So, I’m sure many of us have seen the advertisement packaging for supposedly healthy drinks. One of most popularised is vitamin C. I don’t know about you but when I was growing up, I associated vitamin C with oranges and how they prevent scurvy (as told by parents to ensure I was receiving my 5 a day). So what makes this chemical so important?
Well, it turns out, Vitamin C is a vitamin (duh) which is able to dissolve in water and has the chemical formula of C6 H8 O6. The molecular structure of vitamin C is seen in the diagram below:
On the diagram it can be seen that the molecule contains several –OH parts in red, these are known as hydroxyl functional groups. It is these functional groups which take part in chemical reactions; this is because of the intra-molecular forces present within the group. Oxygen (O) and hydrogen (H) atoms have a relatively large difference between their electronegativity values compared to pure covalent bonds which are non-polar. Based on the Pauling scale on the chemistry data booklet oxygen has a value of 3.5 whereas hydrogen only has a value of 2.2. Electronegativity is how strongly an atom attracts electrons, the higher the value, the stronger the force is exerted on the electrons. In the oxygen to hydrogen bond, the oxygen has a stronger pull on the shared electron with hydrogen and so obtains a permanent slightly negative charge as electrons have a negative charge. The molecule is therefore, overall polar, having an unequal share of electrons. The hydrogen bonds in the hydroxyl group allow vitamin C so be very water-soluble.
Water itself is a polar molecule containing unequal share of electrons between the oxygen and hydrogen bonds similar to hydroxyl groups. Like dissolves like and so the polar parts of vitamin C molecules are able to join with the polar parts of water molecules. In addition, vitamin C is a small molecule so it is even easier to it to dissolve in water as the water molecule are not required to break as many Van der Waal forces or intermolecular forces to surround vitamin C as compared to a larger molecule. This is very important in it functions in the human body. As it is very water soluble the body is unable to store excess quantities of vitamin C, apart from what is used the rest is gotten rid of by the body in waste fluids. To ensure there are enough levels of vitamin C in the body, it is required to be continually replenished through ingestion of vitamin C containing food. Vitamin C is essential to chemical reaction in the body such as converting the substance procollagen into collagen, the long fibrous structural protein. Collagen is in turn used to make vital parts of the body such as growing skin, blood vessels and ligaments.
Vitamin C is also an antioxidant; it slows and even stops the oxidation of other molecules. This is of particular use in halting the unwanted oxidation of food items. When food oxidises, there is an increase in the oxygen to hydrogen ratio. The fats and oils, especially those of unsaturated fats and oils which contain two or more double bonds easily react with other molecules such as the oxygen in the air. Free radicals can be formed from oxidation reactions; they are molecules with an unpaired electron in their outer most electron shell and highly unstable as they wish to achieve a stable energy level by removing an electron from another molecule. This leads to its breakdown which makes it unsafe to consume. Vitamin C stops this process by being oxidised, sacrificing itself to prevent the oxidation of another.
Another use of antioxidants is in sun protection, terminating chain reactions forming free radicals from oxidation reactions. During sun exposure the energy from the sun’s high energy ultraviolet light is able
Vitamin C is commonly referred to as ascorbic acid; this is due to the fact that the molecule contains a double bonded carbonyl functional group, seen above in blue. Although it does not possess carboxyl functional groups present carboxylic acids, the carbonyl group gives vitamin C similar chemical properties as acids. Vitamin C dissolves in water to form a slightly acidic solution with a greater ratio of H+ ions to OH- ions which makes it acidic.
It is possible to determine the concentration of vitamin C in food and drinks by a redox titration with iodine:
C6 H8 O6 (aq) + I2 → C6 H6 O6 + 2H+ + 2e-
I2 (aq) + 2e- → 2I- (aq)
A redox reaction is one which a reduction and an oxidation occur at the same time. The number of electrons gained in reduction equals the number of electrons lost in oxidation. Iodine is added to the vitamin C solution and the vitamin C oxidises in the first equation, whilst at the same time iodine reduces to iodide. The reaction stops when the vitamin C solution turns a pale blue/black colour.
So we can now rest assure when we drink our orange juices.
Author – Jiangmin Hou
Jiangmin is a 5th year high school student currently studying five STEM subjects at Scottish Higher level-Mathematics, Physics, Biology, Computer Science and Chemistry. She is interested in pursuing a degree in Medicine after completion of Secondary Education.