(overview)
 
 

Ozonolysis of Alkenes
Hydration of Alkynes
Oxidation of Alcohols
Friedel-Crafts Acylation of Aromatics

Ozonolysis of Alkenes
(review of Chapter 6)

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.
What would be the products of the ozonolysis reactions of: (a) ethene ?  (b) 1-butene ?	(c) 2-butene ?  (d) 2-methylpropene ?

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.
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)

Summary

• Alkynes can be hydrated to form enols that immediately tautomerise to ketones
• Reagents:  aq. acid, most commonly H₂SO₄, with a mercury salt
• Regioselectivity predicted by Markovnikov's rule (review)
• Reaction proceeds via protonation to give the more stable carbocation intermediate (review).
• Not stereoselective since reactions proceeds via planar carbocation.

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.
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)

Summary

• Common reagents for oxidation of alcohols:

• The outcome of oxidation reactions of alcohols depends on the substituents on the carbinol carbon (C-OH).
• In order for each oxidation step to occur, there must be H on the carbinol carbon.
• Primary alcohols can be oxidized to aldehydes (or further to carboxylic acids).
• In aqueous media, the carboxylic acid is usually the major product.
• PCC or PDC, which are used in dichloromethane, allow the oxidation to be stopped at the aldehyde.

1o alcohol                      aldehyde

• Secondary alcohols can be oxidized to ketones, no further:

2o alcohol                       ketone

• Tertiary alcohols cannot be oxidized (no carbinol C-H)

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.

Questions:
What type of reaction could the oxidation step be described as ? 
What is the oxidation state of Cr in chromic acid ? 
What is the oxidation state of the Cr in the chromate ester intermediate ? 
What is the oxidation state of the Cr in the "leaving group", HCrO₃^- ? 

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

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

Reaction type: Electrophilic Aromatic Substitution

Summary.

• Overall transformation :  Ar-H to Ar-COR (an aryl ketone).
• Reagent : normally the acyl halide (e.g. usually RCOCl) with aluminum trichloride, AlCl₃, a Lewis acid catalyst.
• The AlCl₃ enhances the electrophilicity of the acyl halide by complexing with the halide.
• Electrophilic species : the acyl cation or acylium ion (i.e. RCO⁺ ) formed by the "removal" of the halide by the Lewis acid catalyst.
• The acylium ion is stabilized by resonance as shown below. This extra stability prevents the problems associated with the rearrangement of simple carbocations:

• The reduction of acylation products can be used to give the equivalent of alkylation but avoids the problems of rearrangement  (more details)
• Friedel-Crafts reactions are limited to arenes as or more reactive than mono-halobenzenes
• Other sources of acylium can also be used such as acid anhydrides with AlCl₃

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
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.

Copyright ©2000 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.