In today’s lesson, we will be learning how to name and draw the following organic families (organic families are organic compounds that are categorized into “families” based on similar characteristics, such as having a carbon to oxygen bond, a double bond, a triple bond, a nitrogen group, or more), discussing homologous series, aliphatic vs aromatic compounds, as well as introducing basic Organic Chemistry terminology:
(a) Alkanes
(b) Alkenes
(c) Alkynes
Introduction
Organic chemistry deals with a backbone of Carbon-Hydrogen bonds, with other elements sometimes added in, most commonly Oxygens and Nitrogens. Instead of drawing out compounds the Lewis Structure way (where we write out the letter of each element), we use a “Line-Bond” or “Skeletal” structure to make it easier to draw complex shapes. We draw out stick structure like this, where we start out with a Lewis Structure and convert it to a zig zag structure with each vertice being a carbon. The hydrogens don’t have to be drawn in, so in the last bottom right image I have removed them (but they’re still there, just invisible):
When we draw out these compounds, we usually have a “Main Chain,” basically like the backbone of our compound. We always need to identify this main chain in order to be able to correctly name the compound. I have shown how to identify the main chain with this example below – we highlight the longest continuous chain of carbons without lifting our pen off the paper, so our longest chain here is 9 carbons.
When we have carbons or other elements that are not directly in our main chain, but instead they are hanging off of it, they are called substituents. There are many different substituents which will be discussed in a separate lesson. From the example above, I have circled the substituents in blue. These are anything not directly on our main chain that is in red.
When we have identified the main chain and any substituents, we need to know the ten basic prefixes to name compounds. These prefixes are used to describe how many carbons we have in certain places of our shape. They are as follows (they go further than 10, usually introductory classes don’t require you to know past this):
1 – Meth
2 – Eth
3 – Prop
4 – But
5 – Pent
6 – Hex
7 – Sept
8 – Oct
9 – Non
10 – Dec
Note that sometimes we see “R” in a picture – that means we are talking about an alkyl (hydrocarbon) group. Sometimes we denote “R” in a picture when the alkyl group can take on any hydrocarbon chain, rather than having a specific chain of only a certain number of carbons. In the picture below, if we were told that R is, say, 3 carbons long, that means we can replace it in the picture and we can the following image highlighted in red.
Also note that you might sometimes see an “n” in front of a name – this is used in the common name of a compound, which is a bit different than IUPAC name. Although most places use IUPAC, when there is a straight and unbranched hydrocarbon chain it can have an “n,” (for normal) connected to the chain. See below:
Homologous Series
A homologous series is a sequence of carbon compounds that have the same functional group. Each homologous series has its own formula that differs by one carbon for each compound in the series. For example – methane (CH3), ethane (CH3CH3), propane (CH3CH2CH3), etc are all part of the alkane homologous series. You can get a more complete list at this link.
Aliphatic vs Aromatic Compounds
An aliphatic compound is an organic hydrocarbon compound (which means it is made up of only hydrogens and carbons), which has these atoms joined together in straight chains (when we say “straight chain” in organic chemistry, we mean “zig zag” chains), branched chains, or non-aromatic rings such as cyclopentane, cyclohexane, and so on (which are called cyclic compounds).
A cyclic compound is any organic compound that is in a ring shape. Aromatic compounds, also known as “arenes,” are a special type of cyclic compound. When cyclic compounds exhibit something known as aromaticity, that means their rings consist of a conjugated system with delocalized pi-electron clouds rather than alternating double bonds. (I will have a lesson up under the Organic Chemistry II post to discuss Aromatics further).
Alkanes
Characterization: Only contain saturated (aka single) carbon to carbon bonds. This means no other functional group, such as alcohol, carbonyl, etc, can be on the shape for it to be an alkane.
Alkanes – From Name to Drawing
Let’s try drawing the following example using the rules below, if we are given the name:
2-methylpentane
If given the name of an alkane, we can draw it out by:
(a) First identifying the main chain and draw out the correct number of carbons in a zig-zag pattern
(b) Number it in any direction
(c) Add in the substituents on their respective carbons. Substituents will be given before the name of the main chain and they end in “yl”. There will also be a number indicating which carbon the substituent is on.
Using the rules above, we draw out the shape:
(a) Identify the main chain using the prefix in the name. This is PENT because the end of the chain says PENTane. So we draw out 5 carbons in a zig-zag
(b) We number these carbons 1 to 5
(c) We have a 2-methyl before the main chain of pentane. So that means on carbon 2, we have a methyl group. “Meth” is the prefix for 1 carbon, so “methyl” is a substituent of 1 carbon.
I have highlighted the main chain in red, and this is the final answer.
Alkanes – From Drawing to Name
Let’s try naming the following example using the rules below, if we are given the structure:
Order of Naming Alkanes: Substituents with numbers + main chain + “ane”
If given the drawing of an alkane, we can convert it into a name by:
(a) Identify the main chain by using the trick identified above, under “Main Chain”
(b) Write out the prefix for that number of carbons using the list above
(c) Add “ane” to the end of the prefix
(d) Add any substituents to the beginning of the name, before the main chain. These should include the carbon number that the substituent is located on
(e) Here are rules for adding substituents to a name:
Add dashes in between substituents and their numbers as well as between subsequent substituents. There should be no space or dash between the last substituent and the name of the main chain.
Substituent endings should be changed to -yl endings connected with the appropriate prefix.
Click here for more nomenclature rules in a separate post.
Using the rules above, we will name the structure given above:
(a) The main chain is 5 carbons long, like I’ve highlighted below. We could’ve highlighted it any of these two ways, because they’re both the same.
