Problem 40 - Chemistry: The Central Science in SI Units, Global Edition

Question

Describe the intermediate that is thought to form in the addition of a hydrogen halide to an alkene, using cyclohexene as the alkene in your description.

Step-by-Step Solution

Answer

The intermediate formed in the addition of a hydrogen halide (HX) to cyclohexene is a carbocation. This carbocation has a six-membered ring structure with a hydrogen atom attached to one of the carbons that previously participated in the double bond, and a positive charge localized on this carbon atom. This intermediate is short-lived and reacts with the halogen (X-) ion to form the final product, a haloalkane.

Step 1: Identify the reactants

We have cyclohexene (alkene) and a hydrogen halide. The general formula for hydrogen halides is HX, where X is a halogen. Some common hydrogen halides are HCl, HBr, and HI.

Step 2: Understand the mechanism of the addition of a hydrogen halide to an alkene

The addition of a hydrogen halide to an alkene follows a reaction mechanism known as electrophilic addition. The alkene consists of a double bond with π electrons, which are attracted to the electropositive hydrogen atom in the hydrogen halide. As a result, the double bond in the alkene will break to form a bond with the hydrogen atom, creating a positively charged carbocation intermediate along with the negatively charged halogen (X-) ion.

Step 3: Illustrate the mechanism with cyclohexene and a hydrogen halide

Using cyclohexene, C_6H_10, as the alkene and HX as the hydrogen halide, we can illustrate the mechanism as follows: 1. The π electrons in the double bond of cyclohexene (C─C═C) form a bond with the electropositive hydrogen atom in HX. 2. The H─X bond breaks, resulting in a negatively charged halogen (X-) ion and the formation of a carbocation intermediate with the hydrogen atom attached to one of the carbons that were previously double-bonded.

Step 4: Describe the intermediate formed in the reaction

The intermediate that forms in the reaction is a carbocation, which is a carbon atom carrying a positive charge. In the case of cyclohexene, the carbocation intermediate has a six-membered ring structure with a hydrogen atom attached to one of the carbons that previously participated in the double bond. The positive charge is localized on this carbon atom. This intermediate is short-lived, as it will react quickly with the halogen (X-) ion to form the final product.

Step 5: Formation of the final product

The negatively charged halogen (X-) ion will react with the positively charged carbocation intermediate. This reaction forms a new C─X bond, resulting in the addition of the halogen across the former double bond in the alkene. The final product will be a haloalkane, where the halogen X has been added to the cyclohexane ring.

Key Concepts

Carbocation Intermediate

During the electrophilic addition of a hydrogen halide to an alkene, an important step is the formation of a carbocation intermediate. A carbocation is a species featuring a carbon atom with a positive charge. In the context of alkenes like cyclohexene, this formation occurs when the pi (\(\pi\)) electrons of the double bond interact with the hydrogen atom of the hydrogen halide.

The double bond's electrons are attracted towards the electropositive hydrogen, breaking the bond between hydrogen and halogen, and forming a new bond between the hydrogen and one of the carbon atoms of the alkene. This results in a carbon atom that now carries a positive charge due to the loss of electron sharing – creating the carbocation.

• The carbocation is a reactive species. It is unstable as the carbon atom achieves an incomplete octet of electrons.
• It's highly electrophilic, meaning it will readily react with nucleophiles like the negatively charged halogen ion for stabilization.

The presence and stability of a carbocation intermediate can influence the rate and outcome of the reaction, including the product stability, through rearrangements to more stable carbocations if possible.

Hydrogen Halide

Hydrogen halides are molecules composed of hydrogen and a halogen. These compounds, represented generally as HX, play a vital role in the electrophilic addition reactions involving alkenes. Common hydrogen halides include hydrogen chloride (HCl), hydrogen bromide (HBr), and hydrogen iodide (HI).

These substances are polar, featuring a covalent bond between the hydrogen and halogen atoms. The electropositive hydrogen atom is particularly attracted to the electrons from the alkene's double bond. This attraction initiates the formation of the carbocation intermediate when the alkene's pi bond electrons are drawn to the hydrogen atom.

• In the process, the H-X bond breaks, producing a negatively charged halide ion (X-).
• The resulting halide ion is highly nucleophilic and will attack the carbocation to stabilize the intermediate.

The unique properties of the hydrogen halide, including its bond polarity, are crucial in understanding how they drive the mechanism of electrophilic addition reactions.

Alkene Reaction Mechanism

The reaction mechanism involving alkenes and hydrogen halides is known as electrophilic addition. This mechanism sees the alkene, using its double bond, react with the hydrogen halide.

Here's a basic outline of the alkene reaction mechanism:

• First, the double bond of the alkene serves as a site of high electron density, making it appealing to the electropositive component of the hydrogen halide.
• The alkene's pi bond electrons form a bond with the hydrogen, breaking the H-X bond and transferring the halogen to the alkene to create a carbocation.
• Finally, the halide ion reacts with the unstable carbocation, resulting in the formation of a new carbon-halogen bond.

Understanding this step-by-step mechanism helps rationalize why certain products form and how the structure of the alkene or choice of halogen can influence the reaction pathway and outcomes. This knowledge is crucial for predicting the products and tailoring reactions in organic synthesis.