Section outline

  •             Key unit competency

    To be able to compare the chemical nature of carbonyl compounds to their 

    reactivity and uses. 

     Learning objectives

    • Describe the reactivity of carbonyl compounds
    • State the physical properties of aldehydes and ketones
    • Describe the preparation reactions of ketones and aldehydes
    • Explain the mechanism of nucleophilic addition reactions of carbonyl 
      compounds
    • Prepare ketones from secondary alcohols by oxidation reactions
    • Compare aldehydes and ketones by using Fehling’s solution and Tollens’ 
      reagent
    • Write and name carbonyl compounds and isomers of ketones and aldehydes
    • Write equations for the reactions of carbonyl compounds with other 
      substances
    • Compare the physical properties of carbonyl compounds to those of alcohols 
       and alkenes
    • Differentiate the methyl ketones from other ketones by using the iodoform 
       test
    • Carry out an experiment to distinguish between carbonyl compounds and 
       other organic compounds
    • Carry out an experiment to distinguish between ketones and aldehydes

    • Carry out an experiment to prepare ethanol and propan-2-one.

    6.1. Definition and nomenclature of carbonyl compounds

       Introductory activity 

    Many fruits such as mangoes and honey contained sugar. The following images 

    represent mangoes, honey and some sugars such as fructose and glucose. 

                    

    1. State the functional groups found in fructose and glucose.
    2. Enumerate other foods that contain sugars 
    3. Describe the similarity and difference between the two sugars in term of 

        structure formulae. 

    6.1.1 Definition

         Activity 6.1

    Observe the following molecules and answer to the questions.

                 

    1. Categorize the above molecules

          4. What criteria have you used to categorize?
          5. Name those categories 

          6. Name individual molecules 

    Carbonyl compounds are compounds that contain carbon-oxygen double bond 
    (C=O). Carbonyl compounds are classified into two general categories based on the 
    kinds of chemistry they undergo. In one category there are aldehydes and ketones; 
    in the other category there are carboxylic acids and their derivatives. This unit looks 

    on category of aldehydes and ketones.

    Aldehyde molecules 

    For aldehydes, the carbonyl group is attached to hydrogen atom and alkyl group as 
    shown in the molecule of propanal below. Methanal is the smallest aldehyde, it has 

    two hydrogen atoms attached to carbonyl group.

                       

    If you are going to write this in a condensed form, you write aldehyde as –CHO, 

    don’t write it as -COH, because that looks like an alcohol functional group.

    Ketone molecules

    Ketone has two alkyl groups attached to the carbonyl group. Examples:

                     

    6.1.2 Nomenclature
           Aldehydes
    The systematic name of an aldehyde is obtained by replacing the terminal “e” from 
     the name of the parent hydrocarbon with “al.” In numbering the carbon chain of an 

    aldehyde, the carbonyl carbon is numbered one.

                    


    Ketones

    The systematic name of a ketone is obtained by removing the terminal“e” from the 
    name of the parent hydrocarbon and adding “one.”The chain is numbered in the 
    direction that gives the carbonyl carbon the smallest number.Ketone contains 
    a carbon-oxygen double bond just like aldehyde, but for ketone carbonyl groupis 

    bonded to two alkyl groups.

               

          

    Checking up 6.1

    7. For each of the following structures, justify whether it is an aldehyde or 

        a ketone, and name each.

                

     2. Draw the structural formulas derived from the following names. 

          i. Hexan-3-one 
         ii. Pentan-2-one

        iii. 2-methylpropanal

      6.2. Isomerism

            Activity 6.2

       Look at the molecules below and answer the following questions.

                     

       4. Write molecular formula of A and B 
       5. Compare the molecular formulae of A and B
       6. State a term that can be used to describe relationship between 
          molecules A and B.

      7. Write down other three different examples which are related as A and B.

          6.2.1 Functional isomerism in aldehydes and ketones  

    Isomers are molecules that have the same molecular formula, but have a different 
    arrangement of the atoms in space. Functional group isomers are molecules that 
    have same molecular formula but contain different functional groups, and they 

    belong to different homologous series of compounds.

    Example1;  structural formulae of this molecular formula can be either 

    propanal or propanone, aldehyde or ketone.

