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QUESTION

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

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 are2^4 = 16 stereoisomers. We can limit ourselves to the 8 D isomers. The other 8 isomers are their L .

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

"All" "altr"uists "gl"adly "ma"ke "gu"m "i"n "gal"lon "ta"nks. "All"ose "altr"ose "gl"ucose "ma"nnose "gu"lose "i"dose "gal"actose "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.