Carbonium Ion Rearrangements Flashcards
Pinacol-pinacolne reaction
Completely substituted 2,3 dimethyl 2,3 butane diols are called pinacols. on treatment with mineral acid (HCL) they undergo dehydration and rearrangment to form ketones and this reaction
P-P reaction steps
1- protonation of diol
Completely substituted diol undergoes protonation to form Oxonian ion
2- dehydration of Oxonium ion to form carbocation
Water molecule is Eliminated from oxonium ion and carbocation is formed
3-rearrangement of carbocation
Secondary carbocation undergoes rearrangement to form the more stable compound (oxonium ion)
4- dehydrogenation to form ketone
Hydrogen atom is removed from the unstable oxonium ion to form ketone
P-p application
1- Pinacolone is used in pesticides , fungicides and Herbicides
2. Pinacolone is used to prepare the cyanoguanidine drug pinacidil.
3. Another drug use of pinacolone is it’s use in stiripentol, which is used to treat epilepsy.
4. Pinacolone is used to produce triadimefone which is used to control fungal diseases in agriculture
Wagner meerwein reaction
Group of reactions in which highly branched alcohols or alkyl halides undergoes rearnangment in the presence acids to of acids to form more stable products such as alkenes. It is also called neo-pentyl rearangment
Steps of Wagner meerwein reaction
1- protonation of alcohol
Neo-pentyl alcohol is protonates to form oxonium ion
2- dehydrogenation to form carbocation
Water molecule is eliminated to form unstable secondary carbocation
3- rearrangement of carbocation
The unstable secondary carbocation undergoes rearrangement to form stable tertiary carbocation
4- nucleophilic attack of base
Base nucleophillicaly attacks on the carbocation. If the bases are strong then elimination occurs otherwise nucleophilic substation will occur
Application of Wagner meerwein applications
1-it is widely used in the synthesis of other heterocyclic compounds and polymer
2- It is used in terpene chemistry.
3- used in the preparation of phenanthrene.
4- it is used to prepare chiral auxilary in the organic chemistry.
Wolff rearrangement reaction
A rearrangment reaction inn which hte diazocompound undergoes rearrangement by photolysis or thermolysis in the presence of AgO to form ketone is called Wolff’s reaction
Wolff rearrangement steps
1-delocalization if double bond
The alpha diaazoketone contains two terminal nitrogen’s with double bonds c=n and n=n. The bond is relocalized to form the anion which has a lot stable canonical form
2- removal of nitrogen gas
The nitrogen gas is removed from the anion to form the carbocation
3- carbocation undergoes rearrangement to form the product ketene
Applications of Wolff rearrangement
-The Wolf rearrangement has been used in the total systheses, the most commen use is trapping the ketene intermediate
with nucleophiles to form carboxylic acid devivatives.
2- another common use is in ring-contraction methods, if the alpha diazo ketone is cyclic, the Wolff rearrangment results in a ring contracted product.
3- The wolff rearrangment works well in generating wing-strained systems. where other reactions may fail.
Beckmans reaction
A rearrangement reaction in which oxides are converted into substituted amide by action of acid
Beckmans steps
1- protonation of oxime
Protonation if oxime occurs to form oxonium ion which then undergoes dehydration to form nitronium ion
2- rearrangement
1,2 slight shift occurs in the nitronium ion to form carbocation
3- attack of nucleophile
Water nucleoiphilically attacks the on the carbocation to form oxonium ion
4- dehydrogenation-
The hydrogen atoms from the oxonium ion is removed by in the presence of water to form hydro ion ion and the product then undergoes tautomeresim to form substituted amide
Water nucleophillically attacks on the carbocation to form oxonium ion
Beckmans applications
1- in the synthesis of caprofactram, which is the monomer of nylon, and Nylon 12
2-synthesis of paracetamol by convecting a ketone with the help of hydroxyl amine
3- For the synthesis of various stecords and drugs.
4- in the production of some of the chlorobicyclic factors .
Hoffmanns bromide degradation rxn
reaction in which the Amide is made to react with the solution of bromine in sodium hydroxide, which yields a primary amine having one carbon atoms less that the starting amide
BR rxn steps
1- attack of base
The base attacks on amide and extracts the H-atom from it to form anion and H2O is eliminated
2- halogenation
Halogenation of anion then produces H-bromaamide which is more acidic than amide
3- attack of base
The base again attacks on N-bromamide to form the unstable intermediate acts nitrene which rearranged to an isocyanate
4- nucleophillic attack of water
Water nucleophilicaly adds in the isocyanate to form carbonic acid from which carbon dioxide is removed to form primary amine
Applications HR rxn
- hr can be used to prepare anthranilic acid from pthalimide. Antheranilic acid had a wide range of industrial applications including the preparation of perfumes, asp dyes and saccharin- an aritificial sweetener
- also used to convert nicotinic acid into 3- aminopyridine
