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.
• 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.
C
O
The Carbonyl group
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
bondO
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
H
OOHOOH OH
H
O
O
Br
OH
HOO
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 dipole-dipole 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
-Ketones are resistant to oxidation
Aldehydes can also be oxidized by O2!!
H
O
H2SO4OH
OK2Cr2O7
H
O
O2 OH
O
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.
H
O
Pd, Pt, or Ni
OPd, Pt, or Ni
H2OH
OHH2
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
H2, Pd
OH
O OH
H3O
NaBH4
In Nature
• In Biological systems, nicotinamide adenine dinucleotide, a coenzyme abbreviated NADH, is used to provide the hydride ion to reduce aldehydes and ketones.
H3C COO
ONADH
H3C COO
O
H
H3O
H3C COO
OH
HPyruvate Lactate
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
H
O
HOCH2CH3
O
CH3OH
OH
O CH3
H
OH
O 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
H
OH
O CH2CH3HOCH2CH3
CH3OH
OH
O CH3
O
O CH3
CH3
H
O
O CH2CH3
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
H
O
HO
1234
5
4-Hydroxypentanal
Redraw to show-OH and carbonyl
close to each other
H
OOH
1
23
45
H
OOH
1
23
45
O OH
H1
23
4
5
A Cyclic Hemiacetal
YOUR TURN!!
• Do the same thing with:
H
O
OH
OH
OH
OH
OH
NOTE: Six membered rings are more stable than five memberedRings. If both can form, the six membered ring will form over theFive membered ring
Keto-Enol Tautomerism
• A carbon atom adjacent to a carbonyl group is called an -carbon, and a hydrogen atom bonded to it is called an -hydrogen
H3C
C
CH2
CH3
O
Alpha Carbons
Alpha Hydrogens
Beta Carbon (
Keto-Enol Tautomerism
• A carbonyl compound that has an -hydrogen 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)
H3C CH3
O
H3C CH2
OH
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!
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