                       

                 

    You could draw others possible structural formula of 

    that have alkene and alcohol functional groups.

                 

         6.2.2. Position isomerism in ketones 

    Position isomerism is isomerism where carbon skeleton remains constant, but the 

    functional group takes different positions on carbon skeleton.

                 

         6.2.3. Chain isomerism in aldehydes and ketones

    In chain isomerism the same number of carbons forms different skeletons. Aldehydes 
    with 4 or more carbon atoms and ketones with five or more carbon atoms show 

    chain isomerism. 

                  

     Checking up 6.2

    Draw as many as possible all the structural isomers of 

    6.3. Physical properties of aldehydes and ketones

      Activity 6.3

    • Take 50 ml for each substance: ethanal, butanal and propanone.
    • Mix ethanal with 50ml of water in beaker 
    • Mix butanal with 50ml of water in beaker 
    • Mix propanal with 50ml of water in beaker 
    viii. Compare the solubility of ethanal, butanal and propanone in water.
    ix. State intermolecular forces present in each substances 
    x. Explain what happen in term of intermolecular forces during mixing 
         those above substances with water.
    xi. Explain why some substances have high solubility in water than other.
    xii. how the intermolecular forces present in ethanal, butanal and 
          propanone affect other physical properties like boiling and melting 

           point of these substances.

    6.3.1. Solubility in water aldehydes and ketones

    The small molecules of aldehydes and ketones are soluble in water but solubility 
    decreases with increase of carbon chain. Methanal, ethanal and propanone - the 
    common small aldehydes and ketones are soluble in water at all proportions.
    Even though aldehydes and ketones don’t form hydrogen bond with themselves, 

    they can form hydrogen bond with water molecules.

                             

    The slightly positive hydrogen atoms in a water molecule can be sufficiently attracted 
    to the lone pair on the oxygen atom of an aldehyde or ketone to form a hydrogen 
    bond.

    Other intermolecular forces present between the molecules of aldehyde or ketone 
    and the water are dispersion forces and dipole-dipole attractions.
     
    Forming these attractions releases energy which helps to supply the energy needed 
    to separate the water molecules and aldehyde or ketone molecules from each other 
    before they can mix together.

    Apart from the carbonyl group, hydrocarbon chains are non polar, they don’t dissolve 
    in water. By forcing hydrocarbon chain to mix with water molecules, they break the 
    relatively strong hydrogen bonds between water molecules without replacing them 
    by other attractions good like hydrogen bonds. This makes the process energetically 

    less profitable, and so solubility decrease.

    6.3.2. Boiling points of aldehydes and the ketones

    Methanal is a gas and has a boiling point of 
    and ethanal has a boiling point of The other aldehydes and ketones
    are liquids or solids, with boiling points rising with rising of molecular mass
     hence rising of strength of Van der Waals force. 

    Comparing the physical properties of carbonyl compounds to those of alcohols 

    and alkanes.

    Physical properties of covalent compounds depend on intermolecular forces. 
    Compounds that have similar molecular mass but different intermolecular forces 

    have different physical properties.

    Example of comparison between molecules of similar mass but different 

    compositions.

                

    Alcohols have higher boiling point than aldehydes and ketones of similar lengths. In 
    the alcohol, there is hydrogen bonding, but the molecules of aldehydes and ketones 
    don’t form hydrogen bonds.Aldehydes and ketones are polar molecules but alkanes 

    are non polar molecules.

    Checking up 6.3

    The Table below shows the boiling points of an alkane, an aldehyde and an 

    alcohol.

                    

    m. Explain why the boiling point of an aldehyde is greater than that of the 
          alkane?
    n. Why is the boiling point of the alcohol still higher?
    o. Explain why, unlike the similar-sized alkanes, the small aldehydes and 
          ketones are soluble in water.

    p. Describe the solubility variation of aldehydes and ketones.

    6.4. Chemical properties of carbonyl compounds 

    6.4.1. Nucleophilic addition reactions

           Activity 6.4.1

    • KCN is a reagent used to add HCN to carbonyl compounds. 
       Write equation that show how KCN dissociates in polar solvent 
    • Observe carefully the following carbonyl functional group and answer the 

       following questions.

