Necessary conditions for Fiels-Alder reactions are :

Options

A. `(I),(II),(III)`
B. `(I),(II),(IV)`
C. `(II),(III)`
D. `(II),(IV)`

Answer

### Step-by-Step Solution

1. **Understanding Diels-Alder Reaction**: The Diels-Alder reaction is a cycloaddition reaction between a conjugated diene and a dienophile (an alkene or alkyne) to form a six-membered ring. This reaction is a key method in organic synthesis.

2. **Non-linearity of Dienes**: The diene must be in a non-linear (or s-cis) conformation. This is because the reactive π bonds of the diene need to be oriented properly to overlap with the π bonds of the dienophile during the reaction. If the diene is in a linear (or s-trans) conformation, it will not react effectively.

**Hint**: Remember that the s-cis conformation allows for the necessary orbital overlap during the reaction.

3. **S-cis Conformation of Dienes**: The diene must adopt the s-cis conformation. This conformation allows the two double bonds of the diene to be in the correct position to react with the dienophile.

**Hint**: Visualize the diene in both s-cis and s-trans conformations to understand which one is reactive.

4. **Electron Donating Groups on Dienes**: Dienes should ideally have electron-donating groups. These groups increase the electron density on the diene, making it more nucleophilic and thus more reactive towards the electron-deficient dienophile.

**Hint**: Identify common electron-donating groups, such as -OH, -OCH₃, or -NH₂.

5. **Electron Withdrawing Groups on Dienophiles**: Dienophiles should contain electron-withdrawing groups. These groups decrease the electron density on the dienophile, making it more electrophilic and more reactive towards the electron-rich diene.

**Hint**: Look for groups like -NO₂, -CN, or -COOH that can withdraw electron density.

6. **Conclusion**: The necessary conditions for a Diels-Alder reaction are:
- The diene must be in a non-linear (s-cis) conformation.
- The diene should have electron-donating groups.
- The dienophile should have electron-withdrawing groups.

### Summary of Necessary Conditions:
- Non-linearity of dienes (s-cis conformation)
- Dienes with electron-donating groups
- Dienophiles with electron-withdrawing groups

### Final Answer:
The necessary conditions for Diels-Alder reactions are:
1. Non-linearity of dienes
2. S-cis conformation of dienes
3. Dienes with electron-donating groups
4. Dienophiles with electron-withdrawing groups