Question

Hi, I had a quick question about hydrophobic and hydrophillic regions on a molecule.

I am confused on how to know if a molecule, which has both hydrophillic and hydrophobic regions, will be soluble in water or not? Can someone please explain in clear and simple words, thank you so much!

For example, in the molecule vitamin A, theres OH group which is hydrophillic but the rest of the molecule is hydrophobic, so how would I know if the whole molecule would be soluble in water or not?

Answer 1

The terms hydrophobic and polar refer to the overall distribution of charge in a molecule. If there are no local regions of high or low electron density in the molecule, it is called hydrophobic (Greek for "water-fearing"). This term arises because hydrophobic molecules do not dissolve in water. If a molecule has areas where there is a partial positive or negative charge, it is called polar, or hydrophilic (Greek for "water-loving"). Polar molecules dissolve easily in water. How can you tell if a molecule is polar or hydrophobic? There are two things to consider when trying to decide. 1. If a molecule has polar covalent bonds, then it may be polar. A covalent bond is polar if one of the atoms in it has a significantly higher affinity for electrons (electronegativity) than the other. Some examples of polar covalent bonds often found in biological molecules include C-O, C-N and O-H bonds. If all the bonds in a molecule are nonpolar, then the molecule itself is nonpolar. Some examples of nonpolar covalent bonds are C-C and C-H bonds. 2. Even if a molecule has polar covalent bonds, if these bonds are arranged symmetrically, the molecule overall will be hydrophobic. The best example is carbon tetrachloride (CCl4, see the structure). Even though each of the C-Cl bonds is quite polar, they are arranged symmetrically around the central carbon atom, so the molecule as a whole is nonpolar or hydrophobic.