1)  Detecting the presence of a carbonyl group:

• Brady’s reagent – 2,4-dinitrophenylhydrazine in methanol and sulphuric acid:

• This reaction produces a yellow/orange precipitate specifically with the carbonyl group of an aldehyde or a ketone only:

                                                                                                                                Yellow / Orange crystals

• This is followed by elimination of water and is often described as a condensation reaction.

• The crystalline product can be separated and purified.

• Determining the melting point of the product allows you to find out which aldehyde or ketone was present.

Aldehyde or ketone?

2)  Tollens’ reagent

• This is used to identify whether the molecule is an aldehyde or ketone.

• Often referred to as the ‘silver mirror’ test or 'ammoniacal silver nitrate'

• This is a solution of silver nitrate in an excess of ammonia and is a weak oxidising agent.

• Silver nitrate contains Ag⁺ ions.  

• They can be reduced to silver by a reducing agent / something that will itself be oxidised:

                                      Ag⁺_(aq)        +          e^-             à            Ag_(s)

• Only aldehydes can be oxidised further which means that only aldehydes will produce a silver mirror with Tollens’ reagent:

Summary – how to test / distinguish for aldehydes and ketones:

The orange crystals from 2,4-DNPH can be filtered, recrystallised and melting point determination.  Once you know whether it is an aldehyde or ketone from Tollen's reagent, look up the MPt of the derivative in a table to identify the crystals and hence the aldehyde / ketone.

Qu 1 - 3  P27

Carboxylic acids

Introduction to carboxylic acids

• Carboxylic acids have the general formula:

Has carbonyl group C = O                                 and hydroxyl group C – OH

• These are present in many foods:

                                                  Ethanoic acid – vinegar

                                                  Benzoic acid – used as flavouring in lemonade

                                                  Citric acid – Used as flavouring in citrus drinks

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.

Acid reactions of carboxylic acids:

• Carboxylic acids are weak acids as they partially dissociate in water.

• They react as acids to form salts called carboxylates.

• Less than 1% ionised:-

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

2.  Formation of salts:

• Carboxylic acids are acidic enough to react with:

                   Metals                            Metal hydroxides                          Metal carbonates

• They react in the usually acidic way making salts, hydrogen, water or water and carbon dioxide.

The salt is a carboxylate ion.

a)  With magnesium

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

                                                                               Magnesium ethanoate

b)  With sodium hydroxide

             CH₃CO₂H          +      NaOH            à         CH₃CO₂Na       +      H₂O

                                                                               Sodium ethanoate

c)  With sodium carbonate

             2CH₃CO₂H       +      Na₂CO₃         à        2CH₃CO₂Na     +      H₂O      +      CO₂

                                                                               Sodium ethanoate

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

A salt is formed each time

Qu 1 - 2  P29

Esters

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.

Making esters:

• From AS you made esters from carboxylic acids and alcohols using an acid catalyst and heat.

• The reaction is known as esterification.  It is a reversible reaction (later) and the yield of ester was low:

Esters from acid anhydrides

• Esters can be produced from acid anhydrides with gentle heating.

• This gives a better yield of ester as acid anyhdrides are a lot more reactive than carboxylic acids:

The reaction:

Ester hydrolysis:

• This is the break down of esters with the reaction with water.

Ester                         +          water                       à        Carboxylic              +             Alcohol

                                                                                              acid

• It is the reverse reaction of the esterification reaction.

Carboxylic                   +           Alcohol               à        ester                      +             water

    acid

• It can be done using an acid or alkaline catalyst.  The products formed are slightly different:

1)  Acid hydrolysis:

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

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

2)  Alkaline hydrolysis:

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

• The reaction is non reversible which means that only products are present in the final reaction mixture.

• This reaction is also called 'saponification' as soaps are made in this way by the hydrolysis of fats (later).

Esters as perfumes and flavourings:

• Esters occur naturally in foods and flowers so can be used as perfumes.

• Esters are also found in essential oils, 'oil of wintergreen' used as deep heat.

• Benzyl ethanoate gives and apple and pear flavour / smell.

Qu 1 - 3  P31

Fats and oils - building triglycerides

Fats and oils:

• Fats and oils are basically esters made from propan-1,2,3-triol or glycerol (a triol) and long chain carboxylic acids, fatty acids:

Glycerol	Long chain carboxylic acid, fatty acid - hexadecanoic acid

• Its melting point determines whether it is a fat or oil.

• If its melting point is above room temperature it is a solid which makes it a fat, eg margarine and butter.

• If its melting point is below room temperature it is a solid which makes it an oil, eg olive oil.

Triglycerides - the building blocks:

• Triglycerides are naturally occurring fats.  They are triesters of propan - 1,2,3 - triol, glycerol and fatty acids.

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

• If all 3 join we make a triglyceride (an ester).

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

• Fatty acids are saturated if they contain only C - C and unsaturated if they contain C=C.

• Fatty acids contain an even number of carbon atoms due to their synthesis in nature.

A saturated fatty acid:  Hexadecanoic acid (palmitic acid)

• Has no C = C

A monounsaturated fatty acid: Octadec - 9 - enoic acid (oleic acid)

• Has 1 C = C between Carbon 9 and 10 (common)

A polyunsaturated fatty acid: Octadec - 9,12 - enoic acid (linoleic acid)

• Has 2 C = C between Carbon 9 and 10 / 12 and 13

Fatty acid - shorthand:

• Fatty acids can be written in shorthand:

	16:0
	18:1  (9)
	18:2  (9.12)

Forming triglycerides:

• These are basically esters:

• Most triglycerides (fats) actually form using different fatty acids

• Triglycerides (fats) can be saturated or unsaturated depending on whether the fatty acids they are made of are saturated or not

Qu 1 - 4  P33

Triglycerides, diet and health

Isomerism in unsaturated fatty acids:

• In AS we covered the Cis Tranz (E/Z) isomerism involving a C=C:

	No free rotation around the C=C
Trans but-2-ene (E-but-2-ene)		Cis but-2-ene (Z-but-2-ene)

• Remember the C=C stops free rotation around the C=C.

• Fats and oils come from saturated and unsaturated fatty acids:

Unsaturated packing:

	Non linear - inefficient packing - liquid
	Linear - efficient packing - solid

• The food industry often removes some of the C=C by hydrogenation (adding H₂ across the C=C)

• This makes them more solid = margarine manufacture.

• A side effect of this is that the structure of the fat is changed from a liquid (little health risk) to a solid (Heart risk)

Trans fats cholesterol:

High Density Lipoproteins	Low Density Lipoproteins
These carry cholesterol out of the blood and out of the body. Good	These carry about 65% of cholesterol around the blood End up depositing lipids (fats) onto artery walls This restricts blood flow Bad

Fatty Acids as Biodiesel:

Biodiesel is in fact a methyl or ethyl ester. Fats are essentially 3 methyl esters joined together. This can be made by using waste cooking oil and using a procedure called: TRANSESTERIFICATION

Making Biodiesel - Transesterification:

• The waste oil is filtered then reacted with methanol and sodium hydroxide (catalyst) to form biodiesel.

• This also increases the atom economy of fats.

Qu 1 - 4  P35

Qu 5 - 8  P41  /  4 - 10, 14 P42 - 45