Which of the following gives the correct order of decreasing electron withdrawing effect?

Options

A. `-CN gt C_(6)H_(5)- gt NO_(2) gt F-`
B. `-F gt -CN gt C_(6)H_(5)- gt -NO_(2)`
C. `-NO_(2) gt - CNgt -F gt C_(6)H_(5)-`
D. `C_(6)H_(5)- gt -F gt -CN gt -NO_(2)`

Explanation

To determine the correct order of decreasing electron withdrawing effect among the given species (NO2, CN, C6H5, and fluorine), we need to analyze the electron-withdrawing capabilities of each group based on their electronegativity and the type of effect they exhibit.

### Step-by-Step Solution:

1. **Understanding Electron Withdrawing Effect**:
- Electron withdrawing groups (EWGs) are groups that attract electrons towards themselves, thereby decreasing the electron density on the atom they are attached to. This is often referred to as a -I (negative inductive) effect.

2. **Identifying the Groups**:
- The groups provided are:
- NO2 (Nitro group)
- CN (Cyano group)
- C6H5 (Phenyl group)
- F (Fluorine)

3. **Evaluating the Electron Withdrawing Strength**:
- **NO2**: The nitro group is a strong electron withdrawing group due to the presence of highly electronegative oxygen atoms and resonance effects. It has a significant -I effect.
- **CN**: The cyano group is also a strong electron withdrawing group, but not as strong as NO2. It has a -I effect due to the electronegativity of nitrogen.
- **F (Fluorine)**: Fluorine is highly electronegative and exhibits a -I effect, but it is less effective than NO2 and CN due to its ability to donate electrons through resonance (it can also act as a +I group in some contexts).
- **C6H5 (Phenyl group)**: The phenyl group is generally considered a weak electron withdrawing group. It can exhibit resonance but does not withdraw electrons as effectively as the other groups listed.

4. **Ranking the Groups**:
- Based on the above evaluations, we can rank the groups in order of their electron withdrawing effects:
- 1st: NO2 (strongest)
- 2nd: CN
- 3rd: F
- 4th: C6H5 (weakest)

5. **Final Order**:
- The correct order of decreasing electron withdrawing effect is:
- NO2 > CN > F > C6H5

### Conclusion:
The correct answer is that the order of decreasing electron withdrawing effect is NO2 > CN > F > C6H5.