Chapter 10 - Nucleophilic Addition to Carbonyl Groups

Chapter 10: Nucleophilic Addition to Carbonyl Groups

10.1 Introduction to Carbonyl Groups

  • Carbonyl groups (C=O) are polar and reactive due to the difference in electronegativity between carbon and oxygen.

  • The atoms bonded to carbonyl carbon influence the functional group's identity and reactivity.

  • Focus of the chapter: Chemistry of aldehydes and ketones.

10.2 Nomenclature of Aldehydes

  • IUPAC suffix for aldehydes: –al.

  • The aldehyde carbonyl carbon is always at the end of a chain, numbered as carbon #1.

  • Examples of aldehyde names:

    • Methanal (formaldehyde)

    • Ethanal (acetaldehyde)

    • Benzenecarbaldehyde (benzaldehyde)

    • Cyclohexanecarbaldehyde

    • Hexanal

    • 2,4-dimethylhexanal

    • 3-chlorobenzaldehyde

10.3 Nomenclature of Ketones

  • IUPAC suffix for ketones: –one.

  • The ketone carbonyl carbon can be located anywhere except at the chain's end.

  • Examples of ketone names:

    • 2-Propanone (acetone)

    • Cyclohexanone

    • 2-Hexanone

    • 4-chloro-6-methyl-3-heptanone

    • 2,2-dimethylcyclohexanone

10.4 Structure of the Carbonyl Group

  • Carbon and oxygen are sp² hybridized with bond angles of approximately 120°.

  • The carbonyl bond consists of an sp² hybridized sigma bond and a p bond formed by the overlap of 2p orbitals.

10.5 General Reactivity of Aldehydes and Ketones

  • The carbonyl group’s polarity enables rich nucleophilic addition reactions.

    • Focus on reactivity of weaker nucleophiles under slightly basic or acidic conditions.

  • Reactions can proceed via two mechanisms:

    • Nucleophilic addition-protonation under basic conditions.

    • Electrophilic protonation-addition under acidic conditions.

10.6 Addition of Water to Form Hydrates

  • Aldehydes and ketones can add water across C=O to form carbonyl hydrates (geminal diols).

  • Mechanism of Hydration: Two pathways:

    • Base-catalyzed (alkoxide anion + water).

    • Acid-catalyzed (protonated carbonyl + water).

  • Equilibrium generally favors the carbonyl compound over the hydrate.

  • As the carbonyl carbon becomes more electropositive, the hydrate form is less stable.

10.7 Addition of Alcohols to Form Hemi-Acetals and Acetals

  • Hemi-acetal Formation: Addition of alcohol elements across a carbonyl group, typically acid-catalyzed.

    • Equilibrium favors carbonyl compound not the hemi-acetal.

    • Cyclic hemi-acetals favored when hydroxyl is present in the same molecule as the carbonyl.

  • Acetal Formation: Includes two OR groups attached to the same carbon (C with two R'OH).

    • Exclusively acid-catalyzed reaction.

    • Removal of water favors acetal formation, which can easily be reversed through acetal hydrolysis.

10.8 Carbohydrates: Overview and Nomenclature

  • Carbohydrates are poly-hydroxy aldehydes or ketones, often referred to as sugars or saccharides.

  • Examples include glucose, classified as a monosaccharide with one carbonyl and multiple hydroxyl groups.

    • Monosaccharides combine to form larger polysaccharides (disaccharides form via acetal linkages).

10.9 Chiral Carbons and Stereochemistry

  • Most carbohydrates have one or more chiral carbons and exist mainly as one enantiomer in nature.

  • Glucose has 4 chiral carbons, with multiple stereoisomers possible.

  • Carbohydrates are often named using suffixes:

    • Hexose (6 carbons), pentose (5 carbons).

    • Aldose (aldehydes), ketose (ketones).

10.10 Fischer Projections and D/L Configurations

  • Fischer projections depict the molecule's structure, showing horizontal bonds as coming forward and vertical bonds as going back.

  • Configuration designation:

    • D = hydroxyl on the right (D-glucose).

    • L = hydroxyl on the left (L-glyceraldehyde).

10.11 Cyclic Forms of Carbohydrates

  • Cyclic hemi-acetal formation occurs when an aldehyde reacts with alcohol, forming a ring.

    • Cyclic forms exist predominantly in equilibrium, especially D-glucose which can form pyranose or furanose structures.

10.12 Anomeric Forms and Equilibria

  • D-glucose interconverts between two anomers (α and β forms).

  • The b-form is more stable due to lower steric strain in the chair conformation.

10.13 Glycosides and Polymer Formation

  • Glycosides are acetals of cyclic forms of carbohydrates, formed through acetal linkages between monosaccharides.

    • Configuration and position of acetal bond affect the carbohydrate's properties.

10.14 Hydrolysis of Glycosides

  • Glycosides can undergo hydrolysis back to sugars using aqueous acid, with enzymes mediating the process in biological systems.

10.15 Imines and Enamines Formation

  • Imines are formed when a primary amine reacts with an aldehyde or ketone, releasing water and requiring catalysts.

  • Secondary amines form enamines, with an amine nitrogen directly bound to a double-bonded carbon.

10.16 Disaccharides and Their Importance

  • Disaccharides consist of two monosaccharides joined by acetal linkages, with configurations critical for metabolic processes.

    • Examples include sucrose (glucose + fructose) and lactose.

10.17 Polysaccharides: Cellulose, Starch, Glycogen

  • Common polysaccharides are cellulose (structural role), starch (energy storage in plants), and glycogen (energy storage in animals).

    • These consist of glucose units in various configurations (β-linked in cellulose, α-linked in starch/glycogen).

10.18 Modified Sugars and Drug Structures

  • Modified carbohydrates contain various changes, e.g., N-acetylglucosamine in chitin.

  • Recognition of these modifications is important for understanding biochemical pathways and drug interactions.