5. A compound 'A' with molecular formula C₄H₉Br is treated with aqueous KOH solution. The rate of this reaction depends upon the concentration of compound 'A' only. When another optically active isomer 'B' of this compound was treated with aqueous KOH solution, the rate of reaction was found to be dependent on concentration of compound and KOH both.
(a) Write down the structural formula of both A and B.
(b) Out of these two compounds, which one will be converted to the product with inverted configuration?
Answer (2)
This question pertains to organic chemistry, specifically to the reaction mechanisms called nucleophilic substitution reactions. We are dealing with two types of nucleophilic substitution reactions here: S N 1 and S N 2 .
(a) Structural formulas of A and B:
Compound 'A': The reaction rate of compound 'A' depends only upon the concentration of 'A'. This indicates that 'A' undergoes an S N 1 reaction, which involves the formation of a carbocation intermediate and is unimolecular. A common structure for such reactions is a tertiary alkyl halide. A possible structure for 'A' could be ( C H 3 ) 3 CB r , also known as tert-butyl bromide.
Compound 'B': The reaction rate of compound 'B' depends on both the compound and the concentration of KOH, which signifies an S N 2 reaction. In S N 2 , the reaction is bimolecular and involves a single step where the nucleophile attacks the carbon with the leaving group. For 'B' to be optically active, it must contain an asymmetric carbon. A common isomer of C 4 H 9 B r that fits this description is C H 3 C H B r C H 2 C H 3 , which is 2-bromobutane.
(b) Product with inverted configuration:
In an S N 2 reaction, the nucleophile attacks the carbon atom from the side opposite to the leaving group, which usually results in an inversion of configuration at that carbon center. Therefore, compound 'B', which undergoes an S N 2 process, will be converted to the product with inverted configuration.
Compound 'A' is tert-butyl bromide with structure (CH₃)₃CBr, while compound 'B' is 2-bromobutane with structure CH₃CHBrCH₂CH₃. Compound 'B' will undergo an S_N2 reaction and produce a product with inverted configuration. The S_N1 reaction of compound 'A' does not result in inversion.
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