A. Sources of Alkanes
1. Petroleum
a. Fuel
b. Petrochemicals
2. Coal
B. Alkanes-All Carbons are SP3, no ½ bonds
1. Shapes of Alkanes-SP3 Carbon-most
bond angles are l09.5o
a. “Straight” chain alkanes-each
carbon is bonded to no more than two other carbon atoms.
b. “Branched” chain alkanes-one more carbon atoms is bonded to 3 or 4 other carbon atoms.
c. Constitutional Isomers in Alkanes-
Straight and branched chain isomers are constitutional isomers.
2. General Formula of Alkanes and Cyclo-
alkanes
a. Alkanes=CnH2n+2, where
n=# carbon atoms per molecule.
b. Cycloalkanes=CnH2n, where
n=# carbon atoms in the ring. Applies
to one ring only.
3. Types of C and H in Alkane Molecules
a. Primary Carbon (1o)=a carbon which is attached to only one carbon, the hydrogens attached to this C are also called primary hydrogens.
b. Secondary Carbon (2o)=a carbon which is attached to two carbon, the hydrogens attached to this C are also called secondary hydrogens.
c. Tertiary Carbon (3o)=a carbon which is attached to three carbons, the hydrogens attached to this C are also called tertiary hydrogens.
4. Various Representations of Alkanes
and Cycloalkanes
a. The Dash Formula
b. The Condensed Formula
c. The line-angle Formula
C. The IUPAC System of Nomenclature-1892
International Union of Pure and Applied
Chemistry-Each compound should have a
different name. The names should be based on a set of written rules.
1. Nomenclature of the Alkyl Group
a. Alkane - H = Alkyl Group
CH3CH3 -H = CH3 CH2-
Ethane Ethyl
b. Alkanes containing 3 or more car bons have multiple possibilities for alkyl groups.
(1) C3H7-
CH3CH2CH2-Y =Propyl
(CH3)2CH-Y=Isopropyl(2) C4H9-=The Butyl Group
CH3CH2CH2CH2-Y =Butyl
(CH3)2CHCH2-Y=isobutyl
(CH3)3C-Y
2. Nomenclature of the Straight Chain
Alkanes-Table 2.1-Page 53
Name No. C’s Structure
Methane 1 CH4
Ethane 2 CH3CH3
Propane 3 CH3CH2CH3
Butane 4 CH3CH2CH2CH3
Pentane 5 CH3(CH2)3CH3
Hexane 6 CH3(CH2)4CH3
Heptane 7 CH3(CH2)5CH3
Octane 8 CH3(CH2)6CH3
Nonane 9 CH3(CH2)7CH3
Decane 10 CH3(CH2)8CH3
Undecane 11 CH3(CH2)9CH3
Pentadecane 15 CH3(CH2)13CH3
3. Nomenclature of Branched Chain
Alkanes
Pages 57-58 of Text.
Rules on Handout Sheet-Big Deal
4. Nomenclature of Cycloalkanes
a. One ring=Cyclo + Name of the
alkane possessing the same number of carbons.
b. Substituents are named as in non-
cyclic compounds.
c. Use the lowest series of numbers for polysubstituted cycloalkanes.
d. Stereoisomerism in Cyclic
Compounds-
Stereoisomers-isomers which dif-
fer only in the arrangement of their
atoms in three dimensional space.
Any cyclic compound with two groups on different carbon atoms can exist as a cis or trans stereoisomer.
cis=groups on the same side of
the ring.
trans=groups on the opposite sides of the ring.
The name of the compound must
indicate the stereochemistry of the
compound.
D. Conformational Analysis in Alkanes and
Cycloalkanes
1. The Newman Projection
a. Staggered Conformations-most stable arrangements
(1) Potential Energy-stored
energy. Exists only when an
attractive or repulsive force
exists between objects.
