Rank the following alkenes (A, B, C, D) from least stable to most stable, explaining your reasoning based on alkene stability principles.

To rank the alkenes from least stable to most stable, we need to count the number of alkyl substituents directly attached to the carbons of the double bond. Alkene stability increases with increasing substitution due to hyperconjugation.

• Alkene A: The double bond is between a carbon in the ring and an external carbon. The carbon in the ring is attached to one alkyl group (the rest of the ring). The external carbon is attached to two hydrogens. Therefore, Alkene A has 1 alkyl substituent (monosubstituted).
• Alkene B: The double bond is within the ring. One carbon of the double bond is attached to a methyl group and one carbon of the ring. The other carbon of the double bond is attached to one carbon of the ring and one hydrogen. Therefore, Alkene B has 3 alkyl substituents (trisubstituted).
• Alkene C: The double bond is within the ring. One carbon of the double bond is attached to an ethyl group and one carbon of the ring. The other carbon of the double bond is attached to a methyl group and one carbon of the ring. Therefore, Alkene C has 4 alkyl substituents (tetrasubstituted).
• Alkene D: The double bond is between a carbon in the ring and an external carbon. The carbon in the ring is attached to two alkyl groups (the two adjacent carbons in the ring). The external carbon is attached to two hydrogens. Therefore, Alkene D has 2 alkyl substituents (disubstituted).

Ranking based on the number of alkyl substituents (least stable to most stable):

1. Monosubstituted (1 alkyl group)
2. Disubstituted (2 alkyl groups)
3. Trisubstituted (3 alkyl groups)
4. Tetrasubstituted (4 alkyl groups)

Therefore, the order from least stable to most stable is:

$$\text{A} < \text{D} < \text{B} < \text{C}$$

Justification:
The stability of an alkene is primarily determined by the number of alkyl groups attached to the carbons of the double bond. More substituted alkenes are more stable. This phenomenon is explained by hyperconjugation, where the electron density from the C-H sigma bonds of the adjacent alkyl groups can delocalize into the empty $\pi^*$ antibonding orbital of the double bond. This delocalization stabilizes the alkene. With more alkyl groups, there are more C-H bonds available for hyperconjugation, leading to greater stabilization.

The final ranking is:
$\boxed{\text{A} < \text{D} < \text{B} < \text{C}}$