ch20 - laney college · 4/18/2012 3 • show the nucleophilic attack for some other nucleophiles....
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• Common in biomolecules
• Important in the synthesis of many pharmaceuticals
• The basis upon which much of the remaining concepts in this course will build
20.1 Ketones and Aldehydes
• The carbonyl group is common to both ketones and aldehydes
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20.1 Ketones and Aldehydes –Relevant Examples
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1. Identify and name the parent chain:– For aldehydes, replace the e with an al.
– Example:
20.2 Nomenclature of Aldehydes
– Be sure that the parent chain includes the carbonyl carbon.
– Example:
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1. Identify and name the parent chain:– Numbering the carbonyl group of the aldehyde takes priority
over other groups.
– Example:
20.2 Nomenclature of Aldehydes
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1. Identify and name the parent chain.
2. Identify the name of the substituents (side groups)
3. Assign a locant (number) to each substituents.
4. Assemble the name alphabetically.
20.2 Nomenclature of Aldehydes
p y
• Name the following molecule.
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1. Identify and name the parent chain:– For ketones, replace the e with an one.
– Example:
20.2 Nomenclature of Ketones
– The locant (number showing where the C=O is located) can be expressed before the parent name or before the suffix.
– Example:
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1. Identify and name the parent chain.
2. Identify the name of the substituents (side groups).
3. Assign a locant (number) to each substituents.
4. Assemble the name alphabetically.
20.2 Nomenclature of Ketones
p y
• Name the following molecule.
• Practice with SKILLBUILDER 20.1
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20.3 Preparing Aldehydes and Ketones
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• What makes the carbonyl carbon a good electrophile?1. RESONANCE: There is a minor but significant contributor that
includes a formal 1+ charge on the carbonyl carbon.
20.4 Carbonyls as Electrophiles
– What would the resonance hybrid look like for this carbonyl?
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What makes the carbonyl carbon a good electrophile?
2. INDUCTION: The carbonyl carbon is directly
attached to a very electronegative oxygen atom.
20.4 Carbonyls as Electrophiles
3. STERICS: How does an sp2 carbon compare to an sp3?
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• Consider the factors: resonance, induction, and sterics.
• Which should be MORE REACTIVE as an electrophile, aldehydes or ketones? Explain WHY.
• Example comparison:
20.4 Carbonyls as Electrophiles
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• We want to analyze how nucleophiles attack carbonyls and why some nucleophile react and others don’t.
– Example attack:
20.4 Nucleophilic Attack on a Carbonyl
• If the nucleophile is weak, or if the attacking nucleophile is a good leaving group, the reverse reaction will dominate.
– Reverse reaction:
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• Show the nucleophilic attack for some other nucleophiles. Nucleophiles to consider include OH–, CN–, H–, R–, H2O.
20.4 Nucleophilic Attack on a Carbonyl
• When the nucleophile attacks, is the resulting intermediate relatively stable or unstable? WHY?
• If a nucleophile is also a GOOD LEAVING GROUP, is it likely to react with a carbonyl? Explain WHY.
• Compare attack on a carbonyl with attack on an alkyl halide.
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• If the nucleophile is strong enough to attack and NOT a good leaving group, then the full ADDITION will occur (Mechanism 20.1).
20.4 Nucleophilic Attack on a Carbonyl – Nucleophilic Addition
• The intermediate carries a negative charge, so it will pick up a proton to become more stable.
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• If the nucleophile is weak and reluctant to attack the carbonyl, HOW could we improve its ability to attack?
• We can make the carbonyl more electrophilic:
20.4 Nucleophilic Attack on a Carbonyl
– Adding an acid will help. HOW?
• Consider the factors that make it electrophilic in the first place (resonance, induction, and sterics).
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• With a weak nucleophile, the presence of an acid will make the carbonyl more attractive to the nucleophile so the full ADDITION can occur (Mechanism 20.2).
