NOMENCLATURE (SYSTEM for NAMING COMPOUNDS)

Hydrocarbons are molecules made from hydrogen and carbon. The simplest member is methane, CH₄, with just one carbon, and the other members would be H−(CH₂)_n−H.

The first observation in Organic Chemistry is the chemical reactions of the molecule are dependent on the type of bonds present in the molecule. For the H−(CH₂)_n−H molecules the chemistry can only be of the CH σ-bond and the CC σ-bond. So all the H−(CH₂)_n−H compounds will show the same (not identical) type of reactions.

The reaction is not identical in terms of reactivity as the chemistry of a molecule is dictated not only by the type of bonds present but also by the geometry and size of the molecule. However we can confidently classify the type of reactions according to the type of bonds present in a molecule.

So H−(CH₂)_n−H can be studied as a single class of organic compounds, or a homologous series. This particular homologous series is well known in history. We have methane, ethane, propane and butane. So it is not surprising that we named this series the alkanes.

Let us add a CC π−bond to ethane and we will get ethylene. So the next homologous series of hydrocarbon will be the alkenes, which are hydrocarbons having a CC π−bond in the molecule. Members of this homologous series will exhibit reactions of CH σ−bond, CC σ−bond, and CC π−bond. Again it must be emphasised that they show similar type of reaction, but not identical reactions.

Some mathematically inclined chemists would like to rationalise a homologous series as a series of compounds conforming to a certain molecular formula. The alkane will be C_nH_2n+2 while alkene will be C_nH_2n. With such a definition the molecule CH₂=CH−CH₂−CH₂−CH=CH₂, will not conform. I prefer a homologous series to be based on the types of bonds present in the molecule.

Remember that each carbon atom can share four valence electrons. So far the carbon has share an electron with a hydrogen or form a π−bond with an adjacent carbon. However it can also form a δ−bond with the adjacent carbon. In this case the members will have the reactions of a CH σ−bond, a CC σ−bond, and a CC δ−bond; making it a distinct homologous series. We named this class of hydrocarbons the alkynes. Its lowest member is ethyne (known in the commercial world as acetylene).

In conclusion, the introduction of the term homologous series is to impress upon students that the systematic approach to studying chemistry is to classify the compounds and study their general chemistry as a group. We did this for Inorganic Chemistry (the Periodic Table) and we are doing this here for Organic Chemistry.

IUPAC

In 1982, forty chemists met at Geneva in an "International Congress of Chemists" to set up a systematic approach in naming compounds. It eventually became the present "International Union of Pure and Applied Chemistry (IUPAC)" whose main objective is to make all chemists understand one another in chemistry terminologies, including the names of compounds.

Since Organic Chemistry is a late development in the history we are more organised here. We have already organised it into homologous series and the molecules methane, ethane, propane and butane were very well established. So we take it from here.

Number of carbon 1 2 3 4  5   Pent  6    Hex  7   Hept  8    Oct  9    Non 10     Dec

ALKANE Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane

ALKENE - Ethene Propene Butene Pentene Hexene Heptene Octene Nonene Decene

ALKYNE - Ethyne Propyne Butyne Pentyne Hexyne Heptyne Octyne Nonyne Decyne

For alkenes and alkynes we will also have to specify the carbon at which the π−bond or δ−bond is positioned. The alkene CH₃CH₂CH=CH₂ can be named 1−butene or 3−butene. Traditionally we opted for the lower number. But it is wrong to refer to it as 2−butene.

There are only two rules in the naming of chemical compounds:

• The international chemistry fraternity must know the compound you are referring to.
• One name must refer to only one compound, although one compound may have more than one name. In many instances the commercial sector will use a technical name or historical name. For example. They would not know ethyne but they know what is acetylene.

Other then these two principles there are certain traditions that most of us will follow, but no one is compel to follow them.

BRANCHED ALKANES

Let us start with alkanes. The simple ones would be the straight chain alkanes; H−(CH₂)_n−H. However there are also branched alkanes.

2-Methylbutane

2-methyl-3-ethylhexane

2,3-Dimethylpentane

2,2-Dimethylpropane

2−Methylbutane: The attached side group is generally referred to as the radical. It is named by replacing the ane of the equivalent alkane with yl. So methane becomes methyl, and ethyl becomes ethyl.

2−Methyl−3−ethylhexane can also be named 2−methyl−3−propylpentane, but IUPAC recommended that we pick the longest straight chain as the "backbone" alkane.

If similar groups are attached to the chain, then instead of repeating the name of the radical, like 2−methyl−2−methylpropane, we use the prefix di. The other prefixes are: tri (three groups), tetra (four), penta (5), hexa (6), etc....

CYCLOALKANES

Cyclopentane

Cyclohexane

We can also prepare cycloalkanes. The common cycloalkanes are cyclopentane and cyclohexane. To name a cycloalkane just add the prefix "cyclo" to the alkane forming the ring. Name whatever groups that are attached to the ring accordingly. Just go ahead and picked any carbon in the ring as C1 and the rest will fall in line. Sometimes the cycloalkane may even be named as an alkyl.

Tutorial 1.1

Draw the structure of the following molecules: 5−(2−ethylbutyl)−3,3−dimethyldecane; 1−ethylpropylcyclopropane; and 2−cyclobutyl−3−methylbutane. Answer

There are basically three classes of dienes based on their chemistry. The −C=C=C− an allene), the −C=C−C=C− conjugated diene, and isolated diene. Isolated dienes are alkenes with the two π−bonds separated by more than one σ−bond.

1,2−propadiene, the first member is an allene. 1,3−butadiene is the first member of the conjugated diene. 1,4−pentadiene is the first member of the isolated diene.

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