Question

2,2,3,3 -Tetrabromopentane can be prepared by an addition reaction of excess \(\mathrm{Br}_{2}\) with an alkyne. Draw the structure of the alkyne and name it.

Short answer

The alkyne is 2-pentyne, with structure CH₃-C≡C-CH₂-CH₃.

Step-by-step solution

Understanding the Problem

We want to prepare 2,2,3,3-Tetrabromopentane using an addition reaction with excess Br₂ on an alkyne. This involves adding four bromine atoms to an alkyne.

Determine the Number of Carbons

The product, 2,2,3,3-Tetrabromopentane, suggests there are five carbon atoms in the parent chain of the desired alkyne.

Analyze the Bromine Positions

The '2,2,3,3' part of the name indicates the positions where bromines are added. The alkene equivalent bond (triple) should be between the second and third carbon atoms for symmetrical addition.

Write the Alkyne's Structure

Draw the alkyne: CH₃-C≡C-CH₂-CH₃. This is an alkyne with a triple bond between the second and third carbon of a pentane chain.

Name the Alkyne

The IUPAC name for the alkyne with a triple bond between the second and third carbons of a five-carbon chain is '2-pentyne.'

Key concepts

Alkyne

Alkynes are a fascinating group of hydrocarbons characterized by their carbon-carbon triple bonds. This strong triple bond is what distinguishes alkynes from other hydrocarbons such as alkanes and alkenes. Each carbon atom in the triple bond has two additional bonds, making these molecules linear and highly reactive.
Alkynes follow the general formula \(C_nH_{2n-2}\), highlighting their unique saturations. Due to their triple bond, they can engage in addition reactions, which are reactions where molecules add to the alkyne.

• The simplest alkyne is ethyne, also known as acetylene.
• Alkynes are typically represented by the suffix "-yne" at the end of their names.
• They are less stable than alkanes and alkenes due to strained electronic configurations.

Recognizing the presence of a triple bond helps in predicting reactivity patterns and understanding the basis for chemical transformations like bromination.

Bromination

Bromination is a type of addition reaction where bromine \(Br_2\) molecules react with other compounds. This process usually involves the formation of a dibromo-substituted product if an alkene is considered, but for alkynes, it can go further.
In the case of alkynes, excessive bromine can add across the triple bond, leading to a multi-bromo product. Here's how bromination typically proceeds:

• A bromine molecule splits into two bromine radicals.
• These radicals attack the carbon atoms of the alkyne, breaking the triple bond.
• If excess bromine is present, further bromination can occur, converting double bonds to single bonds with bromine atoms at each site.

Bromination is a useful tool in organic synthesis to extend the molecular complexity and functionality of hydrocarbons.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) provides systematic rules for naming chemical substances, making it easier for chemists around the world to communicate through a universal language.
The IUPAC nomenclature for alkynes incorporates several key guidelines:

• The longest carbon chain containing the triple bond defines the parent name and is numbered to give the triple bond the lowest possible locant.
• Suffix "-yne" is used to indicate the presence of a triple bond.
• When substituents are present, their names and positions are also included in the name.

For example, in '2-pentyne', the "2" specifies the position of the triple bond between the second and third carbon in the five carbon chain. This ensures that any chemist can discern the structure and composition of the molecule simply from its name.

Addition Reaction

An addition reaction is a fundamental type of chemical reaction where elements or compounds add to another compound. For alkynes, these reactions are particularly significant due to their propensity to interact with molecules like \(Br_2\). The triple bond in alkynes is a source of electron-rich character, making them ideal candidates for such reactions.
During an addition reaction involving alkynes:

• The alkyne's triple bond opens up as elements are added, transforming the molecule's structure.
• Alkynes can undergo successive addition, first breaking one pi bond to form a double-bonded alkene.
• Further addition reactions can fully saturate the molecule with single bonds, such as in the formation of 2,2,3,3-tetrabromopentane from 2-pentyne.

This broadens the scope for synthesizing more complex molecules and modifying existing compounds to enhance functionality.