MEDCHEM 11 Notes

Learning Outcomes

Recall general structures of aldehydes and ketones:
Aldehydes: General structure is RCHO, where R can be a hydrogen atom (H) or a hydrocarbon group such as an alkyl or aryl group. Aldehydes are characterized by the presence of a carbonyl group (C=O) positioned at the end of the carbon chain.
Ketones: General structure is RC(=O)R’, where both R and R’ are hydrocarbon groups, indicating that the carbonyl group is located within the carbon chain. Ketones typically possess a central carbonyl carbon flanked by two other carbon groups.
Explain polarity of the carbonyl group:
The carbonyl group (C=O) exhibits polarity due to the significant difference in electronegativity between carbon, which has an electronegativity of about 2.5, and oxygen, which has an electronegativity of approximately 3.5. This results in oxygen carrying a partial negative charge while carbon carries a partial positive charge, making the carbonyl group an electrophilic site that is susceptible to nucleophilic attack.
Recall the relative reactivity of aldehydes and ketones toward nucleophiles:
Aldehydes are generally more reactive than ketones due to two key factors: 1) Aldehydes have less steric hindrance as they have only one alkyl group (R), making the electrophilic carbon more accessible to nucleophiles. 2) Aldehydes are more electrophilic because the electron-donating effect of the alkyl group is less pronounced compared to that in ketones, which have two R groups influencing the carbonyl carbon.
Recall products and mechanisms of reactions of aldehydes and ketones with alcohols:
Aldehyde/ketone + alcohol (in the presence of an acid catalyst) → hemiacetal formation. This process involves the nucleophilic addition of the alcohol to the electrophilic carbonyl carbon, leading to the formation of a hemiacetal, which contains an alcohol and an ether on the same carbon.
Hemiacetal + alcohol (in excess, in presence of acid) → acetal formation. If enough alcohol is present, the hemiacetal can be converted into a more stable acetal, which features two ether groups bonded to the same carbon.
Recall products and mechanisms of reactions of aldehydes and ketones with nitrogen nucleophiles:
Aldehyde/ketone + amine → imine formation, commonly referred to as a Schiff base. In this reaction, the amine acts as a nucleophile, attacking the carbonyl carbon, and releasing water as a byproduct. The imine contains a C=N bond and has substantial importance in biological systems.
Aldehyde/ketone + hydroxylamine → oxime formation, where hydroxylamine serves as the nucleophile, leading to a similar nucleophilic addition reaction that produces an oxime functional group.
Aldehyde/ketone + hydrazine → hydrazone formation, which is used in organic synthesis and analysis as a method to identify carbonyl compounds. This reaction typically produces a stable hydrazone product.

Occurrence of Aldehydes and Ketones

Natural Sources: Aldehydes and ketones are prevalent in nature, often playing critical roles in biological compounds. For example, retinal, crucial for vision in animals, is an aldehyde. Additionally, many fruits like raspberries and bananas contain naturally occurring aldehydes and ketones that contribute to their flavors and aromas.
Synthetic Sources: These compounds are also widely utilized in various industries, including pharmaceuticals, where they serve as intermediates and active ingredients. Notable examples include formaldehyde used as a preservative and vanillin, the primary component of vanilla flavoring, and pentoxifylline, which is used to improve blood flow.

Structural and Physical Properties

Carbonyl Group: The carbonyl carbon is sp2 hybridized, forming a trigonal planar geometry with bond angles around 120°. This configuration allows for unique spatial orientation that facilitates various chemical reactions.
Polarity: The polar C=O bond endows carbonyl compounds with varied solubility and reactivity patterns; they typically dissolve well in polar solvents and act as electrophiles in nucleophilic addition reactions due to the presence of the partial charges created by the difference in electronegativity.

Nucleophilic Addition Reactions

General Mechanism: In nucleophilic addition, nucleophiles (which can be comprised of O, N, or C species) attack the electrophilic carbon of the carbonyl group, leading to varying product formation based on the nature of the nucleophile. The reaction typically results in the formation of a tetrahedral intermediate that can further rearrange to form different functional groups.
Nucleophiles: Common nucleophiles that participate in these reactions include alcohols (which donate oxygen) and amines (which donate nitrogen). These nucleophiles play crucial roles in forming different derivatives of aldehydes and ketones.
Reactivity: Aldehydes' greater reactivity compared to ketones can be attributed to steric hindrance from alkyl groups surrounding the carbonyl carbon, along with electronic effects that affect the electrophilicity of the carbonyl group.

Hemiacetals and Acetals

Hemiacetal Formation:

Protonation of the carbonyl carbon: This step increases its electrophilicity, making it more susceptible to nucleophilic attack.
Nucleophile (alcohol) adds to the activated carbonyl: This process leads to the formation of the hemiacetal, which can exist in equilibrium with the aldehyde or ketone.
Formation of acetal: With the presence of excess alcohol, the hemiacetal can undergo dehydration to form a stable acetal by replacing the hydroxy group with another alkoxy group.

