Question

Ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), reacts with HBr, hydrogen bromide, to give ethyl bromide, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br}\), and water. Ethanol, however, does not react with NaBr. Explain. Suggest a mechanism for the reaction with HBr.

Short answer

Ethanol reacts with HBr but not NaBr because HBr is an electrophile due to its polar covalent bond, allowing it to react with the electron-rich hydroxyl group in ethanol. In contrast, NaBr is an ionic compound without an electrophilic center and doesn't directly react with ethanol. The mechanism for the reaction with HBr involves two steps: (1) proton transfer, forming a protonated ethanol molecule and a bromide ion, and (2) nucleophilic substitution, where the bromide ion attacks the carbon connected to the hydroxyl group, leading to the formation of ethyl bromide and water.

Step-by-step solution

1. Analyze the Structure of the Compounds

Ethanol, HBr, and NaBr are compounds with different chemical properties. Ethanol (\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)) is an alcohol that contains an -OH group. HBr is a hydrogen halide with a polar covalent bond between its hydrogen and bromine atoms, making it acidic when dissolved in water. On the other hand, NaBr is an ionic salt with a stable, ionic bond between the sodium and bromine ions.

2. Identify the Nature of Ethanol and its Reactivity

Ethanol is an organic compound with electron-rich sites that can readily participate in chemical reactions. The primary reactive site of ethanol is its hydroxyl group (-OH), which contains a lone pair on its oxygen atom. This site can participate in proton transfer or nucleophilic reactions, allowing it to react with reagents that exhibit electrophilic behavior.

3. Compare the Reactivity of HBr and NaBr

HBr can act as an electrophile because of its polar covalent bond, which creates a partial positive charge on the hydrogen atom. This electrophilic nature enables it to react with electron-rich sites such as those found in ethanol. On the other hand, NaBr is an ionic compound that does not possess an electrophilic center. Instead, it dissociates into Na+ and Br- ions in the aqueous solution, with no direct reactivity towards ethanol's nucleophilic sites. Thus, NaBr does not react with ethanol.

4. Suggest a Mechanism for the Reaction of Ethanol with HBr

The reaction between ethanol and HBr can be described in the following two steps: Step 1: Proton Transfer In the first step, the electron-rich oxygen atom of the hydroxyl group in ethanol acts as a nucleophile and forms a bond with the electrophilic hydrogen atom of HBr. This process results in the formation of a protonated ethanol molecule and a bromide ion (\(\text{Br}^{-}\)). \[ \text{C}_{2}\text{H}_{5}\text{OH} + \text{HBr} \rightarrow \text{C}_{2}\text{H}_{5}\text{OH}_{2}^{+} + \text{Br}^{-} \] Step 2: Nucleophilic Substitution In the second step, the bromide ion acts as a nucleophile and attacks the electron-poor carbon atom that is connected to the protonated hydroxyl group. This leads to the substitution of the hydroxyl group with the bromine atom and the formation of ethyl bromide and water as products. \[ \text{C}_{2}\text{H}_{5}\text{OH}_{2}^{+} + \text{Br}^{-} \rightarrow \text{C}_{2}\text{H}_{5}\text{Br} + \text{H}_{2}\text{O} \] Overall, the reaction between ethanol and HBr can be represented as follows: \[ \text{C}_{2}\text{H}_{5}\text{OH} + \text{HBr} \rightarrow \text{C}_{2}\text{H}_{5}\text{Br} + \text{H}_{2}\text{O} \]

Key concepts

Ethanol Reactivity

Ethanol, also known as ethyl alcohol, is an organic compound and a type of alcohol. Its chemical formula is \( ext{C}_2 ext{H}_5 ext{OH}\). The reactivity of ethanol largely comes from its hydroxyl group (-OH), which contains an oxygen atom with electrons that can interact with other chemical species.

Ethanol can undergo a variety of reactions, making it a versatile reactant in chemistry. This is because the hydroxyl group can participate in:

• Nucleophilic substitution reactions, where it acts as a site for ionic attack.
• Proton transfer reactions, where it can either donate or accept protons.

When ethanol reacts with reagents that have electrophilic properties, such as hydrogen bromide (HBr), it can lead to the formation of new compounds, such as ethyl bromide. This ability to react with various compounds highlights its significant role in organic chemistry.

Electrophilic Reaction

An electrophilic reaction involves an electrophile, a positively charged or electron-deficient species. Electrophiles seek out electron-rich sites to form chemical bonds. In our reactions, the focus is on hydrogen bromide (HBr) which acts as an electrophile.

HBr contains a polar covalent bond between the hydrogen and bromine. This results in a partial positive charge on the hydrogen atom, due to its affinity to acquire more electrons. When HBr encounters an electron-rich site, such as the hydroxyl group (-OH) in ethanol, it is capable of accepting electrons.

During the reaction, the oxygen atom of the hydroxyl group in ethanol donates its electrons to the hydrogen of HBr, highlighting an electrophilic reaction. This initial contact sets the stage for the protonation of ethanol, a critical first step in converting ethanol to ethyl bromide.

Proton Transfer

Proton transfer is a fundamental concept in chemistry that involves the transfer of a hydrogen ion (proton) from one molecule to another. In the context of ethanol's reaction with HBr, proton transfer is a crucial step.

During this reaction, the oxygen atom in the ethanol molecule donates its electrons to the hydrogen atom of HBr. This electron transfer facilitates the movement of the hydrogen ion toward the oxygen, leading to the formation of protonated ethanol \( ext{C}_2 ext{H}_5 ext{OH}_2^+\).

The creation of this new molecule signifies the completion of the proton transfer step. The protonated alcohol now displays increased reactivity, readying the structure for further transformation, specifically nucleophilic substitution, where a bromide ion will replace the hydroxyl group.

Alcohol Chemistry

Alcohol chemistry focuses on the behavior and characteristics of alcohols, like ethanol, in chemical reactions. Alcohols are defined by the presence of a hydroxyl (-OH) group, which makes them versatile in multiple chemical processes.

In particular, ethanol is a critical example in alcohol chemistry because:

• It can participate in substitution reactions, where the -OH group can be replaced, as seen in reactions with electrophiles like HBr.
• The hydroxyl group is an active site for nucleophilic attacks due to its lone pairs of electrons.
• It can engage in dehydration reactions, proton transfer, and oxidation.

These characteristics make ethanol very reactive, with diverse applications in synthesis and industrial chemistry. Understanding the behavior of ethanol in these reactions demonstrates the broader concepts of alcohol chemistry, showcasing how functional groups lead to significant reactivity and variations in potential chemical outcomes.