Different Potions!

Potion Ester Name Scent Methanol + Butyric Methyl butyrate Pineapple or apple Acid Methanol + Benzanoic Methyl benzoate Fruity-ylang ylang Acid Ethanol + Formic Ethyl formate / propyl Rum isobutyrate Acid / Propanol + Isobutyric Acid

Ethanol + Butyric Ethyl butyrate Pineapple or strawberry Acid Ethanol + Salicylic Ethyl salicylate Oil of wintergreen Acid Ethanol + Heptanoic Ethyl heptanoate Grape Acid

Isobutanol + Formic Isobutyl formate Raspberries Acid

Butanol + Butyric Butyl butyrate Pineapple Acid

Pentanol + Acetic Pentyl acetate Banana Acid

Petanol + Pentanoic Pentyl pentanoate Apple Acid

Pentanol + Butyric Pentyl butyrate Pear or apricot Acid Isopentanol + Acetic Isopentyl acetate Pear or banana Acid Octanol + Acetic Acid Octyl acetate Fruity-orange Benzanol + Acetic Acid Benzyl acetate Slightly of jasmine

What are esters?

Esters have a hydrocarbon group of some sort replacing the hydrogen in the -COOH group of a carboxylic acid. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).

A common ester - ethyl ethanoate

The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:

Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.

Note: In my experience, students starting organic chemistry get more confused about writing names and formulae for esters than for almost anything else - particularly when it comes to less frequently met esters like the ones coming up next. Take time and care to make sure you understand!

A few more esters

In each case, be sure that you can see how the names and formulae relate to each other.

Remember that the acid is named by counting up the total number of carbon atoms in the chain - including the one in the - COOH group. So, for example, CH3CH2COOH is propanoic acid, and CH3CH2COO is the propanoate group.

Note: You can find more about naming acids and esters by following this link to a different part of this site. Use the BACK button on your browser to return to this page.

Making esters The chemistry of the reaction Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). If you are a UK A level student you won't have to worry about these. The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:

So, for example, if you were making ethyl ethanoate from ethanoic acid and ethanol, the equation would be:

Note: The mechanism for the esterification reaction is covered in the catalysis section of this site. It is not required for any UK A level chemistry syllabus. If you follow this link, use the BACK button on your browser to return to this page.

Doing the reactions On a test tube scale Carboxylic acids and alcohols are often warmed together in the presence of a few drops of concentrated sulphuric acid in order to observe the smell of the esters formed. You would normally use small quantities of everything heated in a test tube stood in a hot water bath for a couple of minutes. Because the reactions are slow and reversible, you don't get a lot of ester produced in this time. The smell is often masked or distorted by the smell of the carboxylic acid. A simple way of detecting the smell of the ester is to pour the mixture into some water in a small beaker. Esters are virtually insoluble in water and tend to form a thin layer on the surface. Excess acid and alcohol both dissolve and are tucked safely away under the ester layer. Small esters like ethyl ethanoate smell like typical organic solvents (ethyl ethanoate is a common solvent in, for example, glues). As the esters get bigger, the smells tend towards artificial fruit flavouring - "pear drops", for example. On a larger scale If you want to make a reasonably large sample of an ester, the method used depends to some extent on the size of the ester. Small esters are formed faster than bigger ones. To make a small ester like ethyl ethanoate, you can gently heat a mixture of ethanoic acid and ethanol in the presence of concentrated sulphuric acid, and distil off the ester as soon as it is formed. This prevents the reverse reaction happening. It works well because the ester has the lowest boiling point of anything present. The ester is the only thing in the mixture which doesn't form hydrogen bonds, and so it has the weakest intermolecular forces.

Note: Follow this link if you aren't sure about hydrogen bonding. Use the BACK button on your browser to return to this page.

Larger esters tend to form more slowly. In these cases, it may be necessary to heat the reaction mixture under reflux for some time to produce an equilibrium mixture. The ester can be separated from the carboxylic acid, alcohol, water and sulphuric acid in the mixture by fractional distillation.

