Structure of Alkenes
• Alkenes (and alkynes) are unsaturated hydrocarbons
• Alkenes have one or more double bonds
• The two bonds in a double bond are different:
- one bond is a sigma () bond; these are cylindrical
in shape and are very strong
- the other is a pi (π) bond; these involve sideways
overlap of p-orbitals and are weaker than  bonds
• Alkenes are flat and have a trigonal planar shape
around each of the two C’s in a double bond
Structure of Alkynes
• Alkynes have one or more triple bonds
• A triple bond consists of one  bond and two π bonds
- the two π bonds are orthogonal (perpendicular)
• Alkynes are linear around each of the two C’s in the
triple bond
• Because alkenes and alkynes have π bonds, which are
much weaker than  bonds, they are far more
chemically reactive than alkanes
Naming Alkenes and Alkynes
•
•
•
•
Parent name ends in -ene or -yne
Find longest chain containing double or triple bond
Number C’s starting at end nearest multiple bond
Locate and number substituents and give full name
- use a number to indicate position of multiple bond
- cycloalkenes have cyclo- before the parent name;
numbering begins at double bond, giving
substituents lowest possible numbers
- use a prefix (di-, tri-) to indicate multiple double
bonds in a compound
Cis-Trans Isomers of Alkenes
• The π bond gives an alkene a rigid structure
• Free rotation around the C-C bond is not possible
because the π bond would have to break and re-form
• So, groups attached to the double bond are fixed on
one side or the other
• If each C in the double bond has two different groups
attached, then cis-trans isomers are possible:
- Cis = 2 groups attached to the same side of the
double bond
- Trans = 2 groups attached to opposite sides of the
double bond
Addition Reactions of Alkenes and Alkynes
• Addition (combination) reactions have the form
A + B  AB
• For alkenes the general reaction has the form
R2C=CR2 + A-B  R2AC-CBR2
(where R = any alkyl group or H)
• Addition reactions are the most common types of
reactions for alkenes and alkynes
• The π bonds are easily broken, and that pair of
electrons can form a new  bond
• The reactions are favorable because the products (all
 bonds) are more stable than the reactants
Hydrogenation of Alkenes and Alkynes
• H2 can be added to alkenes or alkynes to form alkanes
• Usually a metal catalyst (Pt, Pd or Ni) is used to speed up the
reaction (the reaction generally doesn’t work without a catalyst)
• Because these reactions take place on a surface, hydrogenation
of substituted cycloalkenes produces cis products.
Examples:
H
H 2C
HC
CH2
+
H2
+
CH
H
H
2H2
H
H
H
H
H
H
H
H
H
H
CH3
CH3
+
H2
CH3
H
CH3
Hydrohalogenation of Alkenes
• Hydrogen halides (HCl, HBr or HI) can add to alkenes to form
haloalkanes
• When a hydrogen halide adds to a substituted alkene, the halide
goes to the more substituted C (Markovnikov’s rule)
Examples:
H
H
H
C
+
C
HBr
H
Br
C
C
H
H
H
H
H
H
H
H
Cl
C
H
+
C
HCl
H
C
H
CH3
C
H
CH3
CH3
CH3
+
HI
I
H
H
H
Mechanism of hydrohalogenation
• Hydrohalogenation takes place in two steps
• In the first step, H+ is transferred from HBr to the alkene to
form a carbocation and bromide ion
• Second, Br- reacts with the carbocation to form a bromoalkane
Example:
H
H
C
H
+
C
H
H
Br
H
CH3
H
H
H
CH3
Br
H
CH3
Br
C
H
+
C
H
+
C
C
H
H
C
H
C
H
CH3
Br
Addition of Water to Alkenes
• In the presence of a strong acid catalyst (HCl, H2SO4 etc.)
alkenes react with H2O to form alcohols
• Recall that acids form H3O+ in water; it is the H3O+ that reacts
with the alkene
• Hydration reactions follow Markovnikov’s rule
Examples:
H
Acid
Cat.
H
C
+
C
H
H
H
CH3
C
H
H 2O
H
Acid
Cat.
+
C
H
C
C
H
H
OH
C
C
H
H
Acid
Cat.
