2.4 NAMING ORGANIC COMPOUNDS, TYPES OF FORMULAE & HOMOLOGOUS SERIES

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Hi everybody! The post is all the basics of organic chemistry. All the things you need to know and remember to be able to answer and understand organic compounds.

There are a lot of different types of organic compounds but nearly all organic compounds contain C and H. This type of organic compounds that only contain C and H are called:

Hydrocarbons- organic compounds containing carbon and hydrogen atoms only.

All organic compounds can be arranged into groups. Each group is called a homologous series.

Each homologous series has compounds with the same functional group.

Functional group - refers to the atom/ group of atoms that gives the compound its characteristic properties.

Firstly, I am going to give you all of the definitions and information and then we'll do some examples.

Here are all of the types of formulae you have to be familiar with for as-level chemistry. In your exam, they are always going to state which type of formulae they will want you to draw. Or otherwise, they will draw it for you and you'll say what type of formulae it is. In this picture, I also included examples for you. Of course, there will be more of them further on this post.

So,

1. Empirical formulae - this is the simplest ratio of elements in a compound. You look at the numbers of carbon and hydrogen and then you write the simplest ratio of these two numbers. This is your empirical formulae.

There are epirical formual calculation questions that can come up under this topic. I already wet through them in one of the earlier posts. It is in Unit 1.3 if you would like to visit it and revise. There is also a youtube video that goes along with it.

2. Molecular formulae- this type shows you how many of each atom there is in a compound. Therefore, this tells you how many of, for example, carbon and hydrogen there is. You would use this formulae to then work out the empirical fomulae.

3. Structural formulae- this formuale shows atoms carbon by carbon and the functional groups that are present. You take the first carbon and the atoms that sorround it and then the next, and next, and so on.

4. Displayed formulae - this demonstrates you how atoms are arranged in a compound and the bonds between atoms. So, what you do is draw every atom that is in the compound. You have to show how the compound looks like.

5. Skeletal formulae - it is the carbon skeletal with only the functional groups shown. They are ups and downs of carbons and then functional groups around them.

Now, we are going to show the functional groups. There are many functional groups and the ones that you need to know for as-level chemistry are here:

I think this photo already gives you everything you need to know. I am not sure if I can explain it in any more detail. The first row show the name of the functional group, then how this functional group looks. Next, how the compound changes its ending depending on this group (or the beginning of the name). In the last row, I included examples of each type of functional group together with the name of this compound and molecular formula.

The last lot of new information that I am going to pass onto you, are the rules for naming organic compounds. There are 5 that you need to know. After this last photo, you will finally be able to draw, name and be fully aware of organic compounds, of course, practice is important as well and that's how you can perfect the skills.

Rule #1

The names are different depending on the number of carbon atoms. On the photo above, I wrote the number of carbon atoms and the start of the name that is connected to this number of carbon atoms. To work out this start to the name of compound, you have to find the longest carbon chain that there is. Be careful with the branches. In the example on the photo, I showed just this. You have three carbons and a branch with another 2 carbons. And this is exactly what you see, hoowever, this branch can be the longest carbon chain. And now we have four carbons in the chain and one branch with just one carbon. I hope this makes sense to you. I really tried my best.

Rule #2

Identify the main functional groups. Look back at the table with functional groups and then at the compound you have and see if you can see any of them. If yes, even circle them just for your knowledge.

Rule #3

Number the carbon atoms in the longest carbon chain. If you have a functional group present remember to number the carbon atoms from the side where the functional group will be on the carbon atom with the smallest number. On the picture above, you have how this should look like and how is shouldn't.

Rule #4

If you have more than one functional group present and they are both identical, then apart from just writing down the functional group in the name, you also have to add di, tri and tetra depending on how many of this group you have.

Rule #5

Always, remember to write down the name of organic compound in alphabetical order. If you have more than one functional group and they are different, you need to write them down in alphabetical order. As always, I got you an example showing this on the picture above.

That's all! No more facts and information to remember. Now, apply this knowledge to the examples I have for you.

