The decreasing order of rate of reaction for the following compounds towards `S_(N)2Th` (bimolecular nucleophilic substitution with tetrahedral intermediate) reaction is
(i) `CH_(3)-CH_(2)-overset(O)overset(||)C-NH_(2)`
(ii) `CH_(3)-CH_(2)-overset(O)overset(||)C-Br`
(iii) `CH_(3)-CH_(2)-overset(O)overset(||)C-O-overset(O)overset(||)C-CH_(2)-CH_(3)`
(iv) `CH_(3)-CH_(2)-overset(O)overset(||)C-O-C_(2)H_(5)`

Options

A. I > II >III >IV
B. II > III > I >IV
C. III >II > IV > I
D. II> III >IV> I

Answer

To determine the decreasing order of the rate of reaction for the given compounds towards the SN2 reaction, we need to analyze how the structure of each compound affects the partial positive charge on the carbon atom that is being attacked by the nucleophile. The more positive the carbon is, the faster the reaction will occur.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- (i) `CH₃-CH₂-C(=O)-NH₂` (Compound 1)
- (ii) `CH₃-CH₂-C(=O)-Br` (Compound 2)
- (iii) `CH₃-CH₂-C(=O)-O-C(=O)-CH₂-CH₃` (Compound 3)
- (iv) `CH₃-CH₂-C(=O)-O-C₂H₅` (Compound 4)

2. **Analyze Compound 1**:
- In Compound 1, the carbonyl (C=O) has a partial positive charge due to the electronegativity of oxygen. The amino group (NH₂) donates electron density to the carbonyl carbon, reducing its positive charge. This makes the carbon less electrophilic and decreases the rate of reaction.

3. **Analyze Compound 2**:
- In Compound 2, the bromine atom (Br) is a good leaving group and is also an electron-withdrawing group due to its electronegativity. This increases the partial positive charge on the carbonyl carbon, making it more electrophilic and increasing the rate of reaction.

4. **Analyze Compound 3**:
- In Compound 3, there are two carbonyl groups (C=O) and an ether linkage. The electron delocalization between the carbonyls and the ether oxygen can stabilize the positive charge but does not significantly enhance it. Thus, the rate of reaction is moderate.

5. **Analyze Compound 4**:
- In Compound 4, the ethoxy group (O-C₂H₅) can also donate electron density to the carbonyl carbon, but not as effectively as the amino group in Compound 1. The partial positive charge is reduced, but the effect is less than in Compound 2.

6. **Rank the Compounds**:
- Based on the analysis:
- Compound 2 has the highest rate due to the strong electron-withdrawing effect of Br.
- Compound 3 has a moderate rate due to the delocalization of charge.
- Compound 4 has a lower rate than Compound 3 due to less effective donation of electron density.
- Compound 1 has the lowest rate due to the strong donation of electron density by NH₂.

7. **Final Order**:
- Therefore, the decreasing order of rate of reaction for the compounds towards SN2 is:
- (ii) > (iii) > (iv) > (i)
- This translates to: **2 > 3 > 4 > 1**.