Acetal Structure: Acetals contain two ether groups bonded to the same carbon atom, distinct from hemiacetals which contain one ether and one hydroxyl (alcohol) functional group.

Reactions with Nitrogen Nucleophiles

Mechanism Overview:
The formation of imines involves the nucleophilic attack of an amine on the electrophilic carbon of the carbonyl group, leading to the elimination of water.
Aldehyde/ketone + hydroxylamine forms an oxime, while the reaction with hydrazine produces a hydrazone. These reactions can often be facilitated by acid catalysis, which accelerates the nucleophilic addition.
Rate of Reaction: Acid catalysis can significantly increase the speed of these reactions, although many nitrogen nucleophiles can still react with carbonyl compounds without the need for an acid catalyst.

Summary of Key Reactions

Aldehyde/Ketone + Alcohol:
1:1 ratio of alcohol to aldehyde/ketone results in the formation of a hemiacetal.
1:2 ratio (excess alcohol) leads to the formation of a more stable acetal.
Aldehyde/Ketone + Nitrogen Nucleophiles:
Reactions with nitrogen nucleophiles yield different products: Imine, Oxime, or Hydrazone, depending on the specific nitrogen species used in the reaction.

Learning Outcomes with Detailed Explanations

Recall general structures of aldehydes and ketones:
Aldehydes: The general structure of aldehydes is represented as RCHO, where R can be either a hydrogen atom (H) or a hydrocarbon group (alkyl or aryl). Aldehydes are uniquely characterized by the carbonyl (C=O) group located at the end of the carbon chain, which makes them distinct from other carbonyl compounds. The hydrogen atom on the carbonyl carbon classifies aldehydes based on their ability to react in nucleophilic addition reactions due to the electrophilic nature of the carbonyl carbon.
Ketones: The general structure of ketones is represented as RC(=O)R’, where R and R’ are hydrocarbon groups (alkyl or aryl). Unlike aldehydes, ketones have their carbonyl group placed within the carbon chain, necessitating that both R and R’ must be carbon-containing groups. This placement significantly influences their chemical reactivity and stability compared to aldehydes, as the central carbonyl carbon is typically flanked by two hydrocarbon groups.
Explain polarity of the carbonyl group:
The carbonyl group (C=O) presents regional polarity due to the substantial difference in electronegativity between carbon (around 2.5) and oxygen (approximately 3.5). Because oxygen is more electronegative, it attracts the shared electron pair more closely than carbon, leading to a partial negative charge (δ-) on oxygen and a partial positive charge (δ+) on carbon. This polarization results in the carbonyl carbon being an electrophilic site, making it highly susceptible to nucleophilic attack during chemical reactions, such as nucleophilic addition.
Recall the relative reactivity of aldehydes and ketones toward nucleophiles:
Reactivity Comparison: Aldehydes exhibit greater reactivity towards nucleophiles in comparison to ketones. This enhanced reactivity can be attributed to two primary factors: 1) Steric Hindrance: Aldehydes possess only one alkyl group (R), which results in less steric hindrance around the carbonyl carbon. This allows nucleophiles easier access to the electrophilic center. 2) Electrophilicity: The electron-donating effect of the single R group on an aldehyde is significantly less than that of the two R groups present in ketones, which diminishes the electrophilic character of the carbonyl carbon in ketones thus making aldehydes more electrophilic.
Recall products and mechanisms of reactions of aldehydes and ketones with alcohols:
Hemiacetal Formation: When an aldehyde or ketone reacts with an alcohol in the presence of an acid catalyst, a hemiacetal is formed through a nucleophilic addition mechanism. The alcohol acts as a nucleophile and attacks the electrophilic carbonyl carbon, producing a hemiacetal which retains one hydroxyl (OH) group and one alkoxy (OR) group on the same carbon. This reaction is reversible and can reach equilibrium with the original carbonyl compound.
Acetal Formation: If more alcohol is provided (in excess) along with acid, the hemiacetal undergoes dehydration (loss of water) and transforms into a more stable acetal. An acetal features two ether (OR) groups attached to the same carbon atom, distinguishing it from hemiacetals, which have one ether and one hydroxyl group.
Recall products and mechanisms of reactions of aldehydes and ketones with nitrogen nucleophiles:
Imine Formation: When an aldehyde or ketone reacts with an amine, an imine forms—a process frequently referred to as the formation of a Schiff base. In this reaction, the amine acts as a nucleophile, attacking the electrophilic carbon of the carbonyl group and leading to water elimination during the reaction, creating a C=N bond within the imine.
Oxime Formation: When hydroxylamine reacts with an aldehyde or ketone, an oxime is synthesized. This occurs through a similar mechanism where hydroxylamine acts as a nucleophile, resulting in the addition reaction to the carbonyl carbon, followed by the release of water, yielding the oxime functional group (-C=N-OH).
Hydrazone Formation: A reaction between aldehydes or ketones and hydrazine produces a hydrazone. These reactions, typically facilitated by conditions that promote nucleophilic addition, produce stable hydrazone products that serve both in organic synthesis and analysis to identify carbonyl compounds based on their distinct properties.

