Carboxylic Acids Edexcel

Carboxylic Acids 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. They can also hydrogen bond to water molecules making them water soluble.

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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 is water soluble, is not correct. As the carbon chain increase in length (non-polar section) the water solubility decreases. This can be applied to any organic molecule.

Preparation Oxidation of a primary alcohol (or aldehyde) using K2Cr2O7/H+ Hydrolysis of a nitrile using HCl/H2O CH3CH2CN + H+ + 2H2O  CH3CH2CO2H + NH4+

Reactions Addition of PCl5  Acyl Chloride

“Steamy white fumes” of HCl are given off which turn damp blue litmus paper red. Also POCl3 is quite a well known/remembered product.

RCOOH + PCl5

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→ RCOCl + HCl(g) + POCl3

this reaction also occurs for alcohols at AS.

Addition of base  a salt You could add NaOH or Na2CO3 CH3COOH + NaOH  CH3COO-Na+ + H2O

2CH3COOH + Na2CO3 → 2CH3COO-Na+ + H2O + CO2

Addition of LiAlH4  a primary alcohol We looked at LiAlH4 is the carbonyl tutorial. Similar to what we saw there, it will reduce carboxylic acids back to primary alcohols. Remember LiAlH4 is a strong reducing agent. CH3COOH + 4[H]  CH3CH2OH + H2O

Addition of alcohol  ester Esters are made in condensation reactions i.e. removal of a small molecule like water or HCl. Carboxylic acid + Alcohol  Ester + Water 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 that high.

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no mechanisms are required for these reactions.

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.

Acyl Chlorides Acyl chlorides are very reactive due to the Cl group being easily substituted. They are nucleophilic reactions but no mechanism is required. The product is very easy to identify, just remove the Cl and attach whatever it being added: 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 + Ammonia  Primary Amide

+

HCl

Acyl Chloride + Amine  Secondary or Tertiary Amide

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+

HCl

can you see the pattern here? You don’t have to memorise all these reactions. It is a simple swap of the Cl with whatever you are attaching.

Ester Hydrolysis As well as making esters, you need to know how to break them in a hydrolysis reaction.

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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 when we made the esters above. We add an OH on to the C=O and an H on to the O.

Conditions Be very careful with the hydrolysis conditions as it can be done under alkaline (using NaOH) or acidic conditions (using HCl). The hydrolysis is still as drawn above but be careful with the final products. Acidic conditions No problem here, the carboxylic acid and the alcohol are the products. This reaction gives the correct 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.

If you want the carboxylic acid, you would then have to add acid. This reaction is not in equilibrium, therefore the yield is much higher than in acidic conditions.

Polyesters The reactions are just the same as making the esters above. The only difference is that we want to make a polymer i.e. a very long chain. To do this we need to double up the groups. Instead of having one carboxylic acid group and one alcohol group, we need two of each. Or we could use two acyl chlorides instead of the carboxylic acids. We need reactions at both ends of the molecule so the polymer can keep growing. 2 x carboxylic acid groups (or acyl chlorides)

+

2 x alcohol groups

You can’t leave the polyester looking as in the reaction above. You either neither to draw the repeating unit or the polymer:

Polymer

Remove another water molecule i.e. remove an H from the OH end and an OH from the carbonyl. Draw lines coming out the end of the chain. Draw brackets with the lines through the brackets. Put a little ‘n’ outside the brackets to show that it is a polymer. The ‘n’ says we don’t know how many units there are.

Repeating unit

No brackets or little ‘n’ are needed. If you do put the brackets in it doesn’t matter but don’t put the little ‘n’ in.