Learning activity 1.2: What’s in a name

Think

Try to imagine an image of a mixed orchard with different kinds of trees. How would you attempt to differentiate and describe each tree or group of trees?

Trees in an apple orchard on a sunny day.

Notebook

Based on the image you've created in your head, answer the following questions then compare to another student's suggested answers. Don't worry if your answers don't match – the goal here is to think about how you would go about categorizing the trees in your imaginary orchard.

Reflect and answer the following questions in your notebook.

1. What are some terms that would be the same for each tree?

fruit tree
tree planted by humans

2. What are some terms that would be the same for a group of trees (but not all trees)?

apple tree
pear tree

3. What are some terms that would be unique for an individual tree?

the single McIntosh apple tree among the apple trees
the only cherry tree in the orchard

An example of a description of an individual tree might be:

"The 100-year old walnut tree planted by my great grandfather, struck and almost burned down by lightning last year, which always loses its leaves last in the fall and blooms earliest at springtime, etc."

In this activity, you will learn how chemists draw and describe an organic compound by the name that they use. Just as you described the features of the trees, chemists describe the features of a compound using terms that are common to a class of compounds as well as unique to the individual compound itself.

The representation of organic compounds

There are a few different ways in which chemists represent organic structures. Three of them are:

complete structural formulas (also referred to as complete formulas or structural formulas)
condensed structural formulas (also referred to as condensed formulas)
line diagrams (also referred to as skeletal formulas)

You need to be familiar with each of these ways of representing organic compounds, in order to be able to recognize and name them.

Notebook

Make sure to include summaries of each type described in your notebook as you press the following phrases.

The complete structural formula of an organic compound shows all of the atoms involved in the compound, represented by their atomic symbols, with bonds between atoms represented by dashed lines. A single dashed line represents a single bond, a double dashed line represents a double bond and a triple dashed line represents a triple bond.

Ethane

Ethene

Ethyne

In condensed structural formulas, the dashes representing bonds are not included. Similar atoms bound to a carbon are grouped together and their number is represented using subscripts. Units that are identical and repeat can be put within brackets and subscripts are used to indicate how many times they repeat. Units that branch off the main chain are also sometimes put in brackets.

For example: pentane $(C_{5} H_{12})$ has the following complete structural formula:

This is represented as $C H_{3} (C H_{2})_{3} C H_{3}$ in its condensed structural formula.

Pentane

In line diagrams, carbon and hydrogen atoms are represented by lines, while all other atoms are represented by their atomic symbol. Carbon atoms are represented at the ends of lines and at the intersection of lines, while the presence of hydrogen atoms is implied by the maximum amount of hydrogen atoms that could be bound to a given carbon atom (remember carbon atoms can have a maximum of 4 bonds). Double bonds are represented by double lines while triple bonds are represented by triple lines.

For example: butane $(C_{4} H_{10})$ , which has the condensed formula of $C H_{3} (C H_{2})_{2} C H_{3}$ is represented in a line drawing as follows:

butane

While 2-butanol, which has the condensed formula of $C H_{3} C H (O H) C H_{2} C H_{3}$ , is represented in a line drawing as:

2-butanol

Try it!

Now that you have learned about complete formulas, condensed formulas and line diagrams, complete the following chart in your notebook. When you are ready, check your answers.

Complete formula	Condensed formula	Line diagram
	$C H_{3} C H (C H_{3}) C H_{2} C H_{3}$
	$C H_{2} (C H)_{2} (C H_{3}) C H (C H_{3}) C H_{3}$
	$C H_{3} C H_{2} C O (C H_{2}) 2 C H_{3} $

Discover more

We experience hydrocarbons (organic compounds containing hydrogen and carbon) everywhere in our life, often without realizing it.