                     

    a. Polarity of carbonyl group

    by comparing carbon-carbon double bond and carbon- oxygen double bond 
    the only difference between bonds C=C and C=O is distribution of electrons. The 
    distribution of electrons in the pi bond is heavily attracted towards the oxygen atom, 

    because oxygen atom is much more electronegative than carbon.

                                    

    During chemical reactions nucleophiles will attack carbon of the carbonyl functional 
    group which bears apartial positive charge. While electrophile will attack oxygen of 

    the carbonyl functional group which bears a partial negative charge.

    b. Reaction of HCN with aldehydes and ketones

         

    Because hydrogen cyanide is a toxic gas, the best way to carry out this reaction is to 
    generate hydrogen cyanide during the reaction by adding HCl to a mixture of the 
    aldehyde or ketone and excess sodium cyanide. Excess sodium cyanide is used in 
    order to make sure that some cyanide ion is available to act as a nucleophile. The 
    solution will contain hydrogen cyanide (from the reaction between the sodium or 
    potassium cyanide and the HCl)

    The pH of the solution is maintained in range 4 - 5, because this gives the fastest 

    reaction. The reaction takes place at room temperature.

    c. The mechanism of reaction between HCN and propanone
    1st Step: A nucleophilic, CN-, attacks on the slightly positive charged carbon of 

    carbonyl group.

                    

    2ndStep: The negative ion formed picks up a hydrogen ion from hydrogen cyanide. 

    Water or the  ions  present in solution can serve as source of the hydrogen ion.

         

      These are examples of nucleophilic addition

                

    e. Reaction of  with aldehydes or ketones

    The aldehyde or ketone is shaken with a saturated solution of sodium hydrogen 
    sulphite in water. Hydrogen sulphite with negative charge act as nucleophile, 
    where the product formed is separated as white crystals. Propanone react hydrogen 

    sulphite, as below:

                

    Impure aldehyde and ketone can be purified by using this reaction. Impure 
    aldehyde or ketone is shaken with a saturated solution of sodium hydrogensulphite 
    to produce the crystals. Impurities don’t form crystals; these crystals formed are 
    filtered and washed to remove any impurities. Addition of dilute acid to filtered 
    crystals regenerates the original aldehyde. Dilute alkali also can be added instead 

    dilute acid. 

    Checking up 6.4.1

    Aldehydes and ketones undergo addition reactions involving hydrogen 
    cyanide in which H and CN add on the carbon-oxygen double bond.
    a. Why isn’t hydrogen cyanide itself normally used in these reactions?
    b. Give a mixture which can be used instead of starting with hydrogen 
         cyanide itself.
    c. Draw the structures and give the names of the products of the 
          reaction between hydrogen cyanide and
           i. (Ethanal
          ii. Propanone
    d. One use of the products of these reactions (known as hydroxy nitriles) 
        is as a part of a sequence of reactions to make more complicated 
        molecules like amino acids from more simple ones. The amino acid 

        valine has the structure:

                 

    i. Write the structure of the hydroxy nitrile which you would have to modify 
       in order to make valine
    ii. Write the structure of the aldehyde or ketone which you would have to 

         react with hydrogen cyanide in order to get that hydroxy nitrile

    6.4.2. Condensation reactions

    Activity.6.4.2

    You are provided with the following: propanal, propanone, alcohol (ethanol), 
    glucose solution and 2,4-dinitrophenylhydrazine (Brady reagent )

    Take about 2ml of each solution; propanal, propanone, alcohol (ethanol) and 
    glucose solution in test tubes. Add 6 drops of the 2,4-dinitrophenylhydrazine 
    to each of the test tubes containing: propanal, propanone, (alcohol)ethanol or 
    glucose solution. If no precipitate forms immediately, warm for 5 minutes in the 

    water bath. Record your observations in the table below.

                 

    a. Experimental reaction

    The procedure of the preparation of Brady’s reagent and carbonyl compounds 
    changesslightly depending on the nature of the aldehyde or ketone, and the solvent 
    in which 2,4-dinitrophenylhydrazine is dissolved in. The Brady’s reagent for activities 

    (6.4.1) is a solution of the 2,4-dinitrophenylhydrazine in methanol and sulphuric acid.

