Transcript Alkenes, Alkynes and Aromatics

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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.
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A catalyst like nickel or
platinum is required
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Alkenes and Addition Reactions
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Alkenes can be added to elemental halogens like bromine
and chlorine to produce substituted alkanes.
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If concentrated sulfuric acid is added, the product will
produce a hydrogen sulfate, which conforms to an alcohol
when reacted with water.
Alkenes plus Unsymmetrical
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Halogens
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Alkenes can have an unsymmetrical halogen compounds
added, like HCl or HBr. However, you would expect a mixture
of products. According to a reaction mechanism (step by
step picture of what bonds are broken and formed), one
product is favored over the other. Why?
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Markovnikov’s Rule
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This rule states that when an unsymmetrical molecule,
such as HCl or HBr, is added to a C-C double bond, the
hydrogen from the HX goes to the carbon atom that has
the greater number of hydrogen atoms.
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The reaction mechanism that goes along with this type of
reaction generates what is known as a carbocation or a
carbon with a temporary positive charge.
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Reaction Mechanism
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Carbocations
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Carbocations are intermediate species shown in mechanisms that
represent carbons with a positive charge.
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There are 3 types of carbocations, depending on how many other
carbons are attached to the one with the positive charge.
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Primary (1°)- attached to one other carbon
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Secondary (2°)- attached to 2 other carbons
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Tertiary (3°)- attached to 3 other carbons
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Cannot have any others!
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The following order represents the most stable carbocations:
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3°>2°>1°>CH3
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Practice
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Write the products formed when 2-methyl-1-butene reacts with:
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A) H2, Pt at 25°C
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B) Cl2
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C) HCl
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D) H2O, H+
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Write equations for the addition of HCl to:
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A) 1-pentene
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B) 2-pentene
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Stereoisomers
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When branches are added as a result of alkene reactions, it is
important to know if they are added to the same side of the
molecule or opposite sides of the molecule.
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If they are added to the same side of the molecule, we call
them syn.
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If they are added to the opposite sides of the molecule, we
call them anti.
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Let's see some examples!
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Oxidation Reaction for Alkenes
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Alkenes can be oxidized by adding cold, dilute potassium
permanganate (Baeyer test) to form a dihydroxy alcohol).
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This test determines the presence of double or triple bonds
in a molecule.
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Alkynes
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Rules for naming:
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Same as rules for alkenes, except main chain ends in –yne.
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Practice:
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1) 3-methyl-1-butyne
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2) 4-ethyl-4-methyl-2-hexyne
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3) cyclohexylethyne
4)
5)
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Preparation
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These compounds are very reactive!
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However, ethyne is relatively easy to make using calcium
carbide and water, or by the cracking of methane in an
electric arc.
Physical and Chemical Properties
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of Alkynes
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Physical:
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All of these properties are of acetylene, which is the common
name for ethyne. It’s the most common alkyne.
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Insoluble in water.
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Gas at normal temperature.
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May decompose violently, spontaneously or from shock.
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Chemical:
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Acetylene is most often used as fuel for welding/cutting torches
or an intermediate in manufacture of other substances.
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Alkyne Reactions
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Addition reactions:
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Burns hot! 2800 °C.
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Undergoes addition reactions like ethane with Cl, Br, HCl,
HBr.
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Alkynes also follow Markovnikov’s rule for addition
reactions.
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Other reactions are possible that alkenes do not do:
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Acetylene with HCN to form acrylonitrile.
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Aromatics
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All structures that contain a benzene ring are considered
aromatic.
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Michael Faraday first isolated benzene in 1825.
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It was a couple of years before the formula, C6H6, was
determined.
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In 1865, Kekule proposed a structure in which each carbon
had 1 H attached and there were three double bonds
alternating with single bonds.
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However, with this model, there should be 2
dibromobenzenes, but there were not. SO…
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He suggested that the bonds
must be in rapid oscillation.
(Like the original idea behind
resonance).
No one could really determine
the correct structure behind
benzene until 1912, when xray diffraction allowed
chemists to measure the
distance between carboncarbon nuclei in bonds.
