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

1 Answer
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.

upload.wikimedia.org

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.

upload.wikimedia.org

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.