Aldehydes & Ketones - University of Toronto · PDF fileIntroduction to Aldehydes and Ketones...
Transcript of Aldehydes & Ketones - University of Toronto · PDF fileIntroduction to Aldehydes and Ketones...
AldehydesAldehydesAldehydes and
Aldehydes andand
Ketonesand
Ketones
β‐damascenone
McMurry: McMurry: ChChMcMurry: McMurry:
ChCh
β damascenone
Chapter Chapter 1919Chapter Chapter 19191
Introduction to Aldehydes and Ketones
Among the various types of carbonyls, aldehydes and ketones are the most widely occurring in nature:
TTypes:aliphatic aromatic α,β‐unsaturated cyclic ketones
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Nomenclature: Aldehydes• Find longest carbon chain containing the aldehyde (C1) and g g y ( )replace the ‐e of the parent chain with –al
• Aldehydes take priority over other substituents (ketones: oxo)Substituents are named in alphabetical order• Substituents are named in alphabetical order
• For aldehydes attached to rings, ‐carbaldehyde suffix is used
49e pp 605–607
Nomenclature: Ketones
Find longest carbon chain containing the ketone and replace the• Find longest carbon chain containing the ketone and replace the ‐e of the parent chain with –one
• The locant indicates the position of the ketone • The substituents are named in alphabetical order
Ketones as substituents:Ketones as substituents:
59e pp 605–607
Nomenclature: Common Ketones and Aldehydes
Some ketones and aldehydes have common names:
69e pp 605–607
Take Home Activity: Nomenclature
Draw the structure for each of the following compounds:
Draw the structure for each of the following compounds:
Draw the structure for each of the following compounds:
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IR Spectroscopy of Aldehydes and Ketones
Aldehydes: C=O Stretch: ~1730 cm‐1 (strong)Aldehydes: C=O Stretch: ~1730 cm 1 (strong)C‐H stretch: 2695–2850 cm‐1 (medium)
Ketones: C=O Stretch: ~1715 cm‐1 (strong)*Conjugation (aryl or α,β‐unsat’d) lowers C=O stretch by 25–30 cm‐1
89e pp 640–643
E b i ll d d ld h d
Preparing Aldehydes: Reduction of Esters
Esters can be partially reduced to access aldehydes:
ester aldehyde
d d h h l h l b i fDIBAL‐H does not reduce the ester to the alcohol because it forms an stable aluminum complex at low temperature until quenched:
stable at low temperature 129e pp 607–608
Preparing Aldehydes: Reduction of Nitriles
Nitriles are also reduced by DIBAL‐H to form aldehydes:
o
2
nitrile aldehyde
Partial Mechanism:
13[not in the textbook]
O (Gil ) C b d l hil
Preparing Ketones: Gilman Addition to Acid Chlorides
Organocuprates (Gilman reagents) are C‐based nucleophiles that can add to acid halides once to give ketones:
*Alkyl lithium (RLi) and Grignard reagents (RMgX) are more reactive then Gilman reagents so they typically add twice to give tertiary alcohols.
149e pp 700–701
Preparing Ketones: Grignard Addition to Nitriles
Grignard reagents readily add to nitriles to form ketones:
159e p 671
Relative Reactivity of Aldehydes and Ketones
Ald h d i h k dAldehydes are more reactive than ketones towards nucleophilic attack due to steric and electronic effects.
1 Steric Effects:1. Steric Effects:
Aldehydes have only one substituent Ketones have two substituents(less hindered, more reactive) (more hindered, less reactive)
179e pp 610–613
Relative Reactivity of Aldehydes and Ketones
2 El i Eff2. Electronic Effects:
Recall that alkyl/aryl substituents are electron donating groups which stabilize the δ+ on the carbonyl:groups, which stabilize the δ+ on the carbonyl:
δ+ δ+
Aldehydes have only Ketones have twoAldehydes have only one donating group
(less stable, more reactive)
Ketones have two donating groups
(more stable, less reactive)
189e pp 610–613
Carbonyl Reactivity: Nucleophilic Addition
Nucleophilic addition reactions to aldehydes and ketones are an important class of reactions:
The electrophilicity is δ‐
derived from resonance and inductive effects: δ+
The angle of attack is 105°to the plane of the sp2orbitals, forming an alkoxide ion:
199e pp 610–613
Types of Nucleophilic Additions
1 Addi i l i i l h l d1. Additions resulting in alcohol products:
a) Acidic conditions:
b) B i ditib) Basic conditions:
209e pp 610–613
Types of Nucleophilic Additions
2. Additions resulting in C=Nuc products:
Some nucleophiles (ex. 1° amines) have similar reactivity with aldehydes/ketones, except the initial product undergoes elimination of H2O to form C=Nuc bonds:
219e pp 610–613
PT = proton transfer
Types of Nucleophiles
A id i f l hil i i i ddi iA wide variety of nucleophiles can participate in addition reactions with aldehydes and ketones.