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Since all of the bonds in benzene are
in-between a single and double bond
length, they must all share the same
bond type.
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Naming Aromatics
Monosubstituted benznes
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These are benzene rings that
have a group replacing one of
the hydrogens on the ring.
Toluene
Most of these are named by
using the substituted group as
a prefix in front of benzene.
Styrene
Some have specific names:
Phenol
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Disubstituted benzenes
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When more than 2 groups
replace hydrogens on a
benzene ring, three isomers
are possible:
1)
Ortho
2)
Meta
3)
Para
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With respect to A, as shown,
there are two ortho positions…
if the 1 carbon is A and the
other branch is B, it would
have to be in the 2 position
either clockwise or
counterclockwise. The meta
would be 1,3, so there are 2 of
those also.
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There is only 1 para position.
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Naming disubstituted benzenes
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Use ortho, meta or para and
then the branch names…
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Example: ortho
dichlorobenzene
If the branches are not the same,
name them alphabetically, but
use ortho, para or meta to describe
their location to each other.
Example: meta bromomethylbenzene
• Dimethyl substituted benzene rings have a special
name: xylene
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Polysubstituted Benzene Rings
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When more than 2 substituents
are on benzene, the carbon
atoms are numbered starting
at one of the substituent
groups.
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If it is named as a derivative of
a special parent compound
(like toluene or phenol), the
group of the parent compound
is carbon #1.
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You must travel in the way that
gives the groups the lowest
numbers of carbons possible,
either clockwise or
counterclockwise.
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Example:
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2,4,6-trinitrotoluene (TNT)
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Practice
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Write formulas and names for
all the isomers of
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A) chloronitrobenzene
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(nitro is a NO2group)
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B) tribromobenzene
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C) chlorophenol
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Name the following:
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A)
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B)
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Polycyclic Aromatic Compounds
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Polycyclic or fused rings
consist of structures where 2
carbons are common to two
rings.
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These fused rings exist in
many of the amino acids and
other natural biochemical
substances.
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Three of the most common in
chemical world are
napthalene, anthracene and
phenanthrene.
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Napthalene
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Anthracene
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Phenanthrene
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Carcinogens or helpful? BOTH
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Carcinogens
A number of benzene and
polycyclic structures have
been found to be
carcinogenic, or cancer
causing.
Some of the more notable ones
are found in coal tar and tar
from cigarette smoke.
Many aromatics were first
derived from coal tar upon
intense heating.
Since our source of coal tar is
limited, we have looked to
petroleum to make aromatics
like benzene, toluene and
xylene.
Helpful
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Used in the production of:
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Dyes
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Pharmaceuticals
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Detergents
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Insecticides
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Plastics
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Rubber
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Explosive
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Physical and Chemical Properties
of Aromatics
Physical properties
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Nonpolar
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Less dense than water
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Miscible in many organic
solvents, but not water
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Most are liquids or solids
Chemical properties
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Most undergo substitution of a
group on the benzene ring for
a hydrogen.
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Typical substitutions:
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1) Halogenation- benzene
reacts with chlorine or
bromine to become
substituted
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More chemical
properties/reactions
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2) Nitration- benzene reacts
with concentrated nitric and
sulfuric acid at 50 Celsius to
form nitrobenzene.
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3) Alkylation- (also called
Friedel –Crafts reaction) the
alkyl group from an alkyl
halide, in the presence of
AlCl3 as a catalyst, adds the
halogen to the benzene ring
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Mechanism of substitution
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Electrophilic substitution-
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3 steps:
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1) An electrophile (electron
seeking group) is formed.
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2) Electrophile is attached to
the benzene ring, forming a
positively charged
carbocation intermediate.
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3) A hydrogen ion is lost from
the carbocation to form the
product.
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Side Chain Oxidation
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Carbon chains attached to
aromatic rings are easy to
oxidize.
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Most common reactants used
to oxidize are KMnO4 or
K2Cr2O7 with sulfuric acid
(H2SO4).
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No matter how long the
carbon chain is of the group
attached to the ring, it will
form a carboxylic acid.
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Carboxylic acid- COOH group