11.5 Alcohols, Aldehydes, and Fatty Acids

11.5.1 Alcohol

• Definition: Alcohols are bio-compounds that contain hydroxyl radicals (-OH). However, some compounds like phenol (CH₆OH) also have hydroxyl groups but are not considered alcohols.

• General Formula: The general formula for alcohols is CₙH₂ₙ₊₁OH, where R represents an alkyl radical. For example, methanol (CH₃OH) and ethanol (C₂H₅OH) are examples of alcohols.

• Nomenclature: Alcohols are named by removing the ‘e’ from the corresponding alkane name and adding ‘-ol’. Example: Ethanol (CH₃CH₂OH).

Preparation of Alcohol:

• From Ethyl Bromide: Ethanol is prepared by reacting ethyl bromide (C₂H₅Br) with sodium hydroxide (NaOH). This process produces ethanol and sodium bromide (NaBr).

• Reaction:

• CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr

• (Bromoethane reacts with sodium hydroxide to produce ethanol).

11.5.2 Aldehyde

• Definition: Aldehydes are bio-compounds that contain an aldehyde group (-CHO). The first member of this class is methanal (HCHO).

• Nomenclature: Aldehydes are named by removing the ‘e’ from the alkane name and adding ‘-al’. Example: Propanal (CH₃CH₂CHO).

Preparation of Aldehyde:

• Addition of Water to Ethyne: Ethanal (acetaldehyde) is produced by adding water (H₂O), 20% sulfuric acid (H₂SO₄), and 2% mercuric sulfate (HgSO₄) to ethyne (C₂H₂) at 80°C.

• Reaction:

• C₂H₂ + H₂O → CH₃CHO

• (Ethyne reacts with water under the conditions to form ethanal).

Formalin: Formalin is a 40% solution of formaldehyde (HCHO) in water. It contains 40% methanol and 60% water and is commonly used to preserve biological specimens in laboratories.

11.5.3 Organic Acids (Fatty Acids)

• Definition: Organic acids, also called fatty acids, contain a carboxyl group (-COOH). The general formula for fatty acids is R-COOH.

• Nomenclature: Fatty acids are named by removing the ‘e’ from the alkane name and adding ‘-oic acid’. Example: Ethanoic acid (CH₃COOH).

Preparation of Fatty Acids:

• From Ethanal: Ethanoic acid is produced by adding potassium dichromate (K₂Cr₂O₇) and sulfuric acid (H₂SO₄) to ethanal (acetaldehyde). The potassium dichromate reacts with sulfuric acid to produce nascent oxygen, which then oxidizes ethanal to form ethanoic acid.

• Reaction:

• K₂Cr₂O₇ + H₂SO₄ → [O] (nascent oxygen)

• CH₃CHO + [O] → CH₃COOH

Chemical Properties of Fatty Acids:

• Acidity: Fatty acids are weak acids that are slightly ionized in solution. They turn blue litmus paper red, indicating their acidic nature.

• Reaction with Alkalis: Fatty acids react with bases (alkalis) to form salts and water. For example, ethanoic acid reacts with sodium hydroxide (NaOH) to form sodium ethanoate (CH₃COONa) and water.

• Reaction:

• CH₃COOH + NaOH → CH₃COONa + H₂O

• (Ethanoic acid reacts with sodium hydroxide to produce sodium ethanoate and water).

Vinegar: Vinegar is a 4% to 10% aqueous solution of ethanoic acid. It is used in cooking and as a preservative due to its mild acidity, which prevents bacterial growth.

11.5.4 Preparation of Alcohol, Aldehyde, and Organic Acids from Hydrocarbons

• Alcohols, aldehydes, and organic acids can be prepared from hydrocarbons, which are the main components of petroleum (alkanes, alkenes, and alkynes).

i) From Alkane (Saturated Hydrocarbon):

• Alkyl Halides: Alkanes react with halogens (e.g., chlorine) in the presence of ultraviolet (UV) light to produce alkyl halides (e.g., chloromethane).

• Alcohol Production: Alkyl halides react with sodium hydroxide (NaOH) to form alcohols. When alcohols are oxidized (using strong oxidants like K₂Cr₂O₇ and H₂SO₄), they first form aldehydes/ketones, which are further oxidized to organic acids.

ii) From Alkene (Unsaturated Hydrocarbon):

• Hydration to Alcohol: Alkenes react with water vapor (H₂O) at high temperature (300°C) and pressure (60 atm) in the presence of phosphoric acid (H₃PO₄) to form alcohols.

• Aldehyde Production from Alkyne: Alkynes (e.g., ethyne) react with water in the presence of mercuric sulfate (HgSO₄) and sulfuric acid (H₂SO₄) to form aldehydes. However, the toxicity of HgSO₄ limits its use in industries.

iii) Oxidation of Hydrocarbons: Alkane hydrocarbons, when oxidized by air (oxygen), at high temperatures and pressures, can produce organic acids.