HS Chemistry - Introduction to Organic Chemistry

Branched Organic Molecules

Overview of The Page

This page will cover:

• How are branched organic molecules named?

In organic molecules, there are often several different "lines" of Carbon atoms, called chains. For example, in the molecule shown below, there are two chains of Carbon atoms. One of the chains $boxed in red below$ contains three Carbon atoms, and the other $boxed in blue below$ contains two Carbon atoms:

Chain 1, which contains three Carbon atoms, is longer than Chain 2, which contains two Carbon atoms. As Chain 1 is the longest chain present in the molecule, it is the main chain $aka the carbon skeleton$. Recall from the previous page that the parent part of an organic molecule's name denotes how many Carbon atoms are in the molecule's main chain - this molecule has 3 Carbon atoms in its main chain, so the parent part of its name will be prop-, despite the molecule having 4 Carbon atoms in total. In this example, Chain 2 is said to branching off of Chain 1.

Given that the molecule shown above is made of Carbon and Hydrogen atoms and only has single bonds, it is still an alkane, which means its suffix is -ane. And given that its main chain contains three Carbon atoms, the parent part of its name is prop-. But the molecule is not propane, as propane doesn't have a second chain. What, then, is the molecule's name?

The prefix$es$ of an organic molecule's name are responsible for noting any chains or other atoms in the molecule. The system for naming an organic molecule is: Prefix$es$-parent-suffix

The prefix$es$ note any branching chains $also known as side\-chains$ in the molecule $as well as any other non\-Hydrogen atom bonded to a Carbon atom$. But to name the prefix, we have to understand the naming convention for side chains.

First we have to look at the entire side chain. That means all the atoms connected to it, not just the Carbon atoms. After boxing the side chain in blue:

We can isolate it and the atoms connected to it:

And since the Carbon on top is part of another chain, in order to properly name this side chain we will need to ignore it:

Now to name this side chain, we'll follow the same rules that we follow for naming organic compounds in general:

Prefix$es$-parent-suffix

Since this side chain consists of only Carbon and Hydrogen atoms, and since this side chain has no further side chains, there won't be any prefixes in this side chain's name.

To name the parent, we need to identify the number of Carbons in the main chain of this side chain - that is, the longest chain within the side chain. Since there's only one chain, we can use the number of Carbons within that chain. That number is 1, as there's only one Carbon atom in this side chain. The parent part of the name of this side chain is therefore "meth-".

Now we need to name the suffix. If the side chain is an alkane $as is the case here$, then the suffix is "-yl", and it is referred to as an alkyl chain or an alkyl group. Otherwise, if the side chain is another functional group $e.g. an alkene$, to form the suffix, simply take the normal suffix of the functional group, remove the ending "e", and add "-yl" after it $e.g. if the side chain is an alkene, you would add "\-yl" after removing the ending "e" from the normal alkene suffix, which is "\-en", and the resulting suffix would be "\-enyl"$.

Following these rules, the name for our side chain:

Is methyl.

Let's return to our organic molecule:

We've almost fully named it - we've already identified the parent part of the name $prop\-$ and the suffix $\-ane$, and we've identified the side chain $methyl$. To complete the organic molecule's name, however, the prefix also needs to note where the side chain is attached along the main chain.

We can start by numbering $in red$ the Carbon atoms in the main chain:

The methyl group is attached to the 2^nd Carbon atom in the main chain. Therefore, we'll append "2-" before "methyl" in the prefix to show that the methyl group is attached to the 2^nd Carbon atom in the main chain.

The name of the molecule, composed of the prefix $2-methyl-$, the parent $prop-$, and the suffix $-ane$, ends up being 2-methylpropane.

In this case, it was easy to determine what number should come before the "methyl" in the prefix, as numbering the main chain's Carbon atoms in the reverse direction would have yielded the same result:

The methyl group is still attached to the 2^nd Carbon atom. But say we had a molecule where this wasn't the case:

It's an alkane $so the suffix is "\-ane"$ with six Carbon atoms in the main chain $so the parent is "hex\-"$, and it has a methyl group $so "\-methy\l-" is part of the prefix$. To complete the prefix, we need to note which Carbon atom the methyl group is attached to.

If we number the Carbon atoms in the main chain from left to right:

The methyl group is attached to the 5^th Carbon atom in the main chain. But if we number the Carbon atoms in the main chain from right to left:

The methyl group is attached to the 2^nd Carbon atom. So is the molecule 5-methylhexane or 2-methylhexane?

In this case, the smaller number is chosen. Therefore, the molecule is named 2-methylhexane, not 5-methylhexane.

Now let's take the following molecule:

It's an alkane $so the suffix is "\-ane"$ with six Carbon atoms in the main chain $so the parent is "hex\-"$, and it has two methyl groups $so "\-methyl\-" is part of the prefix$. However, since there are two methyl side chains instead of one, we need to append the prefix to make note of this $since there are two, we'll add "di\-" before "\-methyl\-"; if there were three, we'd add "tri\-"; if there were four, we'd add "tetra\-"; and so on$. Thus, we can determine that "-dimethyl-" is part of the prefix. To complete the prefix, we need to note which Carbon atom the methyl groups are attached to.

