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Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O



Summary | Aldehydes and Ketones | Preparations of Aldehydes and Ketones | Reactions of Aldehydes and Ketones | Spectroscopic Analysis of Ethers | Self Assessment | Quiz |


Preparations of Aldehydes and Ketones

Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

(overview)
 
 

alkene ozonolysis
alkyne hydration
Friedel-Crafts acylation

Ozonolysis of Alkenes
(review of Chapter 6)

ozonolysis of alkenes
Reaction type: Electrophilic Addition

Summary

  • Overall transformation :  C=C to 2C=O
  • Reagents : ozone, O3,  followed by a reducing work-up, usually Zn in acetic acid. 
  • It is convenient to view the process as cleaving the alkene into two carbonyls:
  • The substituents on the C=O depend on the substituents on the C=C.
how to visualise ozonolysis
 What would be the products of the ozonolysis reactions of:
(a) ethene ? 2 molecules of methanal, H2CO
(b) 1-butene ? two aldehydes, CH3CH2CO and H2CO

(c) 2-butene ? 2 molecules of CH3CHO
(d) 2-methylpropene ? a ketone, propanone CH3COCH3, and an aldehyde, H2CO

 
MECHANISM FOR REACTION OF ALKENES WITH O3
 
Step 1:
The p electrons act as the nucleophile, attacking the ozone at the electrophilic terminal O. A second C-O is formed by the nucleophilic O attacking the other end of the C=C.
ozonolysis of C=C to C=O
Step 2:
The cyclic species called the ozonide rearranges to the malozonide. 
Step 3:
On work-up (usually Zn / acetic acid) the malozonide decomposes to give two carbonyl groups.

 

Hydration of Alkynes
(review of Chapter 9)

hydration of alkynes
Reaction type: Electrophilic Addition

Summary

MECHANISM FOR REACTION OF ALKYNES WITH H3O+
Step 1:
An acid / base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
addition of H2O to alkynes
Step 2:
Attack of the nucleophilic water molecule on the electrophilic carbocation creates an oxonium ion.
Step 3:
An acid / base reaction. Deprotonation by a base generates the alcohol and regenerates the acid catalyst forming an unstable enol.
Step 4:
An acid / base reaction. Reprotonation by the acid catalyst occurs on the carbon. The oxygen atom electrons help facilitate this process generating an oxonium ion.
Step 5:
Another acid / base reaction. Deprotonation of the oxonium ion creates the ketone. Steps 4 and 5 show the acid catalyzed tautomerization of the enol to the ketone.

 

Oxidation of Primary and Secondary Alcohols
(review of Chapter 15)

general scheme for the oxidation of alcohols
Reaction type: Oxidation-Reduction

Summary

reagents for the oxidation of alcohols
oxidation of RCH2OH
1o alcohol                      aldehyde
oxidation of R2CHOH
2o alcohol                       ketone
oxidation of R3COH
3o alcohol
Cr OXIDATION OF ALCOHOLS
The mechanism is not trivial, so attention here is focused on the actual oxidation step. Prior to this, the alcohol reacts to form a chromate ester (shown). A base (here a water molecule) abstracts a proton from the chromate ester, the C=O forms and a Cr species leaves.
This demonstrates the importance of the carbinol H to this mechanism.
oxidation of alcohols
Questions:
What type of reaction could the oxidation step be described as ? an elimination, specifically an E2
What is the oxidation state of Cr in chromic acid ? H2CrO4 = Cr VI
What is the oxidation state of the Cr in the chromate ester intermediate ? still Cr VI
What is the oxidation state of the Cr in the "leaving group", HCrO3- Cr IV

Study Tip: If you see Cr reagents, you are probably looking at an oxidation reaction.

 

Friedel-Crafts Acylation of Benzene
(review of Chapter 12)

acylation of arenes

Reaction type: Electrophilic Aromatic Substitution

Summary.

MECHANISM FOR THE FRIEDEL-CRAFTS ACYLATION OF BENZENE
Step 1:
The acyl halide reacts with the Lewis acid to form a more electrophilic C, an acylium ion
Friedel-Crafts acylation of benzene
Step 2:
The p electrons of the aromatic C=C act as a nucleophile, attacking the electrophilic C. This step destroys the aromaticity giving the cyclohexadienyl cation intermediate.
Step 3:
Removal of the proton from the sp3 C bearing the acyl- group reforms the C=C and the aromatic system, generating HCl and regenerating the active catalyst.


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