Organic Pedagogical Electronic Network Properties of Hydrogen Bonding Created by Max Taggart Edited by Margaret Hilton Honors Organic Chemistry Chem 2321 (Sigman), 2013

Hydrogen Bonding Wiki Page: Overview: Currently IUPAC defines a hydrogen bond as an attractive interaction between a hydrogen atom and an atom that is more electronegative than hydrogen. The nature of hydrogen bonding began to be studied in the early 1900’s and is key to understanding many biological processes, including the structure and function of proteins and DNA. It also plays a vital role in many reactions catalyzed by organic catalysts. Examples in Nature

Bond Strengths: Hydrogen Vs Covalent Hydrogen BondsCovalent Bonds Bond Type Bond Strength (kcal/mol) [F-H—F] - [H 2 O-H—OH 3 ] + [OH-H—OH] - HOH – Cl - COOH – OCOH HOH – OH Bond TypeBond Strength (kcal/mol) H 2 C=CH 2 Me—H Ph—I Et—I HO—OH I—I Although hydrogen bonds are weak compared to covalent bonds, the strongest hydrogen bond strengths begin to resemble weak covalent bond strengths. Steiner, T. Angew. Chem. Int. Ed. 2002, 41, 48.

Catalysis of Diels-Alder Reaction Kelly, T. R. et al. Tet. Lett. 1990, 31, In Diels-Alder reactions involving conjugated dienophiles, hydrogen bonding can help activate the dienophile. Hydrogen bonding increases the partial positive charge on the dienophile promoting ring formation, as shown below.

Problems Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, Akiyama, T. et al. Angew. Chem. Int. 2004, 43, Propose a hydrogen bond-catalyzed mechanism for the following reaction: 2. Rank the following heteroatoms in order of decreasing ability to act as an electron donor in a hydrogen bond. 3. Propose a mechanism for the following reaction. Consider hydrogen bonding interactions between the reagents and the catalyst to explain the stereoselectivity of the product.