Hydrogen Bonding (<------ click here to start)

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A Hydrogen Bond is an unusually strong dipole-dipole interaction

It will only occur between a hydrogen atom attached (covalently bonded) to N, O or F and a lone pair of electrons on a neighboring N, O, or F.

Since the hydrogen-bond is weaker than a covalent bond, the atoms must be fairly close together

****** You cannot form a hydrogen bond with a hydrogen atom covalently bonded to carbon******

Given a set of molecules, you must be able to predict when and where a hydrogen bond will form

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Why only with N, O, and F?

• X-H bond is highly polar
  
  
• Small size of N, O, and F allow the small proton to approach closely

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Examples of compounds with extensive hydrogen bonding interactions

• Water, H₂O
  (accounts for many of the special properties of water, bp, fp, density etc.)
  
  
• Hydrogen fluoride, HF
  
  
• Ammonia, NH₃

  Note that many compounds can exhibit hydrogen bonding

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General Rule of Thumb ....

..... the greater the number of hydrogen bonds that can be formed the stronger the intermolecular forces

each hydrogen bond is worth 4 - 40 kcal/mol, (5-15 kcal/mol typical)

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Hydrogen bonding is critical in biochemistry

• protein folding (alpha helix, beta sheets)

• molecular recognition (enzymatic catalysis, immune response etc.)

• Double Helix formation between two strands of DNA (Watson-Crick Base pairing)
  
  
  
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An X-ray structure of short strand of duplex DNA (5'-D(CpGpCpApApApTpTpGpGpCpG)-3') is shown below. See if you can identify the hydrogen bonding interactions between the G-C and A-T pairs. (right click to change display options, you can zoom in using shift-left click)