C10H12N2O

Hydroxyl Group: A hydroxyl group increases a molecule’s solubility in water (Chemistry II). Because when the OH group is bonded together from O and H, the increased electronegativity in the oxygen activates the partial charges (i.e. partial positive on H and partial negative on O). Water molecules also have this property of partial positive and negative charges, and will be attracted to the opposite charges in the hydroxyl group. Therefore the hydroxyl group is polar and thusly allows the molecule it is part of to be polar/hydrophilic, and therefore water soluble as well (Masterjohn, 2005).

 

Amine Classification and Amino Groups: Small amines are very soluble in water, and serotonin is monoamine (therefore, a small amine). Serotonin is a monoamine (has an amino group in the middle of its carbon chain) and has an amino side group. This amino side group allows it to be slightly more soluble than usually. A special variation of the serotonin molecule is that it is not a linear amine, moreover, it is an aromatic amine (the amine is part of a cycloalkane ring). Aromatic amines are only slightly soluble in water (Amides and Amines, 2007). Serotonin being a monoamine could also affect its solubility. Amines in general are more non-polar than polar, leaving them to be significantly insoluble in water (Ophardt, 2005). That being said, the size of the serotonin molecule changes this. Because the amine in serotonin is only linked to a 5-member carbon ring structure, it is not allowed to make its hydrogen bonds with nitrogen. Without the prominent functional groups – hydroxyl and amino – this would make serotonin a water-insoluble or non-polar substance. However, due to the convenient presence of hydroxyl and amino groups ‘overpowering’ the amine property, serotonin is a polar substance and is water-soluble (Ophardt, 2003). Alcohol Side Group: Some sources describe serotonin as having a phenol side group (benzene ring with a hydroxyl group on one of the carbons of the ring), which is an alcohol(Functional Groups, 2010). Even when analyzed from this perspective, the serotonin molecule will still be polar, because of the hydroxyl functional group it possesses (Hunt).

 

Hydrocarbon Rings: One final note that should be made is the length of the hydrocarbon chains that form serotonin. Serotonin has a 5-member ring structure fused to a 6-member ring. In total serotonin has enough carbon atoms (10) that some of the hydrocarbon properties begin to apply to the overall structure. A long hydrocarbon chain, such as the one serotonin has is usually non-polar and insoluble in water. However, due to the functional groups (hydroxyl and amino), serotonin is polar.