Occurrence of Aldehydes and Ketones

Natural Sources: Aldehydes and ketones occur abundantly in nature due to their integral roles in various biological processes. For instance, retinal, an aldehyde derivative, is essential for vision in many animals. Numerous fruits—such as raspberries and bananas—also contain naturally occurring aldehydes and ketones that impart distinctive flavors and aromas instrumental to fruit sensory attributes.
Synthetic Sources: In synthetic applications, aldehydes and ketones are extensively used across different industries, including pharmacology. For example, formaldehyde serves as a preservative in biological sample treatment, while vanillin is the primary flavor source of vanilla, extracted or synthesized from various sources. Pentoxifylline, a ketone derivative, is clinically utilized to enhance blood flow.

Structural and Physical Properties

Carbonyl Group: The carbonyl carbon is sp2 hybridized, allowing the formation of a trigonal planar geometry with bond angles approximating 120°. This structural arrangement leads to unique spatial orientations which greatly influence the reactivity of carbonyl compounds. This configuration in both aldehydes and ketones facilitates interactions with nucleophiles, contributing to their chemical behavior.
Polarity: Carbonyl compounds are characterized by their polar C=O bond, granting them distinctive solubility and reactivity profiles. This polarity enables them to dissolve favorably in polar solvents and interact as electrophiles in nucleophilic addition reactions, largely driven by the partial positive and negative charges arising from the electronegativity differences between carbon and oxygen.

Nucleophilic Addition Reactions

General Mechanism: Nucleophilic addition reactions involve nucleophiles—often containing oxygen, nitrogen, or carbon species—attacking the electrophilic carbon of the carbonyl group. This process generally forms a tetrahedral intermediate, which can further rearrange into various functional groups (e.g., hemiacetals, acetals, imines) depending upon the nature of the nucleophile and reaction conditions.
Nucleophiles: Common nucleophiles include alcohols, which provide an oxygen atom that forms covalent bonds with the carbonyl carbon, and amines, which bond through nitrogen. These nucleophiles are vital in generating various derivatives of aldehydes and ketones, allowing for varied synthetic applications.
Reactivity: The enhanced reactivity of aldehydes compared to ketones is predominantly due to increased steric hindrance in ketones. The presence of two alkyl groups surrounding the carbonyl carbon in ketones hinders nucleophilic attack, while aldehydes with a single alkyl group are less sterically hindered, allowing for greater electrophilic accessibility during reactions.

Hemiacetals and Acetals

Hemiacetal Formation:

Protonation of the carbonyl carbon: This first step increases the electrophilicity of the carbonyl carbon, rendering it more amenable to nucleophilic attack.
Addition of the nucleophile (alcohol): The alcohol, acting as a nucleophile, adds to the activated carbonyl carbon, forming a hemiacetal that can exist in equilibrium with the original aldehyde or ketone.
Acetal Formation: In the presence of excess alcohol, dehydration occurs, converting the hemiacetal to a more stable acetal by replacing the hydroxy group with another alkoxy group.

Acetal Structure: The acetal structure is characterized by two ether groups (–O–) bonded to the central carbon, contrasting with hemiacetals that possess one ether and one hydroxyl (–OH) group. This structural difference critical enhances acetal stability in various solvolytic and synthetic environments.

Reactions with Nitrogen Nucleophiles

Mechanism Overview: The reaction mechanism involved in imine formation follows the nucleophilic attack of an amine on the electrophilic carbon of the carbonyl group, allowing for the elimination of a water molecule in the process. The creation of imines is a significant reaction as it forms C=N bonds that are central to many biosynthetic processes.
Further reactions: The reaction with hydroxylamine yields oximes, while that with hydrazine allows for hydrazone formation. These reactions, often catalyzed by acid, increase the reaction rate by stabilizing intermediates and facilitating nucleophilic additions, but many nitrogen nucleophiles can also react effectively even in the absence of acid.

Summary of Key Reactions

Aldehyde/Ketone + Alcohol:
A 1:1 molar ratio of alcohol to aldehyde/ketone leads to the formation of a hemiacetal, indicative of the initial nucleophilic addition.
A 1:2 molar ratio (with excess alcohol) promotes the transformation of the hemiacetal into a more stable acetal, showcasing its role in synthetic chemistry.
Aldehyde/Ketone + Nitrogen Nucleophiles:
The interactions with nitrogen nucleophiles yield diverse products (imine, oxime, or hydrazone), driven by the nature of the nucleophile involved, and are fundamental in synthetic organic chemistry processes.