# How can I draw and identify the chiral centers of galactose?

Dec 27, 2014

The easiest way is to draw galactose as a Fischer projection.

Galactose is an aldohexose. The general formula is HOCH₂(CHOH)₄CHO.

It has four chiral centres: the C atoms in the CHOH groups.

Since there are 4 chiral centres, there are${2}^{4} = 16$ stereoisomers. We can limit ourselves to the 8 D isomers. The other 8 isomers are their L enantiomers.

How can we reproduce this diagram and identify D-galactose?

Step 1. Draw 8 skeleton Fischer projections, with horizontal crosses at C-2 to C-5.

Step 2. On C-5, draw all OH groups on the right.

Step 3. On C-4, draw OH groups to the right on the first four structures and to the left on the next four.

Step 4. On C-3, draw OH groups in the order 2 to the right, 2 to the left, 2 to the right, 2 to the left.

Step 5. On C-2, alternate the OH groups right, left, right, left, etc.

Step 6. Assign names to the structures using the mnemonic

$\text{All}$ $\text{altr}$uists $\text{gl}$adly $\text{ma}$ke $\text{gu}$m $\text{i}$n $\text{gal}$lon $\text{ta}$nks.
$\text{All}$ose $\text{altr}$ose $\text{gl}$ucose $\text{ma}$nnose $\text{gu}$lose $\text{i}$dose $\text{gal}$actose $\text{ta}$lose

So galactose is the second structure from the right.

The chiral centres are at C-2, C-3, C-4, and C-5.

The order of OH groups from bottom to top in galactose is R-L-L-R.