1 Hydrogen Bonding is a special type of dipole-dipole interaction involving compounds with O-H or N-H bonds. Hydrogen bonding (or H-bonding) is NOT a covalent.

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Presentation transcript:

1 Hydrogen Bonding is a special type of dipole-dipole interaction involving compounds with O-H or N-H bonds. Hydrogen bonding (or H-bonding) is NOT a covalent bond. It is an intermolecular attraction, stronger than dipole-dipole or London forces.

2 H-bonds are much weaker than covalent bonds, but many times stronger than other intermolecular forces. Why? Look at electronegativity values H = 2.1 N = 3.0 O = 3.5 O-H & N-H bonds are quite polar.

3 E.N. differences O-H = = 1.4 N-H = 3.0 – 2.1= 0.9 Both bonds are significantly polarized, but the O-H bond even more so than N-H.

4 Water is key to life as we know it. H.. O.. H structural formula. O has 4 pairs of e - => tetrahedral e - pair geometry; bond angles about 109°; only two atoms bonded to O => Molecule is Bent, V-shaped, or angular..

5 O H H  + ++ -- unshared e - pairs direction of dipole in O-H bond

6 O H H.. Net dipole in water Molecule -- ++

7 Treating molecule as spherical: H O H.. high e - density side of molecule low e - density side of molecule

8 What is a hydrogen bond then? How does it form? A simple model is shown here: H O H.. H O H.. covalent bond H-bond

9 The H of the O-H bond in one molecule is being “tugged on” or “pulled” by the O of another molecule. The H is the  + end of one dipole and the O is the  - end of a second dipole in a different molecule.  Opposite ends of dipoles attract.

10 The strength of a hydrogen bond is far weaker than that of a covalent bond, but significant. Most molecules the mass of H 2 O are gases. Example, methane or CH 4 boils (liquid  gas) at about -164° C. H 2 O boils at +100° C

11 Why the large difference in boiling point for compounds of similar mass? The CH 4 molecules are non-polar and associate only by very weak London dispersion forces. Each molecule essentially acts as a single low-weight unit.

12 In the case of H 2 O, each molecule can H-bond to several other H 2 O molecules in turn, forming a very large “web” of H 2 O. This makes the water molecule seem heavier. Extra energy to break apart the web before individual molecules can pass into the gas state.

13 The case of ammonia, NH 3, helps make this explanation convincing. NH 3 boils at -33°C. The N-H bond is less polar than O-H, so the H-bond made by N-H groups is weaker than in water. N: H H H HN H H..

14 In any H-bond, the molecule which provides the O-H or N-H is called the H-bond donor. The molecule which provides the N: or :O: attracting the other H is termed the H-bond acceptor.

15 Thus, N: H H H HN H H.. H-bond donor H-bond acceptor H-bond

16 And for water, O H.. H O H.. H H-bond acceptor H-bond donor

17 Crude comparison: strengths of covalent bonds and intermolecular forces (0-100 scale) covalent bond ……………….100 H-bond………………………… 5 dipole-dipole…………………. 0.5 London dispersion force…