SCH 4U
Cycloalkanes, Alkenes, and Aromatics
Text assignment: Remainder of
Page 18 # 3 - 6
Page 21 # 7, 8
Page 22 # 1-4 (every other letter)
Multiple Covalent Bonds
Organic compound arrangements
• Alkanes, alkenes, and alkynes
Nomenclature or names of compounds
• Alkanes
Nomenclature - Alkenes
• Alkanes
Nomenclature of Alkenes and
Cycloalkenes
P Alkenes are named by finding the longest
chain containing the double bond ...
P and changing the name of the
corresponding parent chain from ane to
ene
Nomenclature of Alkenes
The compound is numbered to give one of
the alkene carbons the lowest number
Cycloalkene
P The double bond of a cylcoalkene must
be in position 1 and 2
Alkenes and Alkynes
P Unsaturated (contains one or more double or
triple bonds) hydrocarbons
P Double and triple bonds are more reactive
than single bonds
P Alkenes (CnH2n) are also called olefins
P All atoms around the double bond are in a
plane (flat)
P Atoms around a triple bond are in a line
Alkenes
P Molecules containing a double bond can
have geometric isomers
P There is no rotation around double bonds
P 2-butene can have cis and trans isomers
P Cis - same side of double bond
P Trans - across the double bond
H
H
C
H 3C
H
C
CH 3
C
CH 3
cis-2-butene
H 3C
C
H
trans-2-butene
Geometric isomers
P Cis: If two identical groups occur on the
same side of the double bond
Geometric isomers
P Trans: If two identical groups occur on
opposite sides of the double bond
Geometric isomers
P Some have very different dipoles
P Trans-1,2-dichloroethene is symmetrical and
is not polar
P Movement of electrons adds to zero
d-
d-
Net dipole is zero
Geometric isomers
P Cis-1,2-dichloroethene is not symmetrical
and is polar
dd-
Net dipole is not zero polar molecule
Geometric isomers
Properties of geometric isomers vary because
of the difference in polarity of the molecules
Disubstitued Cycloalkanes
P Cyclic compounds also have fixed shapes
P Geometric isomers can occur in substituted
cyclic compounds
Uses of Alkenes
P Ethene (ethylene) is a major industrial
feedstock
P Ethene used to produce ethanol, ethylene
oxide and the polymer polyethylene
P Propene (propylene) is also very
important in industry
P Propene used to make polypropylene and
acetone
Many alkenes occur naturally
Structures of compounds in gasoline
• Ques. 11:
• 2,2-dimethylbutane
• 2,2,4-trimethylpentane
• 3-methyl-2-pentene
• 1,4-dimethylcyclohexane
Alkynes
P Ethyne (acetylene) is used in welding
torches because it burns at high
temperatures
Many alkynes are of biological
interest
Ethinyl estradiol is a synthetic estrogen
used in oral contraceptives
Benzene-Aromaticity
Aromatic Compounds and
Benzene
Aromatic compounds contain benzene.
Benzene, C6H6 , is represented as a six
carbon ring with 3 double bonds.
Two possible can be drawn to show benzene
in this form.
H
H
H
H
H
H
H
H
H
H
H
H
Resonance structures of benzene
P Resonance theory explains this by
suggesting there are two resonance
hybrids that contribute equally to the real
structure
Resonance structures of benzene
P Single bonds are longer (1.47 Angstroms)
than double bonds
P Double bonds are shorter (1.33 Angstroms)
P All bonds in benzene are 1.39 Angstroms
P Benzene has delocalized electrons
Delocalized electrons in benzene
P Electrons that form double bonds are in p
orbitals (dumbbell shaped)
P Electrons move easily from one p orbital to
the next
Delocalized electrons in benzene
P The electrons move around the entire ring
P Each bond is like 1 and ½ bonds
Benzene Structure
The structures for benzene can also be
written as a single structure where the
“alternating” double bonds are written as a
circle within the ring.
Benzene
structure
Properties of Benzene
P Benzene does not have any double bonds
P Benzene is much less reactive than expected
if it had double bonds
P Benzene does not have any single bonds
Aromatic Compounds in Nature
and Health
Many aromatic compounds are common in nature
and in medicine.
CHO
COOH
COOCH3
CH3
CH3
CH3CHCH2
CHCOOH
OCH3
OH
Aspirin
Vanillin
Ibuprofen
Aromatic Hydrocarbons
•
A cyclic compound containing several degrees of
unsaturation (double bonds).
•
Aromatic compounds have resonance hybrids and thus are
more stable than normal unsaturated compounds. E.g. Cl 2
does not readily add to an aromatic double bond, but reacts
quite rapidly with normal unsaturated compounds.
Benzene
Naphtalene
CH 3
Toluene
Anthracene
Naming Aromatic Compounds
Aromatic compounds are named with
benzene as the parent chain. One side group
is named in front of the name benzene.
CH 3
methylbenzene
(toluene)
Cl
chlorobenzene
Naming Aromatic Compounds
When two groups are attached to benzene, the
ring is numbered to give the lower numbers to the
side groups. The prefixes ortho (1,2), meta (1,3-)
and para (1,4-) are also used.
Cl
CH 3
Cl
CH 3
Cl
1,2-dimethylbenzene
1,3-dichlorobenzene
(ortho-dimethylbenzene)
(meta-dichlorobenzene)
CH 3
1-chloro-4-methylbenzene
(para-chloromethylbenzene)
Some Common Names
Some substituted benzene rings also use a
common name. Then naming with additional
side groups uses the ortho-, meta-, para- system.
CH 3
OH
CH 3
Cl
Toluene
(Methylbenzene)
meta-chlorotoluene
(meta-chloromethylbenzene)
phenol
(hydroxybenzene)
Benzene as a substituent
The C6H5- substituent is called Phenyl
Or phenylbutane
Double bonds have to be in the main chain
Benzene as a substituent
The C6H5- substituent is called Phenyl
t Other Aromatic Compounds
l Benzenoid Aromatic Compounds
Polycyclic benzenoid aromatic compounds have two or more
benzene rings fused together
Learning Check Alk9
Select the names for each structure:
Cl
1. Chlorocyclohexane
2. Chlorobenzene
3. 1-chlorobenzene
CH 3
CH 3
1. Meta-methyltoluene
2. Meta-dimethylbenzene
3. 1,3-dimethylbenzene
Solution Alk9
Select the names for each structure:
Cl
2. Chlorobenzene
CH 3
1. Meta-methyltoluene
2. Meta-dimethylbenzene
3. 1,3-dimethylbenzene
CH 3
Learning Check Alk10
Write the structural formulas for each of the
following:
A. 1,3-dichlorobenzene
B. Ortho-chlorotoluene
Solution Alk10
Write the structural formulas for each of the
Cl
following:
A. 1,3-dichlorobenzene
Cl
CH 3
B. Ortho-chlorotoluene
Cl
l Fullerenes
Buckminsterfullerene is a C60 compound shaped like a soccer ball
with interconnecting pentagons and hexagons
Each carbon is sp2 hybridized and has bonds to 3 other carbons
Buckminsterfullerene is aromatic
Analogs of “Buckyballs” have been synthesized (e.g. C70 )