Alkanoic Acids

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Welcome to class! 

In today’s class, we will be talking about alkanoic acids. Enjoy the class!

Alkanoic Acids

Sources

The alkanoic acid or carboxylic acids are also called fatty acids because some of them are found in natural fats and oils.  They contain the functional group called carboxyl group.

Nomenclature

The IUPAC name of each homologue is obtained by changing the “-e” ending of the corresponding alkane to “-oic” acid e.g. methanoic, ethanoic etc.

Structure

Alkanoic acid has a general molecular formula of CnH₂n + 1COOH where n > 0. or RCOOH.  Thus it has the following structure.

E.g. Ethanoic acid CH₃COOH

Preparation e.g. Ethanoic acid

Ethanoic acid can be prepared by the complete oxidation of ethanol by acidified sodium heptaoxodichromate (VI) solution.  The oxidation reaction is two stages of reaction

1. Ethanol oxidized to ethanol; CH₃CH₂OH         O₃        CH₃CHO

 

1. Ethanol oxidized to ethanoic acid; CH₃CHO             O₃         CH₃COOH

Or

CH₃CH₂OH  +    [O]                 CH₃CHO  +             [O]              CH₃COOH

Physical properties

1. It is a colourless liquid with a sharp and pungent smell.
2. It has a sour taste.
3. It is soluble in water.
4. It freezes into ice-like at a temperature below 17⁰C, therefore, called glacial ethanoic acid (anhydrous ethanoic acid).
5. It has a boiling point of 118⁰C
6. It turns blue litmus papers to red.

Chemical properties

1. It libratescarbon(IV) oxide from either trioxocarbonate (IV) or hydrogen trioxocarbonate (IV) salts. 2CH₃COOH  + Na₂CO₃    →        2CH₃CONa  +  H₂O + CO₂.
2. It librates hydrogen gas when it reacts with highly electropositive metals e.g. Mg &Ca;       2CH₃COOH + Ca       →         (CH₃COO)₂Ca + H₂.
3. As an acid, it neutralizes boxes or alkalis to form salts known as ethanoate and water only CH₃COOH + NaOH        →          CH₃COONa + H₂O.
4. It reacts with alkanols to form ester e.g. CH₃COOH + CH₃CH₂OH    →          CH₃COOCH₂CH₃ + H₂O
5. Reduction:

It can be reduced to ethanol in the presence of lithium tetrahydro aluminate III as a reducing agent (LiAlH₄)

CH₃COOH  + 4H     →       CH₃CH₂OH + H₂O

6. It reacts with chlorine successively to form chloroethanoic acid e.g.

CH₃COOH       +  Cl₂           →            CH₂C_lCOOH   +    HCl

CH₂ClCOOH   +   Cl₂          →           CHCl₂ COOH  +  HCl

CHCl₂ COOH  +   Cl₂          →           CCl₃COOH     +    HCl

Evaluation

1. (a) State four (4) chemical properties of ethanoic acid.

(b) Give two (2) physical properties of ethanoic acid.

2. How would you prepare ethanoic acid in the laboratory?

Classification of alkanoic acid

Alkanoic acids are classified based on the number of carboxylic groups present per molecules.

1. Monocarboxylic Acids: these have one carboxylic group per molecule e.g. methanoic acid (HCOOH)

Ethanoic acid CH₃ COOH

2.  Dicarboxylic Acids: these have two carboxyl groups per molecules e.g. ethan -1, 2-dioe acid

(oxalic acid)

COOH

or

COOH

3. Tricarboxylic acids: these have 3 carboxylic acids per molecule e.g. 2-hydroxy propan 1,2, 3-tricaboxylic acid.

N.B:    Two important aromatic carboxylic acids are

• Benzoic acid
• 2-hydroxy benzoic acid

Uses of ethanoic acid

1. It is used in making compounds like cellulose ethanoate, dyes etc.
2. It is used as an organic solvent.
3. It is used in the food industries for preserving and flavouring food.
4. Used for coagulating rubber latex.

Evaluation

1. Give three (3) classes of alkanoic acid.
2. State four (4) uses of ethanoic acid.

Alkanoates

General molecular formula

The alkanoates are called esters.  They have a general molecular formula of RCOOR’.

Nomenclature

The naming of alkanoates is obtained by substituting “e” ending in an alkane with “oates” in alkanoates.

Preparation e.g. ethyl ethanoate.

Ethyl ethanoate is prepared by the esterification between ethanol and glaciusethanoic acid at 150⁰C in the presence of concentrated tetraoxosulphate (VI) acid

C₂H₅OH + CH₃COOH          →            CH₃COOC₂H₅ + H₂O

Physical properties

1. Ethyl ethanoate is a colourless volatile liquid with a pleasant smell.
2. It is slightly soluble in water.
3. It has a boiling point of 75⁰C.

Chemical properties

1. Hydrolysis:

Ethyl ethanoate can be hydrolysed by water to produce ethanoic acid and ethanol.

CH₃COOC₂H₅  + H₂O        →               CH₃COOH + C₂H₅OH.

N.B:  If an alkali is used instead of water, it will produce the salt of the acid e.g.

CH₃COOC₂H₅ + NaOH       →            CH₃COONa + C₂H₅OH

2. Reaction with ammonia:

Ethyl ethanoate reacts with ammonia to produce ethanol and then amide

CH₃COOC₂H₃ + NH₃       →             C₂H₅OH + CH₃COOH₂

3. Reduction:

Ethyl ethanoate can be reduced by hydrogen from lithium tetrahydridoaluminate (III) as a reducing agent

CH₃COOC₂H₅ + 4[H]        →             2C₂H₅OH

Uses of alkanoates/esters

1. They are used as food flavours.
2. Used in perfumes and cosmetics
3. Used as a solvent for cellulose nitrate.
4. Used for quick-drying substances like paints, nail varnishes etc.

General evaluation

1. Write the general structure of the ester.
2. Write a balanced equation for the reaction between propanol and butanoic acid.

(a) Name the products formed.

(b) What type of reaction is involved.

Reading assignment

New School Chemistry by Osei Yaw Ababio page.504-509

Weekend assignment

1. The name of (CH₃)₂ CHCOOH is  A. Propane acid    B. 2-methylhutanoic acid   C.   Dimethyl butanoic acid D. Propanoic acid
2. Citric acid appears in unripe orange while ethanoic acid appears in  A. Unripe pawpaw B. Carrot    C.  Vinegar     D.  Rice
3. Esters are employed in the following except.  A. Making perfumes   B.  Making cement    C. Nail varnishes    D.  Making yeast
4. An alkanoic acid reacts reversibly with an alkanol to produce.  A. a salt  B.  an ester    C.  a sugar D.  an alkene
5. Ethan-1, 2-dioe acid is   A. a mineral acid    B.  dicarboxylic acid    C.  citric acid    D.  a soap

Theory

1a.      Give the formula of ethanoic acid and indicate its functional group.

b. Ethanoic acid reacts with both sodium hydroxide and ethanol, suing equations to compare the reactions and classify the products.

2a.      Ethylethanoate reacts with both water and alkali; using an equation to compare the reaction.

b. What happens when ethanoic acid is heated strongly with soda-line.

 

In our next class, we will be talking about Fat and Oil as Higher Esters.  We hope you enjoyed the class.

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