Common Sense Organic Chemistry

IUPAC Nomenclature




IUPAC Nomenclature & Functional Groups

Organic compounds, of which over 150 million are currently known and cataloged, are so innumerably mind-boggling that a systematic way of naming them is necessary…one which all peoples of all languages could and would understand. The old system of naming for their odors or for people or any reason at all (common or trivial naming) become so confusing and required so much memorization, that an unequivocal and agreed upon system was inevitably introduced.

This is the IUPAC (International Union of Pure And Applied Chemistry) who, in committee, makes these decisions. They are commonly the best rules since they work for all the organic compounds we can ever possibly imagine. To see how this works:

Take, for example, the simplest alkane hydrocarbon, Methane.

Methane (CH4) is the simplest hydrocarbon that exists. It’s simplicity does not detract from its usefulness as it is the main component in natural gas which nearly powers everything we do in most homes.

CH4 (methane)

CH3-CH3 (ethane)

Ethane (CH3CH3), one carbon longer in the series, is a homologue and also a major component in the mixture of natural gas we use.

Of course, the heavier these become, as an additional carbon is attached, the less gaseous they are, they tend to be require more heat from the environment to evaporate gradually heavier homologues. Put another way, they are less volatile. The largest alkane molecules like waxes don’t evaporate at all at room temperature.

CH3-CH2-CH3 (propane)

Compressed propane (3 carbons) in a steel container, is a liquid. And if you’ve ever barbecued on a grill you understand that propane becomes less like a liquid when released from pressure …it is gas like the first two…(prop- = 3 carbons).

CH3-CH2-CH2-CH3 (butane)

Then there’s butane–or lighter fluid. This is a liquid inside much less volatile and can sit in a small plastic container with an ignition unit or “flint”, the sound is obvious and it burns brilliantly (reddish hues) like larger alkanes. (But- ?4 carbons).

CH3-CH2-CH2-CH3 (butane)

Then there are pentanes: 5 carbon containing systems: These are liquid at roughly room temperature (no pressure but atmospheric pressure alone (1atm or 760mmHg or 760torr at sea level).

CH3-CH2-CH2-CH2-CH3 (pentane)

The larger they get and more cylindrical in shape (more intermolecular dispersion forces and surface area), the less able they are to volatilize (evaporate).

The next molecule in the series, hexane, actually boils at 60°C and are relatively difficult to evaporate completely…and thus this series continues…until you reach paraffins (low affinity compounds) and waxes which are so large they tangle and become solid (and which also burn or undergo combustion) on the wick of a candle.

In other words: All of the alkanes in a homologous series end in the suffix -ane.

By now you realize that all of the “saturated” hydrocarbons are alkanes and that means, that every place on the molecule that can have a C-H bond; indeed has one. Therefore, it is called “saturated” with hydrogen or aliphatic (non-polar and insoluble in water).

And depending on how many carbons they possess, is what determines the prefix of the name.

For example:

If it has two carbons…it is eth___.

If it has 4 carbons, it is but____.

The series gets larger. When they are so long that cease to have names and just known generally as polymers. There can be 120,000 or more in a single chain.

Starting with one carbon and extending the chain as long as the trend continues….we have:

1C-ane – methane

2C-ane – ethane

3C-ane – propane

4C-ane – butane

5C-ane – pentane

6C-ane – hexane

7C-ane – heptane

8C-ane – octane

9C-ane – nonane

10C-ane – decane

11C-ane – undecane

12C-ane – dodecane …


20C-ane – eicosane

Here is roughly the same thing, drawn correctly:

Nomenclature – homologous series – alkanes

Notice that some of the prefixes make sense (5C-10C) as far as geometric shapes go…but before 5 carbons or after 10 carbons…it gets into a territory beyond what you will .

*Rarely, will you ever be called upon to name something with a parent chain longer than 11 carbons. (Really.)

It could happen but that does not mean it will. A good organic professor can make a really hard nomenclature question with just 6 carbons or fewer. But first…

What is a parent chain?

Everything we’ve been drawing or thinking about so far. See the zigzag structure below?….

The homologous series of alkanes is drawn this way rather than abbreviating it as above:

So what is the name of the following compound (the name of its parent chain)? Get used to the skeletal version of things….that’s how everyone writes and draws. That doesn’t mean they always will…they will follow the representations you’re book is using for the moment. **Be able to interconvert from condensed to line-angle drawing**. You never can be sure how things will be written on an exam!

Organic Nomenclature

You should have the answer if you numbered the carbons. Which must give the answer indicated…(below).

Now if things were always this simple, nomenclature would also be simple. But there is more to learn.

For example, the majority of compounds are not like this simple chain. In fact most organic compounds, of any worth, possess branching and substituents (a substituent is anything that takes the place of a hydrogen atom). And it is in that way that we can change or alter the structure of any molecule. But first we need the functional groups which come next.

The compound above is named “nonane”. Its parent chain is the name. (see below (vide infra))

Rule #1:

Find the longest continuous chain of carbons, possessing the major functional groups.

Organic Nomenclature
For this molecule, the name is complete. There are no functional groups, but the longest chain is nine carbons long….it is “non-ane”.

But what if there is branching? That’s the majority of the rest of the procedure….knowing what to call substituents and the most important, reactive parts of the molecule (functional groups).


When substituents are on the chain (or ring) you’ve established as the parent, they must all have have numbers.

Let’s imagine that we have a branch point in the molecule, like this:

Organic nomenclature – branched
Organic nomenclature – branched
What would you call the preceding molecule?

Did you call it 6-methylnonane or 4-methylnonane? The number are “locants” or the positions on the parent chain where the substituents can be found. [Numbers are separated by commas; numbers and letters separated by hyphens–do not separate letters with anything this semester].

If you were typing….you’d never hit the space bar for most correct names.

(A couple exceptions come in the second semester…esters, anhydrides, and acid halides…ignore those for now).

Often people get confused because the structure is written in reverse of what you are used to doing when reading….going from left to right. But no one said we read organic structures from left to right. Not anymore.

However if you do that and go left to right with your numbering, you will end up with 6-methylnonane, the wrong answer. Why is it “wrong”?

Rule #2:

Always give branch points the lowest set of numbers possible.

Since 4-methylnonane has lower numbers in its name than 6-methylnonane, the answer is 4-methylnonane.

In fact, due to the rule, there is no such thing as 6-methylnonane. It’s literally impossible, since you must give all the lowest numbers to all the substituents and functional groups.

So the name of the following compound is what? :

Organic nomenclature

If you follow the first rule: to find the longest continuous carbon chain containing the major functional groups, you will have the parent as nonane.

Nomenclature- parent chain

The parent chain is nonane.

–under construction–

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