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QUESTION

# How do you identify the asymmetrical carbons in glucose and fructose? What do you need to look for in order to identify an asymmetric carbon?

A carbon bound to four different groups.

In sugars, most carbons that bear a hydroxyl will be chiral (asymmetric) carbons, because they are each bound to a hydroxyl, a hydrogen, and the rest of the carbon chain in two directions.

Four different groups means a chiral carbon.

Glucose

One form of glucose is the aldehyde form, shown below.

There is no internal mirror plane, so every carbon is different.

We see that carbon atoms 2 through 5 are chiral.

If two carbons are tied in deciding chirality, we look at the atoms next further out from them until we find a difference.

Each carbon is attached to four different groups.

In the list below, the atom that distinguishes the groups is shown in red.

• bb"C2": "H, OH, CH"color(red)("O"), and "CH("color(red)("O")"H)"
• bb"C3": "H, OH", "CH(OH)CH"color(red)("O"), and "CH(OH)CH(OH)"color(red)("C")"
• bb"C4": "H, OH, CH(OH)CH("color(red)("O")"H)", and "CH(OH)CH"_2color(red)("O")"H"
• bb"C5": "H, OH, CH(OH)"color(red)("C") and "CH"_2color(red)("O")"H"

Fructose

One form of fructose is the ketone form shown below.

We see that carbons 3, 4, and 5 are chiral.

Each carbon is attached to four different groups as follows:

• bb"C3": "H, OH", "C"color(red)("=O"), and "CH(OH)"color(red)("C")
• bb"C4": "H, OH, CH(OH)(C=O)"color(red)("C"), and "CH(OH)CH"_2color(red)("O")"H"
• bb"C5": "H, OH", "CH(OH)"color(red)("C") and "CH"_2color(red)("O")"H"