Organic Nomenclature: Alkanes


Definition How to Represent Organic Molecules, Part I Alkane Table
Normal versus Branched Special Branches Punctuation Rules
Alkane Summary Alkane Practice Worksheet

Alkane Definition

An alkane is a fully saturated carbon or hydrocarbon chain. Fully saturated means that there are as many hydrogens attached to the carbons as possible and only single bonds between carbons exist, like so: H3C–CH3.

Alkanes are also referred to as alkyl groups. They are also aliphatic.

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How to Represent Organic Molecules, Part I

Before we get started, you must know how to draw these organic molecules. There are several ways! Here are the most common forms you will see, for now. Notice that the following molecules are all identical. They are all normal hexanes.

In the Lewis Dot Form all bonds are shown. A single dot is used to represent an electron. A pair of electrons is represented as a dash or hyphen.

The Condensed Form allows a formula to be typed all on one line. The single bonds may be omitted, as shown.

The Carbon Skeleton Form is used when clarity of the main carbon chain is desired.

The line form is a very convenient, but it takes some getting used to. Each end of a line segment represents a carbon. If a carbon does not have four bonds, hydrogens fill in the remaining places.

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Alkane Table

No matter where you learn organic chemistry, you can't get away from having to know the names of the alkanes. So, here is a table of the first 10 alkane chains. You should commit it to memory. The root of each name is in bold because these roots are used in naming alkenes, alkynes, etc.

Number
of
Carbons
Name Formula Normal Structures
Lewis Dot From Line Form
1 methane CH4
2 ethane C2H6
3 propane C3H8
4 butane C4H10
5 pentane C5H12
6 hexane C6H14
7 heptane C7H16
8 octane C8H18
9 nonane C9H20
10 decane C10H22

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Normal versus Branched

The above table showed "normal structures". Why are they "normal"? Does "normal" imply there are "abnormal" structures?

Well, those structures are referred to as normal since they are all straight chains; there is no branching. So, we must learn how to name branched alkanes. Oh, and there are no "abnormal" structures!

The following five molecules (drawn in the skeleton form for clarity) are all hexanes. Therefore, they are called isomers of hexane. These isomers are different molecules with different properties! So, how do we tell them apart? Nomenclature! That's why nomenclature is so important.

So, where did these names come from?

Let's start with the first one, n-hexane. The "n" stands for "normal". Normal, as mentioned before, simply means an unbranched straight chain. It is called a "hexane" because it has six carbons in the chain. If you refer back to the table, you will see that hexane is the name given to chains with six carbons. There are other prefixes besides "n". Those are explained below.

Next is 3-methylpentane. Even though this molecule has six carbons, its carbon chain name is "pentane". Why? Well, this hexane is branched! The longest carbon chain is no longer six carbons long; it is five carbons long. So, we must use the name given to chains with five carbons which is pentane (see table). So, now we have the main chain named, what about that –CH3 on the middle carbon? Well, since it is only one carbon, its name is derived from the name given to one carbon membered chains, "methane" (see table). The "ane" suffix is removed and the "yl" suffix is added; both these suffixes designate alkanes.
To get almost the final name we simply add together the "methyl" and "pentane" giving the name "methylpentane" (no space between the two words). This is only almost the final name because methylpentane is still not specific enough! Notice that the third molecule above is also called a "methylpentane". The way to designate the difference between these two is to state where the methyl is on the pentane chain. In the second molecule, the methyl is on carbon 3 of the pentane chain. Thus, it is called "3-methylpentane". In the third molecule, the methyl chain is on either carbon 2 or 4, depending on how you count. In nomenclature rules, however, you must count so that you get the lowest number. Therefore, the name is "2-methylpentane" and not "4-methylpentane".

The forth molecule above has only four carbons in its longest chain. Thus, "butane" is the name given to it (see table). There are also two –CH3 on the molecule this time. Since there are two of them, the prefix "di" (meaning "two") must be added onto "methyl" giving "dimethyl". Other numerical prefixes are "tri" for three, "tetra" for four and so on. Both of the methyls are on carbon 2 (not carbon 3, use the lowest number) this time. So, in order to specifically tell where both methyls are, the proper name is "2,2-dimethylbutane".

The last molecule above is similar to the forth except that the methyls are on different carbons. So, the name is "2,3-dimethylbutane" this time.

Okay, are you ready to practice what you've learned so far? If so, try the problem below. Don't fret, answers are provided!

Practice Problem 1:

Give the correct name for the following compounds:

Click here to check your answer to Practice Problem 1

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Special Branches

Now, just when you thought you knew how to name alkanes....... How do you name these not so simple skeleton form alkanes?

You should have been able to name at least two of the above correctly. Those two being:

5-propyloctane
5-butyloctane

The rest are a bit tricky!

5-isopropyloctane
5-isobutyloctane
5-sec-butyloctane
5-tert-butyloctane
5-t-butyloctane

To keep naming a bit simpler, certain hydrocarbon chains have gotten special names. Here they are:

Notice that any straight chain is "normal". The normal chains are shown here for comparison. Plus, the prefix "iso" can be given to any hydrocarbon chain of six carbons or less as it was used in isopropyl and isobutyl.

Note that the only difference between normal propyl and isopropyl is where the attachment is to the main chain. The same is true for normal butyl, isobutyl, sec-butyl and t-butyl.

Need a way to remember these prefixes?
Well, see how isopropyl and isobutyl would form an isosceles triangle if we connected the other two carbons; that could be a good way to remember "iso".
Okay, "sec" comes from "secondary". The carbon of the sec-butyl that attaches to the chain is a secondary carbon, in other words, it is attached to two carbons.
Then, "tert" or "t" is short for "tertiary" which means "three". Following the same reasoning from before, the carbon of the t-butyl that attaches to the chain is a tertiary carbon, in other words, it is attached to three carbons.

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Punctuation Rules

Did you notice the use of nomenclature punctuation? This is important to learn as well. In general:

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Alkane Summary

  1. First find the longest continuous carbon chain, the parent chain, and name it.
  2. Determine the names of any side chains. Remember, some side chains have special names: n-, iso, sec-, tert-.
  3. If there are side chains, number the carbon chain using a numbering system that will result in the smallest numbers for the side chains.
  4. If there are multiple side chains of the same length, use the appropriate prefix: di, tri, tetra, etc.
  5. Lastly, if there are multiple side chains to list, list them in alphabetical order. When listing in alphabetical order, ignore any prefixes you may have added. For example, "dipropyl" would be listed after "ethyl".

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