Question

Palmitic acid (hexadecanoic acid) is the source of the hexadecyl (cetyl) group in the following compounds. Each is a mild surface-acting germicide and fungicide and is used as a topical antiseptic and disinfectant. They are examples of quaternary ammonium detergents, commonly called "quats." (a) Cetylpyridinium chloride is prepared by treating pyridine with 1 -chlorohexadecane (cetyl chloride). Show how to convert palmitic acid to cetyl chloride. (b) Benzylcetyldimethylammonium chloride is prepared by treating benzyl chloride with \(N\), \(N\)-dimethyl-1-hexadecanamine. Show how this tertiary amine can be prepared from palmitic acid.

Short answer

Question: Describe the synthesis of two different quaternary ammonium detergents starting from palmitic acid, specifying the reactions to convert the functional groups. Answer: Two quaternary ammonium detergents can be synthesized from palmitic acid by mainly converting its carboxylic acid group to other functional groups. In the first part, palmitic acid is converted to cetyl chloride by treating it with thionyl chloride (SOCl2), resulting in the formation of Cetylpyridinium chloride. In the second part, palmitic acid is reduced to form hexadecanamine using LiAlH4. This primary amine is then converted to a tertiary amine, N, N-dimethyl-1-hexadecanamine, by treating it with excess methyl iodide (CH3I) in the presence of a base. Finally, Benzylcetyldimethylammonium chloride is synthesized.

Step-by-step solution

Write the structures of palmitic acid and cetyl chloride

Palmitic acid: CH3(CH2)14COOH Cetyl chloride: CH3(CH2)15Cl

Convert the carboxylic acid group to the acid chloride group

To convert the carboxylic acid (COOH) group in palmitic acid to the acid chloride (COCl) group in cetyl chloride, we need to treat palmitic acid with thionyl chloride (SOCl2). The reaction proceeds as follows: Palmitic acid + SOCl2 -> Cetyl chloride + SO2 + HCl The balanced reaction is: CH3(CH2)14COOH + SOCl2 -> CH3(CH2)15Cl + SO2 + HCl (b) Preparing N, N-dimethyl-1-hexadecanamine from palmitic acid

Write the structure of N, N-dimethyl-1-hexadecanamine

N, N-dimethyl-1-hexadecanamine: CH3(CH2)15NH(CH3)2

Reduce the carboxylic acid group to an amine group

To convert the carboxylic acid (COOH) group in palmitic acid to the amine (NH2) group in hexadecanamine (1-hexadecanamine), we need to carry out a reduction reaction. One possible method is using LiAlH4 as the reducing agent in the presence of ether solvent, followed by acid workup: Palmitic acid + 4 LiAlH4 -> Hexadecanamine + 4 LiAl(OH)3 Hexadecanamine: CH3(CH2)15NH2

Convert the primary amine to a tertiary amine

To convert hexadecanamine, a primary amine, to N, N-dimethyl-1-hexadecanamine, a tertiary amine, we can treat hexadecanamine with excess CH3I (methyl iodide) in the presence of a base, such as KOH. This will result in a methylation reaction. The reaction proceeds as follows: Hexadecanamine + 2 CH3I -> N, N-dimethyl-1-hexadecanamine + 2 KI CH3(CH2)15NH2 + 2 CH3I -> CH3(CH2)15NH(CH3)2 + 2 KI

Key concepts

Quaternary Ammonium Detergents

Quaternary ammonium compounds, often referred to as 'quats', are a family of compounds that are widely used for their antimicrobial and surfactant properties. These include cetylpyridinium chloride and benzylcetyldimethylammonium chloride, which serve as topical antiseptics and disinfectants. They consist of an ammonium ion where all four hydrogen atoms are replaced by alkyl or aryl groups.

The structure of quaternary ammonium detergents enables them to disrupt microbial cell membranes, leading to the death of the microorganisms. This makes them effective surface cleaners and sterilizing agents. 'Quats' are also known for their ability to lower surface tension, which helps in spreading and wetting properties, hence their classification as surface-acting agents.

Due to their stability and non-volatile nature, they are preferred in various applications ranging from personal care products to industrial cleaning agents. Understanding their synthesis can be crucial for developing products in pharmaceutical and hygiene industries.

Conversion of Carboxylic Acids to Acid Chlorides

The conversion of carboxylic acids to acid chlorides is a pivotal step in organic synthesis. It's a transformation that increases the reactivity of the carboxyl group, facilitating numerous subsequent reactions. To convert a carboxylic acid to an acid chloride, a common reagent used is thionyl chloride (SOCl₂).

The mechanism of this reaction involves the formation of an intermediate acyl chlorosulfite which then decomposes to yield the acid chloride and gaseous by-products, sulfur dioxide (SO₂) and hydrogen chloride (HCl). This reaction is widely appreciated in organic chemistry because the by-products escape as gases, driving the reaction to completion and simplifying purification.

For instance, the synthesis of cetyl chloride from palmitic acid is accomplished using SOCl₂, illustrating the practicality of this method for creating larger, more complex molecules used in quaternary ammonium compounds.

Reduction of Carboxylic Acids to Amines

The reduction of carboxylic acids to amines is an essential transformation in the synthesis of several organic compounds, particularly in the pharmaceutical industry. One common approach is the use of lithium aluminium hydride (LiAlH₄), a powerful reducing agent. When carboxylic acids are treated with LiAlH₄, the carboxyl group is reduced to a primary amine.

The mechanism involves the transfer of hydride ions from LiAlH₄ to the electrophilic carbon of the carboxyl group. This reduction is typically followed by an acid work-up to protonate the amine, resulting in the free amine. The process from palmitic acid to hexadecanamine showcases this procedure. It is important to note that LiAlH₄ should be handled carefully as it reacts vigorously with water, releasing hydrogen gas.

Methylation Reactions in Organic Synthesis

Methylation reactions are a fundamental part of organic synthesis, introducing methyl groups into a molecule. These reactions are often used to increase hydrophobicity or to adjust the biological activity of compounds. A common methylating agent is methyl iodide (CH₃I), which can be used to convert amines into more complex amines through N-methylation.

In the presence of a base like potassium hydroxide (KOH), a primary amine can react with methyl iodide to form a secondary amine, further reacting to form a tertiary amine if excess CH₃I is used. For example, hexadecanamine can be converted to N, N-dimethyl-1-hexadecanamine via N-methylation. This step is crucial in the synthesis of certain quaternary ammonium compounds, which are valuable in various industrial applications.