Question

A three-step mechanism has been suggested for the formation of carbonyl chloride: Step I: \(\mathrm{Cl}_{2} \rightarrow 2 \mathrm{Cl}\) Step II: \(\mathrm{Cl}+\mathrm{CO} \rightarrow \mathrm{COCl}\) Step III: \(\mathrm{COCl}+\mathrm{Cl}_{2} \rightarrow \mathrm{COCl}_{2}+\mathrm{Cl}\) Which species is an intermediate in the mechanism? a. \(\mathrm{COCl}_{2}\) b. \(\mathrm{COCl}\) c. \(\mathrm{Cl}\) d. \(\mathrm{CO}\)

Short answer

The intermediate is \(\mathrm{COCl}\).

Step-by-step solution

Identify Intermediates

An intermediate is a species that is formed in one step of a reaction mechanism and consumed in another. It does not appear in the overall reaction equation.

Analyze Each Reaction Step

- Step I: \(\mathrm{Cl}_{2} \rightarrow 2 \mathrm{Cl}\) - Here \(\mathrm{Cl}\) is formed.- Step II: \(\mathrm{Cl}+\mathrm{CO} \rightarrow \mathrm{COCl}\) - \(\mathrm{COCl}\) is formed.- Step III: \(\mathrm{COCl} + \mathrm{Cl}_{2} \rightarrow \mathrm{COCl}_{2} + \mathrm{Cl}\) - \(\mathrm{COCl}\) is consumed.

Determine Intermediates from Reaction Steps

From Step II and Step III, \(\mathrm{COCl}\) is produced and then immediately consumed, indicating it does not appear in the overall reaction but exists as an intermediate.

Key concepts

Chemical Intermediates

In chemical reactions, intermediates are special species that play a crucial role in multi-step reactions. These molecules or ions form in one step and are used up in subsequent steps, resulting in their absence from the final overall reaction. They are transient, making them difficult to isolate or observe directly. However, understanding intermediates helps predict how a reaction proceeds.
In the formation of carbonyl chloride,

• The intermediate is identified by looking at what is created in one step and consumed in another.
• For example, carbon monoxide (CO) and chlorine (Cl) react to form COCl. Subsequently, COCl reacts again, hinting it’s an intermediate.

This knowledge helps understand the path a reaction takes, allowing chemists to manipulate conditions to favor desired outcomes. It can also help in designing catalysts that stabilize or utilize intermediates to make reactions more efficient.

Stepwise Reaction

Stepwise reactions are processes broken into distinct sequential steps, each with its own reaction conditions and characteristics. These steps have individual reactants, intermediates, and products. Understanding a stepwise mechanism involves analyzing each event to see how the overall transformation occurs.
In our exercise, the conversion of reactants to carbonyl chloride involves three steps:

• Step I: Chlorine molecules (\(\mathrm{Cl}_{2}\)) dissociate into chlorine radicals (\(\mathrm{Cl}\)).
• Step II: The \(\mathrm{Cl}\) reacts with carbon monoxide (\(\mathrm{CO}\)) to form a carbonyl chloride radical (\(\mathrm{COCl}\)).
• Step III: This intermediate radical reacts again with \(\mathrm{Cl}_{2}\) to form the final product carbonyl chloride (\(\mathrm{COCl}_{2}\)).

Understanding each step provides insight into how to optimize a reaction or troubleshoot issues in reaction yield or speed. Identifying the rate-determining step can also highlight where changes can most effectively improve overall reaction speed.

Carbonyl Chloride Formation

The formation of carbonyl chloride is a fascinating reaction due to its industrial relevance and the rich chemistry involved. This compound, known as phosgene, is vital in producing pharmaceuticals and other chemicals. Its synthesis involves converting carbon monoxide and chlorine into a more chemically active form.
The mechanism involves:

• Initial dissociation of chlorine molecules to radicals, which are highly reactive.
• Combination of these radicals with carbon monoxide to form the \(\mathrm{COCl}\) radical compound.
• Subsequent utilization of \(\mathrm{COCl}\) to produce carbonyl chloride in a reaction that regenerates a chlorine radical.

This process exemplifies how understanding intermediates and reaction steps helps control the synthesis. Careful manipulation of reactants and conditions ensures that desired product formation is maximized, a critical factor in scalable industrial procedures. Recognizing the transient nature of species like \(\mathrm{COCl}\) aids in developing safer, more efficient chemical processes.