Transcript Ch 17- Aldehydes and Ketones

Ch 17- Aldehydes and Ketones
Homework: 17.10, 17.13, 17.17,
17.19, 17.28, 17.35, 17.40, 17.41,
17.49, 17.53, 17.69
• In this chapter we study compounds that
contain the carbonyl group.
O
C
The Carbonyl group
• The carbonyl group is one of the most
important functional groups in organic
chemistry and Biochemistry because it is
present in aldehydes, ketones, carboxylic
acid and carboxylic acid derivatives.
Aldehydes & Ketones
• The functional group of an aldehyde is a
carbonyl group bonded to a hydrogen
• In methanal, the simplest aldehyde, the
carbonyl group is bonded to 2 H’s
• In all other aldehydes, it is bonded to one H
and a carbon chain.
• The functional group of a ketone is a
carbonyl group bonded to 2 carbon chains.
Nomenclature
• 1) Find the longest chain that contains the
carbon of the carbonyl and number the
chain to give that carbon the lowest number.
– For aldehydes, drop the -e, and add -al
– Aldehydes will always be at the end of a chain,
so they will always be at carbon #1, so there is
no need to put the 1.
O
H
O
H
O
H
Nomenclature (cont)
• For aldehydes that have carbon-carbon
double bonds in them, the longest chain
must contain the carbon of the carbonyl and
BOTH carbons of the C-C double bond!!
• To name these, we drop the -ane ending and
add -enal
• The -en- shows the double bond, the -al
shows the aldehyde.
– Remember to provide the locant for the double
bond
O
H
O
H
O
H
Nomenclature of Ketones
• For ketones, we find the longest chain that
contains the carbon of the carbonyl group,
and number the chain to give that carbon
the lowest number
• You drop the -e, from the parent name and
add -one.
O
O
O
• In naming aldehydes and ketones that also
have an -OH in the molecule, find the
longest chain that contains both the carbon
of the carbonyl and the carbon bonded to
the -OH group
• Number the chain to give the carbon of the
carbonyl the lowest number
• The -OH will be named as a substituent!
• When the -OH group is named as a
substituent, it is named as a hydroxy, and
numbered and alphabetized will all other
substituents present.
Examples
OH
O
OH
O
O
O
HO
OH
O
H
H
Br
OH
Physical Properties
• Oxygen is more electronegative than carbon
therefore the carbon-oxygen double bond is
polar with the Oxygen bearing a partial
negative charge and the Carbon bearing a
partial positive charge
• The only intermolecular forces are dipoledipole forces and London Forces
• They can not Hydrogen bond to each other!!
Physical Properties
• As the groups bonded to the carbonyl
increase in size, the solubility in water
decreases
• Most aldehydes and ketones have strong
odors and are used in perfumes and
flavoring agents
Reactions
1) Aldehydes can be oxidized to the
carboxylic acids
O
O
K2Cr2O7
OH
H2SO4
H
-Ketones are resistant to oxidation
Aldehydes can also be oxidized by O2!!
O
O
H
O2
OH
Reduction Reactions
• Just like C-C double bonds, C-O double
bonds can be reduced by the addition of H2
with a metal catalyst to produce an alcohol.
OH
O
H2
H
O
Pd, Pt, or Ni
H2
Pd, Pt, or Ni
OH
Sodium Borohydride
• Aldehydes and Ketones can also be reduced
using Sodium Borohydride, NaBH4
• NaBH4 contains hydrogen in the form of
hydride ions, H-
• The advantage of using NaBH4 is that it
does not reduce C-C double bonds!
Examples
O
OH
H 2, Pd
O
OH
NaBH 4
H 3O
In Nature
• In Biological systems, nicotinamide adenine
dinucleotide, a coenzyme abbreviated
NADH, is used to provide the hydride ion to
reduce aldehydes and ketones.
O
H3C
O
NADH
COO
Pyruvate
H3C
H
OH
H 3O
COO
H3C
H
Lactate
COO
Reactions of Alcohols
3) Addition of Alcohols
Addition of one molecule on an alcohol to
an aldehyde or ketone form a hemiacetal
Hemiacetal- a compound with a carbon
bonded to 1 -OH group and 1 -OR group
Examples
O
OH
H
O
+ H O CH CH
2
3
OH
O
+
CH3 OH
O CH3
H
CH2CH3
Reactions of Hemiacetals
• Hemiacetals can react with another
molecule of alcohol to form an acetal
• Acetal- a compound with a carbon bonded
to 2 -OR groups
OH
O
O
H
CH2CH3 +
O
H
CH2CH3
OH
O CH3
O
O
+
CH3 OH
CH2CH3
CH3
O CH3
H
CH2CH3
Info on Hemiacetals
• Hemiacetals are generally unstable and are
only minor components of an equilibrium
mixture
• The only exception is when the -OH group
is part of the same molecule as the carbonyl
group and a five or six member ring can
form
• The compound exists almost entirely in the
cyclic hemiacetal forms
• In this case, the -OH group adds to the C=O
group of the same molecule
Examples
3
O
5
4
3
2
1
Redraw to show
-OH and carbonyl
H
5
2
1
4
OH
close to each other
HO
4-Hydroxypentanal
3
5
2
3
1
4
OH
H
O
4
5
O
2
1
H
OH
A Cyclic Hemiacetal
H
O
YOUR TURN!!
• Do the same thing with:
O
OH
H
OH
OH
OH
OH
NOTE: Six membered rings are more stable than five membered
Rings. If both can form, the six membered ring will form over the
Five membered ring
Keto-Enol Tautomerism
• A carbon atom adjacent to a carbonyl group
is called an a-carbon, and a hydrogen atom
bonded to it is called an a-hydrogen
Alpha Carbons
O
C
H3C
CH3
C
H2
Alpha Hydrogens
Beta Carbon (
Keto-Enol Tautomerism
• A carbonyl compound that has an ahydrogen is in equilibrium with a
constitutional isomer called an enol
• The name enol is derived from the IUPAC
designation of it as having both an
alkene (-en) and an alcohol (-ol)
OH
O
H3C
CH3
H3C
CH2
Keto-Enol Tautomerism
• The Keto and Enol forms are examples of
Tautomers.
• Tautomer- constitutional isomers in
equilibrium with each other that differ in the
location of a hydrogen atom relative to an
oxygen atom
• This type of isomerism is called keto-enol
tautomerism
• For any pair of keto-enol tautomers, the
keto form generally predominates at
equilibrium!