Question

Prepare an alkyne from each alkene precursor by the dehydrohalogenation of alky1 dihalides method.

Short answer

To prepare an alkyne from an alkene precursor using the dehydrohalogenation of alkyl dihalides method, follow these steps: 1. Identify the alkene precursor and desired alkyne product. 2. Convert the alkene precursor to a geminal or vicinal dihalide through halogenation with Cl2 or Br2. 3. Perform dehydrohalogenation using a strong base (such as NaNH2 or t-BuOK) to remove two hydrogen-halogen bonds and form the alkyne. This may require one or two steps, depending on the dihalide type. 4. Review the final product to ensure it matches the desired alkyne structure.

Step-by-step solution

Identify the alkene precursor

First, identify the given alkene precursor and its structure, as well as the desired alkyne product. The structure must be drawn to understand the molecular changes that will occur.

Prepare alkyl dihalide

Convert the alkene precursor to a geminal or vicinal dihalide by adding halogen across the double bond. This is achieved through a halogenation reaction in which the alkene reacts with a molecular halogen (such as Cl2 or Br2). The halogen will add across the double bond and form a dihalogenated product.

Dehydrohalogenation

Carry out the dehydrohalogenation reaction to form the alkyne from the dihalide. In this reaction, a strong base (such as sodium amide, NaNH2, or potassium tert-butoxide, t-BuOK) is required to remove two hydrogen-halogen (HX) bonds. This can be done in one or two steps, depending on whether the dihalide is a geminal or vicinal dihalide. For a geminal dihalide, two successive dehydrohalogenation reactions are needed to remove both hydrogen-halogen bonds and form the alkyne: 1. The base attacks the hydrogen atom bonded to the carbon adjacent to the halogen, and the electrons in the hydrogen-halogen bond will move to form a double bond, thereby releasing the halide ion. This step forms a new alkene. 2. The process is repeated, with the base attacking the new alkene, removing the second hydrogen-halogen bond, and forming a triple bond to create the alkyne product. For a vicinal dihalide, only one dehydrohalogenation reaction is required. The base attacks one of the hydrogen atoms bonded to adjacent carbons and the electrons in the hydrogen-halogen bond form a triple bond, releasing two halide ions in a single step.

Review the final product

Review the final product to ensure that it matches the desired alkyne structure from Step 1.

Key concepts

Alkyne Synthesis

Alkyne synthesis is an essential transformation in organic chemistry, offering routes to create versatile molecules with triple bonds. This process typically involves converting lighter molecules like alkenes into alkynes by sequential reactions. Specifically, it often starts with an alkene precursor, into which halogens are added to form dihalides. These dihalides are then subjected to dehydrohalogenation to ultimately form an alkyne. The presence of a triple bond in alkynes introduces unique chemical properties that are crucial for various applications in chemistry and industry. Understanding this conversion enriches one's ability to design complex organic molecules.

Halogenation Reaction

The halogenation reaction is a pivotal step in transforming alkenes into alkyl dihalides. During this reaction, molecular halogens like chlorine ( \(Cl_2\) ) or bromine ( \(Br_2\) ) are reacted with an alkene. This reaction occurs across the double bond of the alkene, resulting in the addition of halogen atoms to the carbon atoms.

• This step transforms the alkene into a dihalide compound, essential for further reactions.
• The reaction mechanism involves the formation of a cyclic halonium ion intermediate, leading to anti addition of halogens.

For instance, the reaction of ethene with bromine yields 1,2-dibromoethane. This process is general in cases of other alkenes and is a foundation for complex synthetic pathways.

Geminal and Vicinal Dihalides

Geminal and vicinal dihalides are key intermediates in synthesizing alkynes from alkenes. Understanding the difference between these dihalides is crucial for predicting and controlling the outcome of synthetic processes.

• Geminal Dihalides: These have both halogen atoms attached to the same carbon atom. In dehydrohalogenation, two successive removal steps are needed to achieve the alkyne.
• Vicinal Dihalides: These have halogens on adjacent carbon atoms. Here, only one-step removal is necessary to form the alkyne.

The choice between geminal and vicinal configurations influences the strength and number of base-induced elimination reactions needed, thereby affecting the synthesis's efficiency and complexity.

Molecular Structure Drawing

Molecular structure drawing is an indispensable skill in visualizing and executing organic synthesis, such as the transformation of alkenes to alkynes. It involves representing molecules, highlighting the spatial arrangement of atoms and the connectivity of bonds.

• Accurate drawings help predict the course of reactions and determine the stability and reactivity of intermediates like dihalides.
• It assists in confirming the final product, ensuring it adheres to the desired alkyne structure.

Tools such as Lewis structures, and modern software can aid in this task, but understanding the basics of structure drawing remains fundamental for any chemist. They help make informed decision in design and synthesis practices, as well as aid in communicating ideas effectively in a scientific setting.