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Carboxylic Acids & Esters

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Carboxylic Acids & Esters Carboxylic Acids & Esters Carboxylic Acid Properties Carboxylic acids can form intramolecular hydrogen bonds between the carbonyl group and the OH group. This raises their boiling points in comparison to ketones and aldehydes etc. This is called a dimer, which means two of the same thing joining together: They can also hydrogen bond to water molecules making them water soluble. d- O d+ H H d- O d+ H3C C OH ✓ Water solubility depends on two things: the hydrogen bonding (polar) and the carbon chain length (non-polar) i.e. polar versus non-polar. To always say, something can hydrogen bond therefore it is water soluble, is not perfectly correct. As the carbon chain increases in length (non-polar section) the water solubility decreases. It’s a battle between the polar hydrogen bond part and the non-polar carbon chain. This can be applied to other organic molecules such as amines and alcohols. Reactions Addition of base → a salt + water You could add NaOH or Na2CO3 - + CH3COOH + NaOH → CH3COO Na + H2O - + 2CH3COOH + Na2CO3 → 2CH3COO Na + H2O + CO2 Addition of metal → a salt + hydrogen - + 2CH3COOH + 2Na → 2CH3COO Na + H2 Ester Formation From carboxylic Acids Esters are made in condensation reactions i.e. removal of a small molecule like water or HCl. Carboxylic acid + Alcohol → Ester + Water Here is an example of an ester: ester group These reactions require an acid catalyst, usually H2SO4 as it is difficult to remove water. The reaction is also in equilibrium, therefore the yield isn’t very high. ✓ No mechanisms are required for these reactions. To form the ester, simply: remove the OH attached to the C=O group and the H from the alcohol. Then just join up what is remaining and you have an ester. ✓ It is important to remove the OH from the acid and the H from the alcohol as you will do a similar reaction with acyl chlorides, anhydrides, amino acids and amines. Some students want to do the opposite and remove the OH from the alcohol and the H from the acid. In ester formation you will get the correct answer but problems will occur with the other molecules mentioned. The rule is….remove whatever is attached to the carbonyl group (OH above) and an H from the other molecule (the alcohol). Naming esters Esters always end in ‘oate’. For example the ester below is called methyl ethanoate. The ‘methyl’ part is from the original alcohol (methanol) and the ‘ethanoate’ part is from the original carboxylic acid (ethanoic acid). alcohol carboxylic half acid half From anhydrides As an alternative, esters can also be made from acid anhydrides. You can think of anhydrides as being similar to carboxylic acids, hence the name acid anhydrides. O O O So why anhydrides? Simply because they are more reactive than carboxylic acids. They don’t need a catalyst or reflux and give a higher yield as they are not in equilibrium. But it is still a condensation reaction: O O O CH3 + CH COOH H3C O CH3 + HO-CH3 H3C O 3 remove As was mentioned above….remove the H from the alcohol and whatever is attached to the carbonyl of the other molecule (anhydride), CH3COO is removed. This gives us a carboxylic acid instead of water alongside the ester. ✓ Note that acyl chlorides are yet another alternative to carboxylic acids (see below). Ester Hydrolysis As well as making esters, you need to know how to break them in a hydrolysis reaction. ✓ Lysis means to break and hydro means water. Therefore hydrolysis means bond breaking by addition of water. We need to break the bond we made i.e. the C-O in the ester group. So we just do the opposite of what we did to make the ester. We add an OH on to the C=O and an H on to the O. Conditions We also need to take conditions into account as it could alter the product slightly. Hydrolysis can be done under alkaline (using NaOH) or acidic conditions (using HCl). Acidic conditions No problem here, the carboxylic acid and the alcohol are the products. But is in equilibrium, therefore the yield is not as high as for alkaline conditions. Alkaline conditions The carboxylic acid formed will react with the base → salt. This reaction is not in equilibrium, therefore the yield is much higher than in acidic conditions. From this salt, if you want the carboxylic acid, you would then have to add acid. ✓ The conditions cause lost of problems for students. There are similar reactions when forming amides (see polyesters & amides). Acyl Chlorides Acyl chlorides are very reactive due to the Cl group being easily substituted. Cl is good at “leaving” Synthesis of acyl chlorides i.e. very reactive Acyl chlorides can be made from carboxylic acids and thionyl chloride (SOCl2): O O H3C C OH + SOCl2 H3C C Cl + SO2 + HCl Reactions of acyl chlorides All the reactions below are nucleophilic reactions but no mechanism is required. The product is very easy to identify, just remove the Cl from the acyl chloride and H from the other molecule. Exactly the same method as above for carboxylic acids and anhydrides: Acyl Chloride + H2O → Carboxylic Acid + HCl Acyl chloride + Alcohol → Ester + Hydrochloric acid ✓ Acyl chlorides can be used instead of carboxylic acids → esters. Acyl chlorides are much more reactive than carboxylic acids and don’t require an acid catalyst. The reaction is not in equilibrium and therefore gives a better yield. ✓ The downside is the formation of HCl. Acyl chlorides are also quite toxic and expensive. Acyl Chloride + Phenol → Ester + HCl Phenol does not form esters with carboxylic acids; an acyl chloride is required: O OH H3C O C O + HCl H3C C Cl + Acyl Chloride + Ammonia → Primary Amide + HCl Acyl Chloride + Amine → Secondary or Tertiary Amide + HCl ✓ Can you see the pattern here? You don’t have to memorise all these reactions. They are all the same!! It’s a gift from the exam board. Just remove a Cl and an H. Briefly, carboxylic acid → anhydride → acyl chloride. Increasing in order of reactivity. They have their pros and cons for each reaction but they are all condensation reactions and very similar. .
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