Aldehydes & Ketones
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Transcript of Aldehydes & Ketones
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Aldehydes & KetonesO
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In General Fragrant odors Basic building block of housing
materials Hormones Digestion Vision
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In General Carbonyl group
› C=O› Aldehydes
RCH=O Formyl
› Ketones RC=OR’
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Nomenclature Aldehydes
› IUPAC end in “al”› Common end in “aldeyde”› Carbonyl C is always #1› Cyclic cpds
Carbaldehyde is ending for most
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Nomenclature AldehydesCH2 O
O
CH3
O
CH3
O
CH3
methanal(formaldehyde)
ethanal(acetaldehyde)
propanal(propionaldehyde)
butanal(n-butyraldehyde)
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Nomenclature Aldehydes
3-methylbutanal
3-butenal
2,3-dihydropropanal(glyceraldehyde)
CH3
CH3 O
CH2
O
O
OH OH
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Nomenclature Aldehydes
cyclopentanecarbaldehyde(formylcyclopentane)
benzenecarbaldehyde(benzaldehyde)
2-hydroxybenzenecarbaldehyde(salicylaldehyde)
O
O
O
OH
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Nomenclature Ketones
› IUPAC end in “one”› Common end in “ketone”› Carbonyl C is never #1, but always gets
low number preference› Cyclic cpds
Carbaldehyde is ending for most
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Nomenclature Ketones
propanone(acetone)
2-butanone(ethyl methyl ketone)
3-pentanone(diethyl ketone)
CH3
CH3
O
CH3
O CH3
CH3
O CH3
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Nomenclature Ketones
cyclohexanone
2-methylcyclopentanone
3- buten-2-one(methyl vinyl ketone)
O
O
CH3
CH2CH3
O
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Nomenclature Ketones
acetaphenone(methyl phenyl ketone)
benzophenone(diphenyl ketone)
dicyclopropyl ketone
CH3
O
O
O
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Common Aldehydes & Ketones
Formaldehyde› Simplest aldehyde› Manufactured on large scale (8 billion lbs per
annum) from catalyzed oxidation of methanol CH3OH CH2=O + H2
› Gas at RT (bp = -21˚C) but cannot be stored in free state due to polymerization
› Normally 37% soln called formalin (preservative)
› Most used in making of plastics, insulation, particle board, and plywood
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Common Aldehydes & Ketones
Acetaldehyde› Boils close to RT (bp = 20˚C)› Made by catalyzed oxidation of ethylene
2 CH2=CH2 + O2 2 CH3CH=O› ~1/2 is oxidized to acetic acid› Remainder used for production of
1- butanol and others.
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Common Aldehydes & Ketones
Acetone› Simplest ketone› Large scale production like formaldehyde› Produced from oxidation of propene, isopropyl
alcohol, or isopropylbenzene
› ~30% used directly, great solvent, H2O miscible
› Rest used to make stuff like epoxy resins
CH3CH3
O 2
OOHCH3
CH3d ilu te H 2 S O 4
H 2 O
OH
+ CH3CH3
O
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Common Aldehydes & Ketones
Quinones› Cyclic conjugate diketones› Simplest is 1,4-benzoquinone› All are colored and are thus used often as
dyes› Alizarin…used to dye the red coats of the
British Army during American Revolution› Vitamin K is required for normal clotting of
blood
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Common Aldehydes & Ketones
O O
OH
OHO
O1,4-benzoquinonealizarinOH
CH3
OH
CH3
CH3CH3CH3CH3
vitamin K
Vitamin K
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Synthesis of Aldehydes & Ketones
Oxidation› 1˚ ROH gives aldehyde› 2˚ ROH gives ketone› Cr reagents (PCC) are common
CH3 OHo x id iz in g
a g e n tCH2 O
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Synthesis of Aldehydes & Ketones
Friedel-Crafts Acylation› Recall the rxn?› Makes aromatic ketones
+O
Cl A lC l3
O+ ClH
benzophenonebenzyl chloride
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Synthesis of Aldehydes & Ketones
Hydration of terminal alkynes› Gives methyl ketones› Catalyzed by acid and mercuric ion
CH3CH
H + , H 2 O
H g + +CH3
CH3
O
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Aldehydes & Ketones in Nature
Many have pleasant odors Used in the perfume industry Extremely expensive to gather from
natural producers Chanel No. 5 (my mom’s fave perfume)
was first perfume to use synthetic organic chemicals in 1921
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Aldehydes & Ketones in Nature
O
O
O
CH3
O
OH
benzaldehyde cinnamaldehyde vanillin
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The Carbonyl Group C atom is sp2 hybridized Bond angles? C=O bond length is 1.24Å (compared
to 1.43Å for C-O in ROH and ROR O is more EN than C
› Makes a polar bond
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The Carbonyl Group Most carbonyl reactions are
nucleophilic attacks on the carbonyl C C=C usually is attacked by an
electrophile Due to polarization, physical properties
differ from HC’s and ROH’s› bp’s are higher than HC’s, lower than
ROH’s
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The Carbonyl Group C=O is permanently polarized
› Positive part of one molecule is attracted to negative part of another molecule
› Dipole-dipole forces, weaker than H-bonds, stronger than LDF
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The Carbonyl Group C=O’s with low MW are soluble in
water› Can form H-bonds with water or ammonia
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An Overview of Nucleophilic Addition to Carbonyl Groups