There are 5 main nucleophile classes:There are 5 main nucleophile classes:
229e pp 610–613
Addition of H2O (Hydration)
Water can react with aldehydes and ketones to form gem‐diols (or hydrates) in a reversible reaction:
2
gem-diols (hydrates)
2
The equilibrium depends on the type of carbonyl but the h d i ll d i l i h i l ld h d dhydrate typically dominates only with simple aldehydes and other strongly electrophilic carbonyls.
239e pp 614–616
Addition of H2O (Hydration)
Hydration rates increase under an acidic or basic catalysis:
a) Acid‐catalyzed (carbonyl is a better electrophile): y y p
b) Base‐catalyzed (hydroxyl anion is a better nucleophile):
249e pp 614–616
Acetal and Ketal Formation
Aldehydes and ketones will be converted to acetals or ketals with 2 equivalents of alcohol under acidic catalysis (reversible rxn):
ketal ketone
acetal
A id H SO d T l lf i id (T OH)
aldehyde
Acids: H2SO4 and p‐Toluenesulfonic acid (TsOH)
259e pp 626–629
Acetal and Ketal Formation
The mechanism for acetal/ketal formation is similar to hydration:
hemi-acetal or hemi-ketal
acetal or ketal
* Notice how the acid is not consumed! 269e pp 626–629
Acetal and Ketal Formation: Equilibrium
The equilibrium depends on the type of carbonyl and its specific steric and electronic considerations.
For simple aldehydes, the equilibrium favors the acetal:
2
Ketones are favored over ketals, so water is removed during the reaction to push the equilibrium to the product:
2
279e pp 626–629
Acetals and Ketals as Carbonyl Protecting Groups
Since acetal /ketal formation is reversible, they can be used as carbonyl protecting groups with careful selection of conditions:
289e pp 626–629
Primary Amine Additions (Imines)
Nucleophilic addition of primary amines to aldehydes and ketones results in the formation of imines (Schiff bases):
329e pp 619–624
Primary Amine Additions (Imines)
The reaction is reversible and proceeds by an acid‐catalyzed mechanism:
imineimine
339e pp 619–624
Secondary Amine Additions (Enamines)
If secondary amines are used, enamines (unsaturated amines) are formed instead:are formed instead:
349e pp 619–624
Secondary Amine Additions (Enamines)
The mechanism is the same as for imine formation, except for the last step. There is no N‐hydrogen whose loss would lead to a neutral imine, so deprotonation occurs instead at the α‐carbon:imine, so deprotonation occurs instead at the α carbon:
enamine 359e pp 619–624
pH Dependence of Imine and Enamine Formation
Id l i di i H 4 5Ideal reaction conditions: pH 4.5Considering the mechanism, what would happen if the pH was too high or too low?
Too basic (pH >4.5): Hydroxyl will not be protonated (4th step of mechanism)
Too acidic (pH <4.5): Amine will be protonated and will not attack (2nd step of mechanism)
369e pp 619–624
Imines in Sight
Chemistry Connections
Imines in Sight• The chemistry of vision involves imine formation between cis‐retinal and
an opsin protein, forming a new protein complex called rhodopsin.
• When rhodopsin absorbs light, the cis‐alkene isomerizes causing a large protein distortion which send nerveprotein distortion, which send nerve impulses to the brain where it can be interpreted as a visual stimulus.
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Oxime and Hydrazone Synthesis
The substituent on the nitrogen can also be a heteroatom.
If h d l i (NH OH) d i f dIf hydroxylamines (NH2OH) are used, oximes are formed:
oxime
If hydrazines (H2NNH2) are used, hydrazones are made:
hydrazone
409e pp 619–624
Hydrazones in Synthesis: the Wolff‐Kishner Reaction
The Wolff‐Kishner is a very useful reaction used to reduce aldehydes and ketones to alkanes.
