While I was meant to be revising I found out an interesting bit of biochemistry. One strange property of glucose is that it can reduce Tollen’s solution, a test for an aldehyde group, which is a carbon atom double bonded to an oxygen atom and single bonded to a hydrogen atom carbon at the end of a carbon chain. If you look at a ring structure of alpha and beta glucose you will notice that no such aldehyde group exists:
Where an aldehyde group does exist however is in the straight chain version:
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| the straight chain version- aldehyde at the top |
In a solution the two ring isomers are in equilibrium via the straight chain isomer in what is know as ring closure: an internal or intramolecular reaction. Unlike most chemical reactions when one or more reactants make one or more products this is one molecule essentially reacting with itself.
The straight chain isomer isn’t actually that straight at all we just draw it that way to look good on a page. In reality as the molecule flexes the aldehyde group on carbon-1 comes close to the hydroxyl group (OH-) on carbon-5. The hydroxyl group can then attack the electron dense aldehyde double bond. The pi-bond of the aldehyde group breaks. The bond is the hydroxyl group then breaks and the hydrogen ends up on the oxygen of carbon-1. Carbon-1 then bonds to the oxygen on carbon-5 to make a ring.
However there are two ways the hydroxyl group can attack the aldehyde group. If it comes over the carbon atom, alpha-glucose will be formed the OH- below the molecule. If it comes under beta-glucose with the OH- above the molecule will form.
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| Once again things are a bit more interesting and complicated than exam syllabi make things out to be. |
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