Unsaturated hydrocarbons
Chapter 13
Unsaturated hydrocarbons
• Hydrocarbons which contain at least one C-C multiple (double
or triple) bond.
• The multiple bond is a site for chemical reactions in these
molecules. Parts of molecules where reactions can occur are
called functional groups.
Multiple bonds
are examples of
functional groups
Alkenes and cycloalkenes
• Alkenes are unsaturated, acyclic hydrocarbons
that possess at least one C-C double bond.
• The generic formula for an alkene is CnH2n
(note: same as for a cycloalkane).
Ethene
Non IUPAC: "ethylene"
Propene
Non-IUPAC: "propylene"
Alkenes and cycloalkenes
• Cycloalkenes are cyclic hydrocarbons that
possess at least one C-C double bond.
Cyclopentene
Cycloalkenes have a general formula of CnH2n-2
Alkenes and cycloalkenes
• The geometry around the carbon atoms of the multiple bond
is different than the tetrahedral geometry that is always found
in carbon atoms of an alkane.
• There is a trigonal planar arrangement of atoms surrounding
the C-atoms of the double bond.
see: VSEPR theory, Ch-5
120o
109.5o
sp3-hybridized
carbon
Propene
sp2-hybridized
carbon
Alkenes with two double bonds (dienes), three double bonds (trienes) are not uncommon.
Cyclic alkenes usually do not involve more than one C-C double bond.
IUPAC nomenclature for alkenes
and cycloalkenes
• The rules for assigning an IUPAC name for alkenes
are not that different from those for alkanes
(substituent rules, chain numbering pretty much the
same)
• The difference here is that the longest continuous
chain that has the double bond is the parent chain.
correct parent chain
not correct
IUPAC nomenclature for alkenes
and cycloalkenes
• The parent chain is numbered to reflect the
position of the double bond (the lower
number of the two carbons in the bond).
1-Butene
2-Butene
IUPAC nomenclature for alkenes
and cycloalkenes
• For substituted alkenes, the number of the
substituent is indicated as before, at the
beginning of the name.
2-Methyl-2-butene
3-Methyl-1-butene
For numbering, the parent chain is numbered in a way that gives the lowest numbering to
the multiple bond(s). Substituent numbers are then assigned.
IUPAC nomenclature for alkenes
and cycloalkenes
• For dienes, the parent chain that involves both
double bonds is numbered to show the first
carbon in each double bond.
1,4-Hexadiene
3,5-Dimethyl-1,3-hexadiene
IUPAC nomenclature for alkenes
and cycloalkenes
• For cycloalkenes, the double bond in the ring
is numbered only if more than one double
bond exists (it is understood the C-1 is the first
carbon of a double bond in a ring)
3-Ethylcyclohexene
1,3-cyclohexadiene
5-Ethyl-1,3-cyclohexadiene
IUPAC nomenclature for alkenes
and cycloalkenes
• In certain cases, numbering is redundant (and
not shown).
Ethene
Propene
Methylpropene
IUPAC nomenclature for alkenes
and cycloalkenes
• Later, in larger molecules that possess other
groups of atoms (e.g. aromatics), alkene
substituents may be present. The types we
may encounter are named as follows:
methylidene group
Non-IUPAC: methylene
ethenyl group
Non-IUPAC: vinyl group
2-propenyl group
Non-IUPAC: allyl group
Line-angle structural formulas for
alkenes
• Line-angle formulas for alkenes indicate double
bonds with two lines. As before, each carbon must
possess four bonds, so the number of H-atoms on
each position will be able to be found by difference.
1-Butene
Propene
2-Methyl-1,3-butadiene
Non-IUPAC: isoprene
2-Methyl-2-pentene
3,4-Dimethylcyclopentene
Constitutional isomerism in
alkenes
• For a given number of carbon atoms in a chain (> 4 C-atoms),
there are more constitutional isomers for alkenes than for
alkanes (because of the variability of the C-C double bond
position)
Rem: constitutional isomers differ in their atomto-atom connectivity.
Constitutional isomerism in
alkenes
• Two types of constitutional isomers encountered are skeletal isomers and
positional isomers.
– Positional isomers are constitutional isomers that differ in the position
of the multiple bond (or, in general, the functional group)
– Skeletal isomers are constitutional isomers that differ in their C-chain
(and thus H-atom) arrangements.
C5H10
1-Pentene
2-Pentene
positional isomers
skeletal isomers
skeletal isomers
2-Methyl-2-butene
Cis-trans isomerism in alkenes
• We’ve already looked at cycloalkanes and cis-, trans- isomers. In alkenes,
this type of stereoisomerism is possible because a C-C double bond cannot
rotate (like the C-C bonds in a cycloalkane ring).
