Carbanions Rearrangements

Question 1

Also condensation rxn

Answer 1

The condensation rxn in which two aldehydes or ketones, both having a alpha hydrogen condense to form aldol product
-rxn is of general nature and occurs in both aldehydes and ketones having alpha hydrogen
- the product name is aldol becz it contains both aldehyde and alcohol functional group. The term aldol can be used for the specific name of product as well as for the general (B- hydroxy aldehydes or ketones)

Question 2

Aldol condensation mechanism

Answer 2

1- formation of enolate ion
In this step, one of the.aldehyde molecule is altacked by the nucleophile(base) OH to form enolate ion . This enolate ion then forms its equtvalent stable canonical form. As both are equally stable so both canonical forms can be used in reaction
2- attack of enolate ion
Enolate ion attack on the carbonyl carbon of the other aldehyde to form anion
3- formation of aldol
In the presence of water, the anion extracts hydrogen atoms to form aldol
4- formation of condensation product
At concentrate NaOH is used, the molecule of water is extracted from aldol to form the condensation products such as alpha beta unsaturated aldehyde

Question 3

Mix or cross aldol condensation

Answer 3

When an aldehyde having a alpha hydrogen is condensed with the aldehyde having no alpha hydrogen is called mix or cross aldol condensation

Question 4

Aldol condensation application

Answer 4

• very important for the organic synthesis of many compounds becz it form large molecules by the condensation of two smaller molecules with elimination of water and forming C-C bonds
• aldol product contains two functional groups -CHO and -OH which are important for carrying out a number of subsequent chemical rxns

Question 5

Wittig rearrangement

Answer 5

Useful method for preparation of Alkenes in which the aldehyde or ketones are made to react with a phosphorus ylide by forming an intermediate known as betaine to form/produce alkenes

Question 6

Wittig steps

Answer 6

1- preparation of ylide
Phosphorous ylide can easily be prepared by reacting triphenyl phosphorium salt which on reaction with strong base such as sodium hydroxide, butyl lithium or phenyl lithium, in the presence of dry ether or THF form a phosphorane a ylide
2- attack of ylide on ketone
The ylide attack on the carbonyl carbon of the ketones or aldehyde readily at-80c to give a diplomat intermediate “betaine” which forms oxaphosphetane, a neutral species
3- formation of Alkene Oxaphosphophetane is stable in solution at -80C and decomposes at 0c to form alkene and phenyl triphosphoriumoxide as a by product

Question 7

Application of Wittig rxns

Answer 7

-very usefull method for the synthesis of alkene because it is applicable to wide variety of compounds
- any desired alkene can be prepared by selecting appropriate alkyl halide and aldehyde or ketones
- there is no ambiguity about the position of double bond in the product

Question 8

Favorskii rxn

Answer 8

Reaction involves the conversion or rearrangement of a cyclic and cyclic alpha haloketones to carboxylic acids and their derivatives in the presence of a base
- named after Alexei yevgra fouich favorskii
-acidic alpha hydrogens are nessecary in the halo ketones for this rxn

Question 9

Favorskii mechanism for acyclic

Answer 9

1- formation of enolate ion
The oh ion from the base attacks on alpha haloketones on the opposite side of chlorine atom extracts acidic alpha hydrogen to form enolate ion
2- formation of crycloproanone
The chloride atom removes from enolate ion and it undergoes cyclization to form cyclopropanone
3- nucleophillic attack of OH
The Oah attacks on the carbonyl carbon of the cyclopropabe to form the most stable anion
(If the alkoxides are utilized as bases then esters are produced, if amines are used as bases then amides are produced)

Question 10

Favorskii in cyclic mechanism

Answer 10

1- formations of cyclic enolate ion
The OH Timon attacks on the opposite side of chlorine atom to form the enolate ion
2- bucleophilic attacks of OH
The OH attacks on the carbonyl carbon to form the more stable anion which then take hydrogen atom form water to form ring contacted carboxylic acids

Question 11

Favorskii applications

Answer 11

• provides effective methods for the convention of alpha halo ketone to carboxylic acids
  -bicycling esters can be synthesized via this process
  This rxn is involved in synthesis of Cubane
• especially beneficial in the synthesis of branched chain aliphatic carboxylic acids and their derivatives as well as in the ring contraction of cyclic ketones