A to Z Directory – Virginia Commonwealth University
Chapter 18 – Ketones and Aldehydes
• Nomenclature
o Ketones have priority over alcohols
▪ Find the longest chain with the carbonyl in it.
▪ Name the parent and replace the “e” with “one”
▪ Say where the carbonyl is.
[pic]
▪ When the ketone is not the high-priority group, you call the carbonyl “oxo” to name it.
▪ Common names: Name the two pieces coming off the carbonyl and call it “ketone”
[pic]
o Aldehydes have priority over ketones
▪ Find the longest chain with the carbonyl at the end.
▪ Name the parent and replace the “e” with “al.”
[pic]
• Synthesis of ketones and aldehydes
o Oxidation of alcohols
| |Chromic Acid |KMnO4 |PCC |Swern |
|1° ROH |RCOOH |RCOOH |RCHO |RCHO |
|2° ROH |Ketone |Ketone |Ketone |Ketone |
|3° ROH |NR |NR |NR |NR |
o Ozonolysis of alkenes
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▪ See Chapter 8 for review
o Friedel-Crafts Acylation
[pic]
▪ See Chapter 17 for review
o Hydration of alkynes
▪ Acid-catalyzed
[pic]
▪ Hydroboration-oxidation
[pic]
▪ See Chapter 8 for review
o Synthesis from carboxylic acids and acid chlorides
▪ Acids are first converted to acid chlorides.
[pic]
▪ Acid chlorides are reduced to aldehydes with LiAlH(OtBu)3
[pic]
• LiCuR2
o LiCuR2 is like a specialty version of a Grignard that only replaces the Cl of acid chlorides.
[pic]
• Wittig Reactions
o Overall reaction: turn a carbonyl into a carbon-carbon double bond.
▪ The carbon of the alkyl halide replaces the oxygen of the carbonyl.
[pic]
o Step one: Triphenylphosphine attacks an alkyl halide
[pic]
o Step two: Grignard deprotonates to form the ylide
▪ What’s an ylide?
• An ylide is when you have two atoms next to each other where one has a negative charge, the other has a positive charge, and they both have full octets.
[pic]
o Step 3: ylide attacks the carbonyl
[pic]
o Step 4: O- forms bond with P+
[pic]
o Ring collapses – fun with arrows
[pic]
o Considerations: You want alkyl halide to be methyl or primary because they are best for SN2
• Hydration
o Acid-catalyzed
[pic]
o Base-catalyzed
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o Rates
▪ This reaction doesn’t happen to a large degree with most ketones
▪ It happens a little more with aldehydes
▪ Keq for formaldehyde is 40
• Formation of Cyanohydrins
o HCN is a toxic gas, so most often it is made in situ from excess NaCN and HCl.
o Step one: cyanide ion attacks the carbonyl.
[pic]
o Step two: protonation
[pic]
o Things you can do with cyanohydrins.
▪ Catalytic hydration
[pic]
▪ Hydrolysis
[pic]
• Formation of imines
o Overall: The nitrogen replaces the oxygen of the carbonyl, forming a carbon-nitrogen double bond.
▪ It should be mildly acidic; pH between 4 and 5.
[pic]
o Mr. Baker said that you are not responsible for the mechanism, but here it is in case you want to look at it.
o Step one: protonation of the carbonyl
[pic]
o Step two: Amine attacks the activated carbonyl
[pic]
o Step three: Deprotonation
[pic]
o Step four: Protonation of hydroxyl
[pic]
o Step five: Loss of water
[pic]
o Step six: Deprotonation
[pic]
• Formation of acetals
o You could call an acetal a geminal diether
▪ A hemiacetal is what you have when you’re halfway there.
▪ It is one hydroxyl and one alkoxy group coming off the same carbon.
o Step one: protonation of the carbonyl
[pic]
o Step two: the alcohol attacks the activated carbonyl
[pic]
o Step three: deprotonation to give the hemiacetal
[pic]
o Step four: Protonation of the hydroxyl
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o Step five: water falls off
[pic]
o Step six: another alcohol adds
[pic]
o Step seven: deprotonation gives the acetal
[pic]
• Removing acetals
o Use very dilute acid
o The mechanism of hydrolysis of an acetal is just the same thing going backwards.
• Acetals and hemiacetals in sugars
o Sugars exist in their cyclic forms as hemiacetals
▪ The carbon which is the hemiacetal or acetal is called the anomeric carbon.
▪ When the OH is up, it’s Beta; when it’s down, it’s alpha.
[pic]
o When they link up to become polysaccharides, they become acetals.
[pic]
o Something you don’t need to know for this class, but the MCAT expects you to know:
▪ Sugars that differ by only one chiral center are called epimers.
[pic]
• Protecting groups
o Requirements of a good protecting group
▪ Easy to put on
▪ Nonreactive under the reaction conditions
▪ Easily removable
• Oxidation of aldehydes
o Aldehydes are oxidized to carboxylic acids by KMnO4 and chromic acid
• Reductions of ketones and aldehydes
o The following reductions which have already been seen work on ketones and aldehydes:
| |NaBH4 |LiAlH4 |H2/ Raney Ni |Zn(Hg)/HCl (aq) |N2H4/base |
|Ketone |2° ROH |2° ROH |2° ROH |Alkane |Alkane |
|Aldehyde |1° ROH |1° ROH |1° ROH |Alkane |Alkane |
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