Keto esters are bifunctional reactive molecules and are important synthons for the synthesis of alip — Organic Chemistry Chemistry Question
Carbon acids
Keto esters are bifunctional reactive molecules and are important synthons for the synthesis of aliphatic and heterocyclic compounds.
Two isomeric keto esters X and Y have the same molecular formula C5H8O3. Deduce their possible structures
Model Answer
The molecular formula of the keto ester is C5H8O3. Since X and Y are keto esters, they must have the following units- [VISUAL] This accounts for C4O3 .The remaining part comprises of CH8. Thus, only two types of ester groups are possible, methyl or ethyl.
For a methyl ester: CH3 will be a part of the ester moiety. This leaves CH5 to be attached.
For an ethyl ester: CH2CH3 will be a part of the ester group. Therefore H3 unit needs to be accounted for.
Therefore, possible structures of the keto esters are: [VISUAL]
Structure I is Y.
Structure II is X.
Hence the two keto esters are:
CH3–CH2–CO–CO–OCH3 CH3–CO–CH2–CO–OCH3
Compound Y Compound X
(Structure I) (Structure II)
-keto ester -keto ester
Each ester is first reacted with benzyl bromide in the presence of CH3ONa, and the resulting products are treated with 1 or 2 equivalent of a strong base (such as lithium diisopropyl amide, LDA) followed by 1 equivalent of CH3I. Then the products are at the end of the second step hydrolysed by aqueous solution of HCl. Write down the reaction sequences involved.
Model Answer
Reaction sequence for keto esters: [VISUAL]
Structure I gives a keto acid with molecular formula C12H14O3 which matches with the formula of the keto acid obtained from Y. Structure I is Y.
Structure II gives a neutral compound with molecular formula C11H14O that matches with the molecular formula of the neutral acid stated for X. Structure II is X.
Structure III gives a keto acid with molecular formula C11H12O3 that also does not match with any given molecular formula.
Explain why the final product of the reaction of keto ester X is a neutral compound (molecular formula C11H14O) whereas keto ester Y gives a keto acid with a molecular formula of C12H14O3.
Model Answer
The -keto ester gives on hydrolysis a -keto acid. This acid readily undergoes decarboxylation involving a 6-membered transition state, giving a neutral product (Ketone). [VISUAL]
Keto ester X gives different products depending upon the amount of LDA used. Explain what happens when
i. 1 equivalent of LDA is used.
ii. 2 equivalents of LDA are used.
Model Answer
i. When 1 equivalent of LDA is used compound X produces a carbanion (monoanion) as shown below:
Compound X → [VISUAL: Carbanion (monoanion)]
ii. Use of 2 equivalents of LDA leads to the formation of a dianion:
Compound X → [VISUAL: Dianion]