Note: Providing full details for organic preparations (including all the steps necessary in cleaning up the product) is beyond the scope of this site. If you need this sort of detail, you should be looking at an organic practical book.

Where would you like to go now? To the carboxylic acids menu . . . To the menu of other organic compounds . . . To Main Menu . . .

If you want to make a reasonably large sample of an ester, the method used depends to some extent on the size of the ester. Small esters are formed faster than bigger ones.

To make a small ester like ethyl ethanoate, you can gently heat a mixture of ethanoic acid and ethanol in the presence of concentrated sulphuric acid, and distil off the ester as soon as it is formed.

This prevents the reverse reaction happening. It works well because the ester has the lowest boiling point of anything present. The ester is the only thing in the mixture which doesn't form hydrogen bonds, and so it has the weakest intermolecular forces.

Note: Follow this link if you aren't sure about hydrogen bonding.

Use the BACK button on your browser to return to this page.

Making esters using acyl chlorides (acid chlorides) This method will work for alcohols and phenols. In the case of phenols, the reaction is sometimes improved by first converting the phenol into a more reactive form. The basic reaction If you add an acyl chloride to an alcohol, you get a vigorous (even violent) reaction at room temperature producing an ester and clouds of steamy acidic fumes of hydrogen chloride. For example, if you add the liquid ethanoyl chloride to ethanol, you get a burst of hydrogen chloride produced together with the liquid ester ethyl ethanoate.

The substance normally called "phenol" is the simplest of the family of phenols. Phenol has an -OH group attached to a benzene ring - and nothing else. The reaction between ethanoyl chloride and phenol is similar to the ethanol reaction although not so vigorous. Phenyl ethanoate is formed together with hydrogen chloride gas.

Note: These reactions are discussed in rather more detail on a page about reactions of acyl chlorides. If you want the mechanism for the reaction involving alcohols you can find it by following this link. The phenol mechanism is similar, although hindered by the interaction between the lone pair on the oxygen of the -OH group and the ring electrons. If you aren't sure about using this symbol for a benzene ring, you could follow this link to find out all about it. It is likely to take you some time, though, and you may have to visit several other pages as well. It isn't particularly important in the context of the current page. All you need to know is that at each corner of the hexagon there is a carbon atom, together with a hydrogen atom apart from where the oxygen is attached. If you choose to follow any of these links, use the BACK button (or GO menu or HISTORY file) on your browser to return to this page.

Improving the reactions between phenols and some less reactive acyl chlorides Benzoyl chloride has the formula C6H5COCl. The -COCl group is attached directly to a benzene ring. It is much less reactive than simple acyl chlorides like ethanoyl chloride. The phenol is first converted into the ionic compound sodium phenoxide (sodium phenate) by dissolving it in sodium hydroxide solution.

The phenoxide ion reacts more rapidly with benzoyl chloride than the original phenol does, but even so you have to shake it with benzoyl chloride for about 15 minutes. Solid phenyl benzoate is formed.

Making esters using acid anhydrides This reaction can again be used to make esters from both alcohols and phenols. The reactions are slower than the corresponding reactions with acyl chlorides, and you usually need to warm the mixture. In the case of a phenol, you can react the phenol with sodium hydroxide solution first, producing the more reactive phenoxide ion.

Taking ethanol reacting with ethanoic anhydride as a typical reaction involving an alcohol: There is a slow reaction at room temperature (or faster on warming). There is no visible change in the colourless liquids, but a mixture of ethyl ethanoate and ethanoic acid is formed.

The reaction with phenol is similar, but will be slower. Phenyl ethanoate is formed together with ethanoic acid.

This reaction isn't important itself, but a very similar reaction is involved in the manufacture of aspirin (covered in detail on another page - link below). If the phenol is first converted into sodium phenoxide by adding sodium hydroxide solution, the reaction is faster. Phenyl ethanoate is again formed, but this time the other product is sodium ethanoate rather than ethanoic acid.

Note: These reactions (including the formation of aspirin) are discussed in more detail on a page about reactions of acid anhydrides. Unless you are already very familiar with acid anhydrides, it would definitely be a good idea to follow this link. Use the BACK button on your browser to return to this page later if you want to.

