Question

Draw the structure of the product when allene, \(\mathrm{CH}_{2}=\mathrm{C}=\mathrm{CH}_{2}\), reacts with bromine.

Short answer

Bromine adds to the double bond in allene forming \(\mathrm{BrCH}_2-\mathrm{CH}=\mathrm{CHBr}\).

Step-by-step solution

Understanding the Reactants

We begin by understanding the reactants involved in the chemical reaction. Allene is composed of two double-bonded carbon atoms in a linear fashion with the structure: \(\mathrm{H}_2\mathrm{C}=\mathrm{C}=\mathrm{CH}_2\). We are reacting this with bromine (\(\mathrm{Br}_2\)).

Identifying the Reaction

The reaction between allene and bromine is an addition reaction. In these types of reactions, bromine adds across one of the double bonds in allene.

Locating the Site of Reaction

Bromine can add to either double bond in allene, but it will preferentially react with one of the outer \(\mathrm{C} =\) bonds. Typically, it adds to the first double bond (\(\mathrm{C}_1 = \mathrm{C}_2\) or \(\mathrm{C}_2 = \mathrm{C}_3\)) on either end.

Drawing the Structure of the Product

After adding bromine to the \(\mathrm{C}_1 = \mathrm{C}_2\) double bond, the structure becomes \(\mathrm{BrCH}_2-\mathrm{CH}=\mathrm{CHBr}\). The other double bond remains unaffected as only one \(\mathrm{Br}_2\) molecule adds to one double bond.

Key concepts

Understanding Allene

Allene is an organic compound that belongs to the family of cumulenes, which are characterized by having consecutive double bonds. The structure of allene, represented as \( \mathrm{CH}_2=\mathrm{C}=\mathrm{CH}_2 \), shows two double bonds connecting three carbon atoms in a linear formation.
One interesting feature of allene is its unique geometry. While you might expect all atoms to lie in a single plane, allene has distinct perpendicular planes due to its central carbon atom. This gives it a special form where its outer hydrogen atoms form a 90-degree angle.
Because of this linear configuration, allene exhibits different chemical properties. It is often used in reactions where double bonds are key players, and understanding its structure helps predict how it behaves in such reactions. As a reactive molecule, it allows chemists to explore various addition reactions, especially those involving halogens like bromine.

What is an Addition Reaction?

Addition reactions are a fundamental type of chemical reaction where atoms or groups of atoms are added to a molecule without the loss of any atom from the original molecule. This kind of reaction typically occurs where there are double or triple bonds present, as these provide sites for new atoms to attach to.
In the context of the allene and bromine scenario, an addition reaction takes place when bromine adds across one of the double bonds in the allene. Here's how it works:

• The \( \mathrm{Br}_2 \) molecule interacts with the pi electrons of the double bond.
• The electrons from the double bond break, allowing each bromine atom to join the carbons involved in the double bond.
• The result is a saturated product where the double bond once existed.

In summary, because of the rearrangement and use of electrons within the molecule, addition reactions lead to an increase in the connectivity of atoms in a molecule, making them an interesting class of reactions to study.

Exploring the Bromine Reaction

When bromine comes into contact with allene, a specific kind of chemical interaction takes place. This is known as a bromine reaction, where the diatomic bromine molecule \( \mathrm{Br}_2 \) breaks apart to form bonds with the carbon atoms in allene.
The reaction follows several predictable steps:

• Bromine approaches the \( \mathrm{C} = \) double bond.
• The electrons in the double bond repulse the electrons between the bromine atoms, leading to polarization and eventually breaking the \( \mathrm{Br}-\mathrm{Br} \) bond.
• Each bromine atom attaches to a carbon atom from the double bond.

In this way, the bromine reaction with allene forms a dibromo derivative, like \( \mathrm{BrCH}_2-\mathrm{CH}=\mathrm{CHBr} \). This newly formed compound has utilized atoms from the original reactant in a transformation that maintains its carbon skeleton but changes its chemical nature. Understanding the role of bromine in such reactions helps chemists design and predict outcomes in synthetic organic chemistry.