(b) The prefix for 5 is PENT
(c) We add -ane to the end, so we have “Pentane”
(d) We have a substituent on our shape, as highlighted in blue below. The substituent is one carbon (which has a prefix of “meth”) and is on carbon 2.
(e) Now to name the shape, we add the number (2) and name of the substituent (methyl) before the “pentane.” We also have to include dashes in between substituent name and number, and no space or dash between the substituent and the name of the main chain. This gives us:
Alkenes
Characterization: Contains at least one carbon to carbon double bond (aka unsaturated bond – this means a bond that is a double or triple bond)
Alkenes – From Name to Drawing
Let’s try drawing the following example using the rules below, if we are given the name:
4-methylpent-2-ene (same as 4-methyl-2-pentene)
If given the name of an alkene, we can draw it out by:
(a) First identifying the main chain and draw out the correct number of carbons in a zig-zag pattern
(b) Number it in any direction
(c) Add in the double bond on the correct carbon – the number should be indicated in the name.
(d) Add in substituents on their respective carbons. Substituents will be given before the name of the main chain and they end in “yl”. There will also be a number indicating which carbon the substituent is on.
Using the rules above, we draw out the shape:
(a) Identify the main chain using the prefix in the name. This is PENT because the end of the chain says PENTene. So we draw out 5 carbons in a zig-zag
(b) We number these carbons 1 to 5
(c) We have a double bond on carbon 2, because the 2 is before the “ENE” in the name, or it’s right before the main chain (I had written it both ways above)
(d) We have a 4-methyl as a substituent. So that means on carbon 4, we have a methyl group. “Meth” is the prefix for 1 carbon, so “methyl” is a substituent of 1 carbon.
I have highlighted the main chain in red, and this is the final answer.
Alkenes – From Drawing to Name
Let’s try naming the following example using the rules below, if we are given the structure:
Order of Naming Alkenes: Number of double bond with Cis/Trans/E/Z + Substituents with numbers + main chain + “ene”
OR
Cis/Trans/E/Z + Substituents with numbers + main chain + number of double bond + “ene”
OR
Cis/Trans/E/Z + Substituents with numbers + number of double bond + main chain + “ene”
If given the drawing of an alkene, we can convert it into a name by:
(a) Identify the main chain by using the trick identified above, under “Main Chain.” Note when naming alkenes, we must number our carbon main chain THROUGH the double bond, EVEN if it is not the longest continuous carbon chain!
(b) Write out the prefix for that number of carbons using the list above
(c) After this, add the first carbon that the double bond starts on, followed by “ene”
(d) Add any substituents to the beginning of the name, before the main chain. These should include the carbon number that the substituent is located on as well. Use the same rules for adding substituents to the name as it was for alkanes.
Using the rules above, we will name the structure given above:
(a) The main chain is 5 carbons long, like I’ve highlighted below. We could’ve highlighted it any of these two ways, because they’re both the same and put the double bond on carbon 2, and the methyl on carbon 4. The double bond should be on the lowest number compared to the methyl substituent, so I numbered it left to right instead of right to left.
(b) The prefix for 5 is PENT
(c) We add the number of the double bond (2) and “ene” to the end of the prefix, so we have “Pent-2-ene.” We can also have 2-pentene.
(d) We have a substituent on our shape, as highlighted in blue below. The substituent is one carbon (which has a prefix of “meth”) and is on carbon 4. This is a 4-methyl.
(e) Now to name the shape, we put it all together, and we have to make sure to include dashes between numbers and letters
Notes with alkenes: We need to identify the direction of the double bond, and this can be indicated using cis, trans, E or Z in front of the name. However, I will cover this in a separate post so I have not included them in our name today, even though they should be there!
Alkynes
Characterization: Contain at least one carbon to carbon triple bond (aka unsaturated bond – this means a bond that is a double or triple bond) carbon to carbon bonds.
Alkynes – From Name to Drawing
Let’s try drawing the following example using the rules below, if we are given the name:
3-pentyne
If given the name of an alkyne, we can draw it out by:
(a) First identifying the main chain and draw out the correct number of carbons in a linear pattern – alkynes are linear in shape around the triple bond area
(b) Number it in any direction
(c) Add in the triple bond on the respective carbon.
(d) Add in substituents. They will be given before the name of the main chain and they end in “yl”. There will also be a number indicating which carbon the substituent is on.
Using the rules above, we draw out the shape:
(a) Identify the main chain using the prefix in the name. This is PENT because the end of the chain says PENTyne. So we draw out 5 carbons in a linear fashion (shown in picture below)
(b) We number these carbons 1 to 5
(c) We draw a triple bond on the correct carbon. I have drawn the lewis structure as well as the skeletal organic structure to avoid confusion
(d) We have no substituents so we don’t worry about those
This is the final answer:
Alkanes – From Drawing to Name
Let’s try naming the following example using the rules below, if we are given the structure:
Order of Naming Alkanes: Substituents with numbers + main chain + number of triple bond + “yne”
OR
Substituents with numbers + number of triple bond + main chain + “yne”
If given the drawing of an alkyne, we can convert it into a name by:
(a) Identify the main chain by using the trick identified above, under “Main Chain”
(b) Write out the prefix for that number of carbons using the list above
(c) Add “yne” to the end of the prefix
(d) Add any substituents to the beginning of the name, before the main chain. These should include the carbon number that the substituent is located on. Use the same rules for adding substituents that are listed under alkanes.
Using the rules above, we will name the structure given above:
(b) The prefix for 5 is PENT
(c) We add -yne to the end, so we have “Pentyne”
(d) We add in the number of the triple bond, which is on 3. We numbered right to left to put the triple bond on the lower number. (e) Our final name puts everything together (note the dashes between letters and numbers) to give us:
