Since we're naming alkanes we’ll go over 5 different cases you might see on your next exam.
Hello everyone, I’m Melissa Maribel your personal tutor,
and these two tables are extremely extremely important for you to know!
This first table shows the prefixes for the different lengths of carbon chains you may see.
Since we are naming alkanes every structure name will end with ane
This second table shows the most common types of substituents you will need to know.
For naming alkanes we will follow these 3 main steps each time.
STEP 1: Find the longest carbon chain
STEP 2: Number the carbon chain so we have the most substituents and lowest number at each substituent.
STEP 3: Identify the type of substituent
Let’s apply this to Case 1 Case 1: 1 substituent
For 1 substituent this is how we will format the name of the structure
Where the number is the location of the substituent, followed by a dash,
the name of the substituent and the name of the longest carbon chain.
There are no spaces and nothing is ever capitalized!
STEP 1: Find the longest carbon chain
There is 1, 2, 3, 4 so four carbons this way, or
1,2,3,4 carbons this other way, either way the longest carbon chain is 4.
The prefix for four is but- and it ends in ane for alkane so butane.
STEP 2: Number the carbon chain so we have the most substituents and lowest number at each substituent.
So if we numbered the carbon chain from left to right, the substituent would be at 2.
If we numbered it from right to left, the substituent would be at 3.
So we’ll keep it numbered from left to right since we always want the lowest number for our substituent.
We now know the location of the substituent is at 2.
STEP 3: Identify the type of substituent
There is only 1 C that is not already part of the main carbon chain so this is a methyl group. And we get 2-methylbutane
You know we could have numbered this differently by starting at
this end 1, 2, 3, 4 and it would have given us the same exact answer. Since the methyl is still at 2.
CASE 2: The structure has 2 of the same substituents
Here’s how we’ll format the name Whenever a structure has 2 or more
of the same substituent we separate the numbers with a comma, then there’s a dash,
a prefix goes in front of the name of the substituent followed by the longest carbon chain.
STEP 1: Find the longest carbon chain
We have 1, 2, 3, 4 ,5 this way, or 1,2,3,4,5 this other way.
We know that 5 is the longest carbon chain, the prefix for 5 is pent, so pentane.
STEP 2: Number the carbon chain so we have the most substituents and lowest number at each substituent.
Here are some different ways we could have numbered this structure.
This first one shows the substituents at 2 and 3 The second one shows the substituents at 3 and 4
And this third one also is at 3 and 4
The correct numbering is the first one since we get the lowest possible number for substituents.
So 2 comma 3 goes in front
STEP 3: Identify the types of substituents
We have two methyl groups
And since there are two of the same substituent the prefix for 2 is di, so
dimethyl and this is the name of our structure
CASE 3: The structure has 2 different substituents
Here’s how we’ll format the name
With the two different substituent names and the number, they are located at,
followed by the longest carbon chain.
STEP 1: Find the longest carbon chain
We have 1, 2, 3,4 5, 6,7 or 1, 2, 3,4 5, 6, 7
The Longest carbon chain has 7 Carbons, so heptane
STEP 2: Number the carbon chain so we have the most substituents and lowest number at each substituent.
Here are some other possible ways we can number this.
All the ways would give the same amount of substituents but only ways 1 and 2 have the lowest number at each substituent.
Way 1 and 2 will give you the same answer. I’ll pick way 1
STEP 3: Identify the types of substituents
We have a methyl and ethyl group. Note: Always write substituents in alphabetical order
So the ethyl goes first then the methyl and we’ll place the numbers where each substituent is at
and this is the correct name.
CASE 4: The structure has 2 different long carbon chains
Some structures have 2 completely different sized carbon chains, which we’ll see here.
STEP 1: Find the longest carbon chain
Let’s see what we have, 1,2,3,4,5
or 1,2,3,4,5,6 or 1,2,3,4,5,6
And 1,2,3,4,5,6,7 7 is the longest carbon chain, which is heptane
STEP 2: Number the carbon chain so we have the most substituents and lowest number at each substituent.
Here are 2 ways we could number the chains, this first way is the best.
Since substituents are at 3 and 4 rather than 4 and 5.
Because we always want the lowest numbers for each substituent.
STEP 3: Identify the types of substituents
We have a methyl group at 3 and an ethyl group at 4
And we always arrange our substituents in alphabetical order.
When we do that we get this.
CASE 5: The structure has multiple same length carbon chains
STEP 1: Find the longest carbon chain
1,2,3,4,5,6,7,8,9 or 1,2,3,4,5,6,7,8,9
9 carbons is nonane
STEP 2: Number the carbon chain so we have the most substituents and lowest number at each substituent.
Here are the different ways we can number the carbon chain.
Remember we want the way that gives us the most substituents
way 1 gives you only 1 substituent which is t-butyl which as you can tell it literally looks like a T.
But way 2 gives us 3 substituents, an ethyl group and two methyl groups located at the second carbon.
So this is the correct name
Don't forget to download your Organic Chemistry Survival Guide and
to check the resources that I have for you
in the description box all the links are going to be there.
And make sure you know
how to draw the structure when your given the name.
Don't worry that video's right here. And remember stay determined you can do this!