Question

Give structures and names of the organic products expected from the reaction of 1 -butyne with the following: (a) 1 mole \(\mathrm{H}_{2}, \mathrm{~N}\) i (b) 2 moles \(\mathrm{Br}_{2}\) (c) 2 moles \(\mathrm{HC} 1\)

Short answer

The organic products expected from the reaction of 1-butyne with the given reagents are: (a) Reaction with 1 mole \(\mathrm{H}_{2}, \mathrm{~N}\) i: cis-2-butene, with the structure: H H | | H-C=C-C-C-H | | H H (b) Reaction with 2 moles \(\mathrm{Br}_{2}\): 1,1,2,2-tetrabromoethane, with the structure: Br Br | | Br-C-C-C-C-Br | | Br Br (c) Reaction with 2 moles \(\mathrm{HCI}\): 1,1,2-trichloroethane, with the structure: CI CI | | CI-C-C-C-C-H | | CI H

Step-by-step solution

Reaction with \(\mathrm{H}_{2}, \mathrm{~N}\) i

First, we have to react 1-butyne with 1 mole of hydrogen gas in the presence of a nickel catalyst. This reaction is a hydrogenation reaction. Alkynes undergo hydrogenation to form alkenes. In this case, since only 1 mole of \(\mathrm{H}_{2}\) is supplied, it will undergo partial hydrogenation and form a cis-alkene. So in the presence of a nickel catalyst, the triple bond in 1-butyne will be reduced to a double bond. The structure of the product will be: H H | | H-C=C-C-C-H | | H H The name of the product is: cis-2-butene

Reaction with 2 moles of \(\mathrm{Br}_{2}\)

Next, we will react 1-butyne with 2 moles of bromine. This reaction is a halogenation reaction. Alkynes undergo halogenation to form dihalides. In this case, two moles of \(\mathrm{Br}_{2}\) are supplied, so the triple bond will be converted to a single bond with four bromine atoms attached to it. The structure of the product will be: Br Br | | Br-C-C-C-C-Br | | Br Br The name of the product is: 1,1,2,2-tetrabromoethane

Reaction with 2 moles of \(\mathrm{HCI}\)

Finally, we will react 1-butyne with 2 moles of hydrochloric acid. This reaction is a hydrohalogenation reaction. Alkynes undergo hydrohalogenation to form dihalides by breaking the triple bond. In this case, two moles of \(\mathrm{HCI}\) are supplied, so the triple bond will be converted to a single bond with one chlorine attached to the first carbon and two chlorines attached to the second carbon. The structure of the product will be: CI CI | | CI-C-C-C-C-H | | CI H The name of the product is: 1,1,2-trichloroethane

Key concepts

Hydrogenation

In organic chemistry, hydrogenation refers to the addition of hydrogen (H₂) to unsaturated organic compounds, typically facilitated by a catalyst. When we look at alkynes, which contain a carbon-carbon triple bond, hydrogenation can transform these into alkanes or alkenes, depending on the amount of hydrogen used.

• Partial Hydrogenation: If only one mole of H₂ is used, an alkyne such as 1-butyne is converted into an alkene, specifically a cis-alkene due to the configuration of added hydrogens.
• Complete Hydrogenation: If more hydrogen was added, the alkyne would be fully hydrogenated into an alkane.

The catalyst often used in these reactions is nickel, which helps to lower the energy requirement and speed up the reaction. This makes hydrogenation a widely used industrial process, especially in synthetic organic chemistry.

Halogenation

Halogenation is the process where halogens (like Br₂) are added to organic molecules. In the context of alkynes, halogenation results in the conversion of triple bonds to dihalides and sometimes further to tetrahalides if more halogen is available. For 1-butyne, reacting it with two moles of bromine changes its structure dramatically:

• The triple bond of the alkyne becomes fully saturated by forming single bonds with four bromine atoms.
• This results in the creation of a compound known as 1,1,2,2-tetrabromoethane.

The transformation of a triple bond into a single bond by the addition of halogens helps understand the reactivity and stability changes that occur in these molecules, which is a vital concept in organic synthesis.

Hydrohalogenation

During hydrohalogenation, an alkyne reacts with hydrogen halides like HCl, leading to the addition of hydrogen and chlorine across the triple bond. For alkynes like 1-butyne, this reaction occurs in a stepwise manner:

• In the first step, adding a mole of HCl breaks one of the pi bonds to form a vinyl chloride.
• The second mole of HCl saturates the double bond to form a product like 1,1,2-trichloroethane, featuring chlorine atoms occupying not only the previously triple-bonded carbons but also rearranging according to Markovnikov's rule.

Hydrohalogenation is crucial in forming building blocks for more complex chemical synthesis, highlighting its importance in the chemical industry.

Alkynes

Alkynes are hydrocarbons known for their carbon-carbon triple bonds, making them unsaturated. This triple bond is termed acetylenic, and it imparts distinct chemical properties to alkynes, setting them apart from alkenes and alkanes. Key characteristics include:

• The triple bond allows for significant reactivity, permitting a range of addition reactions.
• Alkynes can react with halogens, hydrogen, and hydrogen halides to yield various products like alkenes, alkanes, and dihalides.

Due to these reactions, alkynes serve as pivotal starting materials in organic synthesis. Additionally, the ability of the triple bond to hold multiple groups distinguishes alkynes as versatile petrochemical resources.

Nickel Catalyst

In many chemical reactions, especially those involving hydrogenation, the role of a catalyst is indispensable. Nickel is commonly used as a catalyst to facilitate hydrogenation reactions due to its properties:

• Nickel provides a surface for hydrogen molecules to adsorb and dissociate into active atomic hydrogen.
• This atomic hydrogen is more reactive, allowing it to efficiently attack the π bonds of alkynes and convert them into alkenes or alkanes.

Using nickel not only makes reactions faster but also more selective and efficient than uncatalyzed processes. As a result, nickel catalysts are vital components in the production of vast quantities of industrial chemicals.