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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+
Created by: Yourdanos