Introduction

Carboxylic acids have the general formula:

                                                  Has carbonyl group C = O                                 on one

                                                  and hydroxyl group C – OH                          Carbon atom

These are present in many foods:

                                                  Ethanoic acid – vinegar

                                                  Benzoic acid – used as flavouring in lemonade

                                                  Citric acid – Used as flavouring in citrus drinks

Esters have the general formula:

• Esters are referred to as a derivative of carboxylic acids due to the easy substitution of the hydrogen in the hydroxyl group with an alkyl group.  They are derived from carboxylic acids.

• Again these are found in foods and have a fruity aroma.

Sources of carboxylic acids

Natural

• Naptha from crude oil.

• Natural oils.

• Oils and fats are esters of carboxylic acids.  Hydrolysis provides the carboxylic acids.

• Carboxylic acids from oils and fats usually have long carbon chains and are often referred to as fatty acids.

• They usually contain an even number of carbon atoms and are not branched.

• If a fatty acid contains 1 C=C, we call it monounsaturated.

• With more than 1 C=C, we call them polyunsaturated.  Each double bond will give rise to geometric isomers.

Synthetic

• From alcohols, aldehydes and nitriles

SAQ 4.1 and 4.2

EXPT – The reactions of carboxylic acids

1)  Solubility and pH

• Add 1cm³ of pure ethanoic acid (called glacial) to a test tube (CARE very corrosive).

• Add water dropwise with a pipette, do the liquids mix?

• Spot out some of the mixture on to full range indicator paper.

• Now add a few drops of sodium carbonate solution.

• See how well other carboxylic acids mix with water

• Record all observations.

2)  As an acid

a)  With sodium hydroxide

• Set up apparatus for titration using a small beaker.

• Add 25cm³ of 0.1M ethanoic acid to a small beaker add a pH probe and place on a magnetic stirrer. 

• Make sure to pH probe will not be hit by the stirrer.

• Fill a burette with 0.1M sodium hydroxide.

• Start logging and run all of the sodium hydroxide into the ethanoic acid.

• Sketch the graph.

b)  With magnesium

• Add 10cm³ of ethanoic acid to a test tube.

• Add about a 3cm strip of magnesium ribbon andplace a boiling tube over the top.

• After a minute, test the gas with a lit splint.

• Record all observations.

c)  With sodium carbonate

• Add 5cm³ of sodium carbonate solution to a test tube.

• Add ethanoic acid dropwise to the solution

• Record all observations.

3)  Formation of esters

• Add 1cm³ of pure ethanoic acid to a test tube.

• Add 2 cm³ of ethanol.

• Add 2 – 3 drops of concentrated Sulphuric acid.

• Heat in a water bath for about 5 minutes.

• Add a few drops of sodium carbonate solution until it no longer fizzes.  This should neutralise any excess acid.

• Smell the product; compare it to the smell of ethanoic acid.

• You can repeat this using different alcohols.

Interpretation of the reactions of carboxylic acid

• In chapter 2 we looked at the behaviour of the hydroxyl group in phenol and had to modify its behaviour to take into account the interaction of the lone pair of electrons from the oxygen atom into the benzene ring.

• Again we must look at the properties and modify if necessary to take onto account the interaction of the lone pair of electrons from the oxygen atom and the double bond of the carbonyl group.

1)  Solubility and pH

• Solubility – C₁  - C₄  carboxylic acids mix readily.

• C­₅ à  solubility reduces due to length of insoluble R chain.

• All carboxylic acids H  - bond to each other and with water.

• Carboxylic acids are weak in water.

• Less than 1% ionized:-

                      RCO₂H_(aq)     à     H⁺_(aq)       +       RCO₂^-_(aq)

Ionisation

• Methanoic acid (formic acid) is used in stings by some insects.

• Electron diffraction studies show that the C – O bond lengths in the methanoate ion are equivalent.

• The C – O bonds lengths are shorter than the C – O single bond, but longer than the C =O double bonds.

• This is evidence of delocalisation of the p bond electrons of the carbonyl group (C = O) with the C – O group of the hydroxyl group:-

• The proximity of the carbonyl group allows the O – H bond to partially ionise in water.

• When hydroxyl group ionises, the anion is stabilised by delocalisation of the C = O p electrons.