Using the internet or any other resources available to you, find the complete structural formula for:

Methane – a greenhouse gas that is found in the flatulence of cows (and humans too)
Propane – the gas used in portable barbecues
Butane – the gas used in portable lighters
Hexane – the gas used to extract oils from seeds and vegetables

Notebook

In your notebook, draw or copy and paste a picture of each of the compounds mentioned in Discover more, write their name beside the picture and identify similarities in the names and structures.

Alkanes

The examples – namely methane, propane, butane and hexane – are organic compounds that are classified as alkanes. All alkanes share similar structural properties:

The carbon atoms have single bonds to other carbon atoms.
They have a general formula of $C_{n} H_{2 n + 2}$ .

Just as we could describe the trees in the “Minds On” section at the beginning of this activity, chemists use the name of compounds to describe them.

For example:

Propane	Butane
Prop-: means a three-carbon chain -ane: means all single bonds	But-: means a four-carbon chain -ane: means all single bonds

So you now know that the ending “-ane” means that it is a compound composed of all single bonds, but what are the prefixes that are added to indicate the number of carbons?

Prefixes indicating the number of carbons in an alkane

Prefix	Number of carbons	Prefix	Number of carbons
Meth-	1	Hex-	6
Eth-	2	Hept-	7
Prop-	3	Oct-	8
But-	4	Non-	9
Pent-	5	Dec-	10

So, heptane is a 7-carbon chain made of all single bonds and looks like this:

Notebook

In your notebook, draw the structural forumulas of all the alkane compounds in the table. You may also use a digital drawing tool. When you are ready, press Show Answers to verify your drawings.

Alkanes: Molecular, condensed and structural formulas

Name	Molecular formula	Condensed formula
Methane	$C H_{4}$	$C H_{4}$
Ethane	$C_{2} H_{6}$	$(C H_{3})_{2}$
Propane	$C_{3} H_{8}$	$C H_{3} C H_{2} C H_{3}$
Butane	$C_{4} H_{10}$	$C H_{3} {(C H_{2})}_{2}_{} C H_{3}$
Pentane	$C_{5} H_{12}$	$C H_{3} {(C H_{2})}_{3}_{} C H_{3}$
Hexane	$C_{6} H_{14}$	$C H_{3} {(C H_{2})}_{4}_{} C H_{3}$
Heptane	$C_{7} H_{16}$	$C H_{3} {(C H_{2})}_{5}_{} C H_{3}$
Octane	$C_{8} H_{18}$	$C H_{3} {(C H_{2})}_{6}_{} C H_{3}$
Nonane	$C_{9} H_{20}$	$C H_{3} {(C H_{2})}_{7}_{} C H_{3}$
Decane	$C_{10} H_{22}$	$C H_{3} {(C H_{2})}_{8}_{} C H_{3}$

You now know that the name given to an organic compound describes the structure of the compound, so now, let’s explore more complex structures.

Substituted alkanes

Consider the following structure:

In order to name this structure, we need to describe everything about it including:

the number of carbons that are in the chain
the fact that there is an additional carbon that branches off of the chain

Press the following tabs to examine these pieces step by step.

How do you define a chain?

Is this a chain?

In order to make sure that a name is understood easily, chemists have developed rules that are commonly understood. These rules are codified by the International Union of Pure and Applied Chemistry (IUPAC).

RULE: The rule for determining the number of carbons in a chain is to count the longest, uninterrupted chain of carbons.

With this in mind, we can count the longest, uninterrupted chain of carbons in our compound in two different ways.

Method 1:	OR Method 2:

Either way, since there are 5 carbons in the chain, this is some type of pentane molecule.

When you have something branching off from the main chain, it is called a substituent group.

In this case, it is a single carbon chain.

Using our knowledge from before, to identify it as a single carbon, we will use the prefix “meth-”.

To indicate that it’s a substituent group, it will end in “-yl”.

Therefore this is a methyl group.

Now we need to identify where the methyl group attaches to the pentane molecule.

We will use a number to identify which carbon it is attached to. In this case, there are two possibilities based on where we started numbering our carbon chain.

Consider the following:

Method 1:	OR Method 2:
The methyl group is on carbon 2.	The methyl group is on carbon 4.

RULE: The rule here is that you number the carbon chain so that the substituent groups get the lowest possible number. Where there are multiple substituent groups, you number so that the overall combination is as low as possible.

Using this rule, we say that the methyl group is on carbon 2.

So we have a methyl group attached to carbon 2 of a pentane molecule.

We name this compound therefore: 2-methylpentane.

Try it!

Complete the following table in your notebook. When you are ready, check your answers for understanding.

Structural formula	Name
	4-ethylheptane
	4-propylheptane

Alkanes with more than one substituent group

What about compounds where there is more than one substituent group?

For example this compound:

Line diagram of 3-methyl-4-propylheptane

We will use the same questions and steps as before:

What is the length of the longest chain?
What are the substituent groups?
Where are the substituent groups located?

Press the following tabs for step by step process.

What do you think is the longest chain in this molecule?

This diagram uses different coloured counters to demonstrate the possible chains of carbon. Since the longest possible chain that can be made contains 7 carbons, this compound will be a 7-carbon compound. It doesn’t matter whether you use the red counters or the purple ones, either way you will have the same end result.

What do you think are the substituent groups in this compound? For each of the following questions, select the correct answer from the options provided. Then press Submit to check your understanding. Afterwards, check the explanatory diagram to very them.

There are two ways to number the chain:

For the purple line, the substituents are on carbons 4 and 5.

For the red line, the substituents are on carbons 3 and 4.

The rule is that you use the lowest combination for the substituents, therefore it will be: 3-methyl and 4-propyl.

The rule for assembling the name is that the substituents are listed in alphabetical order, therefore the correct name is: 3-methyl-4-propylheptane.

Important

When there are multiple substituents of the same type (e.g. 2 methyl groups), you identify the carbon number they each branch from, with the numbers separated by commas, and then you use a prefix of di-, tri-, tetra-, etc.

For example:

Example 1

2,3-dimethylpentane

Example 2

3,3-dimethylpentane

In example 2, note that even though the methyl groups are on the same carbon, they are numbered twice (e.g. 3,3). This is to make sure it is clear that both methyl groups are on the same carbon.

The di-, tri-, etc. prefixes don’t affect the alphabetical order of listing the substituent groups.

Consider the following compound:

Line diagram of 4-ethyl-3,5-dimethylheptane

This compound is correctly named 4-ethyl-3,5-dimethylheptane and not 3,5-dimethyl-4-ethylheptane.

Try it!

Name the following compounds. Carefully follow the steps as shown in the previous examples. When you are ready, check your answers.

Note that it is very important to use:

the proper numbering for the main carbon chain
dashes between a number and a name
commas between two or more numbers

Structural formula	Name
	2-methylbutane
	3,5-dimethylnonane
	3-ethyl-2-methylhexane
	4,6-diethyl-2,3,7-trimethyl-5-propylnonane

Join the discussion

In the discussion forum, draw the structural formula for 3-ethyl-5-methylheptane. You may use any drawing tool of your choice. Do you have any advice on tips on using your drawing app? Please share with the class.

Notebook

In your notebook, draw the structural diagram for methylbutane. Alternatively, you may choose to use a digital drawing tool.

Why do you think a number isn’t required in front of the methyl substituent in methylbutane?

Specifying the position of the methyl group in methylbutane isn’t necessary (although it can be done), because there is only one possible place where a methyl side chain can be added: on the second carbon atom. Even if the methyl group is moved one to the right in the diagram, beside the methyl group would still be at carbon 2, as the main carbon chain would have to be numbered from the right to the left. Moving the methyl group to either end of the main carbon chain would now make the longest carbon chain to be five long, and the compound would now be pentane.