Why does the attack occur? If rxn occurs in hyroxylic solvent (water
or ROH), a proton is usually added to the O
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An Overview of Nucleophilic Addition to Carbonyl Groups
Carbonyl cpds are weak Lewis bases due to lone pairs on O
Acids can catalyze the addition of weak nucleophiles to carbonyl cpds through protonation
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An Overview of Nucleophilic Addition to Carbonyl Groups
Nucleophiles add reversibly› Good leaving groups, CB of SA
Nucleophiles add irreversibly› Poor LG, CB of WA
In general, ketones are less reactive than aldehydes› Steric…sp2 v. sp3, R v. H› Electronic…alkyl groups are electron-
donating…ketones have two
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Addition of Alcohols: Formation of Hemiacetals and Acetals
Alcohols are oxygen nucleophiles› OR goes to C, and H goes to O
Because ROH’s are weak nucleophiles, acid catalyst must be used
Product is a hemiacetal› Contains both alcohol and ether on same C
Addition is reversible
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Addition of Alcohols: Formation of Hemiacetals and Acetals
Mechanism of hemiacetal formation has 3 steps› Carbonyl O is protonated by acid catalyst› ROH’s O then attacks carbonyl C› Proton is then lost from resulting +O
Each step is reversible
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Do You Get It? Write an equation for the formation of a
hemiacetal from acetaldehyde, ethanol, and an acid catalyst. Show each step in the rxn mechanism.
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Addition of Alchols: Formation of Hemiacetals and Acetals
Excess ROH means hemiacetals react further to produce acetals
Hydroxyl group of hemiacetal is replaced by an alkoxyl group.
Acetals have two ether groups on same C
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Addition of Alchols: Formation of Hemiacetals and Acetals
Mechanism of acetal formation
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Addition of Alchols: Formation of Hemiacetals and Acetals
Mechanism of acetal formation
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Addition of Alchols: Formation of Hemiacetals and Acetals
Aldehydes that have appropriately located hydroxyl group can exist in equilibrium with a cyclic hemiacetal…5-hydroxypetanal
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Addition of Alchols: Formation of Hemiacetals and Acetals
Aldehydes that have appropriately located hydroxyl group can exist in equilibrium with a cyclic hemiacetal…5-hydroxypetanal
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Addition of Alchols: Formation of Hemiacetals and Acetals
Cpds with hydroxyl group 4 or 5 C’s from the aldehyde group tend to form cyclic hemiacetals and acetals due to lack of strain
Carbohydrates
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Addition of Alchols: Formation of Hemiacetals and Acetals
Ketones also form acetals If a glycol is used, product is cyclic
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Addition of Alchols: Formation of Hemiacetals and Acetals
Summary› Aldehyde or ketone reacts with ROH› Hemiacetal is formed› Further ROH makes acetal
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Addition of Water: Hydration of Aldehydes and Ketones
Water is an oxygen nucleophile, like ROH’s Can add reversibly
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Addition of Water: Hydration of Aldehydes and Ketones
Aside from formaldehyde hydrate most other hydrates cannot by isolated because they lost water…Keq<1
One exception is trichloroacetaldehyde (chloral)› Forms a stable crystalline hydrate, chloral hydrate,
CCl3CH(OH)2
› Used as a sedative
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Addition of Grignard Reagents and Acetylides
Grignard reagents act as carbon nucleophiles toward carbonyl cpds› Grignard reagent adds irreversibly to the
carbonyl carbon, forming a new C-C bond› Favorable because product (an alkoxide) is a
much weaker base than the starting carbanion› The alkoxide can be protonated to give an
ROH
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Addition of Grignard Reagents and Acetylides
Useful route to alcohols› Type of carbonyl determines class of ROH› Formaldehyde gives 1˚ ROH’s
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Addition of Grignard Reagents and Acetylides
Other aldehydes give 2˚ ROH’s
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Addition of Grignard Reagents and Acetylides
Ketones give 3˚ ROH’s
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Addition of Grignard Reagents and Acetylides
Other organometallic cpds like organolithium cpds and aceylides react with carbonyl cpds similarly to Grignard reagents
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Addition of Hydrogen Cyanide: Cyanohydrins
HCN adds reversibly to carbonyl group of aldehydes and ketones to make cyanohydrins› Hydroxyl and cyano group attached to same C› Basic catalyst is needed
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Addition of Hydrogen Cyanide: Cyanohydrins
Acetone reacts as follows:
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Addition of Hydrogen Cyanide: Cyanohydrins
Cyanohydrins play important role in the defense system of the millipede› Two-chambered gland like the bombadier
beetle› Benzaldehyde cyanohydrin is stored and then
converted to a mixture of benzadehyde and hydrogen cyanide and secreted
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Addition of Hydrogen Cyanide: Cyanohydrins
Write an equation for the addition of HCN to benzaldehyde.
