Deduce whether a vitamin is water or fat soluble from its structure.

Water Soluble Vitamins

1. Vitamin C or ascorbic acid
2. the eight B-group vitamins
3. Folic acid

Water soluble vitamins are highly polar due to presence of many hydroxyl (-OH) functional groups and so soluble in water because the hydroxyl groups can form hydrogen bonds with water.
Their structures contain hydrogen attached directly to electronegative oxygen, nitrogen or fluorine atoms that can hydrogen-bond with water molecules.

In the structure of vitamin C alongside H is attached to O in four alcohol or hydroxyl (-OH) groups.

Below is a structure of vitamin B1 (Thiamin). We can see that the hydrogen is attached directly to electronegative nitrogen atom on the animo functional group (-NH2).
external image thiamin.jpg
Thiamin structure
Because water soluble vitamins can dissolve in water, they cannot be stored inside the body for long, so sufficient intake is needed in order to maintain a healthy body. Likewise if an excess of the vitamin is taken what can't be stored will pass out of the body in the urine.

Fat soluble vitamins

1. Vitamin A or retinol
2. Vitamin D or calciferol
3. Vitamin E
4. Vitamin K

Because their structure is comprised mainly of long non-polar hydrocarbon chains or rings without any polar functional groups like -OH or NH2, they will not hydrogen bond with water. Intead these non polar vitamins will from van der waal forces with non polar fats, making them fat soluble.


Here is a structure of vitamin E. We can see that the structure is very long, and the "CH3" at the end of the chain is a non-polar hydrocarbon chain.
external image alphatocopherol.jpg

Vitamins A, D, E, and K are fat-soluble vitamins and as such are more likely to be stored in the body in fatty tissue if they are consumed in excess. The water soluble vitamins if they are in excess tend to pass out of the body in the urine