• Because of this we would expect some acidic behaviour:-

2.  Formation of salts:

• Carboxylic acids are acidic enough to react with:

                                                                                                          Metals

                                                                                                          Metal oxides

                                                                                                          Metal hydroxides

                                                                                                          Metal carbonates

a)  With sodium hydroxide

             CH₃CO₂H          +      NaOH            à         CH₃CO₂Na       +      H₂O

b)  With magnesium

             2CH₃CO₂H       +      Mg                 à         (CH₃CO₂)₂Mg  +      H₂

c)  With sodium carbonate

             2CH₃CO₂H       +      Na₂CO₃         à        2CH₃CO₂Na     +      H₂O      +      CO₂

In each case the carboxylic acid has donated a proton, H⁺

A salt is formed each time

3)  Formation of esters

• Dipole due to C = O group means the carbon atom is electrophylic (d+).

• This means carboxylic acids will be susceptible to nucleaphilic attack:-

You do not need to know a mechanism for these reactions

• Even weak nucleophiles (e.g. ethanol) will attack the carbon atom but require an acid catalyst.

• Carboxylic acids react with alcohols to form esters and water:-

Carboxylic                       +       Alcohol                à                ester                            +    water

    acid

• An acid catalyst (sulphuric acid) + heat is required.

SAQ 4.3 and 4.4

Esters

• The derivatives of carboxylic acids are those where the  – OH group in the  – CO₂H functional group has been replaced with another group:

• In the carboxylic acid section it was possible to make an ester using an alcohol.

• Hydrolysis of any ester will make the corresponding carboxylic acid (and alcohol):

Ester                         +          water                       à        Carboxylic              +             Alcohol

                                                                                              acid

• Compare this reaction with how an ester was made:

Carboxylic                   +           Alcohol               à        ester                      +             water

    acid

• You should notice that they are reverse reactions of each other.

EXPT – Hydrolysis of an ester

1)     Add 2cm³ of methyl benzoate and 30cm³ of 2M sodium hydroxide to a 50cm³ pear shaped flask.  Add a couple of anti bumping granuals.

2)     Assemble the apparatus for reflux and reflux for 30 minutes.

3)     Allow the mixture to cool then acidify with 2M hydrochloric acid.  Spot onto universal indicator paper to check.

4)     Write a balanced chemical reaction to show what has happened.

OPTIONAL – If time allows

5)     Collect your product by suction filtration and wash with some distilled water.

6)     Recrystalise the product with water and determine its melting point.

7)     Use the data book to check purity.

SAQ 4.5

Interpretation of the hydrolysis of an ester:

• The hydrolysis of any carboxylic acid derivative results in the carboxylic acid.

• The reaction undergoes nucleophilic substitution:-

Hydrolysis of an ester

         With acid catalyst

• The reaction is in equilibrium which means molecules of reactants and products are present in the reaction mixture.

        With Base catalyst

• When a base is used, the product is the sodium salt of the carboxylic acid:

SAQ4.6

Fats as natural esters

                                                                    A triglyceride

• Glycerol has 3 alcohol groups which can react with a carboxylic acid (stearic acid) to form an ester.  This is called esterification.

• The carboxylic acids occur naturally and because they make fats (upon esterification) we call them fatty acids.

• If all 3 join we make a triglyceride (an ester).  These are fats and oils and occur naturally:

• If only one of the alcohol groups is esterified with 1 stearic acid we make a mono glyceride, with 2, diglyceride.

• A triglyceride (fat or oil) actually looks like:

SAQ 4.7

The hydrolysis of fats and oils - saponification

• Oils are basically just esters.  This means they can be hydrolysed like any other ester by refluxing with an acid or base catalyst.

• An acid catalyst gives glycerol and the fatty acids (carboxylic acids eg stearic acid).

• With a base you make glycerol and the sodium salts of the fatty acids:

                   3NaOH      +

• The salts of fatty acids are actually soaps.

• The Latin for soap is ‘sapo’ hence this reaction is called saponification.

• After saponification the soap is precipitated out by adding an excess of sodium chloride.  This is known as ‘salting out’.

The uses of esters

• Solvents

• Adhesives

• Nail varnish remover

• White board pens

• Food flavorings

• Essential oils

Essential oils

• The odours from flowers come from volatile organic compounds.

• These are extracted by steam distillation or made synthetically

• They are usually made up from a mixture of asters, aldehydes, phenols and terpenes (a 5 carbon unit molecule).

• Oil of jasmine is extracted from the jasmine plant.  Alternatively it can be synthesised by the esterification of ethanoic acid and phenyl methanol: