Organic Chemistry: ALKANOIC ACIDS (Carboxylic acids)

ALKANOIC ACIDS (Carboxylic acids)

Alkanoic acids belong to a homologous series of organic compounds with a general formula C_nH_{2n +1} COOH and thus  -COOH  as the functional group .The 1^st ten alkanoic acids include:

Alkanoic acids like alkanols /alkanes/alkenes/alkynes form a homologous series where:

(i)the general name of an alkanoic acids is derived from the alkane name then ending with “–oic”  acid as the table above shows.

(ii) the members have R-COOH/R    C-O-H    as the functional  group.

                                                         O

(iii)they have the same general formula represented by R-COOH where R is an alkyl group.

(iv)each member differ by –CH₂– group from the next/previous.

(v)they show a similar and gradual change in their physical properties e.g. boiling and melting point.

(vi)they show similar and gradual change in their chemical properties.

(vii) since they are acids they show similar properties with mineral acids.

  (B) ISOMERS OF ALKANOIC ACIDS.

lkanoic acids exhibit both structural and position isomerism. The isomers are named by using the following basic guidelines

(i)Like alkanes. identify the longest carbon chain to be the parent name.

(ii)Identify the position of the   -C-O-H      functional  group to give it the smallest

                                        O

/lowest position.

 (iii)Identify the type and position of the side group branches.

Practice examples on isomers of alkanoic acids

1.Isomers of butanoic acid C₃H₇COOH

  CH₃ CH₂ CH₂ COOH                    

   Butan-1-oic acid

          CH₃

H₂C    C      COOH   2-methylpropan-1-oic acid

2-methylpropan-1-oic acid and Butan-1-oic acid are structural isomers because the position of the functional group does not change but the arrangement of the atoms in the molecule does.

 2.Isomers of pentanoic acid C₄H₉COOH

CH₃CH₂CH₂CH₂ COOH        pentan-1-oic acid

                                   CH₃

                       CH₃CH₂CH COOH   2-methylbutan-1-oic acid

                              CH₃

H₃C     C    COOH                2,2-dimethylpropan-1-oic acid

                               CH₃

3.Ethan-1,2-dioic acid      

                                                            O    O

HOOC- COOH      //               H – O – C – C – O – H

4.Propan-1,3-dioic acid

  O    H   O

HOOC- CH₂COOH      //                 H – O – C – C – C – O – H

                                                                    H

5.Butan-1,4-dioic acid

                                                            O    H    H   O

     HOOC CH₂ CH₂ COOH              H- O – C – C – C – C –O – H

                                                                   H    H

6.2,2-dichloroethan-1,2-dioic acid

HOOCCHCl₂                                                     Cl                                         

                                      H – O –  C – C – Cl

                                                    O   H

(C) LABORATORY AND INDUSTRIAL PREPARATIONOF ALKANOIC ACIDS.

In a school laboratory, alkanoic acids can be prepared by adding an oxidizing agent (H⁺/KMnO₄ or H⁺/K₂Cr₂O₇)to the corresponding alkanol then warming.

The oxidation converts the alkanol first to an alkanal the alkanoic acid.

NB Acidified KMnO₄ is a stronger oxidizing agent than acidified K₂Cr₂O₇

General equation:

 R- CH₂ – OH      + [O]          –H⁺/KMnO₄–>   R- CH –O   +       H₂O(l)

     (alkanol)                                                                   (alkanal)

R- CH – O +    [O]       –H⁺/KMnO₄–>   R- C –OOH

     (alkanal)                                                         (alkanoic acid)

Examples

1.Ethanol on warming in acidified KMnO₄ is oxidized to ethanal then ethanoic acid .

CH₃– CH₂ – OH + [O]          –H⁺/KMnO₄–>   CH₃– CH –O        +       H₂O(l)

     (ethanol)                                                                   (ethanal)

CH₃– CH – O       +    [O]       –H⁺/KMnO₄–>   CH₃– C –OOH

     (ethanal)                                                         (ethanoic acid)

2Propanol on warming in acidified KMnO₄ is oxidized to propanal then propanoic acid

CH₃– CH₂ CH₂ – OH    + [O] –H⁺/KMnO₄–>   CH₃– CH₂ CH –O          +       H₂O(l)

     (propanol)                                                           (propanal)

CH₃– CH – O       +    [O]       –H⁺/KMnO₄–>   CH₃– C –OOH

     (propanal)                                                      (propanoic acid)

Industrially,large scale manufacture of alkanoic acid like ethanoic acid is obtained from:

(a)Alkenes reacting with steam at high temperatures and pressure in presence of phosphoric(V)acid catalyst and undergo hydrolysis to form alkanols. i.e.

Alkenes + Steam/water — H₂PO₄ Catalyst–> Alkanol

The alkanol is then oxidized by air at 5 atmosphere pressure  with Manganese (II)sulphate(VI) catalyst to form the alkanoic acid.

Alkanol  +  Air    — MnSO₄ Catalyst/5 atm pressure–> Alkanoic acid

Example

Ethene is mixed with steam over a phosphoric(V)acid catalyst,300oC temperature and 60 atmosphere pressure to form ethanol.

CH₂=CH₂  +    H₂O   ->  CH₃ CH₂OH

(Ethene)                        (Ethanol)

This is the industrial large scale method of manufacturing ethanol

Ethanol is then oxidized  by air at 5 atmosphere pressure with Manganese (II)sulphate(VI) catalyst to form the ethanoic acid.

CH₃ CH₂OH        +       [O] — MnSO₄ Catalyst/5 atm pressure–>  CH₃ COOH

(Ethanol)                                                                                      (Ethanoic acid)

(b)Alkynes react with liquid water at high temperatures and pressure in presence of Mercury(II)sulphate(VI)catalyst and 30% concentrated sulphuric(VI)acid to form alkanals.

Alkyne       +       Water — Mercury(II)sulphate(VI)catalyst–> Alkanal

The alkanal is then oxidized by air at 5 atmosphere pressure with Manganese (II) sulphate(VI) catalyst to form the alkanoic acid.

Alkanal      + air/oxygen — Manganese(II)sulphate(VI)catalyst–> Alkanoic acid

Example

Ethyne react with liquid water at high temperature and pressure with Mercury (II) sulphate (VI)catalyst and 30% concentrated sulphuric(VI)acid to form ethanal.

CH   =   CH         +    H₂O   –HgSO₄–>  CH₃ CH₂O

(Ethyne)                                                     (Ethanal)

This is another industrial large scale method of manufacturing ethanol from large quantities of ethyne found in natural gas.

 Ethanal is then oxidized  by air at 5 atmosphere pressure with Manganese (II)sulphate(VI) catalyst to form the ethanoic acid.

CH₃ CH₂O +       [O] — MnSO₄ Catalyst/5 atm pressure–>  CH₃ COOH

(Ethanal)   (Oxygen from air)                                           (Ethanoic acid)

(D) PHYSICAL AND CHEMICAL PROPERTIES OF ALKANOIC ACIDS.

I.Physical properties of alkanoic acids

The table below shows some physical properties of alkanoic acids

From the table note the following:

• Melting and boiling point decrease as the carbon chain increases due to increase in intermolecular forces of attraction between the molecules requiring more energy to separate the molecules.
• The density decreases as the carbon chain increases as the intermolecular forces of attraction increases between the molecules making the molecule very close reducing their volume in unit mass.
• Solubility decreases as the carbon chain increases as the soluble –COOH end is shielded by increasing insoluble alkyl/hydrocarbon chain.
• Like alkanols ,alkanoic acids exist as dimmers due to the hydrogen bonds within the molecule. i.e..

II Chemical properties of alkanoic acids

The following experiments shows the main chemical properties of ethanoic (alkanoic) acid.

          (a)Effect on litmus papers

Experiment

Dip both blue and red litmus papers in ethanoic acid. Repeat with a solution of succinic acid, citric acid, oxalic acid, tartaric acid and dilute nitric(V)acid.

Sample observations

Explanation

All acidic solutions  contains H⁺/H₃O⁺(aq) ions. The H⁺ /H₃O⁺ (aq) ions is responsible for turning blue litmus paper/solution to red

(b)pH

Experiment

Place 2cm3 of ethaoic acid in a test tube. Add 2 drops of universal indicator solution and determine its pH. Repeat with a solution of succinic acid, citric acid, oxalic acid, tartaric acid and dilute sulphuric (VI)acid.

Sample observations

Explanations

Alkanoic acids are weak acids that partially/partly dissociate to release few H⁺ ions in solution. The pH of their solution is thus 4/5/6 showing they form weakly acidic solutions when dissolved in water.

All alkanoic acid dissociate to releases the “H” at the functional group in -COOH to form the alkanoate ion; –COO^–

Mineral acids(Sulphuric(VI)acid, Nitric(V)acid and Hydrochloric acid) are strong acids that wholly/fully dissociate to release many H⁺ ions in solution. The pH of their solution is thus 1/2/3 showing they form strongly acidic solutions when dissolved in water.i.e

Examples

1. CH₃COOH(aq)                       CH₃COO^–(aq)       +      H⁺(aq)

(ethanoic acid)                        (ethanoate ion)              (few H⁺ ion)

• CH₃ CH₂COOH(aq)                         CH₃ CH₂COO^–(aq)  +    H⁺(aq)

(propanoic acid)                               (propanoate ion)         (few H⁺ ion)

• CH₃ CH₂ CH₂COOH(aq)                    CH₃ CH₂ CH₂COO^–(aq)  + H⁺(aq)

(Butanoic acid)                                     (butanoate ion)              (few H⁺ ion)

•  HOOH(aq)                                          HOO^–(aq)          +      H⁺(aq)

(methanoic acid)                       (methanoate ion)                   (few H⁺ ion)

• H₂ SO₄ (aq)                                                SO₄^2- (aq)   +      2H⁺(aq)

(sulphuric(VI) acid)                         (sulphate(VI) ion)         (many  H⁺ ion)

• HNO₃ (aq)                                        NO₃^– (aq)     +      H⁺(aq)

(nitric(V) acid)                            (nitrate(V) ion)                (many  H⁺ ion)

          (c)Reaction with metals

Experiment

Place about 4cm3 of ethanoic acid in a test tube. Put about 1cm length of polished magnesium ribbon. Test any gas produced using a burning splint. Repeat with a solution of succinic acid, citric acid, oxalic acid, tartaric acid and dilute sulphuric (VI) acid.

Sample observations

Explanation

Metals higher in the reactivity series displace the hydrogen in all acids to evolve/produce hydrogen gas and form a salt. Alkanoic acids react with metals with metals to form alkanoates salt and produce/evolve hydrogen gas .Hydrogen extinguishes a burning splint with a pop sound/explosion. Only the “H”in the functional group  -COOH is /are displaced and not in the alkyl hydrocarbon chain.

Alkanoic acid  +  Metal   ->   Alkanoate   +   Hydrogen gas. i.e.

Examples

1. For a monovalent metal with monobasic acid

2R – COOH    +  2M       ->   2R- COOM   +   2H₂(g)

  2.For a divalent metal with monobasic acid

 2R – COOH    +   M       ->   (R- COO) ₂M   +   H₂(g)

 3.For a divalent metal with dibasic acid   

HOOC-R-COOH+   M       ->  MOOC-R-COOM  +   H₂(g)

 4.For a monovalent metal with dibasic acid   

HOOC-R-COOH+   2M       ->  MOOC-R-COOM  +   H₂(g)

5 For mineral acids

  (i)Sulphuric(VI)acid is a dibasic acid 

          H₂ SO₄ (aq)   + 2M     ->  M₂ SO₄ (aq) + H₂(g)

          H₂ SO₄ (aq)   + M     ->    MSO₄ (aq)    + H₂(g)

    (ii)Nitric(V) and hydrochloric acid are  monobasic acid

          HNO₃ (aq)   + 2M     ->  2MNO₃ (aq) + H₂(g)

          HNO₃ (aq)   +  M     ->   M(NO₃ ) ₂ (aq) + H₂(g)

Examples

 1.Sodium reacts with ethanoic acid to form sodium ethanoate and produce. hydrogen gas.

Caution: This reaction is explosive.

         CH₃COOH (aq)   +   Na(s)  ->  CH₃COONa (aq)  + H₂(g)

          (Ethanoic acid)                    (Sodium ethanoate)

2.Calcium reacts with ethanoic acid to form calcium ethanoate and produce. hydrogen gas.

2CH₃COOH (aq)   +   Ca(s)  ->  (CH₃COO) ₂Ca (aq)  + H₂(g)

          (Ethanoic acid)                    (Calcium ethanoate)

3.Sodium reacts with ethan-1,2-dioic acid to form sodium ethan-1,2-dioate and produce. hydrogen gas.

HOOC-COOH+   2Na       ->  NaOOC – COONa  +   H₂(g)

(ethan-1,2-dioic acid)             (sodium ethan-1,2-dioate)

Commercial name of ethan-1,2-dioic acid is oxalic acid. The salt is sodium oxalate.

4.Magnesium reacts with ethan-1,2-dioic acid to form magnesium ethan-1,2-dioate and produce. hydrogen gas.

HOOC-R-COOH+   Mg       ->  ( OOC – COO)Mg  +   H₂(g)

(ethan-1,2-dioic acid)             (magnesium ethan-1,2-dioate)

5.Magnesium reacts with

(i)Sulphuric(VI)acid to form Magnesium sulphate(VI)

          H₂ SO₄ (aq)   + Mg     ->  MgSO₄ (aq) + H₂(g)

(ii)Nitric(V) and hydrochloric acid are  monobasic acid

          2HNO₃ (aq)   +  Mg     ->   M(NO₃ ) ₂ (aq) + H₂(g)

(d)Reaction with hydrogen carbonates and carbonates

Experiment

Place  about 3cm3 of ethanoic acid in a test tube. Add about 0.5g/ ½ spatula end full of sodium hydrogen carbonate/sodium carbonate. Test the gas produced using lime water. Repeat with a solution of succinic acid, citric acid, oxalic acid, tartaric acid and dilute sulphuric (VI) acid.

Sample observations

All acids react with hydrogen carbonate/carbonate to form salt ,water and evolve/produce bubbles of carbon(IV)oxide and water.

Carbon(IV)oxide forms a white precipitate when bubbled in lime water/extinguishes a burning splint.

Alkanoic acids react with hydrogen carbonate/carbonate to form alkanoates ,water and evolve/produce bubbles of carbon(IV)oxide and water.

Alkanoic acid + hydrogen carbonate -> alkanoate + water + carbon(IV)oxide

Alkanoic acid +   carbonate      ->     alkanoate   +   water   + carbon(IV)oxide

Examples

1. Sodium hydrogen carbonate reacts with ethanoic acid to form sodium ethanoate ,water and carbon(IV)oxide gas.

CH₃COOH (aq)   +   NaHCO₃ (s)  ->  CH₃COONa (aq)  + H₂O(l) + CO₂ (g)

          (Ethanoic acid)                    (Sodium ethanoate)

2.Sodium carbonate reacts with ethanoic acid to form sodium ethanoate ,water and carbon(IV)oxide gas.

2CH₃COOH (aq)   +   Na₂CO₃ (s)  -> 2CH₃COONa (aq)  + H₂O(l) + CO₂ (g)

          (Ethanoic acid)                         (Sodium ethanoate)

3.Sodium carbonate reacts with ethan-1,2-dioic acid to form sodium ethanoate ,water and carbon(IV)oxide gas.

HOOC-COOH+   Na₂CO₃ (s)       ->  NaOOC – COONa  + H₂O(l) + CO₂ (g)

(ethan-1,2-dioic acid)             (sodium ethan-1,2-dioate)

4.Sodium hydrogen carbonate reacts with ethan-1,2-dioic acid to form sodium ethanoate ,water and carbon(IV)oxide gas.

HOOC-COOH+   2NaHCO₃ (s)       ->  NaOOC – COONa  + H₂O(l) + 2CO₂ (g)

(ethan-1,2-dioic acid)             (sodium ethan-1,2-dioate)

(e)Esterification

Experiment

Place 4cm3 of ethanol acid in a boiling tube.

 Add equal volume of ethanoic acid. To the mixture, add 2 drops of concentrated sulphuric(VI)acid carefully. Warm/heat gently on Bunsen flame.

Pour the mixture into a beaker containing 50cm3 of water. Smell the products. Repeat with a solution of succinic acid, citric acid, oxalic acid, tartaric acid and dilute sulphuric (VI) acid.

Sample observations

Explanation

Alkanols react with alkanoic acid to form the sweet smelling homologous series of esters and water.The reaction is catalysed by concentrated sulphuric(VI)acid in the laboratory but naturally by sunlight /heat.Each ester has a characteristic smell derived from the many possible combinations of alkanols and alkanoic acids.

          Alkanol      +       Alkanoic acids     ->      Ester +      water

Esters derive their names from the alkanol first then alkanoic acids. The alkanol “becomes” an alkyl group and the alkanoic acid “becomes” alkanoate hence alkylalkanoate. e.g.

          Ethanol       +       Ethanoic acid       ->      Ethylethanoate      +    Water

          Ethanol       +       Propanoic acid     ->      Ethylpropanoate    +    Water

          Ethanol       +       Methanoic acid    ->      Ethylmethanoate   +     Water

          Ethanol       +       butanoic acid       ->      Ethylbutanoate      +     Water

          Propanol     +       Ethanoic acid       ->      Propylethanoate  +     Water

          Methanol    +       Ethanoic acid       ->      Methyethanoate   +     Water

          Methanol    +       Decanoic acid      ->      Methyldecanoate +     Water

          Decanol      +       Methanoic acid    ->      Decylmethanoate          +      Water

During the formation of the ester, the “O” joining the alkanol and alkanoic acid comes from the alkanol.

          R₁  -COOH   +    R₂ –OH     ->    R₁  -COO –R₂    +  H₂O

Examples

1. Ethanol reacts with ethanoic acid to form the ester ethyl ethanoate and water.

          Ethanol       + Ethanoic acid    –Conc. H₂SO₄ –>Ethylethanoate + Water

         C₂H₅OH (l) + CH₃COOH(l) –Conc. H₂SO₄ –>  CH₃COO C₂H₅(aq)   +H₂O(l)

CH₃CH₂OH (l)+ CH₃COOH(l) –Conc. H₂SO₄ –>  CH₃COOCH₂CH₃(aq)   +H₂O(l)

2. Ethanol reacts with propanoic acid to form the ester ethylpropanoate and water.

          Ethanol       + Propanoic acid –Conc. H₂SO₄ –>Ethylethanoate + Water

C₂H₅OH (l)+ CH₃ CH₂COOH(l) –Conc. H₂SO₄ –>CH₃CH₂COO C₂H₅(aq)   +H₂O(l)

CH₃CH₂OH (l)+ CH₃ CH₂COOH(l) –Conc. H₂SO₄ –>

 CH₃ CH₂COOCH₂CH₃(aq)   +H₂O(l)

3. Methanol reacts with ethanoic acid to form the ester methyl ethanoate and water.

          Methanol    + Ethanoic acid    –Conc. H₂SO₄ –>Methylethanoate + Water

         CH₃OH (l)  + CH₃COOH(l) –Conc. H₂SO₄ –>  CH₃COO CH₃(aq)   +H₂O(l)

4. Methanol reacts with propanoic acid to form the ester methyl propanoate and water.

          Methanol    + propanoic acid –Conc. H₂SO₄ –>Methylpropanoate + Water

 CH₃OH (l)+ CH₃ CH₂COOH(l) –Conc. H₂SO₄ –> CH₃ CH₂COO CH₃(aq)   +H₂O(l)

5. Propanol reacts with propanoic acid to form the ester propylpropanoate and water.

          Propanol     + Propanoic acid –Conc. H₂SO₄ –>Ethylethanoate + Water

C₃H₇OH (l)+ CH₃ CH₂COOH(l) –Conc. H₂SO₄ –>CH₃CH₂COO C₃H₇(aq)   +H₂O(l)

CH₃CH₂ CH₂OH (l)+ CH₃ CH₂COOH(l) –Conc. H₂SO₄ –> CH₃ CH₂COOCH₂ CH₂CH₃(aq)   +H₂O(l)

C. DETERGENTS

Detergents are cleaning agents that improve the cleaning power /properties of water.A detergent therefore should be able to:

          (i)dissolve substances which water cannot e.g grease ,oil, fat

          (ii)be washed away after cleaning.

There are two types of detergents:

          (a)Soapy detergents

          (b)Soapless detergents

• SOAPY DETERGENTS

Soapy detergents usually called soap is long chain salt of organic alkanoic acids.Common soap is sodium octadecanoate .It is derived from reacting concentrated sodium hydroxide solution with octadecanoic acid(18 carbon alkanoic acid) i.e.

 Sodium hydroxide  +  octadecanoic acid   ->   Sodium octadecanoate   +   water

NaOH(aq) + CH₃ (CH₂) ₁₆ COOH(aq)       -> CH₃ (CH₂) ₁₆ COO ^–  Na⁺  (aq) +H₂ O(l)

Commonly ,soap can thus be represented ;

                   R-COO ^–  Na⁺      where;

R is a long chain alkyl group and   -COO ^–  Na⁺   is the alkanoate ion.

In a school laboratory and at industrial and domestic level,soap is made by reacting concentrated sodium hydroxide solution with esters from (animal) fat and oil. The process of making soap is called saponification. During saponification ,the ester is hydrolyzed by the alkali to form sodium salt /soap and glycerol/propan-1,2,3-triol is produced.

Fat/oil(ester)+sodium/potassium hydroxide->sodium/potassium salt(soap)+ glycerol

Fats/Oils are esters with fatty acids and glycerol parts in their structure;

                             C₁₇H₃₅COOCH₂

                             C₁₇H₃₅COOCH

                             C₁₇H₃₅COOCH₂

When boiled with concentrated sodium hydroxide solution NaOH;

          (i)NaOH ionizes/dissociates into Na⁺  and  OH^– ions

          (ii)fat/oil split into three C₁₇H₃₅COO^– and one CH₂ CH CH₂

(iii) the three Na⁺ combine with the three C₁₇H₃₅COO^– to form the salt     C₁₇H₃₅COO^– Na⁺

(iv)the three OH^–ions combine with the CH₂ CH CH₂ to form an alkanol with  three functional groups CH₂ OH CH OH CH₂ OH(propan-1,2,3-triol)

C₁₇H₃₅COOCH₂                                                                                                    CH₂OH                                                                                                               

C₁₇H₃₅COOCH    +NaOH -> 3 C₁₇H₃₅COO^– Na⁺  +             CHOH

C₁₇H₃₅COOCH₂                                                                                                    CH₂OH

Ester                    Alkali         Soap                              glycerol

Generally:

C_nH_2n+1COOCH₂                                                                                                                  CH₂OH                                                                                                               

C_nH_2n+1COOCH    +NaOH -> 3 C_nH_2n+1COO^– Na⁺  + CHOH

C_nH_2n+1COOCH₂                                                                                                 CH₂OH

Ester                    Alkali         Soap                              glycerol

      R – COOCH₂                                                                                                   CH₂OH                                                                                                               

      R – COOCH    +NaOH -> 3R-COO^– Na⁺           +                CHOH

       R- COOCH₂                                                                                                  CH₂OH

         Ester             Alkali       Soap                              glycerol

During this process a little sodium chloride is added to precipitate the soap by reducing its solubility. This is called salting out.

The soap is then added colouring agents ,perfumes and herbs of choice.

          School laboratory preparation of soap

Place about 40 g of fatty (animal fat)beef/meat in 100cm3  beaker .Add about 15cm3 of 4.0M sodium hydroxide solution. Boil the mixture for about 15minutes.Stir the mixture .Add about 5.0cm3 of distilled water as you boil to make up for evaporation. Boil for about another 15minutes.Add about four spatula end full of pure sodium chloride crystals. Continue stirring for another five minutes. Allow to cool. Filter of /decant and wash off the residue with distilled water .Transfer the clean residue into a dry beaker. Preserve.

The action of soap

Soapy detergents:

          (i)act by reducing the surface tension of water by forming a thin layer on top of the water.

          (ii)is made of a non-polar alkyl /hydrocarbon tail and a polar  -COO^–Na⁺ head. The non-polar alkyl /hydrocarbon tail is hydrophobic (water hating) and thus does not dissolve in water .It dissolves in non-polar solvent like grease, oil and fat. The polar  -COO^–Na⁺ head is hydrophilic (water loving)and thus dissolve in water. When washing with soapy detergent, the non-polar tail of the soapy detergent surround/dissolve in the dirt on the garment /grease/oil while the polar head dissolve in water.

Through mechanical agitation/stirring/sqeezing/rubbing/beating/kneading, some  grease is dislodged/lifted of the surface of the garment. It is immediately surrounded by more soap molecules It float and spread in the water as tiny droplets that scatter light in form of emulsion making the water cloudy and shinny. It is  removed from the garment by rinsing with fresh water. The repulsion of the soap head prevent /ensure the droplets do not mix. Once removed, the dirt molecules cannot be redeposited back because it is surrounded by soap molecules.

Advantages and disadvantages of using soapy detergents

Soapy detergents are biodegradable. They are acted upon by bacteria and rot. They thus do not cause environmental pollution.

Soapy detergents have the disadvantage in that:

          (i)they are made from fat and oils which are better eaten as food than make soap.

          (ii)forms an insoluble precipitate with hard water called scum. Scum is insoluble calcium octadecanoate and Magnesium octadecanoate  formed when soap reacts with Ca²⁺ and Mg²⁺ present in hard water.

