Question

(a) Upon treatment with sulfuric acid, a mixture of ethyl and n-propyl alcohols yields a mixture of three ethers. What are they? (b) On the other hand, a mixture of tert- butyl alcohol and ethyl alcohol gives a good yield of a single ether. What ether is it likely to be? How can the good yield be accounted for?

Short answer

In part (a), the mixture of ethyl and n-propyl alcohols treated with sulfuric acid yields the following three ethers: diethyl ether (\(\mathrm{CH_3CH_2OCH_2CH_3}\)), ethyl n-propyl ether (\(\mathrm{CH_3CH_2OCH_2CH_2CH_3}\)), and di-n-propyl ether (\(\mathrm{CH_3CH_2CH_2OCH_2CH_2CH_3}\)). In part (b), the mixture of tert-butyl alcohol and ethyl alcohol forms tert-butyl ethyl ether (\(\mathrm{(CH_3)_3COCH_2CH_3}\)). The good yield can be attributed to the lack of an acidic hydrogen in tert-butyl alcohol, reducing side-reactions and the steric hindrance of the tert-butyl group, further increasing the yield of the desired ether.

Step-by-step solution

Part (a): Ethyl and n-Propyl Alcohols Etherification Reaction

In order to determine the possible ethers formed from the mixture of ethyl alcohol and n-propyl alcohol, we write the following reaction scheme: Ethyl alcohol: \(\mathrm{CH_3CH_2OH}\) n-Propyl alcohol: \(\mathrm{CH_3CH_2CH_2OH}\) Now, the possible combinations are: 1. Ethyl alcohol + Ethyl alcohol 2. Ethyl alcohol + n-Propyl alcohol 3. n-Propyl alcohol + n-Propyl alcohol Now, we have to write the corresponding ether for each reaction type: 1. Ethyl - O- ethyl: \(\mathrm{CH_3CH_2OCH_2CH_3}\) (diethyl ether) 2. Ethyl - O - n-Propyl: \(\mathrm{CH_3CH_2OCH_2CH_2CH_3}\) (ethyl n-propyl ether) 3. n-Propyl - O - n-Propyl: \(\mathrm{CH_3CH_2CH_2OCH_2CH_2CH_3}\) (di-n-propyl ether) So the possible ethers formed in part (a) are diethyl ether, ethyl n-propyl ether, and di-n-propyl ether.

Part (b): Tert-Butyl Alcohol and Ethyl Alcohol Etherification Reaction

In order to determine the ether formed from the mixture of tert-butyl alcohol and ethyl alcohol, we write the following reaction scheme: Tert-butyl alcohol: \(\mathrm{(CH_3)_3COH}\) Ethyl alcohol: \(\mathrm{CH_3CH_2OH}\) Now, since tert-butyl alcohol does not have an acidic hydrogen, it can only react as an alkoxide like: Tert-Butyl - O - Ethyl: \(\mathrm{(CH_3)_3COCH_2CH_3}\) (tert-butyl ethyl ether) So, the only possible ether formed in part (b) is tert-butyl ethyl ether. The good yield in the synthesis of tert-butyl ethyl ether can be accounted for by the nature of the starting alcohols. Tert-butyl alcohol lacks an acidic hydrogen, meaning that it can only react as an alkoxide in the etherification reaction, reducing the number of side-reactions (only alcohol which acts as a leaving group). Additionally, the steric hindrance of the tert-butyl group makes it less likely for ethyl alcohol to react with another molecule of ethyl alcohol, further increasing the yield of the desired ether.

Key concepts

Diethyl Ether

Diethyl ether is one of the simplest members of the ether family and is known for its former widespread use as an anesthetic. It is a colorless, volatile liquid with a characteristic ethereal odor.

Chemically, diethyl ether is represented by the formula \( \mathrm{CH_3CH_2OCH_2CH_3} \). It is an example of a symmetrical ether since it contains two identical ethyl groups attached to either side of the oxygen atom. During its formation, through a process known as etherification, two ethanol molecules react in the presence of an acid catalyst, typically sulfuric acid. The acid's role is to protonate the alcohol to create a better leaving group, which in turn facilitates the nucleophilic attack by another alcohol molecule, leading to the formation of diethyl ether and water.

This reaction can occur between two molecules of the same alcohol, which in the case of ethanol, leads to the creation of diethyl ether. The balanced reaction can be written as:\[ 2 \mathrm{CH_3CH_2OH} \rightarrow \mathrm{CH_3CH_2OCH_2CH_3} + \mathrm{H_2O} \]

tert-Butyl Ethyl Ether

The tert-butyl ethyl ether, with the formula \( \mathrm{(CH_3)_3COCH_2CH_3} \), is another example of an asymmetrical ether. It is formed when tert-butyl alcohol reacts with ethyl alcohol. The tert-butyl group, being bulky, introduces steric hindrance which minimizes potential reactions that could lead to other products. Thus, this steric effect plays a crucial role in enhancing the yield of tert-butyl ethyl ether in a reaction.

In terms of production, the reaction is favored as tert-butyl alcohol forms a very stable carbocation due to its three methyl groups. This stability makes the alkoxide intermediate more likely to react with ethyl alcohol rather than with another tert-butyl alcohol molecule, which is less reactive due to steric hindrance. As a result, the reaction is driven towards the formation of tert-butyl ethyl ether as the major product, further explaining why the yield is high in this case.

Alkoxide Formation

Alkoxides are potent nucleophiles formed when alcohols react with a strong base or in the presence of a dehydrating acid catalyst. The general structure of an alkoxide ion is \( \mathrm{R-O^-} \), where R is an alkyl group. They play a key role as intermediates in the synthesis of ethers, such as in the etherification process.

The formation of an alkoxide is a crucial step in ether synthesis. It requires the removal of a proton from the alcohol, which is facilitated by the acid catalyst in acid-catalyzed reactions. For instance, when tert-butyl alcohol \( \mathrm{(CH_3)_3COH} \) is mixed with sulfuric acid, the acidic environment protonates the alcohol, leading to the departure of water and the formation of a resilient tert-butoxide ion \( \mathrm{(CH_3)_3CO^-} \). This ion then readily reacts with other alcohols to form ethers, explaining its importance in the etherification reaction.

Acid-Catalyzed Dehydration

Acid-catalyzed dehydration is a common chemical reaction where an alcohol loses a water molecule to become an alkene or an ether. This process is crucial in the context of etherification where an acid such as sulfuric acid catalyzes the dehydration of alcohols to form ethers.

In this mechanism, the acid protonates the alcohol, transforming it into a more reactive form. For the etherification to occur, two such activated alcohol molecules interact, where one acts as a nucleophile and the other as an electrophile. The outcome is the removal of a molecule of water (dehydration) and the formation of an ether linkage, exemplified by the transition from \( \mathrm{CH_3CH_2OH} \) to diethyl ether \( \mathrm{CH_3CH_2OCH_2CH_3} \).

Common Pitfalls

When learning about acid-catalyzed dehydration, it's essential to understand the difference between dehydration that leads to ethers versus dehydration that yields alkenes. Factors like temperature, the nature of the alcohol, and the type of acid used can sway the reaction towards producing either ethers or alkenes. It is the acid's ability to promote alkoxide ion formation that dictates the path towards ether production.