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Chapter Final

CHEM267 Chapter Notes - Chapter Final: Nucleophilic Acyl Substitution, Haloform Reaction, Enol


Department
Chemistry
Course Code
CHEM267
Professor
Monica Barra
Chapter
Final

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Chem 267 Chapter 22 Notes Winter 2012
1
22.1 Introduction to Alpha Carbon Chemistry: Enols and Enolates
The C positions (indicated by Greek letters) are relative to the position of the carbonyl group
Enols
In the presence of a catalytic acid or base, a ketone will exist in equilibrium with an enol
o Ketone is favoured at equilibrium
o NOTE: tautomers are NOT resonance structures! (different arrangement of atoms)
HOWEVER: in some cases, the enol tautomer is more stabilized than keto (2 reasons):
1. The enol has a conjugated π system
2. The enol can form an intramolecular H-bonding interaction between the hydroxyl proton
and the nearby carbonyl group (shown with the dotted line)
NOTE: The enol tautomer is very reactive
o The α position is very nucleophilic due to resonance
Enolates
When treated with a strong base, the α position of a ketone is deprotonated to give a resonance-
stabilized intermediate called an enolate
Enolates = “ambident nucleophiles” (they possess 2 nucleophilic sites)
NOTE: C-attack is more common than O-attack! (even though O has the lone pairs)
NOTE: Enolates are more reactive than enols (b/c they possess a full negative charge)
NOTE: only the α protons of an aldehyde/ketone are acidic (lead to a resonance stabilized anion)
Choosing a Base for Enolate Formation
When an alkoxy ion is used as the base, an equilibrium is established in which the alkoxide ion
and the enolate ion are BOTH present (usually less enolate ions present at equilibrium)
NOTE: Since the enolate is a nucleophile and the aldehyde is an electrophile, the 2 species will
react with each other when they are both present
other bases, like sodium hydride (NaH), can irreversibly convert the aldehyde into an enolate
o when NaH is used as the base, hydrogen gas (H2) is formed and bubbles out of solution
o THEREFORE: the aldehyde and enolate are NOT both present (only the enolate)
Another base that can be used for irreversible enolate formation is LDA

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2
In a compound with 2 carbonyl groups that are β to each other, the H’s at the α-position are very
acidic (has a pKa of ~9 compared to the pKa of 10-20 for ketones)
o This is b/c the deprotonated compound with 2 carbonyl groups is resonance stabilized
NOTE: due to the high acidity of beta diketones, LDA or NaH is NOT required to irreversibly
deprotonate these compounds! (hydroxide/alkoxide ions also go to complete enolate formation)
NOTE: the large pKa difference indicates that the reaction will go to complete enolate formation
22.2 Alpha Halogenation of Enols and Enolates
Ketones/aldehydes undergo halogenations at the α position under acid-catalyzed conditions
o NOTE: reaction works with chlorine (Cl), bromine (Br), and iodine (I), but NOT fluorine (F)
Mechanism has 2 main steps:
1. Enol formation = ketone undergoes tautomerization to produce an enol (this is the RDS!)
2. Halogenation = enol serves as a nucleophile and the double bond attacks the halogen
a. The halogen adds to the α position
NOTE: when an unsymmetrical ketone is used, bromination occurs primarily at the more
substituted side of the ketone
o This is b/c the reaction proceeds faster through the more substituted enol (in step 1)
The halogenated product can react further and undergo elimination when treated with a base
Alpha Halogenation in Basic Conditions: The Haloform Reaction
Ketones/aldehydes also undergo halogenations at the α position under basic conditions
NOTE: when more than one α proton is present, it is difficult to achieve monobromination b/c the
brominated product is more reactive and rapidly undergoes further bromination
(polybromination occurs)
NOTE: when a methyl ketone is treated with excess base and excess halogen, a reaction occurs in
which a carboxylic acid is produced after acidic workup
After polybromination, the tribromomethyl group can function as a leaving group, resulting in a
nucleophilic acyl substitution reaction (where OH attacks the electrophilic carbon)
NOTE: Even though carbanions almost never act as leaving groups, the negative charge on the
carbon is stabilized in this case by the electron-withdrawing effects of the 3 bromine atoms
the resulting carboxylic acid is then deprotonated, producing a carboxylate ion and CHBr3
(bromoform) therefore, called a haloform reaction (can also occur with Cl and I)
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