CHM136H1 Lecture Notes - Oxonium Ion, Alkoxide, Phenol

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Published on 28 Sep 2012
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Alcohols and phenols
Alcohols and phenols have nearly the same geometry around the oxygen atom as water
R-O-H bond angle has an approximately tetrahedral value and the oxygen is sp3-hybriedized
Higher boiling points
Can hydrogen bonding
A positively polarized OH atom is attracted to a lone pair of electron on the EN oxygen of another
molecule
Intermolecular attraction must be overcome for a molecule to break free from the liquid and enter
vapor state
Boiling temperature is raised
oxonium ions H3O+
Weakly basic: reversibly protonated by strong acids to yield oxonium ions, ROH2+
Weakly acids: dissociate slightly in dilute aqueous solution by donating a proton to water, making H3O+
and an alkoxide ion RO-, or a phenoxide ion, ArO-
Smaller Ka and larger pKa are less acidic
Larger Ka and smaller pKa are more acidic
Substituent groups have a significant effect
Phenols and thiols, the sulfur analogs of alcohols are substantially more acidic than water
alky substitution on alcohol acidity is due to Solvation of the alkoxide ion formed on acid dissociation
the more readily the alkoxide ion is solvated by water the more stable, the more its formation is
energetically favored, and the greater the acidity of the parent alcohol.
Inductive effects are also important in determining alcohol acidities
Electron-withdrawing halogen substituents stabilize an alkoxide ion by spreading out the charge over a
larger volume, thus making the alcohol more acidic
Alcohols are weak acid, they dont react with weak bases, such as amines or bicarbonate ion and they
react to only a limited extent with metal hydroxides phenols are about a million times more acidic than
alcohols therefore soluble in dilute aqueous NaOH
Often be separated from a mixture simply by basic extraction into aqueous solution by reacidification
They are more acidic because the phenoxide anion is resonance-stabilized Delocalization of the negative
charge over the ortho and para positions of the aromatic ring results in increased stability of the
phenoxide anion relative to un-dissociated phenol and in a consequently lower delta G for dissociation
The negative charge in phenoxide ion is delocalized from oxygen to the ring
Substituted phenols can be either more acidic or less
Depending on the substituent is electron-withdrawing (acidic because these substituents delocalize the
negative charge) or electron-donating (substituents concentrate the charge)
Especially in phenols with a nitro group at the ortho or para position
Smaller pKa for phenol and alcohol = stronger acid
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Document Summary

Alcohols and phenols have nearly the same geometry around the oxygen atom as water. R-o-h bond angle has an approximately tetrahedral value and the oxygen is sp3-hybriedized. A positively polarized oh atom is attracted to a lone pair of electron on the en oxygen of another molecule. Intermolecular attraction must be overcome for a molecule to break free from the liquid and enter vapor state. Weakly basic: reversibly protonated by strong acids to yield oxonium ions, roh2+ Weakly acids: dissociate slightly in dilute aqueous solution by donating a proton to water, making h3o+ and an alkoxide ion ro-, or a phenoxide ion, aro- Smaller ka and larger pka are less acidic. Larger ka and smaller pka are more acidic. Inductive effects are also important in determining alcohol acidities. Electron-withdrawing halogen substituents stabilize an alkoxide ion by spreading out the charge over a larger volume, thus making the alcohol more acidic.

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