2.7 ALCOHOLS AND CARBOXYLIC ACIDS & THEIR REACTIONS

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Hi, hi everybody. Here I come with another organic chemistry post and of course a video with past paper questions on youtube (link at the very top of this post). This time we have alcohols and carboxylic acids to talk about. There are a couple of reactions to this topic.

I know for sure, this post is going to be very long. We're not only going to explore alcohols and their reactions, but also carboxylic acids and their reactions. Ok, so let's get into it.

ALCOHOLS

We are starting with alcohols. From previous posts, you should know that alcohols have a functional group -OH. This means that to every chain of carbon atoms there needs to be an OH group for it to be an alcohol as in the example above. Alcohols are also a homologous series (we learned together about functional groups and homologous series, so if you're not sure what I am talking about, go back to the first post on organic chemistry, unit 2.4).

Furthermore, when you name alcohols, you add -ol to the name. Such as when you have a propane molecule and then an -OH is added to it, the name changes to propanOL. The other rules for naming organic compounds also apply (when you have another group attached, e.g. bromine - remember about alphabetical order, etc.).

Properties of alcohols:

• Boiling temperature --> it is higher than for alkanes or alkenes due to the hydrogen bond between hydrogen and oxygen (-OH). Of course, hydrogen bonds are more difficult to break than Van der Waals forces that are present between carbons and C-H. For boiling to occur, these bonds need to be broken first, so a lot of energy is needed (higher temperature).

• Solubility --> the solubility of small alcohols (methanol, ethanol, propanol) in water is higher due to their hydrogen bond. Because of the hydrogen bond in alcohol, the compound can bond with water molecules, which also have O-H bonds. However, the situation changes when the length of the carbon chain in alcohol increases because the OH group has less and less dominance over the molecule.

Now, alcohols can be classified as either primary, secondary or tertiary and the reactions that they go into depend on this. Let's start with:

1. Primary - the alcohol is primary when the OH group is attached to a carbon which is either attached to no other carbons or to just one carbon. I got you an example for each type on the photo above. As you can see we have a propan-1-ol. Here, the OH group is attached to a carbon which is attached to only one other carbon, therefore, it is a primary alcohol.

2. Secondary - the alcohol is secondary when the OH group is attached to a carbon which is attached to two other carbons. Again, the example above. We have butan-2-ol, and the OH group is connected to the second carbon and this carbon is attached to two other carbons.

3. Tertiary - the alcohol is tertiary when the OH group is attached to a carbon which is attached to three other carbons. Can you see the pattern yet? This alcohol is not seen as often in your as-level, but you need to understand it. Lastly, we have an example, and it is clear to see the carbon is bonded to three other carbons (2 carbons from the chain and the third from the methyl group).

You probably realised that the name suggests to how many other carbons our bonding carbon (the one with -OH group) is attached to. Primary --> to one (or none), secondary --> to two and tertiary --> to three.

Preparation of ethanol:

I did these examples focussing on ethanol, but any alcohol could be done this way. Firstly, we can prepare ethanol by:

1. Hydration of ethene --> as the name implies, we are hydrating ethene, so adding water to it in the form of steam. There are a couple of conditions and reagents needed for this reaction: -Steam - 70atm -300 degrees Celcius - Catalyst (phosphoric acid). These conditions and catalyst are used to increase the rate of this reaction. It saves energy and time, which means it also saves money. From the equation, you can see that we are only producing ethanol and no other wastes.

2. Fermentation --> this is a process where sugars are converted into alcohol. I included the equation for this reaction above, so have a peak. We start with glucose and are left with the alcohol (our aim) and carbon dioxide (waste/ pollutant). In this process, carbon dioxide is evaporated (because it is a gas) and ethanol can be separated from the mixture by fractional distillation. Fractional distillation looks like the one on the photo above (you may be asked to draw one in the exam).

Fractional distillation - works by separating mixtures according to their boiling temperatures. If one compound has a lower boiling temperature than the other, it will evaporate and then condensate first before the second one boils.

Ethanol can be used as a biofuel. There are advantages and disadvantages of using biofuels such as bioethanol over the use of fossil fuels. Let's state them.

Advantages:

• Of course, you would imagine there are advantages of using biofuels (even the name seems fancy, we want everything that is bio). The biggest advantage would be that they're renewable, while as we know fossil fuels can run out.

• Plus, our biggest problem, carbon dioxide, which is also produced in the production of biofuels. This would seem like a disadvantage, however, the amount is much smaller and it is believed that the amount of carbon dioxide that is produced from this process is used in photosynthesis (plants are needed for biofuels).

