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Open-chain alkanes (without rings) all have the general formula CnH2n + 2, where n equals the number of carbon atoms.
The following table shows the structures and names for the first 10 unbranched, open-chain alkanes.
Look at the trends in the boiling and melting points and the density of the alkanes as their mass increases.
An alkyl is basically an alkane minus one of its hydrogen atoms.
| remove one H |
H-C-H ============> H-C- or CH3-
H H H H
| | remove one H | |
H-C-C-H ============> H-C-C- or CH3CH2-
| | | |
H H H H
The significance of alkyl groups will be explained in the next section.
IUPAC System of Naming Alkanes
The IUPAC (International Union of Pure and Applied Chemistry) is composed of chemists representing the national chemical societies of several countries.
One committee of the IUPAC, the Commission on Nomenclature of Organic Chemistry, has set a system for naming organic compounds.
The last syllable in the name of a compound designates the family to which it belongs.
The alkanes all end in -ane.
IUPAC Rules for Namining the Alkanes
- The name ending for all alkanes (and cycloalkanes) is -ane.
The parent chain is the longest continuous chain of carbons in the structure.
For example, the branched-chain alkane:
is regarded as being "made" from the following parent:
by replacing an H on the third C from the left with CH3.
A prefix that specifies the number of carbon atoms in the parent chain is attached to the name ending, -ane.
These prefixes up to 10 carbon atoms are and should be learned:
meth- 1 C hex- 6 C
eth- 2 C hept- 7 C
prop- 3 C oct- 8 C
but- 4 C non- 9 C
pent- 5 C dec- 10 C
The parent chain in the example above has 6 carbon atoms, therefore, it is a derivative of hexane.
The carbon atoms of the parent chain are numbered starting from whichever end of the chain gives the location of the first branch the lower of two possible numbers.
For the example above, the correct direction is from left to right.
1 2 3 4 5 6
The branch is therefore located on the 3rd carbon.
If it was numbered from right to left, the branch would be located on the 4th carbon, which is a higher number, which is not allowed by IUPAC.
Name each branch attached to the parent chain according to alkyl groups.
In this case, the branch would be methyl.
Attach the name of the alkyl group to the name of the parent as a prefix.
Place the location number of the alkyl group in front of the resulting name.
In this case, it would by 3-methylhexane.
When two or more groups are attached to the parent, name each and locate each with a number.
The alkyl substituent names are assembled in alphabetical order.
Always use hyphens when separating numbers from words.
The following is 4-ethyl-2-methylheptane:
7 6 5 4 3 2 1
When two or more substituents are identical, use prefixes such as di- (2), tri- (3), tetra- (4), and specifiy the location number of every group.
Always separate a number from another number in a name by a comma.
The following is 2,4-dimethylhexane:
When identical groups are on the same carbon, repeat the number of the carbon in the name.
The following is 2,2-dimethylpentane:
Sometimes, you may need to go around corners and zigzag to find the longest (parent) chain.
The following is 3,4-dimethylheptane (the parent chain is in bold):
Chemical Properties of Alkanes
Alkanes are usually stable at room temperature toward reactants such as concentrated aqueous acids or bases, an even the most reactive metals.
Fluorine, however, attacks virtually all organic compounds, including the alkanes, to give mixtures of products.
Hot nitric acid, chlorine, and bromine can also react with alkanes.
The chlorination of methane can give the following compounds depending on the relative quantities of reactants used:
When heated at high temperatures in the absence of air, alkanes can "crack," meaning that they break up into smaller molecules.
The cracking of methane gives finely powered carbon and hydrogen gas.
CH4 =================> C + 2H2
The controlled cracking of ethane gives ethene ("ethylene"), which is an important raw material in the organic chemicals industry, used to make polyethylene plastics, ethyl alcohol, and ethylene gycol (an antifreeze).
CH3CH3 =================> CH2=CH2 + H2