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Chapter 17 Aldehydes and Ketones

Chapter 17 Aldehydes and Ketones. Properties and Names of Aldehydes and Ketones Nomenclature Carbonyl groups are the highest priority functional group we have seen Aldehydes Common Names modify the name of the corresponding carboxylic acid

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Chapter 17 Aldehydes and Ketones

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  1. Chapter 17 Aldehydes and Ketones • Properties and Names of Aldehydes and Ketones • Nomenclature • Carbonyl groups are the highest priority functional group we have seen • Aldehydes • Common Names modify the name of the corresponding carboxylic acid • IUPAC Names treat aldehydes as derivatives of alkanes • Suffix –al added to the alkane name: alkane becomes alkanal • The carbonyl carbon is #1, but is not numbered in the name • Complex aldehydes are named as “alkanecarbaldehydes”

  2. Ketones • Common Names come from the 2 R groups listed in alphabetical order, followed by “ketone”. Phenyl Ketones have common names ending in “-phenone” • IUPAC Names of ketones modify the alkane name with “-one” • The carbonyl carbon is assigned the lowest possible number • Aromatic ketones are named as aryl substituted alkanones • A ketone with an aldehyde is called an “oxo-” substituent

  3. Carbonyl groups named as a substitutent are called alkanoyl (or acyl) groups • Drawing Aldehydes and Ketones • Aldehyde = RCHO, Alcohol = RCOH • Ketone: RCH2COCH3 • Carbonyl Structure and Physical Properties • C=O bond is similar to C=C bond • C(sp2)—O(sp2) s-bond overlap, with a p-p overlap for the p-bond b) Planar group with 3 120o angles

  4. The C=O bond is fairly strong, 175-180 kcal/mol (ethene = 173 kcal/mol) • Oxygen is electronegative, so the bond is polar • The partially positive charged carbon is electrophilic • The partially negatively charged oxygen is nucleophilic and basic • Resonance structures: • Boiling points are higher than alkanes due to polarity • Small carbonyl compounds (< 7 C) are water soluble due to polarity (acetone)

  5. Spectroscopy • 1H NMR • Aldehyde proton is extremely deshielded, d = 9-10 ppm • Movement of p-electrons reinforces magnetic field • d+ C increases the deshielding beyond that effect • Hydrogens adjacent to the aldehyde are also slightly deshielded • Ketones also slightly deshield adjacent hydrogens

  6. 13C NMR • Carbonyl carbons are observed at around 200 ppm due to d+ C • Adjacent carbons are somewhat effected as well

  7. IR • C=O stretch is intense, 1690-1750 cm-1 • Aldehyde carbonyl usually around 1735 cm-1 • Ketone carbonyl usually around 1715 cm-1 • Conjugation reduces the wavenumber by 30-40 cm-1 • Small rings increase the wavenumber

  8. UV-Vis • Nonbonding oxygen lone pairs give np* • p-bond gives pp* transitions • Acetone: np* = 280 nm (e = 15), pp* = 190 nm (e = 1100) • Conjugation shifts the absorbances to longer wavelenth (lower E) • Preparation of Aldehydes and Ketones • Oxidation of Alcohols • Cr(VI) reagents (like CrO3) oxidize alcohols to carbonyls • Secondary ROH gives ketones

  9. Primary ROH gives aldehydes • Must be done under anhydrous conditions to prevent overoxidation • PCC = pyridinium chlorochromate, pyridine, and CH2Cl2 conditions • Manganese dioxide (MnO2) oxidizes only allylic alcohols; it won’t react with ordinary alcohols. • Ozonolysis of Alkenes • Hydration of Alkynes 1. Markovnikov hydration yields ketones

  10. Anti-Markovnikov hydration yields aldehydes • Aryl Ketones Via Friedel Crafts Alkanoylation (Acylation) • Additions to Carbonyls • Three regions of Carbonyl Reactivity • The :O: is nucleophilic and will attack electrophiles • The C is electrophilic and will be attacked by nucleophiles • The a-C has acidic protons (NEXT CHAPTER)

  11. Hydrogenation • Catalytic Hydrogenation reduces carbonyls • Reaction is slower than for alkenes: higher H2 pressure and temp. needed • C=C bonds can be selectively hydrogenated in the presence of C=O • Ionic Additions • Polar d+X—Yd- molecules will add to C=O • Hydrides add 2 H atoms to the carbonyl, but won’t reduce alkenes

  12. Grignard Reagents add R, H to the carbonyl • Milder Reagents • Hydrides and Grignard Reagents are strong bases. They irreversibly add to the carbonyl • Less basic reagents can also add to carbonyls, but the reactions are reversible: H2O, ROH, RSH, RHN2, etc… • The conditions used in the reaction of these milder bases determine how the reaction proceeds • Nucleophilic Addition-Protonation (Basic Conditions) a) Mechanism

  13. Nucleophile approaches, causing C to rehybridize • p-bond electrons move to Oxygen, producing an alkoxide anion • Protonation from solvent yields the product • The new Nu—C bond has both electrons from Nu- (like in SN2) • An electron pair is the “leaving group” • Strongly basic nucleophiles typically follow this mechanism • Electrophilic Protonation-Addition (Acidic Conditions) • Mechanism • C=O is a weak base (strong acid) so small, but reactive, amount present • Nu attacks carbon electrophile to give product. This removes intermediate and shifts the equilibrium to the right. • Weakly basic nucleophiles typically follow this mechanism. Strongly basic nucleophiles would just get protonated and couldn’t react.

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