Now, we also have some disadvantages, although some people think that these disadvantages are not as important and do not outweigh the advantages.

Disadvantages:

• Certain people argue that although carbon dioxide is used in photosynthesis and there is less of it produced in biofuels, there is still carbon dioxide emitted during transport and building and running of the factories.

• Furthermore, plants need to be grown for the biofuels to be produced and so the land can't be used to produce food.

• Lastly, forests and lands may be destroyed for the production of plants for biofuels.

Reactions of alcohols:

The next part is the reactions of alcohols. There are two reactions that you need to know and be able to write and explain for this topic.

1. Dehydration of primary alcohols: we already had a reaction which was called hydration of ... and in that reaction we added water to the compound. This time we are working with dehydration and as the name suggests, we take away water from the compound. To do this, we need a catalyst and it is sulfuric acid and the reaction proceeds like this:

alcohol --> alkene + water

As you can see alcohols are converted to alkene (C=C) and water is the remaining part.

2. Oxidation of alcohols: in this reaction, there is one reagent needed, which is acidified potassium dichromate (remember to write 'acidified' ). When an alcohol is oxidised the colour of the solution will change from orange to green. Ok, so let's start with:

• Oxidation of primary alcohols ( they are the ones where the OH bond is bonded to a carbon which is bonded to one other carbon, or none) - when heat and acidified potassium dichromate is added to a primary alcohol, two stages happen. The alcohol firstly changes to an aldehyde (and water is also produced), and then it changes to a carboxylic acid (this time water is not produced). As you can see in the picture above, acidified potassium dichromate is shown as a [O] because it oxidises the molecule. This whole process only happens in primary alcohols. If you wanted this reaction to be complete (produce carboxylic acid), you would need to use reflux (continuous evaporation and condensation), otherwise, the reagents needed would evaporate before the reaction would reach producing carboxylic acid. primary alcohol --> aldehyde --> carboxylic acid

• Oxidation of secondary alcohols ( the one where the OH group is connected to a carbon which is connected to two other carbons) - this reaction is different to the one with primary alcohols. There is only one stage and in this stage, secondary alcohol is changed into a ketone and water is also produced. Of course, the colour change is always the same when the alcohol is oxidised. Again [O] is used to represent acidified potassium dichromate. alcohol --> ketone

• Oxidation of tertiary alcohols ( the one where the OH group is connected to a carbon which is connected to three other carbons) - alcohols stay the same when acidified potassium dichromate is added to tertiary alcohols. Therefore, this reaction is sometimes used as a test for alcohols. You would see a colour change for primary and secondary alcohols but not for tertiary.

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CARBOXYLIC ACIDS

Firstly, let's talk about carboxylic acids and what they are. As with alcohols, carboxylic acid is also a homologous series with a functional group COOH and it looks like that (look at the picture above). Carboxylic acids (acid - proton donor) are weak acids (weak acid - only partially ionised in aqueous solution). Let's work on some reactions of carboxylic acids: 1.Acids + base acid + base --> salt and water We put our carboxylic acid where it says acid, then a base, such as sodium hydroxide and we produce water with salt. It is called salt because we have O- Na+ where OH was before. It is no longer an acid because it was replaced by Na+. The other part of NaOH is OH and because we also lost another H from the acid, we are left with water (H2O). 2.Acid+ carbonate acid + carbonate --> salt + carbon dioxide + water Whenever you see that carbonate has been added, this means we will produce carbon dioxide. And because carbon dioxide is produced, we often use this reaction to test for a carboxylic acid. If carbon dioxide is produced, bubbles will be shown. You test it with lime water and it should go cloudy if carbon dioxide is produced. When you see bubbles and cloudy solution (after the addition of a carbonate), that means we are working with an acid. These are the two easy reactions with carboxylic acid, next we are going to have something more tricky and you need to focus to understand it.

Esterification

Esterification - this is a reaction of an alcohol with a carboxylic acid. The reagent needed for this reaction is sulfuric acid and we also need heat.

This reaction is called esterification because when you add alcohol and a carboxylic acid together with the addition of our catalyst, an ester is produced ( and water).

I included an example on the image above. It displays an ethanoic acid and ethanol. We take the OH from carboxylic acid and H from alcohol. This is going to be our water ( keep that in mind, you can lose a mark for not putting this in). The O which is left on our alcohol is the part that will connect both of our molecules, as seen above. When you name esters, you always start with the part that was alcohol before and add -yl to it, such as ethyl ethanoate. In order to separate the ester, you use fractional distillation. You know when the ester is produced, as a sweet smell will be produced.

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!