Where would you like to go now? To the esters menu . . . To the menu of other organic compounds . . . To Main Menu . . .

POLYESTERS

This page looks at the formation, structure and uses of a common polyester sometimes known as Terylene if it is used as a fibre, or PET if it used in, for example, plastic drinks bottles

Poly(ethylene terephthalate) What is a polyester? A polyester is a polymer (a chain of repeating units) where the individual units are held together by ester linkages.

The diagram shows a very small bit of the polymer chain and looks pretty complicated. But it isn't very difficult to work out - and that's the best thing to do: work it out, not try to remember it. You will see how to do that in a moment. The usual name of this common polyester is poly(ethylene terephthalate). The everyday name depends on whether it is being used as a fibre or as a material for making things like bottles for soft drinks. When it is being used as a fibre to make clothes, it is often just called polyester. It may sometimes be known by a brand name like Terylene. When it is being used to make bottles, for example, it is usually called PET.

Making polyesters as an example of condensation polymerisation In condensation polymerisation, when the monomers join together a small molecule gets lost. That's different from addition polymerisation which produces polymers like poly(ethene) - in that case, nothing is lost when the monomers join together. A polyester is made by a reaction involving an acid with two - COOH groups, and an alcohol with two -OH groups. In the common polyester drawn above: The acid is benzene-1,4-dicarboxylic acid (old name: terephthalic acid). The alcohol is ethane-1,2-diol (old name: ethylene glycol).

Now imagine lining these up alternately and making esters with each acid group and each alcohol group, losing a molecule of water every time an ester linkage is made.

That would produce the chain shown above (although this time written without separating out the carbon-oxygen double bond - write it whichever way you like).

Note: This does NOT describe the way the actual reaction happens - it is a way of working out the structure of the polymer. The chemistry of the reaction is more complicated than this (see below). The diagram shows a slightly shorter bit of chain than the corresponding one at the top of the page. However, it is exactly consistent with the loss of water from the last diagram. It was impossible to include another ethane-1,2-diol in that diagram for space reasons. If any of this offends you, draw it again yourself so that everything matches! In fact, it would be good practice to draw a bit of chain starting from a few more monomers. This is what I meant further up the page by working the structure out rather than remembering it. The structures of both monomers are easy to remember. If you line them up and remove water as I have shown, the structure follows automatically.

Manufacturing poly(ethylene terephthalate) The reaction takes place in two main stages: a pre- polymerisation stage and the actual polymerisation.

Warning! This manufacturing process is only currently required by one UK A level Exam Board (WJEC). If you don't need to know about this, skip over the next bit. It is really confusing, because it doesn't relate easily to the way we have used to work out the structure of the polyester. The overall result is the same, but it happens by a much more complicated process.

In the first stage, before polymerisation happens, you get a fairly simple ester formed between the acid and two molecules of ethane-1,2-diol.

In the polymerisation stage, this is heated to a temperature of about 260°C and at a low pressure. A catalyst is needed - there are several possibilities including antimony compounds like antimony(III) oxide. The polyester forms and half of the ethane-1,2-diol is regenerated. This is removed and recycled.

Note: Notice the way the polymer is drawn. This is the minimum amount of chain that you can draw to show the repeating unit.

Hydrolysis of polyesters Simple esters are easily hydrolysed by reaction with dilute acids or alkalis. Polyesters are attacked readily by alkalis, but much more slowly by dilute acids. Hydrolysis by water alone is so slow as to be completely unimportant. (You wouldn't expect your polyester fleece to fall to pieces if you went out in the rain!) If you spill dilute alkali on a fabric made from polyester, the ester linkages are broken. Ethane-1,2-diol is formed together with the salt of the carboxylic acid. Because you produce small molecules rather than the original polymer, the fibres are destroyed, and you end up with a hole! For example, if you react the polyester with sodium hydroxide solution:

Note: Hydrolysis of esters is covered in detail on another page in this section.