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Of the various groups of organic compounds, alkanes have the simplest structure. Alkanes are hydrocarbons in which all the carbons form
hybridized bonds; thus, each carbon atom bonds with four other atoms in the tetrahedral shape. Because each carbon atom in an alkane bonds to four other atoms, chemists say they are
. Saturated hydrocarbons fall into two general types: alkanes and cycloalkanes. Alkanes have the general molecular formula CnH2n+2, and cycloalkanes have the general molecular formula C
. The molecular formula for the seven-carbon alkane, heptane, is C
. The molecular formula for the seven-carbon cycloalkane, cycloheptane, is C
Each carbon atom in a saturated alkane forms bonds to four other atoms.
An unsaturated molecule has multiple bonds between some pairs of atoms in the molecule.
A succession of hydrocarbons that differ from each other by one methylene group (—CH
—) is a homologous series. For example, the first three alkanes, methane (CH
—H), ethane (CH
—H), and propane (CH
—H), are all members of a
. Chemists often call each compound in a homologous series a homolog.
A series of molecules that differ by one carbon atom, but that are otherwise identical, is called a homologous series.
Alkanes that contain up to three carbons form only the straight chain arrangement because that is the only way they can bond together. Alkanes that consist of more than three carbons have more than one possible molecular structure. For example, the molecular formula C
has two possible structural formulas.
Isomers are compounds with the same molecular formula but different molecular structures.
Stereoisomers have different three-dimensional structures, and constitutional isomers have different bond sequences.
The fundamental rule of chemical nomenclature is that each different compound must have its own unique name. The IUPAC system provides a unique name for each of the nearly 10 million known organic compounds, as well as the thousands of new compounds discovered or synthesized each year. The IUPAC rules are simple to learn and easy to use. With these rules you can readily write the name of any compound you might encounter or derive the structure of any given compound from its name.
Chemists adopted the IUPAC system of nomenclature for several reasons:
Chemists everywhere understand it.
They can readily adapt it to the indexing methods used for the chemical literature.
They can easily use it for computerized literature searching operations.
The IUPAC method for naming a straight chain or unbranched alkane is as follows:
Step 1: Start with the prefix that indicates the number of carbons in the compound. The following compound has 8 carbons. The prefix for 8 carbons is oct–.
Step 2: Add the –ane ending. The name of the compound is octane.
The prefixes for most alkanes larger than four carbons come from Greek and Latin origin. Learning these prefixes is like learning to count in another language: one, two, three, four, five, six becomes meth–, eth–, prop–, but–, pent–, hex–.
In a branched alkane the longest continuous chain of carbon atoms does not include all of the carbon atoms in the molecule.
Not all alkanes are unbranched; in fact, most are
. Except for the shorter alkanes, which have only one or two different possible structures, branched alkanes have many different possible structures.
The following step-by-step description is used for naming branched alkanes:
Locate the longest continuous chain of carbon atoms.
a. This chain determines the compound's parent name. The parent name for the following compound is butane because the longest continuous chain contains four carbons. The box indicates the longest chain. The carbon—hydrogen group that is not a part of the parent chain is called a
b. The longest continuous chain is not always immediately obvious from the way the formula is written. For example, the following structure represents hexane because the longest chain contains six carbons. So, count the carbons carefully.
Number the carbons in the parent chain beginning with the end of the chain nearest the substituent.
Use these numbers to designate the position of the substituent, called an
, in the name of the compound.
An alkyl group is any single carbon or carbon chain, along with their hydrogens, attached to the parent chain. An alkyl group is an unbranched alkane with a hydrogen atom removed from the terminal, or end, carbon. To name the alkyl group, replace the –ane ending of the unbranched alkane with –yl. Thus, if you take one hydrogen from CH
, it becomes —CH
, and the name changes from methane to methyl. Below table illustrates this process with several additional examples.
Now you have all the parts of the compound's name. Put them together in the following way.
a. Write down the number of the carbon from which the alkyl group branches. In the example it is carbon 2.
b. Add a hyphen.
c. After the hyphen, place the name of the alkyl group. In this example, it is a methyl group.
d. Finally, add the name of the parent compound. In this example, the parent compound is butane. So the complete name is 2-methylbutane.
When naming a compound with two or more alkyl groups, follow the above steps with these additions:
To name a molecule with multiple substituents use the following rules:
a. When numbering the parent chain, give each substituent a number that corresponds to its location on the longest chain by listing the groups alphabetically1. If a compound contains both an ethyl and a methyl group, list the ethyl before the methyl. The general form of such a name is #-alkyl-#-alkylalkane. As a specific example, the name of the following compound is 4-ethyl-2-methylhexane:
b. If the two alkyl groups are on the same carbon, use the number of that carbon twice. For example, the following compound is called 3-ethyl-3-methylhexane:
c. When multiple alkyl groups are identical, indicate this by the use of prefixes di–, tri–, tetra–, penta–, and so on with the alkyl group name. Also, every identical group must have a number with commas separating those numbers.
The general forms are #,#-dialkylalkane and #,#,#-trialkylalkane. When naming compounds with the prefixes of di–, tri–, and so on, do not alphabetize the prefixes. Thus, dimethyl follows triethyl. The following compounds are 2,2-dimethylbutane, 2,2,3-trimethylbutane, and 2,2,3,3-tetramethylbutane.
Write the structural formulas and the IUPAC names for the nine isomers of C
. (Hint: It is helpful to name them as you go, to quickly eliminate duplication.)
Starting with the simplest structures and working toward the more complex, the first two structures would be heptane and 2-methylhexane.
If two or more chains compete for selection as the parent chain, choose the one with the smaller or simpler alkyl groups attached. This often means the greatest number of alkyl groups. For example, for the following structure, the lower chain has more and simpler attached groups than the upper chain even though the chains are both the same lengths:
Thus, the correct name for the structure is 3,4,6-trimethyl-5-propyloctane.
When branching occurs an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference. Looking again at the previous structure, its name cannot be 3,5,6-trimethyl-4-propyloctane as it would give a higher number at the first point of difference. In the following structure numbering from the right end gives lower numbers than when numbered from the left end. Thus, the structure’s name is 2,3,5-trimethylhexane and not 2,4,5-trimethylhexane.
The same principles apply to naming a compound when its bond-line formula is given. The colored screen on the right indicates the longest chain for the following compound.
The colored screen covers nine carbons. To simplify the way the drawing looks, redraw the longest chain straightened and number the carbon atoms as shown below.
The name of this compound is 5,6-diethyl-2-methylnonane.
A Summary of the Steps Used to Name an Alkane:
Find the longest possible chain of carbon atoms. Name this chain based on the number of carbons in this chain. If there is more than one chain of the same length, choose the one with the simpler substituents.
Number the carbons on the longest chain. Begin numbering from the end closest to a substituent on the chain.
Name any substituents using the alkane name for the number of carbons in the chain, changing the –ane suffix to –yl.
List the names of the substituents alphabetically. Precede each substituent with a number and a dash. Immediately follow the last substituent by the name of the longest chain.
If there are multiple instances of a substituent, list the substituent name with the prefix di–, tri–, etc., indicating the number of identical substituents. Number each substituent and separate the numbers with commas.
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