Question

Chloroform was at one time synthesized commercially by the action of sodium hypochlorite on ethanol. Formulate the reactions that might reasonably be involved. What other types of alcohols might be expected to give haloforms with halogens and base?

Short answer

The synthesis of chloroform from ethanol and sodium hypochlorite occurs through the haloform reaction mechanism, which involves conversion of ethanol to acetaldehyde, followed by the formation of chloroform. The overall balanced reaction for this process is: \(CH_3CH_2OH + 4NaOCl \rightarrow CHCl_3 + NaHCOO + 3NaCl + 2H_2O\). Suitable alcohols for the haloform reaction include primary and secondary alcohols, such as methanol, 1-propanol, 1-butanol, isopropanol, and cyclopentanol, which can be oxidized to carbonyl compounds containing a hydrogen atom bonded to the carbonyl carbon.

Step-by-step solution

Identify the reactants

In this case, we are given the reactants: sodium hypochlorite (NaOCl) and ethanol (CH3CH2OH).

Write the haloform reaction mechanism

The haloform reaction involves the conversion of an alcohol to a haloform through the use of a halogen and a base. For the synthesis of chloroform, the halogen is chlorine (Cl) and the base is sodium hypochlorite (NaOCl). Here is a general mechanism for the haloform reaction: 1. The sodium hypochlorite (NaOCl) dissociates into \(Na^+\) and \(OCl^-\) ions in the solution. 2. The hydroxide ion (from the base) reacts with the alcohol, converting it into a carbonyl compound. 3. The halogen (here, Cl) reacts with the carbonyl compound to form a haloform and a carboxylate ion.

Formulate the reaction for chloroform synthesis

Using the general mechanism from Step 2, we can write the reaction for chloroform synthesis as follows: 1. Dissociation of sodium hypochlorite: \(NaOCl \rightarrow Na^+ + OCl^-\) 2. Conversion of ethanol to acetaldehyde: \(CH_3CH_2OH + OCl^- \rightarrow CH_3CHO + Cl^- + H_2O\) 3. Formation of chloroform: \(CH_3CHO + 3OCl^- \rightarrow CHCl_3 + 3Cl^- + HCOO^-\) 4. Complete balanced reaction: \(CH_3CH_2OH + 4NaOCl \rightarrow CHCl_3 + NaHCOO + 3NaCl + 2H_2O\)

Identify other suitable alcohols

The haloform reaction occurs with alcohols that can be oxidized to carbonyl compounds containing a hydrogen atom bonded to the carbonyl carbon. This includes primary and secondary alcohols. Primary alcohols will yield methyl ketones, while secondary alcohols will yield haloforms directly. Some examples of primary alcohols that might give haloform are methanol, 1-propanol, and 1-butanol. Examples of secondary alcohols include isopropanol and cyclopentanol.

Key concepts

Sodium hypochlorite

Sodium hypochlorite is a chemical compound with the formula NaOCl. Commonly known as bleach, this compound is widely used as a disinfectant, as well as a reagent in various chemical reactions.
In the context of the haloform reaction, sodium hypochlorite acts as a base and a source of chlorine. It dissociates in water to produce hypochlorite ions \(OCl^-\) and sodium ions \(Na^+\).
This dissociation is important because the \(OCl^-\) ion is an active participant in oxidizing alcohols to carbonyl compounds, which are key intermediates in the synthesis of haloforms like chloroform.

• Sodium hypochlorite is inexpensive and readily available, making it a popular choice for haloform reactions in educational and industrial settings.
• Handling sodium hypochlorite requires careful attention to safety, as it is a powerful oxidizing agent and can be corrosive.

Ethanol

Ethanol, often referred to as ethyl alcohol, is a simple alcohol with the chemical formula \(CH_3CH_2OH\). It is widely used in alcoholic beverages, as a solvent, and as a precursor to organic compounds.
Ethanol is classified as a primary alcohol because it has its hydroxyl group \((-OH)\) attached to a terminal carbon atom that is also linked to two hydrogen atoms.
In the haloform reaction, ethanol serves as the starting material that gets oxidized to form acetaldehyde, a carbonyl compound.
The presence of a methyl group \(CH_3\) adjacent to the alcohol group in ethanol is crucial for its ability to undergo the haloform reaction, which ultimately results in the formation of chloroform when treated with sodium hypochlorite.

• Ethanol is flammable, so it must be handled with caution.
• It is miscible with water, meaning it can form a homogeneous mixture, which helps in facilitating reactions such as the haloform reaction.

Carbonyl compound

Carbonyl compounds are organic compounds that contain a carbon-oxygen double bond, \(C=O\). They are a pivotal class of compounds in organic chemistry, which includes aldehydes and ketones.
In the haloform reaction, the carbonyl compound formed is essential as it provides the site for chlorine addition to eventually form a haloform.
For example, when ethanol is oxidized by sodium hypochlorite, it turns into acetaldehyde, which is a simple carbonyl compound.
This is crucial because acetaldehyde, having a hydrogen atom on the carbonyl carbon, can further react with chlorine to form chloroform.

• The ability of a carbonyl compound to undergo further transformation is heavily reliant on its structure, specifically the presence of a hydrogen on the carbonyl carbon.
• Applications of carbonyl compounds extend beyond the haloform reaction, as they are pivotal in many synthetic pathways, including reduction and nucleophilic addition reactions.

Primary alcohols

Primary alcohols are a type of alcohol where the -OH group is attached to a carbon atom that is only connected to one other alkyl group. This makes them distinct in their reactivity compared to other types of alcohols.
In the haloform reaction, primary alcohols like ethanol are oxidized to give aldehydes. These aldehydes can further react to form haloforms.
However, not all primary alcohols can undergo the haloform reaction; they need to have at least one methyl group attached to facilitate the conversion to a methyl ketone.

• Some examples of primary alcohols include methanol, 1-propanol, and 1-butanol.
• Primary alcohols are often used in laboratories and industries due to their reactivity and versatility in forming various derivatives.

Secondary alcohols

Secondary alcohols have their hydroxyl group attached to a carbon atom that is also connected to two other carbons, creating different reactivity profiles compared to primary alcohols.
In the context of the haloform reaction, secondary alcohols are notable because they can directly form haloforms without the need for additional intermediate oxidation steps.
For instance, isopropanol, a common secondary alcohol, can directly yield a haloform when treated with sodium hypochlorite.

• This is because secondary alcohols are generally oxidized to form ketones, which, having the necessary methyl group adjacent to the carbonyl, can proceed through a haloform reaction.
• Examples of secondary alcohols include isopropanol and cyclopentanol, which are commonly used in disinfection and as solvents.