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Chapter 23

CHM 2211 Chapter Notes - Chapter 23: Protonation, Lone Pair, Decarboxylation


Department
Chemistry
Course Code
CHM 2211
Professor
Mohammed Daoudi
Chapter
23

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Chapter 23 Lecture Recordings: Alpha Carbonyl Reactions
- The alpha carbon to the carbonyl carbon.
o The carbon next to the carbonyl group is designated as being the in alpha position.
- One of the alpha hydrogens (hydrogens on the alpha carbon) will be replaced.
- Two ways to react:
o Electrophilic substitution occurs at this position through either an enol (double bond between
carbonyl carbon and alpha carbon) or an enolate ion (lone pair of electrons on alpha carbon).
o Both pathways result in replacement/substitution of a hydrogen with an electrophile.
- A carbonyl compound with alpha hydrogen atom on its alpha carbon rapidly equilibrates with its
corresponding enol.
- Compounds that differ only by the position of a moveable proton are called tautomers.
o Keto and enol tautomers are in equilibrium with each other.
o They are constitutional isomers.
o Does not mean they are in 50:50 ratio, but they are in equilibrium with each other.
o THEY ARE NOT RESONANCE STRUCTURES.
- Tautomers are not resonance forms:
o Tautomers are structural isomers.
o Resonance forms are representations of contributors to a single structure.
o Tautomers interconvert rapidly while ordinary isomers do not.
- Usually the keto form is the dominant one of the tautomers.
o Keto: >99.99%
o Enol: <0.01%
- Keq for keto-enol equilibrium can vary widely:
o The enol tautomer is generally much less stable than the keto tautomer, and thus present at a
much lower concentration.
o For simple aldehydes and ketones, the form dominates.
o When the enol form is particularly stabilized by conjugation, the enol form dominantes.
Keto: 10-30%
Enol: 70-90%
Stabilized by conjugation and intramolecular hydrogen bonding forming a six membered
ring.
- Keto contains C=O, while enol contains C=C.
- Mechanism can take place in two medium:
o Acid catalyzed
Protonation first: ketone is protonated to produce protonated ketone which is in
equilibrium with a carbocation (resonance structure).
Deprotonation second: carbocation is attacked by H2O to form the enol.
o Base catalyzed
Deprotonation first: OH deprotonates alpha carbon, forming an enolate, which is
stabilized by resonance.
Protonation second: protonation of the enolate; H2O protonates the oxygen to produce
the enol.
o Both reactions produce an enol.
- Reactivity of enols:
o Enols are more electron rich than alkenes.
o The OH group has a powerful electron donating resonance effect, which makes it more
electron-rich.
o Enols behave as nucleophiles and react with electrophiles because the double bonds are
electron-rich compared to alkenes.
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- Enolates:
o The enolates are much more nucleophilic than the enols.
o Use base to deprotonate the alpha carbon and produce the enolate, which is resonance
stabilized.
- Esters R’OOR’’ and nitriles an also reat ith a ase to produe an resonane stailized enolate.
- Comparking pka’s:
o Alkane pka = 50 very weak bases
o Alkene pka = 43
o Ketone pka = 19.2
More stable than alkanes and alkenes because one of its resonance structures contains
an oxygen atom (electronegative atom) with a negative charge.
Alkanes don’t hae resonane strutures.
Alkenes resonance structures contain negative charges on carbon (electropositive
atom).
o Alcohol pka = 16
o Carboxylate pka = 4.8
More stable than ketone because it contains two oxygen atoms with negative charges
on its resonance structures.
- Beta-dicarbonyl compounds:
o Have high acidity (low pka).
o Hydrogen is flanked between two oxygen.
o Pka = 9 more acidic than ketone.
o Has three resonance structures: two of the contain negative charges on oxygen.
- The base:
o To form an enolate, a base must be used to remove the alpha protons.
o The appropriate base depends on how acidic the alpha proton is.
o A conjugate acid is produced.
o For reaction to favor products, the conjugate acid (base) must be weaker than the acid
o Base attacks alpha proton.
o Base should be stronger than conjugate base (enolate).
o Use a strong base to deprotonate carbonyl compound and form the enolate.
- LDA (lithium diisopropylamide) a strong nonnucleophilic base
o Very strong base.
o Cannot behave as a nucleophile because it is hindered.
o Pka of conjugate acid is 36.
o Can generate almost 100% of enolate.
- General reactions of enolates:
o Base graps alpha hydrogen and deprotonates.
o Enolate is resonance stabilized.
o Electrophile can attack at:
Alpha carbon (preferred mechanism)
Carbonyl oxygen
- Enolates of unsymmetrical carbonyl compounds:
o Two possible enolates can be produced.
Kinetic enolate
Least stable
Alkene is less substituted so it is least stable
Reaction occurs rapidly at the least substituted alpha carbon
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