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Chapter 17 Aldehydes and Ketones I. O Properties and Names of Aldehydes and Ketones A. C Nomenclature 1) Carbonyl groups are the highest priority functional group we have seen 2) Aldehydes a) Common Names modify the name of the corresponding carboxylic acid O O H3C O C OH Acetic Acid b) O H3C C O C H H Acetaldehyde H C H Formaldehyde Benzaldehyde IUPAC Names treat aldehydes as derivatives of alkanes i. Suffix –al added to the alkane name: alkane becomes alkanal ii. The carbonyl carbon is #1, but is not numbered in the name iii. Complex aldehydes are named as “alkanecarbaldehydes” O O CH3CH2CH ClCH2CH2CH2CH propanal 4-chlorobutanal H 4,6-dimethylheptanal O H cyclohexanecarbaldehyde H 3) Ketones a) Common Names come from the 2 R groups listed in alphabetical order, followed by “ketone”. Phenyl Ketones have common names ending in “O O phenone” O O H3C C CH3 CH3CH2 dimethyl ketone acetone b) H3C C C C CH3 ethyl methyl ketone Acetophenone Benzophenone IUPAC Names of ketones modify the alkane name with “-one” i. The carbonyl carbon is assigned the lowest possible number ii. Aromatic ketones are named as aryl substituted alkanones iii. A ketone with an aldehyde is called an “oxo-” substituent Cl CH3 O O O H3C C CH3 propanone O O 4-chloro-6-methyl-3-heptanone 2,2-dimethyl cyclopentanone 2-pentanone O O OH O 1-phenylethanone O O COOH CH3CCH2CH CH3CCH2CH=CHCH2CCH3 H C C C 3-oxobutanal O H CH3 propynal 4-formylcyclohexane 7-hydroxy-7-methyl-4-octen-2-one carboxylic acid Br 5-bromo-3-ethynyl cycloheptanone 4) Carbonyl groups named as a substitutent are called alkanoyl (or acyl) groups O H3C O C H ethanoylor acetyl- 5) C formyl- Drawing Aldehydes and Ketones a) Aldehyde = RCHO, Alcohol = RCOH b) Ketone: RCH2COCH3 O O CH3CH2CH2CH CH3CH2CH2CHO B. Butanal O O CH3CH2CCH3 H CH3CH2COCH3 2-butanone Carbonyl Structure and Physical Properties 1) C=O bond is similar to C=C bond a) C(sp2)—O(sp2) s-bond overlap, with a p-p overlap for the p-bond b) Planar group with 3 120o angles 2) 3) 4) The C=O bond is fairly strong, 175-180 kcal/mol (ethene = 173 kcal/mol) a) Oxygen is electronegative, so the bond is polar b) The partially positive charged carbon is electrophilic c) The partially negatively charged oxygen is nucleophilic and basic d) Resonance structures: O O C C Boiling points are higher than alkanes due to polarity Small carbonyl compounds (< 7 C) are water soluble due to polarity (acetone) C. Spectroscopy 1. 1H NMR a) Aldehyde proton is extremely deshielded, d = 9-10 ppm i. Movement of p-electrons reinforces magnetic field ii. d+ C increases the deshielding beyond that effect b) Hydrogens adjacent to the aldehyde are also slightly deshielded c) Ketones also slightly deshield adjacent hydrogens O RCH2 2.5 C H 9.8 O R2CH 2.6 C CH3 2.0 2) O CH3CH 31.2 199.6 13C NMR a) Carbonyl carbons are observed at around 200 ppm due to d+ C b) Adjacent carbons are somewhat effected as well O CH3CH2CH 5.2 36.7 201.8 O CH3CCH3 30.2 205.1 O CH3CCH2CH2CH3 29.3 206.6 45.2 17.5 13.5 3) IR a) b) c) d) e) O 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 1680 cm-1 CH3 O 1745 cm-1 4) II. UV-Vis a) Nonbonding oxygen lone pairs give np* b) p-bond gives pp* transitions c) Acetone: np* = 280 nm (e = 15), pp* = 190 nm (e = 1100) d) Conjugation shifts the absorbances to longer wavelenth (lower E) Preparation of Aldehydes and Ketones A. Oxidation of Alcohols 1) Cr(VI) reagents (like CrO3) oxidize alcohols to carbonyls H 2) Secondary ROH gives ketones CrO3 H3C C OH CH3 H3C O C CH3 3) Primary ROH gives aldehydes a) Must be done under anhydrous conditions to prevent overoxidation b) PCC = pyridinium chlorochromate, pyridine, and CH2Cl2 conditions PCC, py CrO3 CH3CHO CH3CH2OH CH3CH3COOH CH3CH2OH CH2Cl2 c) Manganese dioxide (MnO2) oxidizes only allylic alcohols; it won’t react with ordinary alcohols. O MnO2 HOCH2CH2CH=CHCH HOCH2CH2CH=CHCH2OH CHCl3 B. Ozonolysis of Alkenes O CH3 1. O3 O CH3CCH2CH2CH2CH2CH 2. Zn C. Hydration of Alkynes 1. Markovnikov hydration yields ketones + 2+ H2O, H , Hg HO H R O R C C C R C CH tautomerization H CH3 2. Anti-Markovnikov hydration yields aldehydes H R2BH BR2 C C R C CH R D. H H H2O2 HO OH tautomerization RCH2CH C C - R H Aryl Ketones Via Friedel Crafts Alkanoylation (Acylation) O CH3O 1. CH3CCl, AlCl3 CH3O 2. HCl, H2O C CH3 O III. Additions to Carbonyls A. Three regions of Carbonyl Reactivity 1) The :O: is nucleophilic and will attack electrophiles 2) The C is electrophilic and will be attacked by nucleophiles 3) The a-C has acidic protons (NEXT CHAPTER) O O C C CH2 CH2 O B. Hydrogenation 1. Catalytic Hydrogenation reduces carbonyls OH O CH3CCH2CH3 2. 3. H2 Ra Ni CH3CHCH2CH3 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 H2 (1 atm) Pt, 25 oC O O C. O X+Y- OX Ionic Additions C C d+ d1. Polar X—Y molecules will add to C=O Y 2. Hydrides add 2 H atoms to the carbonyl, but won’t reduce alkenes O OH NaBH4 CH3CCH2CH3 CH3CHCH2CH3 1. LiAlH4, Et2O O 2. H+, H2O H OH 3. Grignard Reagents add R, H to the carbonyl OH O CH3CCH2CH3 RMgBr THF CH3CCH2CH3 R 4. Milder Reagents a) Hydrides and Grignard Reagents are strong bases. They irreversibly add to the carbonyl b) Less basic reagents can also add to carbonyls, but the reactions are reversible: H2O, ROH, RSH, RHN2, etc… c) The conditions used in the reaction of these milder bases determine how the reaction proceeds 5. Nucleophilic Addition-Protonation (Basic Conditions) a) Mechanism + - d d C O Nu- C O Nu Alkoxide ion H OH C OH + OH Nu b) c) d) e) f) g) 6. d+ dC O 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) a) Mechanism C OH + H pKa of C=OH is -8 b) c) d) C OH Nu C OH Nu 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.