More about alkanes
linear or straight chain alkanes - each carbon bonded to a maximum of two other carbons
Branched alkanes - certain carbons are bonded to three or four carbons
Branched alkanes can be viewed as straight chain alkanes in which hydrogens along the chain have been replaced by "substituents"
Above, the CH3 replaces a hydrogen and is termed a substituent.
Alkyl groups - When substituents are hydrocarbon groups the substituents are called alkyl groups
The names of alkyl groups follow the names of their parent alkanes Replace the ane ending with yl
Using substituent names and the names of linear alkanes, we can name branched alkanes:
1. Find the longest chain in the molecule. This is considered the parent and for alkanes simply give the appropriate linear alkane name.:
longest chain 5-carbons long - pentane
2. Identify the substituent(s)
3. Number the carbons in the main chain sequence, starting at the end that will give the smallest number at the alkyl substituent.
The methyl group is said to be at the "2" position
4. Construct the name from the above information as follows
General form: #-substituentparent, where # = the substituent position
5. What if there is more than one substituent?
Use di, tri, tetra, penta, hexa, etc... and numbers of positions
6. What if the substituents are different, list each one separately with appropriate numbering. Ordering of substituent list is alphabetical (the prefixes di,tri, etc... are ignored for purposes of alphabetization).
Parent name: octane
Substituents: methyl, propyl, ethyl
Parent name: decane
Substituents: ethyl, methyl (two of them)
dimethyl follows ethyl because the di is ignored in alphabetization
Cycloalkanes - we have talked about linear and branched alkanes, but there is one more common type - cycloalkanes:
What do cycloalkanes look like?
Other types of alkanes?
How bout this isomer of C20H20 - dodecahedrane
The properties of alkanes depend a great deal on the number of carbons and branching pattern.
Increase number of carbons, both melting point and boiling point increases.
Branching generally lowers the boiling point
Effect of branching on melting point more difficult to predict. For a given number of carbons, the more symmetric molecule will have the higher melting point