Structural isomers

Using a line diagram, represent $C_{6} H_{14}$ in as many unique ways as you can in your notebook. When you are ready, check your drawings.

Figure a)

Figure b)

Figure c)

Figure d)

Figure e)

The compounds represented in the figures above are known as structural isomers of each other. Structural isomers are compounds that have the same molecular formula (in this case $C_{6} H_{14}$ ) but different physical arrangements of atoms.

Try to name all the represented isomers represented in the previous figures. Check your answers when you are ready.

Figure a): 3-methylpentane

Figure b): hexane

Figure c): 2-methylpentane

Figure d): 2,3-dimethylbutane

Figure e): 2,2-dimethylbutane

Think

Why isn’t the compound shown in the following Figure f) also one of the possible structural isomers of $C_{6} H_{14}$ ? To help you answer this question, name the compound. Write the name and your reasoning in the space provided. Then, verify your answer.

Line diagram of 2-methylpentane (mirror image)

Figure f)

The molecule represented in Figure e), 2-methylpentane, is the exact same compound as that represented in Figure c), which is also called 2-methylpentane. They are just mirror images of each other. Therefore, since it is the same compound (as evidenced by the shared name) it is only listed once as a possible structural isomer of $C_{6} H_{14}$ .

Other substituent groups

Organic compounds often contain substituent groups that aren't hydrocarbons. To name these compounds we use prefixes to identify the different substituent groups. Complete the following table of prefixes using the internet or other resources available to you.

Substituent	Prefix
Cl	chloro-
Br	bromo-
F	fluoro-
I	iodo-
NO₂ (as a substituent)	nitro-

Notebook

In your notebook, draw the compound and explain why you named it as you did. You can also use a digital drawing tool as well. When you are ready, check your answer.

Using your knowledge of the rules for naming alkanes, what would you name the following compound:

Name: 2-chloropentane.

Explanation: The parent chain of this compound is 5 carbons, therefore the root name is pentane. There is a chlorine atom found on the second carbon atom in the chain, therefore the compound is given the prefix 2-chloro.

Notebook

Practise your ability to name the following compounds. For each compound, select the corresponding name description, from the drop-down menu. Then press Submit to check your understanding.

Notebook

Finally, try to name the compounds in your notebook. When you are ready, verify your answers.

Compound	Name
	1-chloro-2-methylpropane
	1-chloro-3-ethylhexane
	2-fluoro-3,3,6-trimethylnonane

Join the discussion

Refer to Learning Activity 1.1, where you identified an organic compound that was used in the industry. Research the structure of the compound and identify any substituent groups that have been introduced in this unit. Post your structural and list all of the substituent groups that are part of the organic compound.

Reflection

Review the learning goals and success criteria for this learning activity. Revisit any areas you need to strengthen your understanding. Reflect on the following success criteria and make note of your responses in your notebook.

Success criteria

Am I able to...?

represent an organic compound in a variety of ways
name straight alkanes
name alkanes that have substituents attached (e.g. a methyl-group or a chloro-group)
recognize a structural isomer of a compound
create, name and draw structural isomers of a chemical formula

Think

Notebook

The representation of organic compounds

Notebook

Complete structural formulas

Condensed structural formulas

Line diagrams

Try it!

Discover more

Notebook

Alkanes

Prefixes indicating the number of carbons in an alkane

Notebook

Alkanes: Molecular, condensed and structural formulas

Substituted alkanes

Step 1: The number of carbons in the chain

Step 2: Identify the substituent groups

Step 3: Identify where the substituent group is located

Step 4: Putting it together

Try it!

Alkanes with more than one substituent group

Step 1: What is the longest chain?

Step 2: What are the substituent groups?

Step 3: Where are the substituents located?

Step 4: Assemble the name

For example:

Try it!

Join the discussion

Notebook

Structural isomers

Think

Other substituent groups

Notebook

Notebook

Notebook

Join the discussion

Reflection

Success criteria