Question

The reactions which does not correctly match with major product is/are (A) \(\mathrm{R}-\mathrm{OH}+\mathrm{NaBr}+\mathrm{H}_{3} \mathrm{PO}_{4} \longrightarrow \mathrm{R}-\mathrm{Br}+\mathrm{NaH}_{2} \mathrm{PO}_{4}+\mathrm{H}_{2} \mathrm{O}\) (B) \(\mathrm{R}-\mathrm{OH}+\mathrm{NaI}+\mathrm{H}_{3} \mathrm{PO}_{4} \longrightarrow \mathrm{R}-\mathrm{I}+\mathrm{NaH}_{2} \mathrm{PO}_{4}+\mathrm{H}_{2} \mathrm{O}\) (C) \(\mathrm{R}-\mathrm{OH}+\mathrm{NaBr}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{R}-\mathrm{Br}+\mathrm{NaHSO}_{4}+\mathrm{H}_{2} \mathrm{O}\) (D) \(\mathrm{R}-\mathrm{OH}+\mathrm{NaI}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{R}-\mathrm{I}+\mathrm{NaHSO}_{4}+\mathrm{H}_{2} \mathrm{O}\)

Short answer

All the reactions given (A, B, C, and D) correctly match with their major products.

Step-by-step solution

In each case, we see a substitution reaction taking place. A hydroxyl group of the alcohol (\(R-OH\)) is getting replaced by a halogen atom (\(Br\), \(I\)). #Step 2: Analyze the reagents and conditions in each reaction#

The key to determining whether the reactions produce the given major product is analyzing the reagent and reaction conditions, as follows: - Option (A): \(R\)-OH is an alcohol, NaBr is a sodium halide, and \(H_3 PO_4\) is phosphoric acid. - Option (B): \(R\)-OH is an alcohol, NaI is a sodium halide, and \(H_3 PO_4\) is phosphoric acid. - Option (C): \(R\)-OH is an alcohol, NaBr is a sodium halide, and \(H_2 SO_4\) is sulfuric acid. - Option (D): \(R\)-OH is an alcohol, NaI is a sodium halide, and \(H_2 SO_4\) is sulfuric acid. #Step 3: Evaluate the reaction conditions and major products for each option#

We can now evaluate each reaction option based on the reagents and reaction conditions: (A) With \(H_3 PO_4\) present, it works as a catalyst for the nucleophilic substitution reaction (SN1/SN2), depending on the alcohol structure. Thus, this option has a correct major product. (B) Similarly, with \(H_3 PO_4\), the substitution reaction takes place, leading to a correct major product. (C) Since \(H_2 SO_4\) is a strong acid, it protonates the alcohol and makes it a better leaving group. So, the substitution reaction will happen, leading to a correct major product. (D) This reaction will also follow the same mechanism as reaction (C), leading to a correct major product. #Step 4: Identify the reactions that do not match the major product#

Based on our evaluations in Step 3, we find that all the reactions given correctly match with their major products.

Key concepts

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are crucial in organic chemistry, often represented as SN1 or SN2 mechanisms. These involve replacing a leaving group in a molecule with a nucleophile. A nucleophile is an electron-rich species that seeks out positive charges, typically found where there is a deficiency of electrons.

For instance, in alcohol halogenation, a hydroxyl group (OH)) which is a poor leaving group, is transformed into a better one such as a water molecule, which can more readily depart. This transformation is often facilitated by the presence of an acid. The type of nucleophilic substitution reaction, either SN1 or SN2, depends on several factors such as the nature of the alcohol (primary, secondary, or tertiary), the strength of the nucleophile, and the solvent.

In an SN1 reaction, the rate-determining step involves the formation of a carbocation intermediate after the leaving group departs. This mechanism is most common in tertiary alcohols where steric hindrance affects the reaction. Meanwhile, in an SN2 reaction, the nucleophile attacks simultaneously as the leaving group departs, which is one swift step. This reaction is stereospecific and results in the inversion of the stereochemistry at the carbon center. This mechanism is often favored by primary alcohols.

Alcohol Halogenation

Mechanism of Alcohol Halogenation

Alcohol halogenation involves substituting the hydroxyl group of an alcohol with a halogen atom. The hydroxyl group is a bad leaving group due to its strong bond and basicity; however, in the presence of a strong acid, it can be protonated to form water, a much better leaving group. Once the hydroxyl group leaves, a carbocation is formed in an SN1 reaction, or a backside attack happens in an SN2 reaction.

Factors Affecting Halogenation

Several factors affect the efficiency of alcohol halogenation. The nature of the alcohol (primary, secondary, or tertiary) and the strength and size of the nucleophile (halide ion) greatly affect whether an SN1 or SN2 mechanism will occur. Solvent polarity also plays a significant role; polar protic solvents can stabilize carbocations and thus favor SN1 mechanisms, whereas polar aprotic solvents favor SN2 reactions by enabling better nucleophilic attack.

Products of Halogenation

During the halogenation process, the major product is the halogenated compound, where a halogen (Brcor I) replaces the hydroxyl group. By-products may include water and salts ensuing from the other reactants involved in the process, especially in the presence of acids that act as catalysts.

Reagents and Reaction Conditions

Reagents and reaction conditions play pivotal roles in dictating the path and outcome of chemical reactions. In organic transformations like nucleophilic substitution reactions, understanding these variables is essential for predicting or controlling products.

Function of Reagents

Reagents are substances added to cause a chemical reaction. For nucleophilic substitution reactions, the nucleophile and the leaving group, often represented as reagents, are fundamental. Reagents like NaBr or NaI provide the nucleophile (Brcor I), while acid reagents like H3PO4or H2SO4aid in converting the poor leaving group (OH) into a better one like water).

Importance of Reaction Conditions

Reaction conditions, including temperature, solvent, and presence of acids or bases, drastically influence reaction mechanisms. For example, the presence of H3PO4(catalyst) tends to support an SN1 mechanism for tertiary alcohols by promoting carbocation formation. In contrast, strong acids such as sulfuric acid (H2SO4can protonate the alcohol to facilitate the leaving of the hydroxyl group, conducive for both SN1 and SN2 mechanisms depending on other conditions.

Assessing these elements combined with knowledge of organic chemistry principles allows chemists to predict reaction outcomes more accurately. Practical execution, often described in textbooks, demonstrates these concepts through problems that reinforce the understanding that the correct choice and manipulation of reagents and conditions are essential to success in chemical synthesis.