Let's start with example number:

1.Draw pentane using structural formulae- structural formulae is showing carbon by carbon what is happening in the compound. In the picture above, you can see that I drew a displayed formula of this compound first. I find it easier to firstly do this before I draw the structural formula (or any other formula) because it helps me visualise the compound. It is pentane, our functional group is alkane and pent- suggests it is a carbon chain with 5 carbons.

2. Draw 2-bromopentane using skeletal formula - the number in front shows you on which carbon atom, we have our bromine functional group attached to. Furthermore, the ending of this compound, pentane, suggests we have 5 carbon atoms. This is what you start with, the carbon chain and then the functiolnal group on the 2nd carbon (from whatever side of the chain). The rest of the bonds around carbon will be hydrogens. I also draw the displayed formula first as before, because it is just easier for me. Then, draw the skeleton of carbon and only the functional group around.

3.Butanoic acid- displayed formula - displayed formula is the one that you draw out. Firstly our carbon chain, in this example it is but-, so 4 carbon atoms. Our functional group -oic acid means it is a carboxylic acid. At the end of the molecule, you have a COOH group, as shown in the photo above.

4.Name this compound- this time you have the compound in front of you and you need to name it. Firstly, I found the longest carbon chain and counted the number of carbon atoms. It is 6, and our branch C2H5, which is ethyl on the third carbon (not on the 4th carbon if we counted from the other side, because it has to be the smallest). Then, just combine this information and you have 3-ethyl-hexane.

5.Draw displayed, structural and skeletal formulae of these examples - you have 4 examples here. I am not going to go into detail because you cn exactly see what is happening.

-- 3-chloropropanoic acid -- acid group at the end, and chlorine on the 3rd carbon, there are three carbons in total due to prop-.

-- propyl amine -- amine is our functional group, there is no number to it therefore it is on one of the ends. Again prop- suggesting we have 3 carbon atoms.

--1,2di bromo - 3- chloropropane -- we have three functional groups here. Two the same ones (bromine) on the 1st and 2nd carbon, and then chlorine on the 3rd. As you can see we have an alphabetical order in the name of this compound. Prop-, 3 carbon atoms.

--butan-2-one-- the functional group is a ketone here due to the ending which is -one. This functional group is on the 2nd carbon (visible in the name). But- means we have 4 carbon atoms.

Homologous series - conclusion

As we talked about it earlier, we said, a homologous series is a group of organic compounds with the same functional group.

We have encountered a couple of these functional groups already but currently, we are going to focus on 5 of them in particular and we're going to explore a couple of facts about each of them.

Let's start with the first one on my list which is

1.Alkanes (-ane)

They are saturated hydrocarbons, we already know what a hydrocarbon is; a compound containing only carbon and hydrogen atoms. Saturated means however that it can't accept any more atoms/ functional groups, it is full. I've included an example of propane above with three carbon atoms and hydrogens around. Each carbon atom has 4 bonds and can't accept any more. The general formula for alkanes is CnH2n+2.

2.Alkenes (-ene)

They are unsaturated hydrocarbons, meaning they are not full and can accept another atom or a functional group. This is because it has a double bond which can open and bond to something else (becoming a single bond). The general formula of alkenes is CnH2n.

3.Alcohols (-ol)

The functional group of alcohol is OH and I don't think there is anything else to say at this point (there will be more in the next posts about alcohols).

4.Halognoalkanes (bromo-, chloro-, iodo-)

The functional groups of these are Br, Cl and I. There are also examples for these homologous series above in the picture if you want to familiarise yourself with them.

5.Carboxylic acid (-oic acid)

The functional group of carboxylic acid is COOH. When you name carboxylic acids you probably realised that there is no number which tells us on which carbon our carboxylic acid is present. This is because carboxylic acid is always at the end of a molecule (either end).

This is all for this topic! I hope this is a nice start to organic chemistry. I am going to do many more posts on organic chemistry, so be prepared it is coming your way. If you're new, visit my youtube channel where I post past paper questions for every single topic. Links are at the top of this post, but you can find them everywhere on my website. Don't forget to share this website with your friends and family if you liked it. Please also leave me a comment on how you're finding this website and the posts. I would like to hear what you think because it is a great chance for me to improve.

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