• For certain alkenes (which possess one H-atom on each carbon of the C-C
double bond) there are two stereoisomers: cis- and transFor cis-/trans- isomerism,
there must be a H-atom
and another group
attached to each C-atom
of the double bond
H-atoms on same side
of C-C double bond
H-atoms on opposite
sides of C-C double bond
cis: H-atoms on same side of C-C double bond
trans: H-atoms on opposite sides of C-C double bond
Cis-trans isomerism in alkenes
• For cis-, trans- isomerism, the alkene double
bond cannot be located at the end of a carbon
chain:
Cis-trans isomerism in alkenes
• You can differentiate cis-/trans- isomers in
line-angle structures:
=
=
trans-2-Pentene
=
=
cis-2-Pentene
Cis-trans isomerism in alkenes
• For dienes, each bond is labeled as cis- or
trans-, as required:
trans-trans-2,4-Heptadiene
cis-trans-2,4-Heptadiene
trans-cis-2,4-Heptadiene
cis-cis-2,4-Heptadiene
Cis-trans isomerism in alkenes
• In some cases, you’ll encounter alkenes that have only one or
no H-atoms bound to the C-atoms of the double bond.
• For these cases, instead of cis- and trans- labels, (Z)- and (E)labels (respectively) are used.
CH3-CH2- substituent
higher priority than
CH3- substitutent
(E similar to trans- and Z similar to cis-)
(E)-3-Methyl-3-hexene
(text calls this
trans-3-Methyl-3-hexene)
This system works
for more than just
alkyl substituents,
but we will stick to
these cases for now.
(Z)-3-Methyl-3-hexene
For both higher priority substituents on same side of double bond, (Z)For higher priority substituents on opposite sides of double bond: (E)-
Physical properties of alkenes and
cycloalkenes
• Alkenes and cycloalkenes have solubilities similar to
what was discussed for alkanes and cycloalkanes
• Generally, alkenes have melting points that are lower
than for corresponding alkanes
Chemical properties of alkenes and
cycloalkenes
• Like alkanes, combustion reactions can occur,
producing H2O and CO2
• Other reactions of alkenes tend to involve the
C-C double bond. These are addition-type
reactions
alkene
alkane
A-B “adds across” the C-C double bond. The double bond becomes transformed to a C-C single
bond in the process
Chemical reactions of alkenes and
cycloalkenes
• Addition reactions can be symmetrical or unsymmetrical,
depending on what is being added to the double bond.
• In a symmetrical addition, the atoms (or groups) added to
each carbon of the double bond are identical.
Hydrogenation of an alkene
Ni or Pt
H2
150oC
12-15 atm
pressure
Propene
Propane
Cl2
2,3-Dichloropentane
trans-2-Pentene
Halogenation of an alkene
Chemical reactions of alkenes and
cycloalkenes
• Unsymmetrical addition reactions occur when different atoms
(or groups) are added across a double bond.
• Several examples of unsymmetrical addition reactions follow:
– Hydrohalogenation of a double bond
– Hydration of a double bond
Chemical reactions of alkenes and
cycloalkenes
• Hydrohalogenation: a hydrogen halide is added to a
double bond; one C-atom becomes bound to the
halogen and the other C-atom to a hydrogen:
HBr
In general:
HX
Chemical reactions of alkenes and
cycloalkenes
• Hydration reactions add a molecule of water to a
double bond. The water molecule adds as HO-H:
HO-H
An alcohol (R-OH)
Chemical reactions of alkenes and
cycloalkenes
• In unsymmetrical addition reactions, if the alkene
itself is not symmetrical, there will be more than one
possible product. An unsymmetrical alkene is one
for which the two C-atoms of the double bond are
not equivalent.
H-OH
Chemical reactions of alkenes and
cycloalkenes
• There will typically be one product in these cases that is
favored (produced in greater yield). Markovnikov’s Rule states
that when an unsymmetrical addition involves an
unsymmetrical alkene, the H-atom of HX adds to the carbon
of the double bond that has the most hydrogens.
Major product
H-OH
Minor product
Chemical reactions of alkenes and
cycloalkenes
• For dienes and trienes, addition reactions (e.g.
hydrogenation) will involve more than one of the
double bonds, provided enough of the reactant (e.g.
H2) is added:
H2
Heptane
1-Heptene
2H2
1,3-Heptadiene
3H2
1,3,5-Heptatriene
Polymerization of alkenes
• Alkenes (and alkynes) are able to undergo reactions that create long
chains of atoms called polymers. In general, these reactions are called
polymerization reactions.
