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.

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▪ 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”

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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.”

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• 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

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▪ See Chapter 17 for review

o Hydration of alkynes

▪ Acid-catalyzed

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▪ Hydroboration-oxidation

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▪ See Chapter 8 for review

o Synthesis from carboxylic acids and acid chlorides

▪ Acids are first converted to acid chlorides.

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▪ Acid chlorides are reduced to aldehydes with LiAlH(OtBu)3

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• LiCuR2

o LiCuR2 is like a specialty version of a Grignard that only replaces the Cl of acid chlorides.

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• 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.

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o Step one: Triphenylphosphine attacks an alkyl halide

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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.

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o Step 3: ylide attacks the carbonyl

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o Step 4: O- forms bond with P+

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o Ring collapses – fun with arrows

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o Considerations: You want alkyl halide to be methyl or primary because they are best for SN2

• Hydration

o Acid-catalyzed

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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.

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o Step two: protonation

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o Things you can do with cyanohydrins.

▪ Catalytic hydration

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▪ Hydrolysis

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• 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.

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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

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o Step two: Amine attacks the activated carbonyl

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o Step three: Deprotonation

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o Step four: Protonation of hydroxyl

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o Step five: Loss of water

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o Step six: Deprotonation

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• 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

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o Step two: the alcohol attacks the activated carbonyl

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o Step three: deprotonation to give the hemiacetal

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o Step four: Protonation of the hydroxyl

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o Step five: water falls off

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o Step six: another alcohol adds

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o Step seven: deprotonation gives the acetal

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• 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.

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o When they link up to become polysaccharides, they become acetals.

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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.

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• 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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