Problem 72 - General Chemistry: Principles and Modern Applications

Question

The cyanide anion is a nucleophile and when attached to a carbon atom, undergoes hydrolysis under basic conditions to the carboxylate anion. Suggest a method of preparing sodium butanoate from chloropropane. How can sodium butanoate be converted into butanoic acid?

Step-by-Step Solution

Answer

Sodium butanoate can be prepared from chloropropane by first reacting chloropropane with sodium cyanide to produce cyanopropane. Cyanopropane is then hydrolyzed under basic conditions to create sodium butanoate. Sodium butanoate can be converted to butanoic acid by reacting with hydrochloric acid.

Step 1: Formation of Cyanopropane

React chloropropane with sodium cyanide (\(NaCN\)) in a nucleophilic substitution reaction to form cyanopropane. Sodium cyanide is a good nucleophile and will replace the chlorine atom in the chloropropane molecule. The equation for this reaction is \(CH_3CH_2CH_2Cl + NaCN \rightarrow CH_3CH_2CH_2CN + NaCl\).

Step 2: Hydrolysis of Cyanopropane

Next, undergo hydrolysis under basic conditions (sodium hydroxide solution, \(NaOH\)) to convert cyanopropane to sodium butanoate. The cyano group (\(-CN\)) will be hydrolyzed to a carboxylate group (\(-COO^-\)). The reaction is \(CH_3CH_2CH_2CN + NaOH \rightarrow CH_3CH_2CH_2COO^-Na^+ + NH_3\).

Step 3: Conversion of Sodium Butanoate to Butanoic Acid

Sodium butanoate can be converted into butanoic acid by an acid-base reaction. React the sodium butanoate with an acid (like hydrochloric acid, \(HCl\)). The reaction is \(CH_3CH_2CH_2COO^-Na^+ + HCl \rightarrow CH_3CH_2CH_2COOH + NaCl\).

Key Concepts

Understanding Cyanide Anion as Nucleophile

In chemistry, a nucleophile is a substance that donates an electron pair to an electrophile to form a chemical bond in a reaction. Among the variety of nucleophiles, the cyanide anion ( (CN^-)) is one of the most potent, due to its strong affinity for positive charges. This ion consists of a negatively charged carbon atom triple-bonded to a nitrogen atom, and it's often employed in nucleophilic substitution reactions to replace leaving groups like chlorine.

The nucleophilic nature of cyanide allows it to attack electrophilic carbon atoms, particularly in alkyl halides. For instance, when chloropropane comes into contact with sodium cyanide, the cyanide anion, being a good nucleophile, displaces the chlorine atom. The result is the formation of cyanopropane, an intermediate step towards producing compounds like carboxylic acids and their salts upon further reactions.

Key Reactions Featuring Cyanide Anion:

• Alkyl halide substitution (CH₃CH₂CH₂Cl + CN^- → CH₃CH₂CH₂CN + Cl^-)
• Synthesis of carboxylic acids and their salts through additional reactions.

This particular property makes the cyanide ion an essential tool for building complex organic molecules in both industrial and laboratory settings.

Hydrolysis Under Basic Conditions

Hydrolysis is a chemical process where water is used to break down a compound; when this occurs under basic conditions, it often results in the formation of a carboxylate anion among other products. In the context of organic chemistry, hydrolysis of nitriles, such as cyanopropane, in the presence of a strong base like sodium hydroxide ( NaOH) is a crucial step in synthesizing carboxylate salts.

When cyanopropane undergoes hydrolysis with NaOH, the cyano group ( -CN) is converted into a carboxylate group ( -COO^-). This transformation is made possible by the fact that the basic conditions favor the breakdown of the nitrile group, paving the way for the nitrogen to be removed as ammonia ( NH3), and subsequently replaced by a hydroxyl group to form the carboxylate salt.

Advantages of Hydrolysis Under Basic Conditions:

• Transforms nitriles into carboxylic acid salts efficiently.
• Facilitates the removal of nitrogen as ammonia, which is an easily separable byproduct.

Therefore, hydrolysis under basic conditions is a straightforward and convenient method to derive carboxylate anions from nitriles in organic synthesis.

Synthesis of Sodium Butanoate

Sodium butanoate is an example of a carboxylate salt that can be synthesized through the hydrolysis of nitriles – specifically, from cyanopropane. The journey to sodium butanoate involves a nucleophilic substitution reaction followed by hydrolysis, as previously detailed. The initial step of reacting chloropropane with sodium cyanide to form cyanopropane sets the stage.

This is then followed by the hydrolysis of the cyanide group in cyanopropane using NaOH under basic conditions, ultimately yielding sodium butanoate (CH₃CH₂CH₂COO^-Na^+). This series of reactions harnesses the fundamental principles of organic chemistry to transform a simple starting material into a more complex organic salt with a wide range of applications, from industrial manufacturing to the production of pharmaceuticals.

Steps to Sodium Butanoate:

• React chloropropane with sodium cyanide to form cyanopropane.
• Hydrolyze cyanopropane with NaOH to yield sodium butanoate.

The value of the sodium butanoate lies in its versatile nature, as it forms the backbone for further chemical modifications, such as the production of butanoic acid.

Conversion to Butanoic Acid

After synthesizing sodium butanoate, one might wish to convert this salt into its corresponding acid, butanoic acid. This is accomplished through an acid-base reaction, commonly using hydrochloric acid ( HCl). When sodium butanoate is treated with HCl, the sodium ion ( Na^+) is replaced by a hydrogen atom, thus yielding butanoic acid ( CH₃CH₂CH₂COOH).

This simple yet elegant reaction highlights the interplay between acids and bases, which is central to many processes in synthetic chemistry. Notably, the choice of HCl is practical as it is a strong acid that effectively protonates the carboxylate anion, and the resulting byproduct, sodium chloride ( NaCl), is an innocuous salt.

Benefits of Converting to Butanoic Acid:

• Enables access to the acid form, important for various applications.
• Utilizes user-friendly reagents, and resulting byproducts are benign.

Through this conversion, one can obtain butanoic acid, an important molecule in both industrial chemistry and biochemistry, serving as a building block for more complex molecules and as a flavoring agent in the food industry.