Question

Write a balanced chemical equation for the reaction that is described and then classify the reaction as a substitution, an elimination, an addition, or a rearrangement reaction: (a) Ethene and \(\mathrm{Br}_{2}\) react in carbon tetrachloride to give 1,2 -dibromoethane. (b) Iodoethane reacts with KOH(aq) yielding ethene, water, and potassium bromide. (c) Chloromethane reacts with \(\mathrm{NaOH}(\) aq) to give methanol and sodium chloride.

Short answer

For part (a), the reaction type is addition and the balanced chemical equation would be \( \mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{4}\mathrm{Br}_{2} \). For part (b), it's an elimination reaction and the balanced equation is \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{I} + \mathrm{KOH} \rightarrow \mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{H}_{2}\mathrm{O} + \mathrm{KBr} \). Lastly, part (c) represents a substitution reaction and its balanced equation is \( \mathrm{CH}_{3}\mathrm{Cl} + \mathrm{NaOH} \rightarrow \mathrm{CH}_{3}\mathrm{OH} + \mathrm{NaCl} \).

Step-by-step solution

Identifying the reaction type (a)

For part (a), ethene and \( \mathrm{Br}_{2} \) react to give 1,2 -dibromoethane. This is an addition reaction because a molecule of \( \mathrm{Br}_{2} \) is added across the double bond of ethene to form 1,2 -dibromoethane. The balanced equation for the reaction is: \( \mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{Br}_{2} \rightarrow \mathrm{C}_{2}\mathrm{H}_{4}\mathrm{Br}_{2} \)

Identifying the reaction type (b)

In part (b), iodoethane reacts with KOH to yield ethene, water, and potassium bromide. Given that we started with a haloalkane and ended up with an alkene, water, and potassium halide, it's an elimination reaction. The balanced reaction equation here is: \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{I} + \mathrm{KOH} \rightarrow \mathrm{C}_{2}\mathrm{H}_{4} + \mathrm{H}_{2}\mathrm{O} + \mathrm{KBr} \)

Identifying the reaction type (c)

In part (c), chloromethane reacts with sodium hydroxide to give methanol and sodium chloride. This is a nucleophilic substitution reaction where a hydroxyl ion (\( \mathrm{OH}^{-} \)) replaces a chlorine atom in chloromethane. The balanced chemical equation for the reaction would be: \( \mathrm{CH}_{3}\mathrm{Cl} + \mathrm{NaOH} \rightarrow \mathrm{CH}_{3}\mathrm{OH} + \mathrm{NaCl} \)

Key concepts

Addition Reaction

Imagine combining two pieces to create a new whole; that's essentially what happens in an addition reaction. It involves two or more reactants joining together to form a single product. For example, if we have ethene (\texttt{C2H4}), a simple hydrocarbon with a double bond, and we react it with bromine (\texttt{Br2}), the atoms of bromine add across the double bond.

this transformative process results in 1,2-dibromoethane, where each carbon atom in the original ethene now forms a bond with a bromine atom. The general equation for this specific addition reaction is:
\[ \texttt{C2H4} + \texttt{Br2} \rightarrow \texttt{C2H4Br2} \]
Notice how the double bond 'opens up' to accommodate the bromine atoms. So, in the realm of organic chemistry, addition reactions are particularly common with compounds containing double or triple bonds, allowing for direct attachment of atoms or groups of atoms.

Elimination Reaction

Think of an elimination reaction as a 'trimming' process in the chemical world. It's where a molecule loses elements from its structure, leading to the formation of a double or triple bond. For the reaction in our exercise, iodoethane reacts in the presence of potassium hydroxide (\texttt{KOH}), resulting in the loss of a hydrogen atom and an iodine atom, which come together to form hydrogen iodide.As a result, the remaining carbon atoms form a double bond, yielding ethene. The by-products of this reaction are potassium bromide (\texttt{KBr}) and water (\texttt{H2O}). The balanced equation that mirrors the elimination process in this instance is:
\[ \texttt{CH3CH2I} + \texttt{KOH} \rightarrow \texttt{C2H4} + \texttt{H2O} + \texttt{KBr} \]
Elimination reactions are a key pathway for transforming halogenated organic compounds into alkenes, which are quite valuable in chemical synthesis.

Nucleophilic Substitution

Nucleophilic substitution is like a dance of molecules where one partner is swapped for another. This reaction is characterized by the exchange of a nucleophile (an electron pair donor) for a leaving group (The portion of the molecule that departs. In the given example, chloromethane undergoes a reaction with sodium hydroxide (\texttt{NaOH}). The \texttt{OH-} ion, which is the nucleophile, effectively displaces the chlorine atom in chloromethane.This substitution gives rise to methanol and sodium chloride (table salt). Thus, our balanced chemical equation becomes:
\[ \texttt{CH3Cl} + \texttt{NaOH} \rightarrow \texttt{CH3OH} + \texttt{NaCl} \]
Such reactions are fundamental in organic chemistry, often leading to the creation of different alcohols from simple alkyl halides, and are influenced by factors like the nature of the nucleophile, the solvent, and the leaving group.

Balanced Chemical Equation

To understand the language of chemistry, one must understand balanced chemical equations, which is akin to balancing scales. The principle of conservation of mass dictates that the number of atoms of each element must remain equal on both sides of the equation. In our exercise, each reaction must be balanced to reflect this principle.For instance, the addition reaction between ethene and bromine leads to a product with the same amount of carbon, hydrogen, and bromine atoms as the reactants:
\[ \texttt{1 \times C2H4} + \texttt{1 \times Br2} \rightarrow \texttt{1 \times C2H4Br2} \]
Similarly, the elimination and nucleophilic substitution reactions are also balanced to ensure atom conservation. Balancing chemical equations is vital, as it allows chemists to predict the amounts of products formed from given reactants, ensuring precise and efficient reactions in both laboratory and industrial settings.