Question

Draw structural formulas of all chloroalkanes that undergo dehydrohalogenation when treated with KOH to give each alkene as the major product. For some parts, only one chloroalkane gives the desired alkene as the major product. For other parts, two chloroalkanes may work.

Short answer

Short Answer: The chloroalkanes that undergo dehydrohalogenation to give alkenes as the major product when treated with KOH are chloroethane, 1-chloropropane, 2-chloropropane, 1-chlorobutane, and 2-chlorobutane. These chloroalkanes form ethene, propene, 1-butene, and 2-butene as the major products upon dehydrohalogenation.

Step-by-step solution

Identify possible starting chloroalkanes for dehydrohalogenation

To find the chloroalkanes that can undergo dehydrohalogenation to produce each alkene as the major product, we must first look for chloroalkanes where the chlorine atom is attached to a carbon with a hydrogen atom on the adjacent carbon. These chloroalkanes have the potential to form alkenes through dehydrohalogenation.

Determine the resulting alkene for each chloroalkane

For each identified chloroalkane, we need to determine the corresponding alkene that would be formed as the major product when it undergoes dehydrohalogenation with KOH. This will help us verify if the chloroalkane is indeed a suitable candidate for the given reaction.

Draw structural formulas for the selected chloroalkanes

Once we have determined the chloroalkanes that can produce alkenes as the major product in a dehydrohalogenation reaction with KOH, we will draw the structural formulas for each of these chloroalkanes. Here are the chloroalkanes that undergo dehydrohalogenation to give alkenes as the major product, along with their structural formulas: 1. Chloroethane (\(\mathrm{CH_3 CH_2 Cl}\)) - Produces ethene (\(\mathrm{CH_2=CH_2}\)) Structural Formula: ``` H H H | | | -C-C-Cl | | H H ``` 2. 1-Chloropropane (\(\mathrm{CH_3 CH_2 CH_2 Cl}\)) - Produces propene (\(\mathrm{CH_3 CH=CH_2}\)) Structural Formula: ``` H H H H | | | | -C-C-C-Cl | | | H H H ``` 3. 2-Chloropropane (\(\mathrm{CH_3 CHCl CH_3}\)) - Produces propene (\(\mathrm{CH_3 CH=CH_2}\)) Structural Formula: ``` H H H | | | -C-C-Cl | | | H-C-H | H ``` 4. 1-Chlorobutane (\(\mathrm{CH_3 CH_2 CH_2 CH_2 Cl}\)) - Produces 1-butene (\(\mathrm{CH_3 CH_2 CH=CH_2}\)) or 2-butene (\(\mathrm{CH_3 CH=CH CH_3}\)) Structural Formula: ``` H H H H H | | | | | -C-C-C-C-Cl | | | | H H H H ``` 5. 2-Chlorobutane (\(\mathrm{CH_3 CHCl CH_2 CH_3}\)) - Produces 2-butene (\(\mathrm{CH_3 CH=CH CH_3}\)) Structural Formula: ``` H H H | | | -C-C-Cl | | | H-C-H | H ```

Key concepts

Chloroalkane Structural Formulas

Chloroalkanes, also known as haloalkanes, are a group of chemical compounds consisting of an alkane with one or more halogen atoms, such as chlorine, attached to its carbon chain. The structure of chloroalkanes greatly influences their reactivity and properties.

For example, the structural formula of chloroethane is \( \mathrm{CH_3CH_2Cl} \), with chlorine attached to a carbon that is also connected to another carbon atom. Structural formulas are crucial because they give us information on the connectivity of atoms in a molecule. They help us predict how a molecule will react under certain conditions, like in the dehydrohalogenation process where chloroalkanes are converted to alkenes.

The position of the chlorine atom can also affect the outcome. In chloroethane, the chlorine is on an end carbon, making it primary, whereas in 2-chloropropane, the chlorine is attached to a secondary carbon. Students must learn to draw and interpret these structural formulas to understand reaction outcomes.

Alkene Formation

Alkene formation from chloroalkanes occurs through a process called dehydrohalogenation. This involves the removal of a hydrogen atom and a halogen atom, resulting in the formation of a double bond between two carbon atoms.

One common method for creating alkenes involves using a strong base such as potassium hydroxide (KOH) to abstract a hydrogen atom from a carbon next to the carbon bearing the halogen. This creates an alkene plus water and a halide salt. The reaction tends to follow Zaitsev's rule, which states that the more substituted alkene will often be the major product.

Example Reactions

• Chloroethane \( \mathrm{CH_3CH_2Cl} \) can be converted to ethene \( \mathrm{CH_2=CH_2} \).
• 1-Chloropropane \( \mathrm{CH_3CH_2CH_2Cl} \) and 2-chloropropane \( \mathrm{CH_3CHClCH_3} \) both give rise to propene \( \mathrm{CH_3CH=CH_2} \).

It's noteworthy that these reactions must be carefully controlled, as alkenes can readily form a variety of isomers depending on the structure of the starting chloroalkane.

Organic Reaction Mechanisms

Organic reaction mechanisms are the step-by-step descriptions of chemical reactions that allow chemists to understand and predict the outcome of organic transformations. These mechanisms typically involve bond-making and bond-breaking processes that follow the movement of electrons within molecules.

Understanding the mechanism behind dehydrohalogenation of chloroalkanes is pivotal for students, as it not only illustrates how alkenes are formed but also conveys fundamental principles of organic chemistry such as nucleophilicity, basicity, and the stability of intermediates.

Dehydrohalogenation Mechanism

During this reaction, a base abstracts a hydrogen atom, and electrons from the carbon-hydrogen bond move to form the double bond, while the carbon-halogen bond breaks, leading to the departure of the halide ion. Students often encounter the E2 mechanism in this case, where the elimination of the leaving group (halogen) and the proton abstraction occur simultaneously.

Key points in understanding reaction mechanisms include knowledge of electron-pair donors, such as bases, and electron-pair acceptors, such as the carbon atoms involved in bond formation.