    Add a few drops of Brady’s reagent to either aldehyde or ketone. A bright orange or 
    yellow precipitate indicates the presence of the carbonyl group in an aldehyde or 

    ketone. 

    b. Structural formula of 2,4-dinitrophenylhydrazine.

    The carbon of benzene attached to hydrazine is counted as number one.
    In     2,4-dinitrophenylhydrazine, there are two nitro groups,   
    ,attached to the phenyl group in the 2- and 4- positions.

                           

    2,4-dinitrophenylhydrazine is often abbreviated as 2,4-DNP or 2,4-DNPH.

    c. The reaction of carbonyl compounds with 2,4-dinitrophenylhydrazine
    Brady’s reagent is a solution of the 2,4-dinitrophenylhydrazine in methanol 
    and sulphuric acid. The overall reaction of carbonyl compounds with 

    2,4-dinitrophenylhydrazine is:

                             

    Where R and R’ represent alkyl groups or hydrogen(s); if both or only one is hydrogens 
    the starting carbonyl compound is an aldehyde. If both R and R’ are alkyl groups 
    the carbonyl compound is a ketone. The following molecule shows clearly how the 

    product is formed.

                        

    The product formed is named”2,4-dinitrophenylhydrazone”. The simple difference 
    consists in replacing suffix “-ine” by “-one”.
    The reaction of 2,4-dinitrophenylhydrazine with ethanal produces ethanal 
    2,4-dinitrophenylhydrazone; The reaction of 2,4-dinitrophenylhydrazine with 
    butanal produces butanal 2,4-dinitrophenylhydrazone. This is an example of 

    condensation reaction.

    During the chemical reaction, the change takes place only on nitrogen  of 
    hydrazine in 2,4-dinitrophenylhydrazine. If the group is attached to other 

    groups a similar reaction as that of 2,4-dinitrophenylhydrazine will take place:

               

                 

    • Phenylhydrazine reacts with carbonyl compound to form“phenylhydrazone”.

                     

    Checking up 6.4.2

    a. Brady’s reagent is a solution of 2,4-dinitrophenylhydrazine in a mixture 
        of methanol and sulphuric acid.
     i. How is Brady’s reagent used to test for an aldehyde or ketone?
    b. Draw the structural formulae for 
     ii. Propanone hydra zone 
     iii. Propanone phenylhydrazone

    6.4.3. Oxidation reactions using 

       Activity 6.4.3

       Materials:

    • Test tubes 
    • Test tubes holder 
    • Test tube racks
    • Count droppers
    • Beakers 
    You are provided with the following: propanal, propanone and potassium dichromate 
    (VI) solution acidified with dilute sulphuric acid.
    Take about 2ml of each solution; propanal and propanone; add 6 drops of the 
    potassiumdichromate(VI) solution acidified with dilute sulphuric acid.

    Record your observations in the table below.

         

            a. Difference in reactivity of ketones and aldehydes with 
        By considering the structural formulae of aldehydes and ketones, the difference is 
        only the presence of a hydrogen atom attached to the carbonyl functional group in 

        the aldehyde whereas ketones have a alkyl group instead.

                     

        During chemical reaction aldehydes react with oxidizing agent; hydrogen on 
       carbonyl functional group is replaced by oxygen, look on figure below. The presence 
       of hydrogen atom makes aldehydes very easy to oxidize, in other words aldehydes 

       are strong reducing agents.

    For ketone, absence of hydrogen on carbonyl functional group makes ketones to 
    resist oxidation. But very strong oxidising agents like potassium permanganate 
    solution oxidize ketones - and they do it in a destructive way, by breaking carbon bonds.

    Aldehyde oxidation can take place in acidic or alkaline solutions. Under acidic 
    solutions, the aldehyde is oxidized to a carboxylic acid. Under alkaline solutions, acid 

    formed react with base to form a salt of carboxylic acid. 

    Add few drops of the aldehyde or ketone to a solution of potassium dichromate 
    (VI) acidified with dilute sulphuric acid. If the color doesn’t change in the cold, the 

    mixture is warmed gently in a beaker containing hot water.


                   


    b. Oxidation of aldehyde by solution

    Add few drops of the aldehyde or ketone to a solution of potassium dichromate 
    (VI) acidified with dilute sulphuric acid. If the color doesn’t change in the cold, the 

    mixture is warmed gently in a beaker containing hot water.

                

                

    Checking up 6.4.3

    1. If you react ethanal with acidified potassium dichromate (VI) solution, 
        what organic product would you get?
    2. Write a half-equation for the formation of that product from ethanal.
    3. iii.Write a half-equation for the dichromate(VI) ion acting as an oxidising 

        agent is

             

    Use this equation and the one you wrote in part (ii) to work out the ionic 

    equation for the reaction.

    6.4.4. Oxidation reactions using Tollens’ reagent

    Activity 6.4.4

    Materials:

    • Test tubes 
    • Test tubes holder 
    • Test tube racks
    • Count droppers
    • Beakers 
    • Bunsen burner 

    You are provided with the following: 
    • propanal, 
    • propanone 
    • Tollens’ reagent. 
    Take about 2ml of each solution; propanal and propanone. Add 6 drops of 
    the Tollens’ reagent to each of the following in the test tubes; propanal or 
    propanone. Warm gently the mixture in a hot water bath for a few minutes. 

    Record your observations in the table below. 

           

    a. Difference in reactivity of Ketones and Aldehydes with Tollens’ reagent

    Aldehydes can also be oxidized into carboxylic ions in basic medium.Tollens’ reagent 
    is a solution of diamminesilver (I) ion, .In order to identify if a 
    substance is aldehyde or ketone, add few drops of Tollens reagent to test tubes 
    containing aldehyde or ketone and warm gently in a hot water bath for a few 
    minutes. The formations of sliver mirror or grey precipitate is an indication of the 

    presence of aldehyde.

                

                   Refer to experimental student book:

                 

    Checking up 6.4.4

    1. Tollens’ reagent is alkaline because of the sodium hydroxide solution 
        and ammonia solution used to make it. What organic product would you 
        get in this case if you reacted propanal with Tollens’ reagent?
    2. Write half equation for the formation of that product from propanal. 
    3. Write the half-equation for the reaction of the   ion when it 
        forms the visible product of the reaction.

     Combine these two half-equations to give an ionic equation for the reaction of 

     Tollens’ reagent with ethanal.

    6.4.5. Oxidation reactions using Fehling ;or Benedict; solution

    Activity 6.4.5

    Materials:
    • Test tubes 
    • Test tubes holder 
    • Test tube racks
    • Count droppers
    • Beakers 
    • Bunsen burner 
    You are provided with the following: ethanal, propanone, Fehling’s solution and 
    Benedict’s solution.
     
    Take about 2ml of each solution. Add 6 drops of the Fehling’s solution 
    or Benedict’s solution to each of the tubes containing 2ml of ethanal or 
    propanone to be tested. Warm gently the mixture in a hot water bath for a few 

    minutes. Record your observations in the table below.

            

    a. Difference in reactivity of Ketones and Aldehydes with Fehling or Benedict 

         solution.

     Fehling’s solution and Benedict’s solution react with aldehyde in the same way;
    both     solutions contain  . In Fehling’s solution  is complexed with tartrate 
    ligand butin Benedict’s solution is complexed with citrate ligand.

    Don’t worry about ligands, important reagents are  ligands tartrate and 
    citrate are used to prevent formation of precipitate copper (II) hydroxide or copper 
    (II) carbonate.

    A few drops of Fehling’s solution or Benedict’s solution is added to the aldehyde or 

    ketone and the mixture is warmed gently in a hot water bath for a few minutes.

            

    Fehling’s solution and Benedict’s solution are oxidizing agent, they oxidize aldehydes 
    to carboxylic acid. Remember that reaction takes place in basic solutions, acid 
    formed is neutralized by base, and hence the products area salt of carboxylic acid 

    instead of carboxylic acid. Equations of reaction.

