Question

Predict the major products, if any, for the reaction of isobuty1 n-propyl ether with: (a) \(\mathrm{H}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), room temperature (b) Dilute, aqueous \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) Hot, concentrated HBr.

Short answer

(a) No significant reaction (b) No significant reaction (c) Isobutyl bromide and n-propyl bromide

Step-by-step solution

Draw the structure of isobutyl n-propyl ether

Isobutyl n-propyl ether is an ether with an isobutyl group on one side and a propyl group on the other side. Its structure can be drawn as follows: R-O-R' where R = isobutyl (CH3)2CHCH2 and R' = propyl (CH3)2CH2

Reaction with \(\mathrm{H}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) at room temperature

The given reagent, \(\mathrm{H}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), is a strong oxidizing agent. Ethers don't typically undergo oxidation with \(\mathrm{H}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\). Thus, there will be no major reaction under these conditions, and no significant products will be formed.

Reaction with dilute, aqueous \(\mathrm{H}_{2} \mathrm{SO}_{4}\)

Reaction with dilute \(\mathrm{H}_{2}\mathrm{SO}_{4}\) involves protonation of the ether oxygen atom followed by attack from a nucleophile. However, this reaction pathway is typically slow for most ethers (except acid-sensitive groups like epoxides) and therefore, there won't be any major reaction or significant product formed under these conditions.

Reaction with hot, concentrated HBr

In the presence of hot, concentrated HBr, the ether undergoes acid-catalyzed cleavage. This reaction involves the protonation of the ether oxygen atom, followed by a nucleophilic attack from the bromide ion. The ether bond is cleaved, and we can form two alkyl halides as the major products. For our case, isobutyl n-propyl ether will form isobutyl bromide and n-propyl bromide as the major products. The final products for each reaction are: (a) No significant reaction (b) No significant reaction (c) Isobutyl bromide and n-propyl bromide

Key concepts

Ether Cleavage

Ether cleavage is a type of chemical reaction where an ether is split into two different fragments. Ethers generally have a structure of R-O-R', where R and R' are alkyl or aryl groups. Ethers tend to be quite stable and resistant to many common chemical reagents. However, when subjected to certain strong acids, especially at high temperatures, they can undergo cleavage.

• Cleavage often involves protonation of the oxygen atom by a strong acid, making the molecule more susceptible to nucleophilic attacks.
• In acid-catalyzed ether cleavage, the bonds nearest to the oxygen are the ones usually broken.
• Most commonly, strong acids like hydrobromic acid (HBr) or hydroiodic acid (HI) at elevated temperatures are used to cleave ethers effectively.

In our example, the use of hot, concentrated HBr cleaves isobutyl n-propyl ether into isobutyl bromide and n-propyl bromide. This is because the bromide ion attacks the protonated ether, breaking the C-O bond and forming the resulting products.

Oxidation Reactions

Oxidation reactions involve the increase in oxidation state of a molecule, typically through the loss of electrons. In organic chemistry, oxidizing agents like \(\mathrm{H}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) are used to add oxygen to or remove hydrogen from a molecule. However, ethers are quite resistant to oxidation because they lack abstractable hydrogen atoms adjacent to the ether oxygen.

• Oxidizing agents such as chromic acid ( \(\mathrm{H}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\)) are effective at oxidizing alcohols, especially primary and secondary ones, but not ethers.
• This reactivity pattern ensures that under normal conditions, ethers remain unchanged in the presence of strong oxidizers.

In the case of isobutyl n-propyl ether, no significant product is formed when reacted with \(\mathrm{H}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}\) at room temperature, highlighting the inert nature of ethers to oxidation processes.

Nucleophilic Substitution

Nucleophilic substitution is a reaction type where a nucleophile forms a bond with a carbon atom, displacing a leaving group. Ethers can undergo nucleophilic substitution, especially when the oxygen atom is protonated, but typically, they are not very reactive under normal conditions without the presence of strong acid or heat.

• The protonation of an ether converts the oxygen atom into a better leaving group, making substitution more feasible.
• In an ether, the nucleophile usually attacks carbon atoms directly bonded to the electronegative oxygen atom, especially if electron-deficient centers exist.

With isobutyl n-propyl ether reacting with HBr, the nucleophile in action is the bromide ion. It attacks the positively charged carbon atoms (due to protonation), leading to nucleophilic substitution and eventual cleavage of the C-O bond, forming two alkyl bromides.

Acid-Catalyzed Reaction

Acid-catalyzed reactions are those where an acid serves to accelerate or allow the reaction to proceed more efficiently. In organic chemistry, acids can donate protons to various functional groups, transforming them into highly reactive sites for subsequent reactions. In the context of ethers, acid-catalyzed reactions are crucial for promoting cleavage.

• These reactions generally begin with the protonation of the ether oxygen, increasing the positive charge and rendering the molecule more susceptible to nucleophilic attacks.
• The increased positive charge on the molecule not only contributes to more efficient cleavage but also regulates the reaction pathway via charge distribution.

During the reaction of an ether with a strong acid like HBr, the acidic environment results in the cleavage and transformation of ethers into easier-to-react organic components, such as alkyl halides. This is how the products, isobutyl bromide and n-propyl bromide, are conveniently formed through acid-catalyzed ether cleavage.