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ALKANES
HYDROCARBONS
HISTORY OF ORGANIC
Early 19th century chemists were only familiar with inorganic,
ionic compounds.
They were baffled by the fact that several compounds that
were found in fruits and vegetables were composed of many
carbon, hydrogen, nitrogen and oxygen atoms in different
combinations were stable.
The “vital force theory” grew out of the idea that these
molecules could only come from living organisms.
In 1828, Wohler did an experiment that destroyed this
idea…he made urea from ammonia and cyanic acid.
Now, we know that there are virtually a limitless number of
organic compounds that can be synthesized.
ORGANIC
COMPOUNDS
Properties of organic substances are related to their
structure. Most compounds contain carbon, hydrogen and
any of the other following elements: oxygen, nitrogen,
phosphorus, sulfur and the halogens.
Many of these atoms follow typical bonding patterns:
C has 4 bonds and no lone pairs.
N has 3 bonds and 1 lone pair.
O and S have 2 bonds and 2 lone pairs.
H has 1 bond and no lone pairs
Halogens (F, Cl, Br, I) have 1 bond and 3 lone pairs.
HYDROCARBONS
2 categories:
1) Aromatic- compounds with carbon and hydrogen that
contain benzene rings
2) Aliphatic- compounds with carbon and hydrogen that are
not aromatic (alkanes, alkenes, alkynes, and
cycloalkanes)
Benzene
ALKANES
Alkane is the term used for saturated hydrocarbons, meaning
that no multiple bonds exist between carbons.
These molecules are nonpolar, due to electronegativity
differences.
Weak intermolecular forces exist, and so these molecules
have relatively low boiling points.
Typical alkanes follow this pattern for formulas:
CnH2n+2
ISOMERS
In some cases, it is possible to draw molecules with 2
different structures that have the same chemical formula.
That is an isomer, by definition.
Isomers are common among organic compounds.
Examples:
n-butane and isobutane
IUPAC NAMING
IUPAC stands for International Union of Pure and Applied
Chemistry.
This group of professional chemists set guidelines for
naming chemical compounds.
Different naming procedures are used for organic molecules
than what are used for inorganic compounds.
In order to name compounds, it is important to recognize
many common alkyl groups.
Alkyl refers to a group with one less hydrogen than an alkane
(CnH2n+1).
IUPAC NAMING
Three prefixes used commonly indicate structural info:
Iso- has this structure at one end of a carbon chain
Sec- (for secondary)
Tert- or t- (for tertiary)
PRIMARY, SECONDARY
AND TERTIARY
Primary (1°) refers to a carbon that is bonded to 1 other
carbon.
Secondary (2°) refers to a carbon that is bonded to 2 other
carbons.
Tertiary (3°) refers to a carbon that is bonded to 3 other
carbons.
ALKYL GROUPS
RULES FOR NAMING
ALKANES
1)
Select the longest continuous chain of carbon atoms as the parent
compound, and consider all alkyl groups attached to it as branch
chains that have replaced hydrogen atoms of the parent
hydrocarbon. (The name of the alkane consists of the name of the
parent compound prefixed by the alkyl groups attached to it.)
2)
Number the carbon atoms in the parent carbon chain from one end
to the other starting with the end closest to the first carbon atom
that has a branch chain.
3)
Name each branch-chain alkyl group and designate its position on
the parent carbon chain by a number.
4)
When the same branch alkyl group occurs more than once,
indicate this by a prefix (di, tri, tetra, etc) written in front of the
alkyl group name.
5)
The numbers indicating the carbons that the groups are on are
separated by commas, then a hyphen in front of the group name.
SUMMARY
1) Find the longest chain! Be sure to check groups
attached…they may be part of the main chain!
2) Number the carbons from the end where an attached
group is nearest.
3) Use numbers to tell which carbon the group is attached
to.
4) Use prefixes if there are more than one of the same
group.
5) Always list branches in alphabetical order.
LINE STRUCTURE
In organic, we often write structure with
lines to look less confusing than structural
formulas.
Remember..
Each point, or end of a line segment
represents a carbon atom. It is assumed
that as many hydrogens are attached as
possible to give the carbon an octet.
EXAMPLES
Write the structural formula for the line
structures shown.
1)
2)
PRACTICE
Name the following:
Draw the following:
1)
3) 2-methylhexane
4) 3,4-dibromoheptane
2)
5) 2-chloro-3-ethylpentane
REACTIONS OF
CARBON
1) Oxidation-reduction reactionWhen carbon atoms are oxidized (lose electrons),
they often form additional bonds to oxygen.
When carbon atoms are reduced, they often form
additional bonds to hydrogen.
2) Substitution reactionMany involve halogens…
Methane gas plus bromine liquid
REACTIONS OF
CARBON
3) Elimination reactions (often called
dehydrogenation)Single reactant is split into two products, and one
is eliminated.
These reactions form multiple bonds.
4) Addition reactions- reverse of an elimination
reaction…two reactants come together to form one
product.
Ethene + HBr 
SOURCES OF ALKANES
Natural gas and petroleum are the two main
sources of alkanes.
Main component of natural gas is methane (8095%).
Petroleum is refined into useful products like
gasoline, kerosene, jet fuel, diesel fuel, paraffin
wax, and petroleum jelly.
Alternative fuel sources must be developed
because these sources will run out and are already
in shorter supply.
CYCLOALKANES
General formula- CnH2n
Named by adding cyclo to the main chain name…
For example, a six carbon cyclic alkane is called
cyclohexane. Can you draw it?
Cyclopropane and cyclobutane are more reactive
than their main chain constituents, but the longer
chains are similarly reactive as their cyclic
cousins.
CYCLOPROPANE AND
CYCLOBUTANE
Normally carbon molecules
have a tetrahedral geometry
with a 109.5 degree bond angle.
However in cyclopropane, the
angles are strained and forced
to be 60 degrees!
Cyclobutane forces carbon to
have bond angles of 90
degrees, thus it is also strained.
The strain causes it to be really
reactive because the bonds are
weakened, so they are easily
broken as a result.
OTHER THINGS
ABOUT RINGS
The conformation,
or shape in 3-D, of
rings will assume
the most stable
arrangement
possible.
This depends on
the distance
between
hydrogens.
Equatorial –(blue)
the hydrogens in
the same plane as
carbon
Axial- (red)the
hydrogens are at
right angles to the
carbon
CONFORMATIONS OF
CYCLOHEXANE
There are 2
conformations of
cyclohexane.
Chair and Boatchair is most
stable
NAMING
CYCLOALKANES
The base of the
name of a
cycloalkane is …
Cyclo- plus the root
that describes the
number of carbons
present in the ring.
If they are
substituted, meaning
they have a group
branched off of the
ring, then the
carbons are
numbered either
clockwise or
counterclockwise so
that the branches are
off of the lowest
numbered carbons
possible.