Chemical equation

2C₁₇H₃₅COO^– Na⁺ (aq)    +   Ca²⁺(aq)    -> (C₁₇H₃₅COO^– )Ca²⁺ (s)  +     2Na⁺(aq)

                                                    (insoluble Calcium octadecanote/scum)

2C₁₇H₃₅COO^– Na⁺ (aq)    +   Mg²⁺(aq)    -> (C₁₇H₃₅COO^– )Mg²⁺ (s)  +     2Na⁺(aq)

                                                    (insoluble Magnesium octadecanote/scum)

This causes wastage of soap.

Potassium soaps are better than Sodium soap. Potassium is more expensive than sodium and thus its soap is also more expensive.

(b)SOAPLESS DETERGENTS

Soapless detergent usually called detergent is a long chain salt fromed from by-products of fractional distillation of crude oil.Commonly used soaps include:

          (i)washing agents

          (ii)toothpaste

          (iii)emulsifiers/wetting agents/shampoo

 Soapless detergents are derived from reacting:

          (i)concentrated sulphuric(VI)acid with a long chain alkanol e.g. Octadecanol(18 carbon alkanol) to form alkyl hydrogen sulphate(VI)

Alkanol   +  Conc sulphuric(VI)acid  ->  alkyl hydrogen sulphate(VI)  + Water

    R –OH  +         H₂SO₄           -> R –O-SO₃H     +      H₂O

          (ii)the alkyl hydrogen sulphate(VI) is then neutralized with sodium/potassium hydroxide to form sodium/potassium alkyl hydrogen sulphate(VI)

Sodium/potassium alkyl hydrogen sulphate(VI) is the soapless detergent.

alkyl hydrogen         +    Potassium/sodium  ->        Sodium/potassium      +      Water

 sulphate(VI)                     hydroxide                  alkyl hydrogen sulphate(VI)

R –O-SO₃H              +      NaOH                   ->          R –O-SO₃^– Na⁺        +      H₂O

Example

Step I : Reaction of Octadecanol with Conc.H₂SO₄

   C₁₇H₃₅CH₂OH(aq)  +  H₂SO₄          ->  C₁₇H₃₅CH₂–O- SO₃^– H⁺(aq)   +   H₂O(l)

     octadecanol + sulphuric(VI)acid -> Octadecyl hydrogen sulphate(VI) + water

Step II: Neutralization  by an alkali

 C₁₇H₃₅CH₂–O- SO₃^– H⁺(aq) +  NaOH       ->  C₁₇H₃₅CH₂–O- SO₃^– Na⁺(aq)  +   H₂O(l)

Octadecyl hydrogen  + sodium/potassium ->     sodium/potassium octadecyl+Water

sulphate(VI)                            hydroxide                 hydrogen sulphate(VI)

School laboratory preparation of soapless detergent

Place about 20g of olive oil in a 100cm3 beaker. Put  it in a trough containing ice cold water.

Add dropwise carefully 18M concentrated sulphuric(VI)acid stirring continuously into the olive oil until the oil turns brown.Add 30cm3 of 6M sodium hydroxide solution.Stir.This is a soapless detergent.

The action of soapless detergents

The action of soapless detergents is similar to that of soapy detergents.The soapless detergents contain the hydrophilic head and a long hydrophobic tail. i.e.

 vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv-COO^–Na⁺

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv-O-SO₃^– Na⁺

(long hydrophobic /non-polar alkyl tail)     (hydrophilic/polar/ionic head)

The tail dissolves in fat/grease/oil while the ionic/polar/ionic head dissolves in water.

The tail stick to the dirt which is removed by the attraction of water molecules and the polar/ionic/hydrophilic head by mechanical agitation /squeezing/kneading/ beating/rubbing/scrubbing/scatching.

The suspended dirt is then surrounded by detergent molecules and repulsion of the anion head preventing the dirt from sticking on the material garment.

The tiny droplets of dirt emulsion makes the water cloudy. On rinsing the cloudy emulsion is washed away.

Advantages and disadvantages of using soapless detergents

 Soapless detergents are non-biodegradable unlike soapy detergents.

They persist in water during sewage treatment by causing foaming in rivers ,lakes and streams leading to marine /aquatic death.

Soapless detergents have the advantage in that they:

          (i)do not form scum with hard water.

          (ii)are cheap to manufacture/buying

(iii)are made from petroleum products but soapis made from fats/oil for    human consumption.