Chapter 18: Alcohols, Carboxylic Acids, Esters and Macromolecules

Organic Chemistry 2 (Alcohols and Carboxylic Acids)

 

Alcohols

They are a homologous series containing carbon, hydrogen and oxygen with a hydroxyl (-OH) functional group and the general formula C_nH_2n+1OH.

Characteristics

⦁ low melting / boiling points (but higher than alkanes/alkenes)
⦁ soluble in water (but decreasing solubility as carbon chain length increases)
⦁ soluble in organic solvents
⦁ volatile

Formation of alcohols

 

⦁ Addition of steam (Hydration)­­

⦁ Fermentation of glucose

   ⦁  C₆H₁₂O₆ (aq) → 2CO₂ (g) + 2C₂H₅OH (aq)
Reactants: Yeast in aqueous solution
Conditions: 37°C, no light, anaerobic condition

Anaerobic condition is used because yeast will adopt aerobic respiration when oxygen is present, resulting in formation of water and carbon dioxide instead of ethanol and carbon dioxide.

Chemical properties of alcohols

⦁ Combustion

Examples:

⦁ Complete combustion: C₂H₄ (g) + 3O₂ (g) → 2CO₂ (g) + 2H₂O (𝑙)
Reactants: Oxygen
Conditions: –

⦁ Oxidation

Oxidation by acidified potassium manganate(VII)

⦁ C₂H₅OH (aq) + 2[O] → CH₃COOH (aq) + H₂O (𝑙)
*[O] represents oxygen from oxidising agent*

Reactants: Acidified KMnO₄
Conditions: Heat

*Observation: Purple KMnO₄ turns colourless in the presence of alcohols.
Hence, addition of acidified potassium manganate(VII) is a distinguishing test for alcohols.

Oxidation by oxygen in air using bacteria

⦁ C₂H₅OH (aq) + 2[O] → CH₃COOH (aq) + H₂O (𝑙)
*[O] represents oxygen from oxidising agent*

Reactants: Oxygen in the air
Conditions: Action by bacteria in the air

 

Carboxylic Acids

They are a homologous series containing carbon, hydrogen and oxygen with a carboxyl (-COOH) functional group and the general formula C_nH_2n+1COOH.

Characteristics

⦁ low melting / boiling points (but higher than alkanes/alkenes)
⦁ very soluble in water (but decreasing solubility as carbon chain length increases)
⦁ soluble in organic solvents
⦁ weak acid (pH 5 – 6)

Formation of carboxylic acids

 

Oxidation by acidified potassium manganate(VII)

⦁ C₂H₅OH (aq) + 2[O] → CH₃COOH (aq) + H₂O (𝑙)
*[O] represents oxygen from oxidising agent*

Reactants: Acidified KMnO₄
Conditions: Heat

Observation: Purple KMnO₄ turns colourless in the presence of alcohols.

 

Oxidation by oxygen in air using bacteria

⦁ C₂H₅OH (aq) + 2[O] → CH₃COOH (aq) + H₂O (𝑙)
*[O] represents oxygen from oxidising agent*

Reactants: Oxygen in the air

Conditions: Action by bacteria in the air

Chemical properties of carboxylic acids

Carboxylic acids are weak acids which dissociates partially in water to produce low concentrations of H⁺ ions. Hence, they can also undergo the same reactions that strong acids like HC𝑙 and H₂SO₄ react in.

Reaction with metals

⦁ 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

Reactants: Reactive metal
Conditions: –

Reaction with carbonates

⦁ 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Reactants: Any carbonate
Conditions: –

*Observation: Effervescence is observed when a gaseous product is formed.
Hence, addition of a reactive metal OR a carbonate is a distinguishing test for carboxylic acids.

Reaction with bases

⦁ 2CH₃COOH + MgO → (CH₃COO)₂Mg + H₂O

Reactants: Any base
Conditions: –

Question 1 (Bukit Batok Sec)

The table shows the properties of four hydrocarbons.

Which of the following is true?

A Hydrocarbon 3 could be ethene.

B Hydrocarbons 2 and 3 are unsaturated.

C Hydrocarbon 1 is less viscous than hydrocarbon 3.

D Hydrocarbons 1 and 2 are in the same homologous series.

Answer

A is false as although hydrocarbon 3 can decolourise bromine, indicating it belongs to the alkene homologous series, it is not ethene as short-chain alkenes (first 4 members) are gases with low boiling points.

B is false as hydrocarbon 2 cannot decolourise bromine, indicating the absence of C=C bonds, thus it is saturated.

C is true as hydrocarbon 3, which is a liquid at room temperature, would be more difficult to flow and thus more viscous than hydrocarbon 1, which is a gas at room temperature.

D is false as hydrocarbon 1 can decolourise bromine, indicating presence of C=C bonds, while hydrocarbon 2 cannot decolourise bromine, indicating no C=C bonds are present. Thus, hydrocarbon 1 is an alkene while hydrocarbon 2 is not.

Esters

Esters are sweet smelling, colourless liquid with the ester (-COO-) functional group. They are formed when reacting a carboxylic acid and an alcohol together. This reaction is known as esterification.

The naming of esters is derived from the carboxylic acid and alcohol used to make the ester. The first part comes from the alcohol and the second part comes from the carboxylic acid. Using the reaction above, the esterification of methanol and ethanoic acid will form an ester called methyl ethanoate.

 

Question (Singapore Chinese Girls Sec)

Aspirin is a drug which is used as a general painkiller. The structural formula of aspirin is shown below:

Answer

A is not the answer. Aspirin contains oxygen, hydrogen and carbon. Thus, it is not a hydrocarbon.

B is the answer as without an hydroxyl (-OH) functional group present, aspirin cannot decolourise purple acidified aqueous potassium manganate (VII).

C is not the answer as there is an ester linkage (-COO-) present, indicating that aspirin is formed by joining an alcohol and a carboxylic acid together.

D is not the answer as the presence of a carboxyl (-COOH) functional group gives aspirin acidic properties, allowing it to react with sodium carbonate to form effervescence in the form of CO₂.

 

Macromolecules

It is a large molecule formed by linking many small repeating units (monomers).

 

The process of joining many monomers to form macromolecules is known as polymerisation.

 

Addition polymerisation

A process whereby a large number of monomers join together without losing any molecules or atoms to form a polymer. Typically, only one type of monomer is used and alkenes are used as monomers for addition polymerisation.

Condensation polymerisation

A process whereby a large number of monomers join together with the elimination of small molecules (e.g. H₂O, HC𝑙) to form a polymer. Typically, two different monomers with reactive functional groups at the ends of the monomers are used, forming either polyamides or polyesters.

 

Nylon is a man-made fiber that is light yet strong, allowing it to be stretched, and waterproof.
Uses: ropes, fishing lines, zippers, tents, parachutes and raincoats.

 

Terylene is a man-made fiber that is shrink-proof and crease-proof, allowing it to be washed and dried easily
Uses: woven into cloth for clothes and curtains

 

Question (Singapore Chinese Girls Sec)

The structure of the polymer is shown below.

Answer

The polymer does not have any amide or ester linkage in its main carbon chain, suggesting that it was formed via addition polymerisation of alkenes, thus A and C cannot be the answer.

D is the answer as by splitting the monomer in half right at the center, we can obtain 2 repeating units of the polymer, which has 4 carbon atoms per repeating unit (C₄H₈).