Download Hydrogen Bonding • Aldehydes and ketones don`t hydrogen bond

Aldehydes and Ketones | DAT
Hydrogen Bonding
• Aldehydes and ketones don’t
hydrogen bond with themselves
or each other
Boiling Points of Different Functional Groups
Aldehydes’ and ketones’ polarity gives them
higher boiling points than alkanes and alkenes.
However, their inability to hydrogen bond makes
their boiling points lower than alcohols.
•
Aldehydes and Ketones Won’t Hydrogen
Bond Together
Aldehydes and ketones don’t hydrogen bond
because alkyl hydrogens don’t participate in
hydrogen bonds.
•
Like alcohols, they are water
soluble
o But not if their non-polar
alkyl chains are too long
Aldehyde and Ketone Behavior
• Aldehydes and ketones behave in
either of 2 ways during reactions
• First, as we’ve seen, they can
behave as substrates
o The target of nucleophilic
attack
Aldehydes and ketones will,
however, hydrogen bond with
molecules that hydrogen bond
o Like alcohols
Alcohols Do Hydrogen Bond with Aldehydes
and Ketones
Boiling Point & Solubility
• Aldehydes and ketones are polar
o Dipole-dipole attractions
make their boiling points
higher than that of
alkanes and alkenes
• But they cannot hydrogen bond
o So their boiling points are
less than alcohols
Ketone Behaving as a Substrate for
Nucleophilic Attack
As we’ve previously seen, aldehydes and ketones
can serve as the substrates of nucleophilic attack.
The entering nucleophile attacks the carbonyl
carbon, sending the C=O pi electrons up to the
oxygen.
•
Alternatively, they can also
behave as acids by donating a
hydrogen
o But what hydrogen?
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Aldehydes and Ketones | DAT
•
•
All the hydrogens on the
molecule are alkyl hydrogens
o From what we’ve learned
thus far, alkyl hydrogens
are not acidic
Before we talk about which
hydrogens are acidic, let’s
address a common notation
o Carbons are labeled using
Greek letters
o Carbons that are 1 bond
away from the carbonyl
are alpha (α)
o Carbons 2 bonds from the
carbonyl are beta (β), etc.
o Hydrogens are given the
letter of the carbon to
which they are bonded
Labeling Carbons
In carbonyl-containing molecules, carbons
adjacent to the carbonyl are dubbed alpha.
Carbons next to those are beta, followed by
gamma, delta, etc. Hydrogens are referenced
using the Greek letter on their carbon. So, for
example, the hydrogens on a beta carbon are all
beta hydrogens.
•
•
It turns out that one alpha
hydrogen will be acidic
To understand why, consider
what happens when an alpha
hydrogen is donated
o A lone pair is left on the
carbon
o That lone pair can drop
into a C=C double bond
Thus pushing the
C=O pi electrons
up to the oxygen
Donating an α-Hydrogen
When the α-hydrogen is donated, it leaves a lone
pair on the α-carbon. That lone pair can form a
C=C double bond, thereby pushing the C=O pi
electrons up to the oxygen.
•
In other words, we have
resonance
o And resonance stabilizes
the molecule
The Deprotonated Ketone has Resonance
The ketone was willing to lose an α-hydrogen
because doing so allowed the molecule to form a
resonance structure, and resonance stabilizes a
molecule.
•
•
The second of the two resonance
structures shown above is known
as an enolate ion
o En- because it’s an alkene
o –ol- because the C-O
bond would form an
alcohol, if protonated
o –ate basically tells us the
alcohol is deprotonated
Once the ketone has donated its
α-hydrogen, it can behave as a
nucleophile
o That is, it can attack an
electrophile to form a
bond
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Aldehydes and Ketones | DAT
Deprotonated Ketone Acting as a Nucleophile
Here, the ketone acted as an acid and donated an
α-hydrogen. Its lone pair allows the molecule to
behave as a nucleophile by attacking an
electrophile.
Dicarbonyls
• An α-hydrogen is extremely
acidic if it is flanked by 2
carbonyls
o The molecule is even
more stabilized by
resonance
Resonance Structures of the Deprotonated
Dicarbonyl
When an α-hydrogen between the carbonyls is
donated, we form three resonance structures. The
red arrows on each structure show the electron
path that produces the following structure. The
red arrows on the third structure indicate the
electron path that produces the first structure.
•
α-Hydrogens between Two Carbonyls
When flanked by two carbonyls, an α-hydrogen
is very acidic. Loss of either (but not both) of
these hydrogens will produce three resonance
structures, pictured below. Remember, more
resonance = more stability.
•
•
Before, losing an α-hydrogen
gave us two resonance structures
Losing an α-hydrogen flanked by
two carbonyls will actually give
us three
o So the deprotonated
molecule will be more
stable
o And so it won’t “mind”
losing that proton
If put in acid, the structure can be
protonated to form an alcohol
o The alcohol will also be
stabilized via hydrogen
bonding
Hydrogen Bonding of the Alcohol
If the dicarbonyl loses a flanked α-hydrogen, the
molecule can then be protonated by an acid.
Hydrogen bonding between the alcoholic
hydrogen and the carbonyl aids in stabilizing the
molecule.
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Aldehydes and Ketones | DAT
Keto-Enol Tautomerization
• Ketones (not deprotonated
ketones) undergo a spontaneous
reaction
o An α-hydrogen moves
from the α-carbon to the
carbonyl
o This forms an enol
(a)
(b)
Keto-Enol Tautomerization
The ketone in (a) undergoes a spontaneous
reaction, whereby it loses an alpha proton to the
carbonyl, forming an alkene and an alcohol- (b)
an enol. The reaction will occur rapidly back and
forth under standard conditions.
•
Note that this is not resonance;
it’s a reaction
o Resonance structures
involve only the
movement of electrons
o Here, we are actually
moving an atom, the H+
Important Aldehydes and Ketones
• Know the most common
aldehyde and ketone:
Acidity of Aldehydes vs. Ketones
• Suppose an aldehyde and ketone
each lose an α-hydrogen
(a)
(b)
Ketones and Aldehydes as Acids
The (a) ketone and (b) aldehyde have lost an αhydrogen. In the resulting molecule, the ketone
has an alkyl group, while the aldehyde has a
hydrogen. Alkyl groups are electron donors, and
so it tries to push more electrons to the negative
carbanion (red arrow). This makes the molecule
less stable than the deprotonated aldehyde, in
which the hydrogen does not behave as an
electron donor.
•
•
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The alkyl groups of ketones are
electron donors
o So they try to put more
negative charge on the
carbanion
• Not stable
The hydrogen of the aldehyde is
not an electron donor
o So it doesn’t force more
negative charge to the
carbanion
Since the carbanion is more
stable on the aldehyde than the
ketone, the aldehyde is more
willing to release that α-proton
o That is, the aldehyde is
more acidic
Formaldehyde and Acetone
The most common aldehyde you will see is
formaldehyde, and the most common ketone you
will come across is acetone.
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