(2) Stability of a molecule is
inversely related to its
potential energy.
b. Eclipsed Conformations-Less
stable arrangements
2. Conformational Analysis in Butane
a. Conformational Analysis-an
analysis of the energy changes
that a molecule undergoes as
groups or atoms rotate about a
SINGLE BOND.
b. Types of Unfavorable Interactions
(1) Torsional Strain-strain which
results from a NONSTAGGERED
arrangements of atoms about
a C-C single bond.
(2) Nonbonded interaction strain= Steric Strain =(van der Waals strain)-Strain which results when two atoms or groups are brought too close to eachother.
c. Let’s examine rotations about the
C2-C3 bond in butane, and evaluate any unfavorable interactions which result.
(1) Anti Conformation-a stag gered conformation in which the arger groups (atoms)are l80o
apart.
(2) Gauche Conformation-a
staggered conformation in
which the larger groups are
60o apart.
3. Relative Stabilities of Cycloalkane
Angle Strain-any deviation from
C-C-C bond angle of 109.5o causes strain in a cycloalkane. The greater the deviation-the greater the strain.
a. Cyclopropane is the least stable ring (60o). It has both angle strain and torsional strain.
b. Cyclobutane has 88o bond angles due to some puckered conformation which reduces the torsional strain.
c. Cyclopentane adops an “encelope” conformation (105o) which decreases the torsional strain in the molecule.
4. Conformations of Cyclohexane
a. The Chair Form of Cyclohexane-
Most stable conformation
(1) No angle strain
(2) No torsional strain
(3) No nonbonded strain
b. The Boat Form of Cyclohexane-
least stable
(1) No angle strain
(2) Some torsional strain
(3) Some nonbonded strain-due to the “flagpole” hydrogen interac tions. Trans annular interactions
=across the ring.
c. Twist-boat conformation-an intermediate arrangement.
AT 25oC >99% OF THE CYCLOHEXANE MOLECULES ARE IN THE CHAIR FORM. The
various forms of cyclohexane can not be separated because they intercon vert rapidly.
d. Axial and Equatorial Substituents
in Cyclohexane
(1) Axial bonds-bonds above or
below the plane of the bonds above or below the plane of the ring.
(2) Equatorial Bonds-bonds about
the equator of the ring. They
alternate up and down.
e. A group prefers the equatorial
position.
The larger the group, the more it will prefer the equatorial positon.
Table 2.4 Gives an estimate of preference-p. 77
f. Although cyclohexane is not planar, cis-trans isomerism is possible.
See Board For:
1,3-dimethylcyclohexane
1,4-dimethylcyclohexane
E. Polycyclic Compounds-many organic
compounds contain more than one ring.
1. Bicyclic compounds-compounds
containing two fused rings.
2. Two important polycyclic systems
(a) The steriods
(b) Tetracycline
F. Physical Properties of Alkanes and
Cycloalkanes
Homologous series-a group of com-
pounds with similar structure, dif-
fering from one another by the number of carbons in the structural backbone, and which have graded physi cal properties.
1. Straight Chain and unsubstituted
cycloalkanes form a homologous series. They differ by a -CH2-.
a. The boiling point increases about 30oC per -CH2-.
2. The Branched chain alkanes do NOT
constitute a homologous series.
a. The boiling point is less than that
of straight chain alkanes of the
same molecular weight.
b. As branching increases the
boiling point decreases for
constitutional isomers.
3. All alkanes and cycloalkanes are
LESS dense than water (<1g/mL) the density increases with increasing
molecular weight.
4. Alkanes and cycloalkanes are
relatively nonpolar, and thus are insoluble in water and other polar solvents.
G. Chemical Properties
Inert to most other chemical compounds.
a. Undergo combustion: form CO2 and H2O
CH4 + 2 O2 -----> CO2 + 2 H2O + ENERGY
(natural gas)
b. Heat of Combustion-The quantity of heat LIBERATED when one mole of a
hydrocarbon is combusted to form CO2 and H2O.
CH4 + 2 O2 ----> CO2 + 2 H2O
Ho= -192 Kcal/mole
CH3(CH2)2CH3 + 6.5 O2 --> 4 CO2 + 5 H2O
Ho= -687.5 Kcal/mole