20.4 Nucleophilic Attack on a Carbonyl – Nucleophilic Addition
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• Is there a reason why acid is not used with strong nucleophiles?
20.4 Nucleophilic Attack on a Carbonyl – Nucleophilic Addition
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• Is water generally a strong or weak nucleophile?
• Show a generic mechanism for water attacking an aldehyde or ketone.
20.5 Water as a Nucleophile
• Predict whether the nucleophilic attack is product favored or reactant favored. WHY?
• Would the presence of an acid improve the reaction?
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• If water were to attack the carbonyl, what likely mechanism steps would follow?
• Will the overall process be fast or slow?
20.5 Water as a Nucleophile
• Will the overall process be product or reactant favored?
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20.5 Water as a Nucleophile
Acetone
Formaldehyde
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Formaldehyde
Hexafluoroacetone
20.5 Water as a Nucleophile
Acetone
Formaldehyde
• How do the following factors affect the equilibria: entropy, induction, sterics?
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Formaldehyde
Hexafluoroacetone
• To avoid the unstable intermediate with two formal charges, the reaction can be catalyzed by a base (Mechanism 20.3).
20.5 Water as a Nucleophile
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20.5 Water as a Nucleophile
• How does the base increase the rate of reaction? Will it make the reaction more product‐favored?
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• The reaction can also be catalyzed by an acid (Mechanism 20.4).
20.5 Water as a Nucleophile
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20.5 Water as a Nucleophile
• How does the acid increase the rate of reaction? Will it make the reaction more product‐favored?
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• An alcohol acts as the nucleophile instead of water.
20.5 Acetals – Formation
• Notice that the reaction is under equilibrium and that it is acid catalyzed.
• Analyze the complete mechanism (Mechanism 20.5) on the next slide.
• Analyze how the acid allows the reaction to proceed through lower energy intermediates.
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20.5 Acetals – Formation
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• After the hemiacetal is protonated in Mechanism 20.5, the water leaving group leaves. Why is the water leaving group pushed out INTRAMOLECULARLY?
20.5 Acetals – Formation
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• You might imagine an INTERMOLECULAR collision that causes the water to leave.
20.5 Acetals – Formation
• Why is the INTERMOLECULAR step unlikely?
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20.5 Acetals – Formation
• Practice with SKILLBUILDER 20.2.Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e20 -30
5 and 6-membered cyclic acetals are generally product favored
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20.5 Acetals – Formation
• How do entropy, induction, sterics, and Le Châtelier’s principle affect the equilibrium?
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5 and 6-membered cyclic acetals are generally product favored
• Acetals can be attached and removed fairly easily.
• Example:
20.5 Acetals – Equilibrium Control
• Both the forward and reverse reactions are acid catalyzed.
• How does the presence of water affect which side the equilibrium will favor?
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• We can use an acetal to selectively protect an aldehyde or ketone from reacting in the presence of other electrophiles.
• Fill in necessary reagents or intermediates.
20.5 Acetals – Protecting Groups
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• Fill in necessary reagents or intermediates.
20.5 Acetals – Protecting Groups
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• As a nucleophile, are amines stronger or weaker than water?
– If you want an amine to attack a carbonyl carbon, will a catalyst be necessary?
• Will an acid (H+) or a base (OH‐) catal st be most likel
20.6 Primary Amine Nucleophiles
• Will an acid (H+) or a base (OH‐) catalyst be most likely to work? WHY?
• What will the product most likely look like? Keep in mind that entropy disfavors processes in which two molecules combine to form one.
• Analyze the complete mechanism (Mechanism 20.6) on the next slide.
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20.6 Primary Amine Nucleophiles
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• The mechanism requires an acid catalyst. Note that the optimal pH to achieve a fast reaction is around 4 or 5.
20.6 Primary Amine Nucleophiles
• Practice with SKILLBUILDER 20.3.
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• Why does the reaction slow down below pH 4?
