Question

Why is it preferable to produce chloroethane by the reaction of HCI(g) with ethene than by the reaction of \(\mathrm{Cl}_{2}(g)\) with ethane? (See Exercise 62.)

Short answer

Producing chloroethane by the reaction of HCl(g) with ethene is more preferable because it has a faster one-step addition reaction mechanism, consumes less energy due to breaking weaker bonds, and doesn't produce additional by-products, simplifying the process and reducing costs compared to the substitution reaction of Cl\(_2\)(g) with ethane.

Step-by-step solution

Write down the chemical reactions

First, let's write down the two reactions mentioned in the exercise. Reaction 1: HCl(g) with ethene \[ C_2H_4 + HCl \rightarrow C_2H_5Cl \] Reaction 2: Cl\(_2\)(g) with ethane \[ C_2H_6 + Cl_2 \rightarrow C_2H_5Cl + HCl \]

Compare the reaction mechanisms

Now, let's compare the reaction mechanisms of the two reactions. For Reaction 1, it is an addition reaction where the HCl molecule adds across the double bond in ethene. This reaction is faster than Reaction 2 because it proceeds through a single step mechanism. For Reaction 2, it is a substitution reaction where one hydrogen atom in ethane is replaced by a chlorine atom from the Cl\(_2\) molecule. This reaction is slower than Reaction 1 because it proceeds through a two-step mechanism where first a chlorine radical attacks the ethane and then a hydrogen atom is replaced by a chlorine atom.

Compare energy requirements

We will also compare the energy requirements of the two reactions to produce chloroethane. Reaction 1 consumes less energy compared to Reaction 2 as it only involves breaking the strong π bond of ethene and a H-Cl bond. The breaking of the π bond requires less energy than breaking a σ bond, which is involved in Reaction 2. Reaction 2 requires more energy as it involves breaking two strong σ bonds in ethane and Cl\(_2\). Moreover, the reaction proceeds through an intermediate step with higher energy species, which further increases the energy requirement for the reaction.

Check for by-products

Both reactions produce chloroethane as the desired product but let's check for any additional by-products. Reaction 1 produces only chloroethane with no additional by-products. Reaction 2 produces chloroethane and also HCl as a by-product. This will require additional separation processes to obtain pure chloroethane, increasing the overall cost and complexity of the process.

Conclude the comparison

Based on the comparison of reaction mechanisms, energy requirements, and by-products: - Reaction 1 is faster, as it is an addition reaction requiring a single step mechanism - Reaction 1 consumes less energy because it involves relatively weaker bonds and fewer bond-breakings than Reaction 2 - Reaction 1 produces no additional by-products, simplifying the process and reducing costs Therefore, it is more preferable to produce chloroethane by the reaction of HCl(g) with ethene than by the reaction of Cl\(_2\)(g) with ethane.

Key concepts

Addition Reactions

In organic chemistry, addition reactions are a major type of reaction where atoms are added to a molecule without taking anything away. In the context of producing chloroethane by reacting HCl with ethene, an addition reaction occurs across the double bond in ethene.
This type of reaction is generally faster because it involves a simple mechanism where HCl adds across the double bond in one swift action.

• The double bond in ethene opens up, allowing new atoms to join.
• New bonding occurs without eliminating any existing atoms from the molecule.
• This reaction helps create chloroethane efficiently and swiftly.

This efficiency in addition reactions results in fewer steps and potentially lower energy requirements.

Substitution Reactions

Substitution reactions, on the other hand, involve the replacement of an atom or group in a molecule. In the reaction of Cl _2 with ethane, a substitution reaction takes place. A hydrogen atom in the ethane is replaced by a chlorine atom.
Since substitution reactions often occur in multiple steps, they can be slower. The chlorine molecule splits into radicals first, which then attack the ethane molecule. Here's what happens:

• A bond between two molecules breaks, creating free radicals.
• A radical then replaces a hydrogen atom in ethane with a chlorine atom.

These additional steps make substitution reactions more complex and generally slower compared to addition reactions.

Reaction Mechanisms

Understanding the underlying mechanism of a reaction is vital as it reveals the path taken by chemicals when they react. The reaction mechanism for an addition reaction, like the one between HCl and ethene, is straightforward.
In a single step, the reaction occurs with the direct addition of atoms across a double bond. This simplicity often translates to speed. In contrast, substitution reactions follow a more complex route.

• They typically start with bond cleavage, forming reactive intermediates like radicals.
• These intermediates perform the substitution, which involves further bonds forming and breaking.

The additional steps in substitution reactions contribute to a longer reaction time and can increase the chance of by-products forming.

Energy Requirements

The energy required to drive a chemical reaction is an important factor that affects efficiency and cost. Addition reactions often require less energy compared to substitution reactions.
For instance, breaking a relatively weak π bond in ethene as part of an addition reaction takes less energy than breaking two strong σ bonds in ethane and Cl _2 during a substitution reaction.

• Less energy consumed means the reaction is more cost-effective and quicker.
• Lower energy reactions also mean there's a lower chance of unwanted side reactions occurring.

This makes addition reactions not only efficient but also easier to control.

By-products in Chemical Reactions

The presence of by-products in chemical reactions is a crucial factor determining the practicality of a chemical process. When producing chloroethane via the addition reaction of HCl and ethene, there are no by-products.
This makes the process simpler and more economical as no extra steps are required for purification. On the other hand, the substitution reaction of Cl _2 with ethane creates additional HCl as a by-product.

• By-products can complicate the reaction process.
• They require secondary processes to isolate the desired chemical, increasing costs.

No by-products mean fewer complexities, making the addition reaction a cleaner choice.