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Addition of Nitrogen Nucleophiles
Ammonia, amines, and other related cpds have a lone pair on the N and thus act as a nucleophile toward a carbonyl C
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Reduction of Carbonyl Cpds Aldehydes and Ketones are easily reduced to
1˚ and 2˚ alcohols, respectively Metal hydrides used to reduce
› Irreversible nucleophilic attack› LiAlH4 or NaBH4
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Reduction of Carbonyl Cpds The original product is an aluminum alkoxide Then hydrolyzed by water and acid to give
ROH Net result is addition of H across the C=O
O1 . L iA lH 4
2 . H + , H 2 OOH
H
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Oxidation of Carbonyl Cpds Aldehydes are more easily oxidized than are
ketones Oxidation of an aldehyde gives an acid with
the same number of C’s Oxidizing agents include KMnO4, CrO3, Ag2O
CH3O
C rO 3 , H +
Jo n e s r e a g e n tCH3
O
OH
OA g 2 O O
OH
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Oxidation of Carbonyl Cpds Tollens Silver Test
› Silver-ammonia complex ion is reduced by aldehydes but not by ketones
› If test tube is clean, a mirror forms by the metallic Ag
› Used to “silver” glass using formaldehyde (cheap)
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Keto-Enol Tautomerism Aldehydes and Ketones may exist as an
equilibrium mixture of two forms› Keto and enol forms› Differ in the location of a proton and a double
bond Tautomerism (Greek…same part)
› Structural isomers› Not resonance contributors
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Keto-Enol Tautomerism In order for an enol form to exist carbonyl C
must have an H attached to the carbon adjacent to the carbonyl group› Known as the -hydrogen and is attached to
the -carbon
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Keto-Enol Tautomerism Most simple aldehydes and ketones exist
primarily in the keto form› Keto more stable› Acetone, 99.9997% keto form
Phenols have mainly enol form
OH
HH
O
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Keto-Enol Tautomerism Carbonyl cpds that do not have an -
hydrogen cannot form enols and exist only in keto form
H
O
H
H
O O
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Acidity of -Hydrogens: the Enolate Anion
-Hydrogen is more acidic than normal H attached to a C› Carbonyl C carries a partial + charge, attracting
bonding electrons away from the -H…make it easy for a base to remove the -H
› Resulting anion is stabilized by resonance…enolate anion
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The Aldol Condensation Enolate anions may act as carbon
nucleophiles Enolate can add reversibly to the carbonyl
group of another aldehyde or ketone…known as aldol condensation
Simplest is the combination of two acetaldhyde molecules due to treatment with an aqueous base
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The Aldol Condensation
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The Aldol Condensation Step 1…base removes -H to form enolate
anion Step 2…enolate anion adds to the carbonyl
carbon of another acetaldehyde molecule, making a new C-C bond
Step 3…alkoxide ion form in step 2 accepts a proton from the solvent, thus regenerating the OH- needed for the first step
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The Aldol Condensation 3-hydroxyaldehydes are always formed Since -C acts as a nucleophile, the product
always has just one C between the aldehyde and alcohol C’s
Does not matter how long the C chain is in the starting aldehyde
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The Mixed Aldol Condensation
Enolate anion of one carbonyl cpd can be made to add to the carbonyl carbon of another cpd
Consider acetaldehyde and benzaldehyde (has no -H) when treated with base