Organic Chemistry Structure and Stereochemistry of Alkanes
I. Basics:
A. Hydrocarbons:
1. Alkanes-hydrocarbons that only contain single bonds.
2. Alkenes-hydrocarons that contain a double bond.
3. Alkynes-hydrocarbons that contain a triple bond.
B. Saturated:
1. Alkane that has the maximum number of hydrogens.
a) Contains no double or triple bonds.
C. Molecular Formulas:
1. The basic formula of an alkane is C2Hn+2
2. Homologs- n-alkanes that differ in the number of carbons in the chain.
3. Common names indicate connectivity. Eg. Iso- branched
II. IUPAC naming rules:
A. Rules:
1. Find the longest chain and use the number of carbons for the base name.
a) If there multiple chains with the same number carbons, choose the one with the
greater number of substituents on the main chain.
See ex. of Heptane drawn 2 ways on pg. 84 in the 6th edition of Wade text.
2. Number the longest chain, beginning with the end of the chain nearest a
substituent. The substituted chains should have numbering that results in the
lowest numbering possible for the substituted carbons.
3. Name the substituent groups attached to the longest cahin as alkyl groups,
methyl, ethyl etc., Listing the carbon number to which it is attached.
4. When 2 or more substituents are present, list them in alphabetical order. Using
prefixes of di, tri, penta etc.
*Complex substituents are named by selecting the longest alkyl chain as a base
group, the numbering starts with the carbon that is attached to the main chain.
III. Physical Properties:
A. Basic:
1. Used fuels, solvents, and lubricants.
2. Natural gas, gasoline, kerosene, heating oil
B. Density and Solubility:
1. Alkanes are nonpolar, dissolve in nonpolar or weakly polar organic solvents.
a) Hydrophobic- don’t like water because they do not dissolve in water.
b) Boiling Points-the more the number of carbons the higher the boiling point. Ie
the bigger the radius the higher the melting point.
1. Therefore, branched alkanes boil @ boiling points lower than that of an
unbranched alkane. This is due to the fact that branching causes an
increase in compactness that results in less surface area for London Force
interactions.
c) Melting points- the melting point increases with the number of carbons and
increasing molecular weight.
1. Even number alkanes pack better into a solid structure.
i)this results in a higher temperature to melt them and therefore not a
smooth curve, a sawtone graph is observed as a result.
2. Branching causes an increase in melting point.
ii)this is due to the fact that branching gives it a more compact structure,
which packs more easily into a solid structure thereby increasing melting pt.
IV. Conformations and Structure:
A. Methane-
1. simplest of all the structures, perfectly tetrahedral with bond angles of 109.5 .
2. sp3 hybridized carbon.
3. covalent bonding to the hydrogen that resulted in bond lengths of 1.09 A.
B. Ethane-
1. Two –carbon alkane with overlapping sp3 forming a sigma bond between them.
2. Can have rotation about the sigma bond connecting carbons
a) Conformations- the different arrangements formed from the different rotations
about the sigma bond.
b) Newman Projection- a way of looking at a molecule straight down the bond
connecting the two carbon atoms. The front of the carbon is represented by
three lines coming together in a Y shape. The back carbon is represented by
similar lines coming out behind the circle to where only the ends of the lines can
be seen.
c) Dihedral angle(Θ-theta)- the angle that results between the C-H bonds of the
front carbon and the same bonds of the back carbon.
d) Eclipsed conformation-(Θ-theta=0)- the conformation in which the rear
hydrogen are”eclipsed” by those in the front.
e) Staggered conformation-(Θ-theta=60)- the conformation in which the rear
hydrogens are halfway between the hydrogens on the front carbon.
f) Torsional Strain- the resistance to rotation due to an increase in the potential
energy attained while approaching the eclipsed conformation.
• Note- from the ethane molecule to higher number carbons, you substitute H’s for methyl
groups. The adding of carbon allows for a couple more conformations.
g) Gauche- any staggered conformation in which theta is between 0 and 180.
h) Anti- the conformation in which theta is 180. Usually the lowest energy
conformation due to minimal interferance of attached groups overlapping
electron densities.
i) Steric Hinderance- Structural limitations that arise due to the overlapping of
electron densities of adjacent substituents. Makes certain conformations
impossible and others more favorable.
II. Cycloalkanes:
A. Cyclic- contain a continuous ring of atoms.
1. Cycloalkanes- alkanes that contain rings of atoms.
a) CnH2n is the general molecular formula.
B. Physical properties-
1. Most resemble their acyclic counterpart in properties.
a) Nonpolar.
b) Relatively inert.
c) Melting and Boiling point depend on molecular weight.
C. Nomenclature-
1. Use the number of carbons of the rings for the base name of the cyclic molecule.
2. Number the substituents in the same manner of acyclic n-alkanes.
a) if only one substituent then no numbers are needed.
3. More than one substituent , give the lowest possible numbers for the substituted
carbons.
4. If the acyclic portion contains a longer chain of carbons then the cyclic molecule then
becomes a substituent and is refered to as a cycloalkyl group depending on the number
of atoms on the chain.
5. If there is more than one carbon substituted,
a) If both groups are pointing to the same face then they are said to be cis.
b) If both groups are pointing to opposite directions the they are said to be trans.
D. Stability-
1. Five and six membered rings are more stable and hence more commonly found.
a) If a cycloalkane requires bond angles other than 109.5, the orbitals of the carbon-
carbon bond are not permitted to overlap fully resulting in angle strain(Baeyer
strain)
b) Ring strain- the sum of angle strain and torsional strain.
2. Heat of combustion- is the amount of heat released when a compound is burned with
an excess of oxygen in a sealed container called a bomb calorimeter.
a) Then the heat is measured via measuring the temperature of a water bath
surrounding the calorimeter.