
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Iodo
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Methyl
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Ethyl
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Propyl
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If you have a C2H3 – group off
of the main chain, it is called a
vinyl group.
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An allyl group
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Practice Naming and Drawing
Alkenes
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1) 3-hexene
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2) 4-ethyl-2-heptene
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3) 3,4-dimethyl-2-pentene
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4) 2-cyclohexyl-3-hexene
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5)
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6)
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Multiple Double Bonds in Alkenes
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Many compounds contain two
or more double bonds and are
known as alkadienes,
alkatrienes, alkatetraenes, and
so on, the suffix denoting the
number of double bonds. The
location of each double bond
is specified by appropriate
numbers, as illustrated to the
right:
1,2-butadiene
1,2,3-butatriene
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Geometric Isomers
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Geometric isomers are
present in substances that
have double bonds because of
the nature of the double bond.
Single bonds allow free
rotation around the bond.
However, double bonds have
more of a rigid/locked
formation so rotation is not
free…it is restricted.
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This restricted nature comes from the
fact that not only are sigma bonds
involved, but p-orbital overlap occurs
above and below the sigma bond.
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These isomers are called either cisor trans- depending on the location of
groups branching off the double
bond carbons.
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These isomers are not
superimposable, meaning that they
have different arrangements in space
when placed on top of one another.
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They also have different physical
properties as well.
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Cis- vs. Trans
The green atoms are Cl.
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You can see how the groups
are called cis- if they are on
the same, or adjacent side of
the molecule.
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They are called trans- if they
are on opposite sides of the
molecule.
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This is works for other
groups/branches that come off
of double bonds.
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Practice
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Draw these structures:
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Identify these as cis/trans:
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1) trans-3-heptene
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1)
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2) cis-5-chloro-2-hexene
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Orient the cis/trans
designation of the longest
carbon chain that
surrounds(includes) the
double bond.
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2)
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Cycloalkenes
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These are cyclic compounds
that contain a C-C double
bond in the ring.
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The carbons of the double
bond are numbered 1 and 2,
so no numbers are needed.
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Most common cycloalkenes:
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Cyclopentene
Other branches are numbered
with the lowest number
possible, after numbering the
carbons of the double bond
first.
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Cyclohexene
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Practice:
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Preparation of Alkenes
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Most are prepared from
saturated hydrocarbons.
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Cracking- process in which
saturated hydrocarbons are
heated to very high temps.
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In the presence of a catalyst
(usually silica-alumina), the
hydrocarbon breaks into
smaller molecules. Some
eliminate hydrogen to form
alkenes.
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Results in mixtures of
hydrocarbons…
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Dehydration of Alcohols-
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Water is removed from an
alcohol molecule
(characterized by an OH
group) in the presence of
concentrated sulfuric acid.
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See below…
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Physical Properties of Alkenes
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Boiling points are slightly
lower than corresponding
alkanes.
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Melting points of isomers are
wildly different than boiling
points.
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Chains with 4 or fewer
carbons are gases at room
temp.
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This is because the structure of
the isomer would affect the
crystal structure.
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5-17 C chain length are
liquids.
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Higher than 17, usually a solid.
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Densities are less than water.
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Chemical Reactions involving
Alkenes
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Alkenes are more reactive
than alkanes, mostly because
there are fewer than 4 bonds
to C in alkenes and alkynes.
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Addition Reaction- most
common
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Hydrogen, Halogens,
Hydrogen halides, sulfuric
acid and water are some
substances that can be added
to alkenes.
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Can be converted to saturated
molecules (alkanes) by
addition with hydrogen.