Question

The compound \(\mathrm{C}_{2} \mathrm{H}_{5}\) Br reacts slowly with the compound \(\mathrm{CH}_{4} \mathrm{O}\) to yield a single substance of formula \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\). Assuming normal valences throughout, write structural formulas for \(\mathrm{CH}_{4} \mathrm{O}\) and the three different possible structural (not rotational) isomers of \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\) and show how the principle of least structural change favors one of them as the reaction product. What would you expect to be formed from each of these three \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}\) isomers with strong hydrobromic acid?

Short answer

Methoxyethane is the favored product; it reacts with hydrobromic acid to form ethyl bromide and methanol.

Step-by-step solution

Write the Structural Formula for \( \mathrm{CH}_{4} \mathrm{O} \)

The compound \( \mathrm{CH}_{4} \mathrm{O} \) is methanol, which consists of a single carbon atom bonded to three hydrogen atoms and a hydroxyl group. Thus, the structural formula is \( \mathrm{CH}_{3} \mathrm{OH} \).

Identify Possible Isomers of \( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \)

\( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \) can have three structural isomers:1. Propan-1-ol (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \)) where the hydroxyl group is attached to the end carbon of a 3-carbon chain.2. Propan-2-ol (\( \mathrm{CH}_{3} \mathrm{CH(OH)} \mathrm{CH}_{3} \)), where the hydroxyl group is attached to the middle carbon of a 3-carbon chain.3. Methoxyethane (\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3} \)), which is an ether.

Apply the Principle of Least Structural Change

The principle of least structural change suggests that the product will have a structure closely resembling the reactants. \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br} \) is ethyl bromide, and it's reasonable that the oxygen from methanol simply replaces the bromine in ethyl bromide, forming an ether, which is methoxyethane.

Predict Reaction Products with Hydrobromic Acid

For each isomer of \( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \), predict the product with strong hydrobromic acid:1. Propan-1-ol (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} \)) typically forms 1-bromopropane (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} \)) by replacing the OH group.2. Propan-2-ol (\( \mathrm{CH}_{3} \mathrm{CH(OH)} \mathrm{CH}_{3} \)) forms 2-bromopropane (\( \mathrm{CH}_{3} \mathrm{CHBr} \mathrm{CH}_{3} \)) in a similar way.3. Methoxyethane (\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{3} \)) forms ethyl bromide (\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br} \)) and methanol (\( \mathrm{CH}_{3} \mathrm{OH} \)) when the ether bond is cleaved.

Key concepts

Isomerism

Isomerism in organic chemistry refers to the phenomenon where compounds with the same molecular formula have different structural or spatial arrangements. This means two or more compounds, while having identical numbers and types of atoms, can have different physical or chemical properties due to variations in their connectivity or the way their atoms take up space.
One type of isomerism is structural isomerism, also known as constitutional isomerism. Here, the atoms and functional groups are connected in different orders. Think of them as differently arranged puzzles that use the same pieces. For example, for the compound with the formula \( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \), we've identified three possible structural isomers: propan-1-ol, propan-2-ol, and methoxyethane. Each has the same formula but different connectivity.
It's key to note that isomers can have distinct chemical reactivities and properties, making understanding their differences crucial in fields like pharmaceuticals and materials science.

• Propan-1-ol: alcohol with hydroxyl at the end carbon.
• Propan-2-ol: alcohol with hydroxyl on the central carbon.
• Methoxyethane: an ether with an oxygen bridge between two carbon chains.

Structural Formulas

A structural formula in organic chemistry is a visual representation of how atoms are arranged and bonded in a molecule. While molecular formulas show only the number and types of atoms present, structural formulas provide a clearer picture of the bond types and molecular architecture. This is crucial for understanding chemical reactions and properties.
When you look at a structural formula, you're seeing a two-dimensional drawing that represents the three-dimensional molecule. For example, methanol, \( \mathrm{CH}_{4} \mathrm{O} \), is depicted as \( \mathrm{CH}_{3} \mathrm{OH} \), showing how one carbon atom is bonded to three hydrogen atoms and a hydroxyl group. This gives insight into how methanol might react under various conditions.
Structural formulas help chemists understand potential reactions, predict the behavior of a substance, and design synthetic pathways for new molecules.

• Show connectivity: reveal which atoms are bonded.
• Functional group location: crucial for understanding reactivity.
• Determine isomer possibilities: visualize different arrangements.

Reaction Mechanisms

Reaction mechanisms in organic chemistry depict the step-by-step sequence of elementary reactions by which overall chemical change occurs. They help us understand how and why reactions proceed, allowing chemists to trace the movement of electrons and predict reaction intermediates.
Take for instance the reaction of \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br} \) and methanol (\( \mathrm{CH}_{3} \mathrm{OH} \)). Here, the bromine atom in ethyl bromide is replaced by the methanol's oxygen, forming methoxyethane. This is a subtler reaction, known as a nucleophilic substitution, where the nucleophile (oxygen atom) attacks an electron-deficient carbon atom, displacing the leaving group (bromine in this case).
Understanding these steps is important for predicting products in reactions, such as the behavior of each isomer of \( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \) with hydrobromic acid. Reactants and products are often determined by the reaction path taken.

• Identify nucleophiles and electrophiles: crucial players in reaction pathways.
• Predict reaction kinetics and thermodynamics: assist in understanding reaction speed and favorability.
• Highlight transition states and intermediates: temporary molecules in reaction progress.

Functional Groups

Functional groups are specific groups of atoms within molecules that have characteristic properties and chemical reactivity. These groups play a huge role in the behavior of organic compounds, dictating many of their properties and reactions. Recognizing functional groups helps chemists categorize compounds and infer reactivity patterns.
An example includes the hydroxyl group (\(-\mathrm{OH}\)) found in alcohols. It's present in propan-1-ol and propan-2-ol, making these compounds prototypical alcohols that can engage in reactions typical to this group. Another example is the ether group (\(-\mathrm{O}-\)) in methoxyethane, distinguishing it from alcohols and assigning different reactivity.
Identifying the functional group allows us to predict the behavior and transformations a molecule may undergo in reactions. Moreover, when considering the reaction of these \( \mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O} \) isomers with hydrobromic acid, the nature of the functional group present is pivotal in determining the resultant products.

• Hydroxyl (alcohol) group: \(-\mathrm{OH}\) determines solubility and reactivity.
• Ether group: \(-\mathrm{O}-\) influences boiling points and reaction types.
• Carboxylates, amines, hydrocarbons: just a few more types to explore.