         

    Checking up 6.4.5

    Fehling’s solution and Benedict’s solution both contain copper (II) complexes 
    in an alkaline solution. The copper (II) complex can be simplified to  (in 

    complex), and the electron-half-equation given as


                 

    a. Write the electron-half-equation for the oxidation of propanal in an 
        alkaline solution.
    b. Combine this with the equation above to give the ionic equation for the 

        reaction between Fehling’s or Benedict’s solution with propanal

    6.4.6. Iodoform reaction with aldehydes and ketones

    Activity 6.4.6
    Materials:
    • Test tubes 
    • Test tubes holder 
    • Test tube racks
    • Count droppers
    • Beakers 
    • Bunsen burner 
    You are provided with the following: propanone, propanal, 6M NaOH solution 
    and  solution
    Put 4 drops of each tested substances, propanone, propanal, into different test 
    tubes. 
    Add to this 0.5 mL distilled water to each test tube. 

    Add 0.25mL 6M NaOH and 0.25 mL of water to each test tube.

    Add 6 drops of -s  solution to each test tube.
    If no precipitate forms immediately, warm the mixture very gently.

    Record your observations in the table below. 

               

    a. Reagents for iodoform reaction 

    There are two different mixtures that can be used to do iodoform test, these 
    mixture are:
    • Iodine and sodium hydroxide solution
    • Potassium iodide and sodium chlorate (I) solutions

    Don’t worry about Potassium iodide and sodium chlorate(I) solutions, Potassium 
    iodide and sodium chlorate(I) react to form final solution 
    .Both mixtures contain the same reagents. 

    Each of these mixtures contains important reagent  which react
    with     aldehyde or ketone. When  is added to a carbonyl compound
    containing     the group (blue in the cycle) as shown below, pale yellow precipitate 

    (triiodomethane) is formed.

                           

    a. Description of iodoform test

    For iodine and sodium hydroxide solution
    Iodine solution, is added to aldehyde or ketone, followed by just enough sodium 

    hydroxide solution to remove the colour of the iodine. If pale yellow precipitate 

    doesn’t form in the cold, it may be necessary to warm the mixture very gently.
    The positive result is pale yellow precipitate of   

    For potassium iodide and sodium chlorate (I) solutions
    Potassium iodide solution is added to a small amount of aldehyde or ketone, followed 
    by sodium chlorate (I) solution. If pale yellow precipitate doesn’t form in the cold, 

    warm the mixture very gently. The positive result is pale yellow precipitate of 

    Reaction of iodoform test

    The reagents of iodoform test are  and OH solution. The reaction takes place into 
    two main steps: 
    • Three hydroxides, OH-, remove three hydrogens from methyl group and the 

    place of hydrogen is taken by iodide

                   

    • group is a good leaving group; is replaced by OH-to form carboxylic 
    acid, because is a base according to Bronsted-Lowry, it reacts with acid to 

    form the following product:

             

     The overall equation for reaction of iodoform test:

           

    The same reaction takes place for other halogen elements in the same way.
    Refer to experimental student book.
         

                 

    When methyl ketones or methyl aldehyde, ethanal, are treated with the halogen 
    element in basic solution, hydrogens of the methyl group are replaced by halogen 
    element followed by cleavage of the methyl group. The products are the salt of 
    carboxylic acid and trihalomethane. The reaction is fast until the 3 hydrogens at the 

    methyl group have been replaced by a halogen.

    Checking up 6.4.6

    A has the formula   Its oxidation gives B with the formula  
    B reacts with 2,4-dinitrophenylhydrazine to give a positive test. A is dehydrated by 
    concentrated . Reductive of C gives butanal.

     Identify the compound A

    6.5. Preparation methods of aldehydes and ketones

    6.5.1. Oxidation of alcohols

    Activity 6.5.1

    The set up below represents the method of preparation of ethanal from ethanol. 

    Look it carefully and answer the following questions.

                     

    Ethanol reacts with  does ethanol undergoes oxidation or 
    reduction in this reaction?     
    3. Write down chemical equation that takes place in this experiment
    4. Explain why it is necessary to heat and explain the point of choosing 
         temperature at which reaction takes place. 

