Draw a structural formula for an alkene and dichloroalkane with the given molecular formula that yields the indicated alkyne by each reaction sequence.

Converting a carbon-carbon double bond (C=C) of an alkene into a carbon-carbon triple bond (C≡C) of an alkyne involves a key chemical process known as an elimination reaction.

In this type of organic chemical transformation, we can't simply add more carbon to create a triple bond; instead, we start by adding halogens to the existing double bond through a process called halogenation. After the alkene has been fully halogenated to form a dihaloalkane - in which each carbon of the original double bond now bears a halogen - we need to eliminate these halogen atoms.

To achieve this, we employ a strong base, like sodium amide (NaNH₂), which can selectively remove the halogens, typically in an anti-addition manner. This dehalogenation results in the formation of additional pi bonds between the carbons, converting our original alkene into an alkyne with a characteristic triple bond.

Starting with a dichloroalkane, where two chlorine atoms are bound to adjacent carbon atoms in the hydrocarbon chain, the goal is to synthesise an alkyne. This reaction is a bit more straightforward than the one starting with an alkene because we already have a halogenated intermediate.

The elimination reaction or dehalogenation comes into play once again, as we apply a strong base such as sodium amide (NaNH₂). It helps to eject the chlorine atoms as chloride ions (Cl^-), and in the process, it forms the desired carbon-carbon triple bond. The positioning of the two chlorines and the subsequent elimination are critical in determining the site of alkyne formation, as the triple bond will form where these chlorines were once located.

Halogenation is a chemical reaction that involves the addition of halogen atoms (like chlorine or bromine) to another compound. In the context of converting alkenes to alkynes, halogenation is used to saturate the double bonds with halogens, turning them into dihalides.

Halogenation Process

The first step is the addition of a molecule of dihalogen (X₂) across the double bond of the alkene in a process often facilitated by a solvent like methylene chloride (CH₂Cl₂). This step is highly stereospecific, usually resulting in a vicinal dihalide.

Elimination Reaction

After halogenation, the next step is an elimination reaction. Using a strong base, both halogens are removed, simultaneously forming a triple bond to yield the alkyne. This type of elimination is a dehalogenation and is generally regioselective, meaning the base removes the halogens in such a way that the triple bond forms in a particular location based on the structure of the dihalide.

Drawing chemical structures, whether they're alkenes, alkynes, or any other molecule, is foundational for understanding and performing organic chemical reactions.

To draw a structural formula, you must know the molecular formula or the general bonding patterns characteristic of the molecule's functional groups. For example, alkenes have at least one C=C bond, while alkynes have a C≡C bond. With dichloroalkanes, two chlorine atoms are bonded to carbon atoms.

When drawing structures, care should be taken to represent all single, double, and triple bonds accurately, and all implicit hydrogen atoms should be included to ensure correct molecular geometry is communicated. Moreover, the depiction of stereochemistry with wedges (solid for out of page and dashed for into the page) is extremely important when showing the 3-dimensional arrangement of atoms around a chiral center or across a double bond.