Question

Aldehydes and ketones undergo acid-catalyzed reaction with alcohols to yield hemiacetals, compounds that have one alcohol-like oxygen and one ether-like oxygen bonded to the same carbon. Further reaction of a hemiacetal with alcohol then yields an acetal, a compound that has two ether-like oxygens bonded to the same carbon. (a) Show the structures of the hemiacetal and acetal you would obtain by reaction of cyclohexanone with ethanol. (b) Propose a mechanism for the conversion of a hemiacetal into an acetal.

Short answer

Hemiacetal has one OH and one O-Et group; acetal has two O-Et groups.

Step-by-step solution

Structure of Cyclohexanone

Begin with cyclohexanone which is a cyclic ketone. Draw the structure: a six-membered carbon ring with a double-bonded oxygen (carbonyl group =CO) attached to the top carbon atom.

Reaction with Ethanol to Form Hemiacetal

Add one molecule of ethanol to cyclohexanone. The oxygen of the hydroxyl group in ethanol will attack the carbonyl carbon of cyclohexanone, resulting in the formation of a hemiacetal. The structure has one alcohol-like hydroxyl (OH) and one ether-like group bonded to the same carbon.

Structure of Hemiacetal

The hemiacetal of cyclohexanone and ethanol has a six-membered ring with a new OH group and an OR group (where R is the ethyl group, C2H5) attached to the former carbonyl carbon.

Further Reaction to Form Acetal

React the hemiacetal with another mole of ethanol to undergo nucleophilic substitution: the newly formed OH group is replaced by an ethoxy (O-Et) group from ethanol, forming an acetal. This results in a structure with two ether-like groups attached to the former carbonyl carbon.

Mechanism for Conversion of Hemiacetal into Acetal

Protonate the OH group in the hemiacetal to make it a better leaving group (forming water), and then form a carbocation. The carbocation is attacked by the electron-pair from another ethanol molecule resulting in the formation of the acetal where two ethoxy groups (O-Et) are bonded to the former carbonyl carbon.

Key concepts

Cyclohexanone

Cyclohexanone is a crucial cyclic ketone in organic chemistry, playing an essential role in various chemical reactions. Its structure consists of a six-membered carbon ring with a carbonyl group (a carbon double-bonded to oxygen) at the top. The carbonyl group gives cyclohexanone its reactive nature, making it susceptible to nucleophilic attacks. This property is vital in reactions involving aldehydes and ketones, such as the formation of hemiacetals and acetals. Understanding the structure of cyclohexanone helps in predicting its behavior when reacting with alcohols, such as ethanol.

Aldehyde and Ketone Reactions

Aldehydes and ketones are key players in organic reactions, particularly when interacting with alcohols. They undergo acid-catalyzed reactions that transform them into hemiacetals and acetals. In the case of cyclohexanone, the reaction with ethanol leads initially to a hemiacetal. Here, the hydroxyl group from the ethanol attacks the electrophilic carbonyl carbon, creating a molecule with both an alcohol-like oxygen and an ether-like oxygen. Subsequent reaction with another ethanol molecule results in an acetal, where two ether-like oxygens bond to the same carbon. This transformation showcases the versatility of aldehyde and ketone reactions in synthesizing new organic compounds.

Nucleophilic Substitution

Nucleophilic substitution is a fundamental mechanism in organic chemistry, where a nucleophile replaces a leaving group in a molecule. In the conversion of hemiacetals to acetals, this concept is at play. The hydroxyl group in the hemiacetal becomes protonated, making it a good leaving group in the form of water. As the hydroxyl group departs, a carbocation intermediate forms. This carbocation is highly reactive and quickly attacked by ethanol's nucleophilic oxygen, leading to the formation of an acetal. Understanding nucleophilic substitution is crucial for grasping more complex organic reaction mechanisms.

Organic Reaction Mechanism

The organic reaction mechanism describes step-by-step how chemical reactions proceed on a molecular level. In the transformation of hemiacetals to acetals, each step follows a distinct path governed by molecular interactions. The initial protonation of the hydroxyl group in the hemiacetal initiates the reaction. This step enhances the leaving group's ability to depart, generating a reactive carbocation. The subsequent attack by an ethanol molecule's electron-rich oxygen is the key step in forming the stable acetal. By understanding these mechanisms, chemists can manipulate reaction conditions to favor desired products, highlighting the importance of mastering reaction pathways in organic chemistry.