Hydrogenation of the first double bond in a conjugated diene evolves less heat than hydrogenation of an isolated double bond. Similarly, the enthalpy change of the catalytic hydrogenation of benzene is significantly lower than would be expected with a hypothetical cylohexatriene that is not aromatic. What leads to the lower than expected internal energy of resonant and aromatic systems is resonance stabilization. Aromatic systems are especially stable. The 2p orbitals of benzene carbon atoms are in alignment for π overlap, which generates the π system of delocalized electron density above and below the plane of the ring. The MCAT is fond of testing whether students are aware of the connection between greater stability and reduced heat of hydrogenation.

The WikiPremed MCAT Course is a comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum. Please read our policies on Privacy and Shipping & Returns.  Contact Us. MCAT is a registered trademark of the Association of American Medical Colleges, which does not endorse the WikiPremed Course. WikiPremed offers the customers of our publications or our teaching services no guarantees regarding eventual performance on the MCAT.

Creative Commons License
WikiPremed is a trademark of Wisebridge Learning Systems LLC. The work of WikiPremed is published under a Creative Commons Attribution NonCommercial ShareAlike License. There are elements of work here, such as a subset of the images in the archive from WikiPedia, that originated as GNU General Public License works, so take care to follow the unique stipulations of that license in printed reproductions.