Product obtained by 1, 3-alkyl substitution (sigmatropic shift) in 1-R-cyclohexa-1,3-diene will be :

Question

Accepted Answer

The question involves a 1,3-alkyl substitution (sigmatropic shift) in 1-R-cyclohexa-1,3-diene. Let's analyze why Option 3, "1-R-cyclohexa-1,3-diene," is the correct answer.

### Explanation for Option 3:

1. **Sigmatropic Shift Basics**: A sigmatropic shift is a type of pericyclic reaction where a sigma bond adjacent to one or more pi systems migrates to a new position in the molecule. In the case of 1,3-alkyl substitution, the shift involves the movement of an alkyl group within the molecule.

2. **1,3-Shift Specifics**: In a 1,3-shift, the alkyl group moves from one position to another within the same pi system. For 1-R-cyclohexa-1,3-diene, this means the R group moves within the diene system but does not alter the overall structure of the diene significantly.

3. **Retention of Structure**: The sigmatropic shift in this context does not change the position of the double bonds significantly, and the R group remains in a position that maintains the original structure of the molecule. Therefore, the product is still 1-R-cyclohexa-1,3-diene.

### Why Other Options are Incorrect:

1. **Option 1: 1-R-cyclohexa-1,4-diene**:
   - This option suggests a change in the position of the double bonds, which is not typical for a simple 1,3-alkyl sigmatropic shift. The shift does not alter the conjugation pattern of the diene to form a 1,4-diene.

2. **Option 2: 3-R-cyclohexa-1,4-diene**:
   - Similar to Option 1, this option implies a change in the double bond positions and the location of the R group that is not consistent with a 1,3-sigmatropic shift. The shift does not lead to a 1,4-diene structure.

3. **Option 4: 5-R-cyclohexa-1,3-diene**:
   - This option suggests a migration of the R group to a position that is not typical for a 1,3-sigmatropic shift. The R group remains in a position that maintains the original structure, not moving to the 5-position.

In conclusion, Option 3, "1-R-cyclohexa-1,3-diene," is the correct answer because the 1,3-alkyl substitution (sigmatropic shift) does not significantly alter the structure of the original diene, and the R group remains in a position consistent with the original molecule.