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Ch.21 Alds & Ketones
Nucleophilic Addition of Aldehyde & Ketones
| Two structural feature of aldehydes & ketones | • The carbonyl group is sp2 hybridized and trigonal planar, making it relatively uncrowded. • The electronegative oxygen atom polarizes the carbonyl group, making the carbonyl carbon ELECTROPHILIC. They react with nucleophiles. |
| Reactivity of the carbonyl group is determined by the number of R groups bonded to it | As the number of R groups around the carbonyl carbon increases, the reactivity of the carbonyl compound decreases. |
| Naming Aldehydes in the IUPAC System | [1] If the CHO is bonded to long chain, change -e ending of the parent alkane to the suffi x -al. If bonded to a ring, name the ring & add the suffi x -carbaldehyde. [2] Number to put CHO group at C1, omit the number from name |
| Common Names for Aldehydes | -formaldehyde (methanal) -acetaldehyde (ethanal) -benzaldehyde (benzenecarbaldehyde) |
| Naming Ketones in the IUPAC System | [1] Find the longest chain containing the carbonyl group,change the -e ending of the parent alkane to the suffi x -one. [2] Number to give the carbonyl carbon the lower number. |
| Common Names for Ketones | -acetone -acetophenone -benzophenone |
| Additional Nomenclature Facts | -formyl group -acetyl group -benzoyl group -benzyl group -Compounds containing bothC–C double bond & an aldehyde are named as enals. -compounds containing both a C–C double bond & a ketone are named as enones. |
| Physical Properties of Ald & Keys | -dipole–dipole interactions- due to polar c=o - no O – H bond: incapable of intermolecular Hbonding, making them less polar than alcohols and carboxylic acids. |
| IR Spectra Properties | -strong peak at ~1700 cm–1 due to the C=O (aldehyde, 1730: Ketone,1715) -sp2 hybridized C –H bond of an aldehyde shows one or two peaks at ~2700–2830 cm–1 |
| What two structural factors affect the location of the carbonyl absorption? | -RING SIZE: c=o absorption of cyclic ketones shifts to higher wavenumber as the size of the ring decreases and the ring strain increases -CONJUGATION:Conj.c=o group with a C=C or a benzene ring shifts the absorption to lower wavenumber by ~30 cm–1 |
| How does resonance explain the effect of conjugation on the frequency of the C=O absorption? (Weaker bonds absorb at lower freq/w#.) | An α,β-unsat'd c=o cmpd has 3resonance structures, two of which place a single bond b/w the C&O atms of the c=o. Thus, the π bond of the c=o group is delocalized, giving the c=o single bond character,making it weaker than an unconjugated C=O. |
| NMR Spectra Properties | -The sp2 hybridized C-H proton of an aldehyde is highly deshielded and absorbs far downfield at 9–10 ppm. - Alpha protons absorb at 2–2.5 ppm. -13C NMR, carbonyl carbon is highly deshielded, appearing in the 190–215 ppm region |
| Common Methods to Synthesize Aldehydes | -By oxidation of 1°OHs with PCC -Reduction of esters & acidCl w/ 1.DIBAL-H/LiAlH[OC(CH3)3]3 2. H2O -By hydroboration-oxidation (1.BH3 2.H2O2,-OH) of terminal alkyne -Oxi. cleavage of alkene (O3/Zn,H2O) |
| Common Methods to Synthesize Ketones | - By oxidation of 2°OHs with Cr6+ reagents (CrO3/Na2CrO7/PCC/K2Cr2O7) -By reaction of acidCl with organocuprates (1.R'CuLi 2.H2O) -By F-C acylation (Benzene+acidCl +AlCl3) -By hydration(H20, H2SO4,HgSO4) of an alkyne -Oxi. cleavage of alkene (O3/Zn,H2 |
| General Reaction Sites of Aldehydes & Ketones | [1] Reaction at the carbonyl carbon (e-philic site, nucleophilic addition-H&Nu added) [2] Reaction at the alpha carbon (acidic C-H bond:rxn w/ base to form resonance-stablized enolate anion) |
| Define Enolate | Enolates are nucleophiles, and so they react with electrophiles to form new bonds on the alpha carbon. |
