CHAPTER 3 - ALKANES
CLASSIFICATION BY STRUCTURE: THE FAMILY
Structural theory relies on the structures of the molecules. These structures are divided into families. Alkanes are also referred to as paraffins.
STRUCTURE OF ETHANE
FREE ROTATION ABOUT THE CARBON-CARBON SINGLE BOND
Conformations - different arrangement of atoms that can be converted into another about single bonds.
Torsional energy - energy required to rotate about the carbon-carbon bond.
Staggered has the least energy, therefore, is the most stable.
Eclipsed has the most energy, therefore, is the least stable.
Skewed conformations are between the eclipsed and staggered.
Two systems of naming organic compounds are normally used:
common - derived from various origins
IUPAC - International Union of Pure and Applied Chemistry
alkyl groups are derived from the alkanes (one less hydrogen) - see table above
Drop the -ane and replace with -yl
example: methane (CH4) becomes methyl (CH3-)
CH4 to CH3-
Isomers are compounds with the same molecular formula but different structures.
Alkyls can also form isomers:
COMMON NAMES OF ALKANES
n - alkanes - compounds of carbon which form one continuous chain.
isoalkanes - compounds of six carbons or fewer in which all carbons except one form a continuous chain and that one carbon is attached to the next- to-end carbon.
IUPAC NAMES OF ALKANES
Steps to name (IUPAC) alkanes:
1. Trace the longest continuous chain of carbons (parent chain)
six carbons = hexane
2. Locate the side group and circle it.
one side group which is a methyl
3. Starting from each end of the parent chain, number from the end to give the first side group you come to the lowest possible number.
If you number from left to right, the methyl would be on the 5th carbon
If you number from right to left, the methyl would be on the 2nd carbon
Choose the direction that gives the methyl the lowest carbon number which would be right to left. Therefore, the methyl is on the second carbon.
The name of the compound above would be 2 - methyl hexane (note that a "-" goes between numbers and words)
4. If there are two side groups, number from the direction which gives the lowest number for the first side group encountered.
The methyl would be on the 2nd carbon numbering from right to left and the bromo would be on the 3rd carbon numbering from left to right. The lowest number would be the methyl on the 2nd carbon. Therefore, number from right to left.
To name the compound, list the side groups in alphabetical order.
4 - bromo - 2 - methyl hexane
5. If numbering from each end of the parent chain gives the same initial number (note below that the bromo and the methyl would be on the second carbon), default to the second group of side groups (in this case the chloro side group).
The chloro side group gives the next lowest set of numbers, so we would number from left to right.
We would then name the compound based on the parent name of hexane and order the side groups in alphabetical order.
2 - bromo - 3 - chloro - 5 - methyl hexane
If this doesn't give a second set of lowest numbers, try a third set of lowest numbers, etc.
6. If there is no difference in numbering to find the lowest numbered side group (both the bromo and methyl group below would be on the second carbon),
default to the alphabetical names of the side groups.
Since bromo is alphabetically before methyl, we would number from left to right,
2 - bromo - 5 - methyl hexane
7. If more than one side group is the same, use di (2) - tri (3) - tetra (4) - etc.
We would name this 2,5 - dimethyl hexane.
Notice that we separate numbers using a comma (,) and numbers and words using a dash (-).
Br - bromo
Cl - chloro
F - fluoro
I - iodo
The symbol X in an organic compound indicates a halogen.
CLASSES OF HYDROGENS AND CARBONS
Classes of carbons are based on how many carbons they themselves are directly attached to. For example a carbon directly attached to only one other carbon would be called a primary carbon.
Classes of hydrogen are based on the class of carbon they are attached to. For example, if the hydrogen is attached to a primary carbon, that hydrogen would be called a primary hydrogen.
Alkanes are nonpolar (Van der Waals forces)
As the molecular weight increases, boiling point increases.
Except for smaller alkanes, the boiling or melting point increases approximately 20 - 30oC per carbon increase.
C1 - C4 = gas
C5 - C17 = liquid
C18 and above = solid
As the branching on a molecule increases (given the same molecular weight), the boiling and melting points decrease.
Alkanes are less dense than water and are relatively insoluble.
Principle source is petroleum and natural gas.
Two common processes to convert alkanes into other alkanes.
isomerization - straight chains into branched chains
cracking - long chains into smaller chains
MECHANISM OF HALOGENATION
REACTIONS OF ALKANES