20.6 Primary Amine Nucleophiles
• Why does the reaction slow down when the pH is greater than 5?
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• A proton transfer alleviates the +1 charge in both mechanisms. The difference occurs in the LAST step.
– For 1° amines (Mechanism
20.6 Primary Amine Nucleophiles vs. Secondary Amine Nucleophiles
For 1 amines (Mechanism 20.6): the NITROGEN atom loses a proton directly.
– For 2° amines (Mechanism 20.7): a neighboring CARBON atom loses a proton.
• Practice with SKILLBUILDER 20.4.
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• Reduction of a carbonyl to an alkane:
20.7 Wolff‐Kishner Reduction
• Hydrazine attacks the carbonyl via Mechanism 20.6 to form the hydrazone, which is structurally similar to an imine.
• The second part of the mechanism is shown on the next slide (Mechanism 20.8).
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• In general, carbanions are unstable and reluctant to form.
20.7 Wolff‐Kishner Reduction
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• What drives this reaction forward?
• Is OH‐ a catalyst in the mechanism?
20.7 Wolff‐Kishner Reduction
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• Note the many similarities between the acid catalyzed mechanisms we have discussed:– Carbonyl is protonated first:
• Makes the carbonyl more electrophilic
• Avoids negative formal charge on the intermediate
– Avoid high energy intermediate with two formal charges
20.8 Mechanism Strategies
– Acid protonates leaving group so that it is stable and neutral upon leaving
– Last step of mechanism involves a proton transfer forming a neutral product
• Overall: under acidic conditions, reaction species should either be neutral or have a +1 formal charge.
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• Under acidic conditions, thiols react nearly the same as alcohols. Examples:
20.8 Mechanism Strategies –Sulfur Nucleophiles
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• Conditions to convert a ketone into an alkane:
20.8 Mechanism Strategies –Alternative to Wolff‐Kishner
1. A thioacetal is formed via an acid catalyzed nucleophilic addition mechanism.
2. Raney Ni transfers H2 molecules to the thioacetal converting it into an alkane.
• Recall the Clemmenson (Section 19.6) reduction can also be used to promote this conversion.
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• We rarely see hydrogen acting as a nucleophile. WHY? What role does hydrogen normally play in mechanisms?
• To be a nucleophile, hydrogen must have a pair of l H 1 i ll d h d id
20.9 Hydrogen Nucleophiles
electrons. H:1‐ is called hydride
• Reagents that produce hydride ions include LiAlH4
(LAH) and NaBH4. Hydrides will react readily with carbonyls.
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• Identify the nucleophile.
• Will the reaction be more effective under acidic or
20.9 Hydrogen Nucleophiles
under basic conditions? WHY?
• Show a complete mechanism (Mechanism 20.9).
• Analyze the reversibility (or irreversibility )of each step.
• Describe necessary experimental conditions. Why are there two steps in the reaction?
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• Carbon doesn’t often act as a nucleophile. WHY? What ROLE does carbon most often play in mechanisms?
• To be a nucleophile, carbon must have a pair of electrons it can use to attract an electrophile:
20.10 Carbon Nucleophiles
1. A carbanion with a ‐1 charge and an available pair of electrons. However, carbanions are relatively unstable and reluctant to form.
2. A carbon attached to a very low electronegativity atom such as a Grignard. Analyze the electrostatics of the Grignard reagent.
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• Identify the nucleophile.
• Will the reaction be more effective under acidic or d b i diti ? WHY?
20.10 Grignard Example1)
2) dilute HCl (aq)
OH
O
O
BrMg3 equivalents
under basic conditions? WHY?
• Show a complete mechanism (Mechanism 20.10). Three equivalents of the Grignard are necessary.
• Analyze the reversibility or irreversibility of each step.
• Describe necessary experimental conditions. Why are there two steps in the reaction?
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• The cyanide ion can act as a nucleophile.
