2.4 ISOMERISM & PROPERTIES OF ORGANIC COMPOUNDS

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Hi all! How are you doing today? This is our second post on organic chemistry. I realised that many people struggle with organic chemistry the most, therefore, I decided I will continue all through Unit 2, which is organic chemistry first. Remember, that if you struggle with any questions you can let me know any time. I also created a facebook group, which is linked on the main page of this blog where people share their questions and I try and answer them.

In today's post, we are going to explore isomerism two properties of organic compounds, which are boiling and melting temperatures and solubility.

We're starting with isomerism first though.

So, isomers split into structural and E-Z isomerism in this unit. Of course, there are more types of isomers but you're going to learn about them in A2-level.

I believe it would be good to firstly define isomers.

Isomers - molecules with the same molecular formula, Mr, but different arrangement of atoms.

Ok, so as I said they split into two types:

Structural isomerism - compounds with the same molecular formula (Mr) but different arrangement of atoms.

And:

E-Z isomerism - which occurs in alkenes due to restricted rotation about C=C. Also, the groups on each C=C must be different.

As you probably saw in the previous picture, you can find structural isomerism in 3 different ways:

1. Chain isomerism - in this type, compounds have a different arrangement of the carbon skeleton.

I prepared an example for you above. You can find how the compound would look initially and then how would the chain isomer of it look like. In this example, we have a straight-chain initially and then we have a branch (e.g. methyl or ethyl) in the second, which means we changed the carbon skeleton. The name also changes as the longest chain has no longer 5 carbons but 4.

2. Position isomerism - in this type of isomerism, you have the same skeleton and the same functional group. However, you change the position of this functional group.

Again, we have an example. This time it is a butane and it has a chlorine group. Initially, you have the chlorine group on the second carbon from left and the position isomer of this compound has chlorine on the first carbon. Meaning, it changed its position.

3. Functional group isomerism - this type of isomerism has the same molecular formula, Mr, but different functional group.

And an example now. We have an alcohol and methyl group on the first compound. In the functional group isomer, the OH group is changed and we no longer have an OH group. This is what a functional group isomer is when we have one functional group and we change it into a different one.

You don't have to be able to name this isomer but you need to be able to draw it.

Some organic compounds have a lot of isomerism possible. It usually works that the longer the chain, the more isomers there are.

In the exam, you may be asked to draw possible isomers of some compound and name them.

I prepared an example of this type of question. I hope this will help you. Of course, if you want more practice you can visit my youtube channel for which I left the link at the top of this post.

So, we have a compound C5H12 and you're asked to draw 3 isomers of it. So here we are; the easiest one is just a straight chain, which is called pentane. Next, we can draw one with a branch or two branches. When you draw isomers, it is the best decision to add branches. Remember, if you add a branch on the outer carbon, this is just a straight chain and the name remains the same.

All different structural isomers will have different properties because although they have the same Mr, the arrangement of their atoms change.

E-Z isomerism

We already explained what E-Z isomerism is above in this post. And now, we are going to get deeper. Just as a reminder, it occurs in alkenes where you have a C=C bond and it must have two different functional groups.

So, in E-Z isomerism, you have an E-isomer and a Z-isomer.

E-isomer - when the two functional groups are on opposite sides. Look at the example above, the aldehyde groups are on opposite sides of the C=C bond.

My tip for remembering that E-isomer is the one where functional groups are on opposite sides is 'E for either'.

Z-isomer - when the two functional groups are on the same side of the C=C bond. On the example above, you have two carboxylic acid groups on the same side of C=C bond.

My tip for remembering this one is 'Z for zame'.

Properties of E-Z isomers:

1.Melting point -> E-isomers are more stable and have a higher melting point than Z-isomers. This is because, in an E-isomer, the molecules can fit together more closely.

2.Solubility -> the solubility of Z-isomer is greater than that of E-isomer. The reason for this is that the bonds are easier to break. The molecules are not as close together as in E-isomer.

Now, we're going to learn how to name E-Z isomers. To decide if an isomer is an E or Z isomer, you need to decide what group has priority.

To do that, remember that the group with the highest atomic number takes priority.

I have prepared 3 examples for you with different groups taking priority.

1. We have two OH groups and two H. Obviously, the alcohol group takes priority because it has a higher atomic number (you add the atomic number of oxygen and the atomic number of hydrogen) than the hydrogen. The OH groups are on the same side, therefore, it is a Z-isomer (z for zame)

When you do this, look at one C and the groups attached and then you look at the other C and the groups there. And that's how you compare them.

2. Now, we also have two OH groups and two hydrogens in this example. And as we got to it before, the alcohol group has a higher atomic number than the hydrogen, so it has priority. Because it is on either side of the C=C bond, the compound is an E-isomer.

You can also find that you have a couple of different functional groups. They don't have to be the same.

3. In this 3rd example, you have 3 different functional groups: CH3, Br2 and COOH. Bromine and carboxylic acid have higher priority than the methyl group due to their atomic number being higher. This is why we take these two into consideration when trying to work out if it is a Z-isomer or an E-isomer. These two groups are on opposite sides of the C=C bond and therefore this is an E-isomer.

Physical properties of organic compounds

The properties we are going to explore are melting and boiling points and the solubilities. What we are going to do is explore the effect on physical properties of three different changes to organic compounds.

1.Effect of increasing chain length:

--> Melting and boiling point - as chain length increases, the melting and boiling points increase. This is because the number of forces increases. Therefore, there is more of them to break. Furthermore, long chains can pack closely together.

--> Solubility - hydrocarbons are not soluble because they have no polar bonds as water has. Also, the Van der Waals do not exceed the hydrogen bonds in water, so they can't break them.

2.Effect of branched chains:

-->Melting and boiling point - as branching increases the melting and boiling points decrease. The shape suggests they can't pack as closely together. Therefore, the strength of forces decreases due to this shape.

--> Solubility - the same reason as for increasing the chain length; they are still unsoluble.

3.Effect of adding polar group (-OH, -halogen, -COOH):

-->Melting and boiling point - polar groups increase the melting and boiling points of organic compounds. For example, alcohols have hydrogen bonding which considerably increases the points. Hydrogen bonds are much stronger than Van der Waals, so harder to break. (The bonds have to be broken for them to melt or boil).

--> Solubility - polar molecules are more soluble. For example, alcohols hydrogen bond with water molecules.

That's all I have for you today. Another organic compounds post is coming your way. For now, you're invited to my youtube channel where I just posted past paper questions on this topic. Remember, the more you practice them, the easier it will be for you in the exam. The questions tend to be quite repetitive!

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!