CH3
+
H
H
H
H 2O
OH
CH3
H
CH3
OH
H 2O
H
H
H
Mechanism of Acid-Catalyzed Alkene Hydration
• First, the alkene reacts with H3O+ to form a carbocation
• Next an H2O quickly reacts with the carbocation to form a protonated alcohol
• In the last step the proton is removed by an H2O to form an alcohol
+
C
C
H
O
H
H
H
H
H
+
C
C
H
O
H
H
CH3
H
H
CH3
H
H
C
H
O
+
C
H
H
H
H
CH3
C
C
H
H
H
H
O
H
CH3
H
H
O
H
C
H
H
H
CH3
C
H
H
+
O
H
H
O
C
H
H
CH3
C
H
H
H
+
H
O
H
Halogenation of Alkenes and Alkynes
• Halogens (Cl2 or Br2) can add to alkenes or alkynes to form
haloalkanes
• Alkenes form dihaloalkanes; alkynes form tetrahaloalkanes
• Reaction with cycloalkenes produces a trans product
Examples:
H
H
C
+
C
H
Br
Br2
H
H
H
C
C
H
H
Br
Br
+
Br2
Br
Br
H
C
C
CH3
+
2Br2
H
Br
Br
C
C
CH3
Br
Mechanism of Bromonation of Ethene
• First, a Br+ is transferred from Br2 to the alkene to form a
bromonium ion and a bromide ion
• Next, the bromide ion reacts with the bromonium ion to form
the product
H
H
Br
H
C
H
Br
Br
H
C
H
+
Br
+
C
H
Br
Br
C
C
H
H
H
H
+
C
H
H
H
C
H
C
H
Br
Br
Polymers
• A polymer is a long chain of repeating subunits called
monomers
- examples of natural polymers: DNA, protein, starch
- example of synthetic polymers: polyethylene
• Many synthetic polymers are made from alkenes,
although other functional groups are also used
• The monomers are added to the chain through a series
of addition reactions
• Polymerization reactions usually require high
temperature and pressure and are often radical
reactions carried out with a catalyst
Conjugated Alkenes and Aromatic Compounds
• Recall that a double bond consists of one  bond and one 
bond; a  bond is formed by sideways overlap of two p
orbitals (one electron comes from each orbital)
• A conjugated alkene has alternating double and single bonds
• The p orbitals overlap in a conjugated system (the  electrons
are “delocalized” throughout the system), making conjugated
alkenes more stable than non-conjugated alkenes
• An aromatic hydrocarbon consists of alternating double and
single bonds in a flat ring system
• Benzene (C6H6) is the most common aromatic hydrocarbon
• In benzene all the double bonds are conjugated, and so the 
electrons can circulate around the ring, making benzene more
stable than 1,3,5-hexatriene (the p orbitals on the end of a
chain can not overlap)
1,3,5-hexatriene
Resonance Structures
• There are two ways to write the structure of benzene
• These are called “resonance structures”
• However, neither of these represents the true structure of
benzene since benzene has only one structure, with all C-C
bonds being equivalent
• The true structure is a hybrid of the the two resonance
structures; this can be represented by drawing the  bonds as a
circle
• We use the individual resonance structures when we write
reaction mechanisms involving benzene to show more clearly
the bond formation and bond breaking in the reaction
Naming Monosubstituted Benzene Compounds
• Benzene compounds with a single substituent are named by
writing the substituent name followed by benzene
• Many of these compounds also have common names that are
accepted by IUPAC (you should know those listed here)
CH3
OH
Toluene
(methylbenzene)
O
NH2
Phenol
(hydroxybenzene)
H
Analine
(aminobenzene)
O
OCH3
Anisole
(methoxybenzene)
OH
C
C
Benzaldehyde
(benzenecarbaldehyde
Benzoic Acid
(benzenecarboxylic acid)
Styrene
(phenylethene)
Naming Multisubstituted Benzene Compounds
• When there are 2 or more substituents, they are numbered to
give the lowest numbers (alphabetical if same both ways)
• Disubsituted benzenes are also named by the common prefixes
ortho, meta and para
Examples:
Br
CH3
Cl
Br
ortho -chlorotoluene
(1-chloro-2-methylbenzene)
meta-dibromobenzene
(1,3-dibromobenzene)
F
NH2
OH
Br
para-ethylphenol
(1-hydroxy-4-ethylbenzene)
4-bromo-2-fluoroanaline
(1-amino-4-bromo-2-fluorobenzene)
Physical Properties of Aromatic Compounds
• Because aromatic compounds (like benzene) are flat, they
stack well, and so have higher melting and boiling points than
corresponding alkanes and alkenes (similar to cycloalkanes)
• Substituted aromatic compounds can have higher or lower
melting and boiling points than benzene
- para-xylene has a higher m.p. than benzene
- ortho and meta-xylene have lower m.p.’s than benzene
• Aromatic compounds are more dense than other hydrocarbons,
but less dense than water (halogenated aromatics can be more
dense than water, as can haloalkanes)
• Aromatic compounds are insoluble in water, and are
commonly used as solvents for organic reactions
• Aromatic compounds are also flammable, and many are
carcinogenic
Chemical Reactivity of Aromatic Compounds
• Aromatic compounds do not undergo addition reactions because
they would lose their special stability (aromaticity)
• Instead, they undergo substitution reactions, which allow them
to retain their aromaticity
• We will study three types of substitution reactions of benzene:
halogenation, nitration and sulfonation
Addition:
Br
+
Br 2
Br
Loses aromaticity
Aromatic
Substitution:
+
Aromatic
Br 2
FeBr3
Br
+
Retains Aromaticity
HBr
Halogenation of Benzene and Toluene
• Br2 or Cl2 can react with benzene, using a catalyst, to form
bromobenzene or chlorobenzene
• Only the monohalogenation product is produced
• When Br2 or Cl2 reacts with toluene, a mixture of isomers
is produced
- Ortho and para isomers are the major products, and meta
isomer is the minor product
Examples:
+
Cl2
Cl
FeCl3
+
CH3
CH3
CH3
+
Br2
H Cl
CH3
Br
FeBr3
+
+
Br
+
H Br
Br
(Minor)
Mechanism of Bromonation of Benzene
• First, a Br+ is transferred from Br2 to benzene, forming a
carbocation and a chloride ion
• Next, the chloride ion removes an H+ from the carbocation
to form chlorobenzene and HBr
Br
FeBr3
+
Br
H
Br
+
Br
+
H Br
Br
H
Br
+
Br
Nitration and Sulfonation of Benzene
• Nitric acid can react with benzene, using sulfuric acid as a
catalyst, to form nitrobenzene plus water
• First H2SO4 donates a proton to HNO3, which then
decomposes to form H2O and NO2+ (the reactive species)
• Sulfur trioxide plus sulfuric acid (fuming sulfuric acid) can
react with benzene to produce benzenesulfonic acid
• First H2SO4 donates a proton to SO3 to produce HSO3+ (the
reactive species)
+
+
HNO 3
SO3
H2SO4
NO2
+
H2SO4
SO3 H
H 2O