20.10 Cyanohydrin
• Disadvantage: EXTREME toxicity and volatility of hydrogen cyanide.
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• Advantage: synthetic utility
20.10 Cyanohydrin
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• Like the Grignard and the cyanohydrin, the Wittig reaction can be very synthetically useful. What do these three reactions have in common?
• Example:
20.10 Wittig Reaction
• Similar to the Grignard, one carbon is a nucleophile and the other is an electrophile.
• Identify which is which.
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• The ylide carries a formal negative charge on a carbon.
20.10 Wittig Reaction –Wittig Reagent or Ylide
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• In general, carbons are not good at stabilizing a
20.10 Wittig Reaction –Wittig Reagent or Ylide
g , g gnegative charge. Are there any factors that allow the ylide to stabilize its formal negative charge?
• Why is the charged resonance contributor the major contributor?
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• The Wittig mechanism (Mechanism 20.12):
20.10 Wittig Reaction
• Which of the steps in the reaction is mostly likely the slowest? WHY?
• The formation of the especially stable triphenylphoshine oxide drives the equilibrium forward.
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• To make an ylide, you start with an alkyl halide and triphenylphosphine.
• Example:
20.10 Wittig Reaction –Formation of an ylide
• The first step is a simple substitution. The second step is a proton transfer.
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• Is the base used in the second step strong or weak? Why is such a base used?
20.10 Wittig Reaction –Formation of an Ylide
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• Overall, the Wittig reaction allows two molecular segments to be connected through a C=C.
• Example:
20.10 Wittig Reaction – Overall
O
– Describe the reagents and conditions necessary for the reaction to take place.
– Give a mechanism.
– Note how the colored segments are connected.
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Br
• Overall, the Wittig reaction allows two molecular segments to be connected through a C=C.
20.10 Wittig Reaction – Overall
retro1.
• Use a retrosynthetic analysis to determine a different set of reactants that could be used to make the target.
• Practice with SKILLBUILDER 20.6.Copyright 2012 John Wiley & Sons, Inc. Klein, Organic Chemistry 1e20 -59
2.
• An oxygen is inserted between a carbonyl carbon and neighboring group.
• Mechanism 20.13 shows the movement of electrons.
20.11 Baeyer‐Villiger
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• Which step in the equilibrium is most likely the slowest? WHY?
20.11 Baeyer‐Villiger
• Note the last step is not reversible. WHY?
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• If the carbonyl is asymmetrical, use the following chart to determine which group migrates most readily.
• Predict the product of the reaction, and give a
20.11 Baeyer‐Villiger Example
complete mechanism.
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• Recall the questions we ask to aid our analysis‐1. Is there a change in the carbon skeleton?
2. Is there a change in the functional group?
20.12 Synthetic Strategies
• Changes to the carbon skeleton: C–C bond formation
• Name each reaction.
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• Recall the questions we ask to aid our analysis1. Is there a change in the carbon skeleton?
2. Is there a change in the functional group?
• Changes to the
20.12 Synthetic Strategies
carbon skeleton: C–C bond cleavage
• Name the reaction.
• Practice with SKILLBUILDER 20.7.
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• STRONG peak for the C=O stretch:
20.13 Spectroscopic Analysis –Infrared Spectroscopy
typical carbonyl typical conjugated carbonyl
• Aldehydes also give WEAK peaks around 2700–2800 cm‐1 for the C–H stretch.
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• Protons neighboring a carbonyl are weakly deshielded by the oxygen.
20.13 Spectroscopic Analysis –NMR Spectroscopy
• Aldehyde protons are strongly deshielded, usually appearing around 9 or 10 ppm.
– Why is the aldehyde proton shifted so far downfield?
• In the 13C NMR, the carbonyl carbon generally appears around 200 ppm.
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• Predict 1H NMR shifts, splitting, and integration and 13C shifts for the following molecule.
20.13 Spectroscopic Analysis –NMR Spectroscopy
O
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O
HO