    5. Write a balanced equation of the reaction between propan-2-ol and 

    a. alcohol by 

    Pdichromate (VI) ions is reduced to chromium (III) ions, Cr3+ which is green.otassium dichromate (VI) acidified with dilute sulphuric acid is used as oxidizing 
    agent during the preparation of aldehyde or ketone. Primary alcohol is oxidized to 
    aldehyde, oxygen atom from the oxidising agent removes two hydrogens; one from 
    the -OH group of the alcohol and the other hydrogen comes from the carbon that is 

    attached to hydroxide functional group.

    • Primary alcohol undergoes oxidation to produce aldehyde

          

             • Secondary alcohol undergoes oxidation to produce ketone

                  

    • Tertiary alcohol doesn’t undergo oxidation because the carbon bonded to 

        hydroxide doesn’t have hydrogen to be removed.

                   

    The solution of dichromate (VI) ions,, is orange, during chemical reaction

     dichromate (VI) ions is reduced to chromium (III) ions,  which is green.

            

    b. Technique of stopping oxidation of aldehyde

     The aldehyde produced by oxidation of alcohol could make further oxidation to a 
    carboxylic acid if the acidified potassium dichromate (VI) is still present in solution 
    where reaction takes place. In order to prevent this further oxidation of aldehyde to 
    carboxylic the following technique are used. 

    • Use an excess of the alcohol than potassium dichromate (VI). Potassium 
    dichromate (VI) is limiting reactant hence there isn’t enough oxidising agent 
    present to carry out the second stage of oxidizing the aldehyde formed to a 
    carboxylic acid.

    • Distil off the aldehyde as soon as it forms. Removing the aldehyde as soon as it 
    is formed this means that aldehyde is removed from solution where oxidizing 
    agent is, to prevent further oxidation. Ethanol produces ethanal as shown by 

    the following reaction.

            

      To simplify the writing of the reaction, [O] represents oxygen from an oxidising 

       agent. Then the reaction is written as follows:      

            

    1. If you want to oxidize ethanol to ethanal without further oxidation to 
       ethanoic acid, how do you proceed?
    2. Which oxidising agent is used to oxidize alcohols to either aldehydes or 

         ketones, and what would you observe during the reaction?

    Checking up 6.5

    1. Draw the structure of the aldehyde or ketone that would be formed 
    if each of the following alcohols is oxidised. You can assume that 
    conditions are fulfilled to avoid further oxidation of the aldehyde to a 

    carboxylic acid.

                 

    2. Draw the structure of the alcohol you would oxidize in order to obtain 
    each of the following compounds.
    i. pentan-2-one

    ii. Butanal

    c. Oxidation of alkene by 

    Oxidation of alkenes with hot concentrated acidified potassium manganate (VII) 
    solution produces carbonyl compounds. Consider the general formula of alkene 

    below:

                          

    Where  represent alkyl groups or hydrogen atoms
    Carbon-carbon double bond of alkene is broken by acidified potassium manganate 
    (VII) and is replaced by two carbon-oxygen double bonds to each carbon from 

    double bond. General equation: 

                     

                 

    If acidified potassium manganate (VII) is still present in solution, aldehyde makes 

    further oxidation to carboxylic acid.

                 

           

    Methanoic acid has hydrogen attached on carbonyl group hence it makes further 

    oxidation to carbon dioxide. Final equation is written as below:

        

    6.5.2. Preparation of ketone by distillation of calcium acetate

              

    Procedure: Transfer 15g of calcium acetate in 50ml round bottom flask fixed on 
    a stand, and place it on a heating mantle fitted with a condenser and a receiver 
    flask. Adjust the temperature until the condensation starts. Use the aluminium 
    foil to insulate the flask. Heat the flask and collect the acetone in receiver flask. The 
    obtained product is a crude acetone and needs to be purified. Set up a distillation 
    apparatus and distil the crude product to obtain pure acetone  Do not forget 
    to use stirrer bar which must be placed in the round bottom flask containing the 

    acetone.

