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Di-alpha-halogenation of ketones in acidic medium
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Di-alpha-halogenation of ketones in acidic medium
Acid Catalysed Halogenation of ketone
In the halogenation of ketones, both in acidic and basic medium, enols are reactive intermediates. After addition of first halogen to double bond, deprotanation occurs immediately (Scheme 1). The second halogen does not add$\ce{^1}$.
Scheme 1
The introduction of second Bromine is slower then the first ,since cabocation is destabilized by the electron-attracting polar effect of two Bromines (**Scheme 2).
Scheme 2
Consequently, reprotonation of the monobrominated ketone does not occur.
Base Catalysed Halogenation of ketone
The mechanism of this reaction involves the formation of an enolate ion as a reactive intermediate. After mono bromination, the enolate ion of the alpha-bromo ketone is even more stable the enolate ion of the starting ketone.(Scheme 3). Consequently, a second bromination occurs$\ce{^2}$.
Scheme 3
As discussed above, in acid catalysed halogenation the intermediate is unstable ,hence reprotonation to give dihalo products is less (If one equivalent of Bromine is taken).
Note$\ce{^3}$ :
Comparatively, in base calalysis the intermediate (Bromo enolate anion) is quite stable leading to di and tri bromination (provided acidic H is present).
Reference
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 5th Edition by Prof. Marc Loudon
In the halogenation of ketones, both in acidic and basic medium, enols are reactive intermediates. After addition of first halogen to double bond, deprotanation occurs immediately (Scheme 1). The second halogen does not add$\ce{^1}$.
Scheme 1
The introduction of second Bromine is slower then the first ,since cabocation is destabilized by the electron-attracting polar effect of two Bromines (**Scheme 2).
Scheme 2
Consequently, reprotonation of the monobrominated ketone does not occur.
Base Catalysed Halogenation of ketone
The mechanism of this reaction involves the formation of an enolate ion as a reactive intermediate. After mono bromination, the enolate ion of the alpha-bromo ketone is even more stable the enolate ion of the starting ketone.(Scheme 3). Consequently, a second bromination occurs$\ce{^2}$.
Scheme 3
As discussed above, in acid catalysed halogenation the intermediate is unstable ,hence reprotonation to give dihalo products is less (If one equivalent of Bromine is taken).
Note$\ce{^3}$ :
Comparatively, in base calalysis the intermediate (Bromo enolate anion) is quite stable leading to di and tri bromination (provided acidic H is present).
Reference
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 5th Edition by Prof. Marc Loudon
@orthocresol the question was aswered without assuming ton of Bromine .I have assumed conditions as given in the question and ventured an answer.However,excess usage pf Bromine will react .More
Yes, but you claim that "reprotonation of the monobrominated ketone does not occur". That is not correct. It does not logically follow from "introduction of second bromine is slower than the first".More
Of course the product can be protonated. It can also form an enol. It is just that the enol of the product is less prevalent than the enol of the starting material, and so More
Acid Catalysed Halogenation of ketone
In the halogenation of ketones, both in acidic and basic medium, enols are reactive intermediates. After addition of first halogen to double bond, deprotanation occurs immediately (Scheme 1). The second halogen does not add$\ce{^1}$.
Scheme 1
The introduction of second Bromine is slower then the first ,since cabocation is destabilized by the electron-attracting polar effect of two Bromines (**Scheme 2).
Scheme 2
Consequently, reprotonation of the monobrominated ketone does not occur.
Base Catalysed Halogenation of ketone
The mechanism of this reaction involves the formation of an enolate ion as a reactive intermediate. After mono bromination, the enolate ion of the alpha-bromo ketone is even more stable the enolate ion of the starting ketone.(Scheme 3). Consequently, a second bromination occurs$\ce{^2}$.
Scheme 3
As discussed above, in acid catalysed halogenation the intermediate is unstable ,hence reprotonation to give dihalo products is less (If one equivalent of Bromine is taken).
Note$\ce{^3}$ :
Comparatively, in base calalysis the intermediate (Bromo enolate anion) is quite stable leading to di and tri bromination (provided acidic H is present).
Reference
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 2nd Edition ,Jonathan Clayden , Nick Greeves ,Stuart Warren.
Acid Catalysed Halogenation of ketone
In the halogenation of ketones, both in acidic and basic medium, enols are reactive intermediates. After addition of first halogen to double bond, deprotanation occurs immediately (Scheme 1). The second halogen does not add$\ce{^1}$.
Scheme 1
The introduction of second Bromine is slower then the first ,since cabocation is destabilized by the electron-attracting polar effect of two Bromines (**Scheme 2).
Scheme 2
Consequently, reprotonation of the monobrominated ketone does not occur.
Base Catalysed Halogenation of ketone
The mechanism of this reaction involves the formation of an enolate ion as a reactive intermediate. After mono bromination, the enolate ion of the alpha-bromo ketone is even more stable the enolate ion of the starting ketone.(Scheme 3). Consequently, a second bromination occurs$\ce{^2}$.
Scheme 3
As discussed above, in acid catalysed halogenation the intermediate is unstable ,hence reprotonation to give dihalo products is less (If one equivalent of Bromine is taken).
Note$\ce{^3}$ :
Comparatively, in base calalysis the intermediate (Bromo enolate anion) is quite stable leading to di and tri bromination (provided acidic H is present).
Reference
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 5th Edition by Prof. Marc Loudon
Organic Chemistry, 2nd Edition ,Jonathan Clayden , Nick Greeves ,Stuart Warren.
More
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