If we number the Carbon atoms in the main chain from left to right:

One of the methyl groups is attached to the 3^rd Carbon atom, while the other is attached to the 5^th Carbon atom. But if we number the Carbon atoms in the main chain from right to left:

One of the methyl groups is attached to the 2^nd Carbon atom, while the other is attached to the 4^th Carbon atom. So how do we number these? If we're supposed to pick the smallest numbers, do we label it 2,3-dimethylhexane?

We start numbering from the side that gives us the smallest number, but then continue on from that side. If we number from right to left, our first methyl group is at the 2^nd Carbon atom, while if we number from left to right, our first methyl group is at the 3^rd Carbon atom. Therefore, we number from right to left, but then we continue numbering in that direction rather than switching directions. That way, the second methyl group is attached to the 4^th Carbon atom.

After we've found what numbers we need to add in the prefix, we need to put them in the correct order. When writing multiple numbers in the prefix, we arrange them from smallest to greatest, separating them by commas. Thus, the molecule is named 2,4-dimethylhexane.

What if we have something like:

This is quite a complex one to name, but it can be broken down into smaller parts. There are 3 alkyl side chains in this molecule:

As well as an Iodine atom bonded to the 1^st Carbon atom from the left:

This tells us that the molecule is a halogenoalkane $covered more in depth in [Halogenoalkanes](../Unit-10/6-Halogenoalkanes.md)$. Halogenoalkanes follow almost the same naming convention as normal alkanes, except that the prefix notes what halogen is present and where in the molecule it is present $what atom it is attached to$.

Since we have a halogen bonded to the first Carbon atom from the left, our numbering of the Carbon atoms in the main chain will go from left to right.

There are nine Carbon atoms in the main chain, so the parent part of the molecule is "non-". Since the molecule is a halogenoalkane, the suffix is "-ane".

To form the prefix of a halogenoalkane, we order the halogen prefixes alphabetically $Bromine would come first in the name, then Chlorine, then Fluorine, then Iodine$. First, however, we'll look at the alkyl side chains.

In the drawing below:

What type of alkyl group is the one boxed in green? ethyl methyl propyl

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What is the position number of the side chain in green? Enter your answer as a number to indicate which Carbon atom it is attached to (e.g. enter 1 if it is attached to the 1^st Carbon atom as numbered in the drawing above):

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What type of alkyl groups are the ones boxed in red? One of the side chains is a methyl side chain, and the other is an ethyl side chain They're both ethyl side chains They're both methyl side chains

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What are the positions numbers of the two side chains boxed in red? One of the side chains boxed in red is attached to the 3^rd Carbon atom, and the other is attached to the 4^th Carbon atom Both of the side chains boxed in red are attached to the 4^th Carbon atom Both of the side chains boxed in red are attached to the 5^th Carbon atom

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What is the prefix for all three alkyl groups combined? Enter your answer as just the prefix (e.g. if you had one ethyl side chain at the 7^th Carbon atom and two methyl side chains at the 3^rd and 6^th Carbon atoms, you would enter "8-ethyl-3,6-dimethyl" as your answer, without the quotation marks):

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After we've found the prefix notation for the alkyl groups, we then look at the prefix notation for the halogen $in this case, Iodine$. It's at position 1, so the prefix is "1-iodo"

Now we need to order them correctly. We need to decide whether "1-iodo" or "4,4,8-trimethyl" will come first.

We'll use alphabetical order to decide which comes first. When using alphabetical order, parts of the prefix that note where in the atom the side chain appears $like the numbers$ or how many instances there are of that alkyl group $like the "di" in "dimethyl" or the "tri" in "trimethyl"$ are ignored. The exception to this is when the affix "iso" is used.

Following that rule, "1-iodo" will come before "4,4,8-trimethyl", as "i" comes before "m".

Now that we've ordered our prefixes, we'll combine them. The different prefixes are separated by hyphens. Our resulting, combined prefix is:

1-iodo-4,4,8-trimethyl

We then add that prefix to the beginning of our organic compound's name, and the resulting name of our organic compound is 1-iodo-4,4,8-trimethylnonane.

But now that we've named it and drawn its displayed formula, how would we represent its other formulae?

Our molecule is made of 1 Iodine atom, 12 Carbon atoms, and 25 Hydrogen atoms.

It's molecular formula is therefore C₁₂H₂₅I. However, as you can see this doesn't tell us much about it. We can't tell the structure of 1-iodo-4,4,8-trimethylnonane from its molecular formula.

A better option would be to use its structural formula:

CH₂ICH₂CH₂CCH₃CH₃CH₂CH₂CH₂CHCH₃CH₃

Which tells us that the Iodine is attached to the first Carbon atom, but because its so cluttered, it's difficult to get much information out of it.

An even better option would be to use its displayed formula:

This allows us to more easily distinguish the molecule's structure and identify where the iodine, methyl and ethyl groups are. But it takes a lot of time to create. We could use the 3D displayed formula instead, but that is also time-consuming.

The best option in this case is probably to use the skeletal formula:

Each unlabeled point on the graph represents a Carbon atom, and the branches sticking out of the main chain represent the side chains $notice how the methyl groups only extend one point forward, while the ethyl group extends two points forward$. You'll notice that the skeleton has been extended downwards on the left side, and Iodine is added at the edge. In a skeletal formula, atoms that are not Carbon or Hydrogen are represented as chains of their own, even if there is nothing else attached to that atom.