80. The number of equatorial hydrogens in the following structure is
Number of Equatorial Hydrogens in trans-Decalin
Correct Answer
✅ Correct Answer: 8
Identifying the Structure
The given compound is trans-decalin, a bicyclic hydrocarbon consisting of two fused cyclohexane rings. The two hydrogen atoms shown at the ring junction are on opposite faces of the fused rings, confirming the trans configuration.
Trans-decalin is conformationally rigid because ring flipping is not possible without breaking the fused ring junction. Therefore, the axial and equatorial positions remain fixed.
Counting the Equatorial Hydrogens
Decalin has the molecular formula C10H18, giving a total of 18 hydrogen atoms.
The two bridgehead carbon atoms each bear one hydrogen atom. In trans-decalin, these two bridgehead hydrogens are axial and therefore are not equatorial.
The remaining eight carbon atoms are methylene (CH2) carbons. Each CH2 carbon contains:
- One axial hydrogen
- One equatorial hydrogen
Since there are eight CH2 carbons, the total number of equatorial hydrogens is:
8 × 1 = 8
Why the Bridgehead Hydrogens Are Not Counted
The hydrogen atoms located at the ring junction occupy axial positions in the trans-fused system. They do not have corresponding equatorial orientations because the fusion locks the chair conformations of both cyclohexane rings. Consequently, only the hydrogens on the CH2 carbons contribute to the equatorial hydrogen count.
Concept Behind the Question
This question tests the understanding of chair conformations and fused bicyclic systems. Students should recognize trans-decalin, identify the bridgehead hydrogens, and remember that each non-bridgehead carbon contributes one equatorial hydrogen. Such questions are commonly asked because they combine stereochemistry, conformational analysis, and structural visualization.
Final Answer
The given structure is trans-decalin. It contains eight methylene carbons, each contributing one equatorial hydrogen, while the two bridgehead hydrogens are axial.
✅ Correct Answer: 8
