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Answer: OH (hydroxide) is a polar molecule due to the great electronegativity difference between oxygen and hydrogen.  

This electronegativity difference between oxygen (3.44) and hydrogen (2.20) qualifies this bond as polar covalent and therefore leads to the dipole. Due to the great strength of these dipoles, hydrogen bonds would be formed between these molecules with a strength closer to that of HF than H2O due to the presence of the extra lone pair electrons on the oxygen. OH is typically a base due to its affinity for hydrogen atoms (i.e. more positive charge) to balance out the extreme nature of the oxygen's negative charge; it is also found as a constituent of strong bases such as NaOH. These salts often aid in the formation of basic solutions by disassociating with Na+ and OH- ions.  

OH is related to H2O and only requires the addition of a single hydrogen to convert from one form to another. You can learn more about the closely related lewis structure of H2O or the polarity of H2O at the hyperlinked pages, respectively. 

OH Ball and Stick Model
OH Ball and Stick Model. Created with Avagadro
How is -OH utilized in organic chemistry?

In the context of these applications -OH is considered to be a catalyst of reactions by reacting with "positively charged" structures such as electron-deficient core "central" atoms within a complex organic molecule. These so-called hydroxyl groups can either stably be attached to the molecule or simply remove a hydrogen/other positive element during a reaction. One of the most famous hydroxyl groups in all of Biology appears at the 2nd carbon of the ribose/deoxyribose sugar constituting RNA and DNA, respectively. The presence of an OH group in ribose but not in deoxyribose explains the wide range of structural and functional differences between RNA and DNA (such as the single-stranded vs. double stranded nature and the capability of RNA to become a ribozyme).

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