                      

    6.6. Uses of aldehydes and ketones

    Aldehydes and ketones have many uses for example in industries such as 

    pharmaceutical industry and in medicine.

    a. Formaldehyde: 

    Formaldehyde is a gas at room temperature but is sold as a 37 percent solution in water.
    Formaldehyde is used as preservative and germicide, fungicide, and insecticide 
    for plants and vegetables. Formaldehyde is mainly used in production of certain 
    polymers like Bakelite (Figure 6.1). Bakelite and formaldehyde is used as 

    monomers in production of Bakelite.

                  

    b. Acetone as solvent:
    Acetone  is soluble in water at all proportions and also dissolves in many organic 
    compounds. Boiling point of acetone is low, 56 °C, which makes it easier to be removed by 
    evaporation. Acetone is an industrial solvent that is used in products such as paints, 
    varnishes, resins, coatings, and nail polish removers. 

    c. Aldehydes and ketones
    Organic molecules that contain ketones or aldehydes functional group are found in 
    different foods such as irish potatoes, yellow bananas. 

    Aldehydes and ketones perform essential functions in humans and other living organisms.
     For     examples sugars, starch, and cellulose, which are formed from simple molecules that
    have aldehyde or ketone functional group.

    d. Aldehydes and ketones in human’s body
                   


    Aldehydes and ketones functional group are found in humans hormones like 

    progesterone, testosterone. 

    6.7. End Unit Assessment 

     1. An aliphatic aldehyde A has the formula RCHO.
    a. A reacts with 2,4-dinitrophenylhydrazine. Explain what happens and 
        name the type of reaction. 

     Say how the product of reaction could be used to identify A

    b. When A is treated with warm, acidified  solution, B is formed. 
    Give the structural formula of B.
    c. When A is treated with lithium tetrahydridoaluminate (reducing agent) 
    in ethoxyethane solution C is formed. Give the structural formula of C. 

    d. A is warmed gently with ammoniacal silver nitrate. Explain what 
        happens, and say what is observed. 
    e. B and C react to form D. Write the structural formula of D.
    f. From the compounds A, B, C, and D, which would you predict to 
       possess:
    i. Highest boiling point 

    ii. Lowest boiling point

    2. Three compounds E, F, and G all have the molecular formula 

          E is an alcohol, F is ketone and G is aldehyde.

    i. Draw all possible structural formulae for E, F, and G. 
    ii. Describe tests (reagents, conditions and observations with each 
    compound) that would allow you to show that. 
    3. E is an alcohol whereas F and G aren’t 
    4. F and G are carbonyl compounds whereas E isn’t
    5. G is aldehyde, whereas E and F aren’t. 
    6. Write balanced equations for all reactions that occur.
    a. One of the compounds responsible for the flavor of butter is butan-2,3-
        dione.
    i. Give the structural formula of butan-2,3-dione.
       Give the structural formula of the organic products formed when butan-2,3-

       dione reacts with 

                

    3. Carbonyl compounds X undergoes the following reactions 

    X gives an orange precipitate with 2, 4-dinitrophenylhydrazine.

    X gives pale yellow precipitate with mixture of potassium iodide and sodium 

    iodate (I) X Doesn’t react with warm acidified  solution.

    X doesn’t react with aqueous bromine.
    X is reduced by hydrogen in the presence of catalyst to a mixture of isomers
    Y and Z of formula  Identify X, and give the structural formulae of X, Y

    and Z.

    4. P has the formula . It forms a compound by reaction with hydrogen 
       cyanide which has the formula . P gives a positive iodoform test, a 
       silver mirror with Tollens’ reagent and can be reduced to pentane. What is P?

    5. a. The carbon-oxygen double bond present in aldehydes and ketones is very 
             polar. What does this mean and how does it arise?
       b. The carbon-oxygen double bond is readily attacked by nucleophiles like 
        cyanide ions or ammonia.

    i. What do you understand by the term nucleophile?
    ii. Which part of the carbon-oxygen double bond is attractive to 
       nucleophiles?

    6. Warfarin is an oral anticoagulant, a drug that inhibits the clotting of blood. 
    It prevents the formation of blood clots by reducing the production of factors 
    by the liver that promote clotting, factors II, VII, IX, and X, and the anticoagulant 

    proteins C and S. The structural formula of Warfarin is:

                        

          a. Name any three different functional groups present in the Warfarin 

               molecule.