| Why are Cl-, Br-, and I- ineffective nucleophiles in addition but good in substitution reactions? | They are ineffective in addition b/c when added to c=o sp2 carbonyl group, they cleave the bond, forming alkoxide. Because X- = weaker base than the alkoxide formed, equilibrium favors the starting material, not the addition product. |
| Define Imines | -Also known as Schiff Base. -Formed through treatment of an ald/ketone with a primary amine (carbinolamine +iminium ion still has an H on N atm ) -Replacement of the C=O by C=NR(sp2 hybridized, 120 degrees. -Formed faster under mildly acidic conditions |
| Define Enamines | -Enamine = (alkene + amine) N atm bonded to a double bond. -Formed through treatment of ald/ketone with 2ndary amine. -Elimination of H2O occurs across two adj Cs(carbinolamine +iminium ion w/ no H on N atm) |
| Treatment of a carbonyl compound with H2O in the presence of an acid or base catalyst adds the elements of H and OH across the carbon–oxygen o bond, forming a gem-diol or hydrate. What determines this? (reversible) | -Less stable c=o SM, equilibrium favors the hydrate product. More stable, eq favors the SM. -R groups stabilize a c=o group: increasing R group/EDG on c=o decrease amount of Hydrate Product. -EWG- destablize=increase hydrate @equil. |
| Acid and base increase the rate of reaction (in hydration- addition of water?) for different reasons: | • Acid protonates the c=o group, making it more electrophilic towards nucleophilic attack. • Base converts H2O into –OH, a stronger nucleophile. These catalysts increase the rate of the reaction, but they do not affect the equilibrium constant. |
| Why is an imine formed fastest under mildly acidic conditions (pH 4-5) and not strongly acidic conditions? | Imine formation is fastest when the rxn medium is weakly acidic because: -the mild acid is needed to protonate the -OH into a good LG. -Under SAic conditions, rxn rate decrease because the amine nucleophile is protonated. |
| Define Acetals (Good Protecting groups for alds & ketones) | -Formed through treatment of ald/ketones by two equiv of alcohol. -Acetal: c=o carbon is singly bonded to two OR(alkoxy) groups. -Formation catalyzed by acids(TsOH). -Cyclic acetal formed w/ ethylene glycol(HOCH2CH2OH). water byproduct.reversible. |
| Define Hemiacetal | The addition of one equivalent of alcohol forms a hemiacetal, which is converted to an acetal. A hemiacetal has a carbon atom bonded to one OH group and one OR group. Higer in ene than SM. |
| What are the 7 steps of acetal formation (addition of alcohol to aldehyde or ketone)? Can be reversered through hydrolysis using large excess of water& a sttrong acid | 1. Protonation of C=O (O lp takes H of TsOH) 2. Nucleophilic Attack 3. Deprotonation (Hemiacetal) by TsO- 4. Protonation of -OH to good LG by TsOH 5. Elimination of H2O 6. Nucleophilic attack (2nd equiv) 7. Deprotonation (Acetal) |
| Explain why acetals are NOT considered ethers. | Acetals are not ethers, even though both functional groups contain a C – O σ bond. -Having two C – O σ bonds on the same carbon atom makes an acetal very different from an ether. |
| Define Cyclic Hemiacetals (Lactols) | - Hemiacetals r unstable, but 5/6membered cyclic hemiacetals R stable & isolated. - Formed by intramolecular cyclization of hydroxy aldehydes. - The two reacting functional groups, in this case OH and C–– O in close proximity, increas prob'ility of rxn. |
| Stereochemistry of formation of Cyclic Hemiacetals & conversion to acetal | Intramolec cyclization of a hydroxy aldehyde= hemiacetal w/ new stereogenic center, so that an equal amount of two enantiomers results. -Converted to acetals by treatment w/ alcohol & acid.(replacement of OH by OR) occurs readily due to res stablized C+ |
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