• Polymers are large molecules that are made up of repeated sequences of
smaller units. The small molecules used to make the polymer are called
monomers.
• One of the bonds in the double bond is used to add the monomer
structures into a growing polymer chain. The reaction is called addition
polymerization.
Ethylene
Polyethylene
n
Polyethylene
“n” expresses the
average chain length
Polymerization of alkenes
• Substituted alkenes can also undergo this type of reaction, yielding
polymer chains that possess branches (substituents)
polymerization
n
substituted
ethylene
substituted polyethylene
• For dienes, polymerization yields polymers that contain double bonds:
polymerization
1,3-Butadiene
Polybutadiene
n
• In cases where two different monomers are involved, copolymer
(containing both monomer units) are obtained.
monomer 1
monomer 2
polymerization
n
Vinyl
chloride
1,1-Dichloroethene
Saran Wrap
Polymerization of alkenes
• Polymers find many uses (plastics are polymers).
• However, because they consist of alkane-type carbon
chains, they are also unreactive. This means they
don’t decompose readily in a landfill site.
Monomer
Alkynes
• Saturated hydrocarbons that possess at least
one C-C triple bond are called alkynes.
• For naming, the rules that were followed for
alkenes are used, except that the name of the
parent chain now ends in “yne”.
General formula for alkyne: CnH2n-2
Ethyne
(Acetylene)
Propyne
(Methylacetylene)
6,6-Dimethyl-3-heptyne
Alkynes
• Because C-atoms only possess four covalent bonds,
the C-atoms involved in the C-C triple bonds of
alkynes possess local, linear molecular geometries.
• This means that cis-, trans- isomers are not possible
for alkynes (at the C-C triple bond).
sp-hybridized carbons
Alkynes
• However, constitutional isomers exist.
Positional isomers
C4H6
2-Butyne
1-Butyne
Skeletal isomers
C5H8
1-Pentyne
3-Methyl-1-butyne
Alkynes
• The triple bond in an alkyne can undergo
addition reactions similar to the double bond
of an alkene:
H2
alkyne
Ni (catalyst)
H2
alkene
Ni (catalyst)
alkane
Two equivalent amounts of hydrogen added to an alkyne will make an alkane
Aromatic hydrocarbons
• Aromatic hydrocarbons: a special class of
cyclic, unsaturated hydrocarbons which do not
readily undergo addition reactions.
Benzene (C6H6) is an example of an aromatic hydrocarbon
Aromatic hydrocarbons
• Benzene is a cyclic triene which possesses alternating C-C
double and single bonds.
• Because there are two ways the structure could be drawn,
benzene is often represented with a circle-in-a-hexagon
formula, showing the delocalization of the bonds.
=
C6H6
= set of three
delocalized bonds
Names for aromatic hydrocarbons
• Benzene derivatives with one substituent
Chlorobenzene
tert-Butylbenzene
Isopropylbenzene
• Certain cases have specific names
Toluene (not
Methylbenzene)
Styrene (not
Vinylbenzene)
Names for aromatic hydrocarbons
• In cases where a substituent name is not easily
obtained, the benzene is called a “phenyl”
substituent and the name is assigned using the
alkane/alkene as the parent:
2-Phenyl-2-butene
3-Phenylhexane
Names for aromatic hydrocarbons
• Benzene derivatives with two substituents will
have a bonding pattern that will fit one of the
following schemes:
1,2-dibsubstituted
“ortho”
1,3-dibsubstituted
“meta”
1,4-dibsubstituted
“para”
Names for aromatic hydrocarbons
• This enables one of two possible naming schemes:
1,2-Dichlorobenzene
(ortho-Dichlorobenzene)
1,3-Dichlorobenzene
(meta-Dichlorobenzene)
1,4-Dichlorobenzene
(para-Dichlorobenzene)
ortho-Bromoiodobenzene
meta-Bromopropylbenzene
Names for aromatic hydrocarbons
• When one of the special case compounds (e.g.
toluene) is involved, the compound is named
as a derivative of the special compound.
3-Bromotoluene
2-Ethyltoluene
2-Chlorostyrene
Names for aromatic hydrocarbons
• In cases where disubstituted benzenes occur where
substituents are not the same and where no special cases are
involved, the substituent that has alphabetic priority also gets
numbered on C-1.
1-Bromo-3-ethylbenzene
1-Bromo-2-chlorobenzene
Names for aromatic hydrocarbons
• Disubstituted benzenes possessing two methyl substituents
are a special case themselves. They are not called dimethyl
benzenes or methyl toluenes, but instead are called xylenes:
ortho-Xylene
meta-Xylene
para-Xylene
Fun fact! Three methyl groups on a benzene ring: named as a trimethylbenzene, not a methyl xylene.
Names for aromatic hydrocarbons
• Three substituents: numbered to give the lowest possible
numbering. Given a choice, alphabetic priority would dictate
which substituent is on C-1.
1,2,4-Tribromobenzene
1-Bromo-3,5-dichlorobenzene
Physical properties and sources of
aromatic hydrocarbons
• Similar to what we’ve seen for other hydrocarbons, aromatics
are generally water-insoluble and have densities less than
that of water.
• Benzene is pretty good at dissolving other organic molecules
(can serve as solvents for chemical reactions).
• Industrially, aromatics are produced from saturated
hydrocarbons:
catalyst
4H2
high-temperatures
Chemical reactions of aromatics
• The double bonds of aromatic hydrocarbons are resistant to
addition-type reactions. Instead, with the aid of catalysts,
they can undergo substitution reactions:
• Alkylation of benzene:
Benzene
+
Chloroethane
Ethylbenzene
AlCl3
HCl
AlCl3
HCl
in general:
Benzene
alkyl halide
substituted
benzene
HCl
Chemical reactions of aromatics
• Halogenation of benzene:
FeBr 3
HBr
Br2
Bromobenzene
bromine
Benzene
HBr
FeBr 3
in general:
HX
X2
Benzene
halogen
halogenated
benzene
HBr
Fused-ring aromatics
• There are common cases of aromatic
structures involving fused benzene rings:
Napthalene
Anthracene
Phenanthrene
Hybrid orbitals and bonding in
organic compounds
• We have seen that carbon adopts several bonding formats:
tetrahedral
trigonal planar
linear
• Carbon’s electron configuration is 1s22s22p2. Its valence
orbitals are the 2s and 2p orbitals.
Hybrid orbitals and bonding in
organic compounds
• Bonds are created between atoms when electrons are shared.
In order for electron-sharing to occur, the orbitals containing
these electrons (on each atom) must overlap.
• This model can’t explain the observed bond angles in
molecules like the ones shown below (shapes and
orientations of the valence orbitals are incorrect):
109.5o
120o
180o
Hybrid orbitals and bonding in
organic compounds
• To explain bonding in these cases, a new model is used (called
“Valence Bond Theory”) in which atomic orbitals (2s, 2p, etc.)
are mixed to produce hybrid orbitals, which have directions
that depend on the number of atomic orbitals mixed.
109.5o
sp3-hybrid
orbitals
120o
180o
sp-hybrid orbitals
sp2-hybrid orbitals
Hybrid orbitals and bonding in
aromatic compounds
• For a tetrahedral
carbon (e.g. an alkane
carbon), there are four
other atoms bound to
the C-atom. The
molecular geometry
around carbon is
tetrahedral.
• A sp3-hybrid orbital set
is used for explaining
the tetrahedral
arrangement. Hybrid orbitals point in the
same directions as electron
groups in VSEPR theory
Hybrid orbitals and bonding in
aromatic compounds
• For a trigonal planar carbon, three atomic
orbitals are combined to make three, sp2hybrid orbitals.
120o
Hybrid orbitals and bonding in
organic compounds
• Linear carbons: involved in a triple bonds.
Two atomic orbitals are combined to make a
new hybrid orbital set (two sp-hybrid orbitals)
180o
Hybrid orbitals and bonding in
organic compounds
• In C-C single bonds, the bond is created by the overlap of
orbitals in a head-on fashion. The situation is similar to what
occurs when two H-atoms bond (or H and Cl-atoms):
• This is called a sigma bond (s-bond) (strong)
• What about multiple bonds? How do they form?
Hybrid orbitals and bonding in
organic compounds
• Multiple bonds involve one s-bond, plus at
least one pi-bond (p-bond) (one p-bond in a
double bond or two p-bonds in a triple bond)
p-bonds are created by the sideways
overlap of parallel, atomic p-orbitals
Hybrid orbitals and bonding in
organic compounds
• In a molecule that contains a double bond,
like H2CO:
double bond = s-bond + p-bond
s-bonds
s-bond
sp2-hybrid orbitals are used to create the trigonal
planar molecular geometry and the unused p-orbital
is used to make the p-bond
Hybrid orbitals and bonding in
organic compounds
• For a molecule with a triple bond, there are
two, unused p-orbitals that can be used to
make p-bonds:
triple bond = s-bond + two p-bonds
s-bond
s-bond
sp-hybrid orbitals create the linear molecular geometry around the C-atoms and two,
unused p-orbitals on each C-atom can be used to make two p-bonds with the second carbon