Chapter 17 Lecture Notes - CEM 252 at MSU Spring 1997

Chapter 17 - Aldehydes and Ketones

• Aldehydes have at least one H on the carbonyl
• Both aldehydes and ketones are polar but have no hydrogen bond donor
  Boiling points fall between similar molecular weight alkanes (low boiling) and alcohols (high boiling)
• Aldehydes and ketones have hydrogen bond acceptors
• More water soluble than non-polar compounds of similar MW

Chapter 17-Slide 1

Syllabus Change

Skip section 17.8, 17.10, 17.15B, 17.16C

Skip problems 17.5, 17.30, and 17.31

Chapter 17-Slide 2

Nomenclature - Aldehydes

• Aldehydes use suffix -al on parent alkane
  
• Unsaturated aldehydes include -en- or -yn and number
  
• -CHO as a ring substituent: use suffix -carbaldehyde
  

Chapter 17-Slide 3

Nomenclature - Ketones

• Ketones use suffix -one with parent chain with a number to indicate position
  
• prefix oxo- is used to represent carbonyls if functional group with higher precedence present
  

Chapter 17-Slide 4

Nomenclature - Common Names in the IUPAC System

Chapter 17-Slide 5

Preparation

• A. Oxidation of alcohols
  pyridinium chlorochromate (PCC) commonly used
• B. Oxidation of 1,2-glycols (accompanied by C-C cleavage)
  HIO4 or Pb(OAc)4 commonly used
• C. Friedel-Crafts acylation
  Lewis acid catalyzed reaction between aromatic ring and acyl chloride
• D. Hydration of alkynes
• E. Hydroboration-oxidation of alkynes
• F. Oxidation of alkenes (accompanied by C-C cleavage)
  ozone or osmium tetroxide followed by B above common

Chapter 17-Slide 6

Reactivity of Carbonyl Group

Remember that nucleophiles must have a lone pair - and the greater the negative charge the stronger the nucleophile

Chapter 17-Slide 7

Representative Reactions of Carbonyl Group

Draw the initial intermediate for the following reactions.

Discuss your intermediates with the people around you.

Chapter 17-Slide 8

Nucleophiles Covered by Sections 17.7-17.11 (skip 17.8 and 17.10)

Chapter 17-Slide 9

Common Mechanistic Step

These reactions differ only in small details after this point - recognize the similarity and learning reactions will come easier

Chapter 17-Slide 10

Formation of Organometallic Compounds

Chapter 17-Slide 11

A Limitation of Organometallic Reagents

The partial negative charge on carbon confers a strong basic nature - only useful nucleophiles when acidic hydrogen is not available

Chapter 17-Slide 12

Alcohol Formation with Organometallic Reagents

Chapter 17-Slide 13

Addition of Oxygen Nucleophiles

• Addition of water - hydrate formation
• Addition of alcohols - hemiacetal and acetal formation

Chapter 17-Slide 14

Acetal and Hemiacetal Formation

• This equilibrium is acid-catalyzed (both directions)
  Oxygen is protonated, making the carbon more reactive
• The most abundant species at equilibrium is usually the ketone or aldehyde
  exception: cyclic acetals

Chapter 17-Slide 15

Acetals as Protecting Groups

Carbonyl groups are reactive toward nucleophiles but acetals are not - acetals can be formed to prevent unwanted reactions

Chapter 17-Slide 16

Nitrogen Nucleophiles

Chapter 17-Slide 17

Another Reactive Site

Hydrogens adjacent to carbonyl groups are acidic - their pKa falls between alcohols and terminal alkynes

Chapter 17-Slide 18

Stability of Keto and Enol Tautomers

The Keto form typically is more stable

Two Exceptions: 1,3-diones and phenols

Chapter 17-Slide 19

Reactions at the a-Carbon - I

• Racemization
  
• Deuterium Exchange
  
• Halogenation
  

Chapter 17-Slide 20

Reactions at the a-Carbon - II

• The aldol reaction: an enolate reacting with a carbonyl group
  
• Note: an enolate can form from any carbonyl with a-hydrogens and enolates can react with any carbonyl

Chapter 17-Slide 21

The Aldol Reaction: Multiple Reactants

Which of the following sets of reactants will give a single aldol product and what is that product?

(Hint: how many enolates can form and what can they react with?)

Chapter 17-Slide 22

Dehydration of the Aldol Product

• Dehydration forms a conjugated product
• Will occur under acidic or basic conditions when heated

Chapter 17-Slide 23

Ring Formation with the Aldol

Formation of 5 and 6-membered rings is favored when multiple sizes are possible

Chapter 17-Slide 24

Oxidation of Aldehydes

Aldehydes are very easily oxidized

Sources of Ag(I) include Ag₂O and AgNO₃

Chapter 17-Slide 25

Reductions

• Catalytic Reduction
  
• Metal Hydride Reductions
  

Chapter 17-Slide 26

Selectivity of Reductions

What is the product of the following reduction?

Chapter 17-Slide 27