The 2‐step reaction involves a hydrazone intermediate:
aldehyde hydrazone alkaneor ketone
419e pp 624–626
Wolff‐Kishner Reaction: Mechanism
The Wolff‐Kishner mechanism:
If generating the carbanion is thermodynamically unfavorable, whatIf generating the carbanion is thermodynamically unfavorable, what is the driving force of this reaction?
429e pp 619–624
Synthesis Activity
Complete the following reactivity scheme Name the newComplete the following reactivity scheme. Name the new functional group in each product:
O2
NH
2 4
2 22 2
2 2
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Hydrolysis Reactions
Hydrates, acetals, ketals, imines, enamines, oximes, and hydrazones are formed through reversible reactions, so the ketone or aldehyde can be regenerated with water and acidketone or aldehyde can be regenerated with water and acid (hydrolysis):
Th h i i h b i h di i !The mechanism is the same, but in the reverse direction!
459e pp 619–624
P h d f h f ll i h d l i i
Take‐Home Activity
Propose the products of the following hydrolysis reactions:
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Reactivity of Aldehydes and Ketones (Review)
Ald h d d k b d i fAldehydes and ketones can be converted to a variety of alcohols through reduction and Grignard addition:
479e pp 617–619
The Wittig Reaction
Th Wi i i i f l h d f iThe Wittig reaction is a useful method for converting aldehydes and ketones into alkenes:
Georg WittigNobel Prize 1979
Ylid t l d ith dj t iti dYlides are neutral compounds with adjacent positive and negatively charged atoms. Wittig reactions use a phosphorane (Wittig reagent):
489e pp 630–632
The Wittig Reaction
The mechanism can be described in two ways, depending on the specific reaction conditions and reagents:
oxaphosphetaneoxaphosphetane
betaine49
9e pp 630–632
Making Wittig Reagents
Th d i 2These reagents are made in a 2‐step process:
Step 1: SN2 reaction with alkyl halides and triphenylphosphine
Step 2: Deprotonation to make the ylide
509e pp 630–632
Wittig ReactionsIdentify the reagents required for the 2 possible Wittig reactions y g q p gand evaluate which method is better:
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Conjugate Addition to α,β‐Unsaturated Carbonyls
The addition reactions we have explored have all attacked the carbonyl directly:
1,2‐Addition reaction:–
How would an addition reaction proceed with an α β unsaturated βproceed with an α,β‐unsaturated
aldehyde or ketone?α
β
529e pp 635–636
Addition to α,β‐Unsaturated Carbonyls
An α,β‐unsaturated carbonyl is electrophilic at 2 sites:
δ‐
th b l th β b
δ+
δ+the carbonyl the β‐carbon (1,2‐addition) (1,4‐addition)
539e pp 635–636
F l i l f l hil di ddi i (1 2)
Direct Addition (1,2‐Addition)
For several important classes of nucleophiles, direct addition (1,2) occurs at a higher rate and produces the kinetic product:
–
549e pp 635–636
I i f h h j ddi i (1 4) d h
Conjugate Addition (1,4‐Addition)
It is often the case that conjugate addition (1,4) produces the more thermodynamically stable product:
559e pp 635–637
1,2‐Addition (Direct Addition)
Strongly basic nucleophiles undergo irreversible 1,2‐addition (kinetic control)
ex. Grignard reagents, organolithium reagents, hydride sources
569e pp 637–638
1,4‐Addition (Conjugate Addition)
Weakly basic nucleophiles undergo 1,4‐addition (often under thermodynamic control)y )
ex. Gilman reagents (organocuprates), cyanide andammonia/amines (NH3, H2NR, HNR2)
579e pp 636–638
Conjugate Addition Reactions in Cancer Drugs
Chemistry Connections
Conjugate Addition Reactions in Cancer Drugs• Cancer cells grow and divide significantly faster than normal cells,
leading to errors during replication. Many cancer drugs target these ll b di i h i DNA f icells by disrupting their DNA function.
• Eriolangin is an example of a cancer fighting drug that inactivates DNA polymerase soinactivates DNA polymerase so that it can no longer replicate DNA, and does so via a conjugate addition reactionconjugate addition reaction.
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