🥼Organic Chemistry Unit 19 – Aldehydes & Ketones: Nucleophilic Addition

Key Concepts

• Aldehydes and ketones contain a carbonyl group (C=O) where the carbon atom is bonded to one or two alkyl or aryl groups, respectively
• Nucleophilic addition is a key reaction mechanism for aldehydes and ketones involving the addition of a nucleophile to the electrophilic carbonyl carbon
• The reactivity of aldehydes and ketones is influenced by the electron-withdrawing nature of the carbonyl group and the steric hindrance of the attached substituents
• Common nucleophiles in these reactions include water, alcohols, amines, and organometallic reagents (Grignard reagents)
• The addition of a nucleophile to the carbonyl group forms a tetrahedral intermediate, which can then undergo proton transfer and elimination to yield the final product
• Aldehydes are generally more reactive than ketones due to reduced steric hindrance and increased electrophilicity of the carbonyl carbon
• The stereochemistry of the nucleophilic addition can be influenced by the structure of the substrate and the reaction conditions

Structure and Properties

• Aldehydes have the general formula RCHO, where R can be an alkyl or aryl group, and the carbonyl carbon is bonded to one hydrogen atom
• Ketones have the general formula R1COR2, where R1 and R2 can be alkyl or aryl groups, and the carbonyl carbon is bonded to two carbon atoms
• The carbonyl group is polar due to the difference in electronegativity between carbon and oxygen, with a partial positive charge on the carbon and a partial negative charge on the oxygen
• The polarity of the carbonyl group allows for hydrogen bonding with water, making low molecular weight aldehydes and ketones soluble in water
• The carbonyl group is planar, with the carbon and oxygen atoms lying in the same plane as the attached substituents
• The presence of the carbonyl group influences the physical properties of aldehydes and ketones, such as boiling point and solubility
• Aldehydes and ketones can engage in resonance, with the lone pair on the oxygen atom delocalizing to form a carbon-oxygen double bond

Nomenclature

• Aldehydes are named by replacing the -e ending of the corresponding alkane with -al (methanal, ethanal)
• Ketones are named by replacing the -e ending of the corresponding alkane with -one and indicating the position of the carbonyl group with a number (propanone, 2-butanone)
• Common names for simple aldehydes and ketones are often used (formaldehyde, acetone)
• The prefix "oxo-" is used to indicate the presence of a carbonyl group in complex molecules
• When the carbonyl group is the primary functional group, it is given priority in naming the compound
• In cyclic ketones, the carbonyl group is assigned position 1, and the ring is numbered to give the lowest possible numbers to the other substituents
• Greek letters (α, β, γ) are sometimes used to indicate the position of substituents relative to the carbonyl group

Reactivity and Mechanisms

• The electrophilic nature of the carbonyl carbon makes it susceptible to nucleophilic attack
• The reactivity of aldehydes and ketones is influenced by the electron-withdrawing effect of the carbonyl group and the steric hindrance of the attached substituents
• Nucleophilic addition to aldehydes and ketones involves two steps: addition of the nucleophile to form a tetrahedral intermediate and proton transfer followed by elimination of a leaving group
• The tetrahedral intermediate is a high-energy species that can be stabilized by hydrogen bonding or coordination with a metal ion
• The rate-determining step in nucleophilic addition reactions is typically the formation of the tetrahedral intermediate
• The stereochemistry of the nucleophilic addition can be influenced by the structure of the substrate and the reaction conditions
• In some cases, the tetrahedral intermediate can undergo a subsequent reaction, such as dehydration or rearrangement, to yield a different final product

Common Reactions

• Reduction of aldehydes and ketones to alcohols using hydride reagents (sodium borohydride, lithium aluminum hydride)
• Oxidation of aldehydes to carboxylic acids using mild oxidizing agents (Tollens' reagent, Fehling's solution)
• Nucleophilic addition of water to form hydrates (geminal diols)
• Nucleophilic addition of alcohols to form hemiacetals and acetals
• Nucleophilic addition of amines to form imines and enamines
• Nucleophilic addition of organometallic reagents (Grignard reagents) to form alcohols
• Aldol condensation between two aldehydes or ketones to form β-hydroxy carbonyl compounds
• Wittig reaction between an aldehyde or ketone and a phosphonium ylide to form alkenes
• Cyanohydrin formation by the addition of hydrogen cyanide to aldehydes or ketones

Synthesis and Applications

• Aldehydes and ketones are versatile building blocks in organic synthesis due to their reactivity and the variety of nucleophilic addition reactions they undergo
• Aldehydes and ketones can be synthesized by the oxidation of alcohols or the ozonolysis of alkenes
• Aldehydes and ketones are used in the production of plastics, pharmaceuticals, fragrances, and flavors
• Formaldehyde is used in the production of resins and adhesives, as well as in the preservation of biological specimens
• Acetone is a common solvent and is used in the production of polymers and cleaning agents
• Aldehydes and ketones are important intermediates in the synthesis of more complex molecules, such as sugars, steroids, and alkaloids
• The reactivity of aldehydes and ketones can be exploited in the design of drug molecules and the synthesis of natural products

Lab Techniques and Safety

• Aldehydes and ketones should be handled with caution due to their reactivity and potential toxicity
• Many aldehydes and ketones are flammable and should be kept away from heat and ignition sources
• Proper ventilation is essential when working with aldehydes and ketones, as they can have strong odors and may cause respiratory irritation
• Personal protective equipment, such as gloves and safety glasses, should be worn when handling aldehydes and ketones
• Aldehydes and ketones should be stored in a cool, dry place away from direct sunlight and incompatible materials
• Purification of aldehydes and ketones can be achieved by distillation, recrystallization, or column chromatography
• Characterization of aldehydes and ketones can be performed using spectroscopic techniques, such as infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy

Practice Problems and Tips

• Practice drawing the structures of aldehydes and ketones, paying attention to the connectivity of the atoms and the presence of the carbonyl group
• Work through examples of naming aldehydes and ketones using the IUPAC system and common names
• Practice predicting the products of nucleophilic addition reactions by considering the structure of the substrate and the nature of the nucleophile
• Use curved arrow notation to show the flow of electrons in the mechanism of nucleophilic addition reactions
• Pay attention to the stereochemistry of the products formed in nucleophilic addition reactions, especially when chiral centers are present
• Practice proposing synthetic routes to target molecules using aldehydes and ketones as starting materials or intermediates
• Work through multi-step synthesis problems that involve the use of aldehydes and ketones in combination with other organic reactions
• Review the key concepts and mechanisms regularly to reinforce your understanding of the material