I am in high school and haven't had a formal chemistry course, however I am fascinated with biology and picked up a biology book more advanced than the one the school is using to read on my own. The book is: Biology by Peter Raven (2007) ISBN: 0073227390

I am confused about something in the organic chemistry section.

I don't understand the convention that is used to number carbon atoms in for example, glucose.

In case you don't know what I'm talking about, here is an example: a structural diagram of an a-glucose molecule courtesy of Wikipedia.

enter image description here

The numbers next to the carbon atoms, I don't understand how they are ordered. Can someone please explain? I know this is an elementary question for most of you, but the help is appreciated.


closed as too broad by Jan, M.A.R. ಠ_ಠ, Todd Minehardt, bon, Wildcat Oct 16 '15 at 22:02

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  • $\begingroup$ Um, you know, IUPAC (The International Union of Pure and Applied Chemistry) writes a 1,500-page reference book to answer this question. $\endgroup$ – M.A.R. ಠ_ಠ Oct 16 '15 at 17:21

Glucose occurs as the open chain form aldehyde that determines atom numbering, and as the epimeric cyclic hemiacetal(s) you provided,

Glucose open and cyclic

The "ether" is a hemiacetal. C-1's -OH can be up or down. Either way, it opens and reversibly dehydrates to the aldehyde R-C(=O)H. Number one atom in a chain is the end that bears the highest total mass. Double bonds count double their other end atom. The cyclic form's C=1 also has the largest summed mass attached, two oxygens plus a carbon and a hydrogen.


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  • $\begingroup$ So they essentially are numbered from greatest mass to least? $\endgroup$ – Jason Mills Apr 17 '14 at 3:05
  • $\begingroup$ And why do double covalent bonds cause the other end atom to count twice as much? $\endgroup$ – Jason Mills Apr 17 '14 at 3:12
  • $\begingroup$ @JasonMills For a double covalent bond, you have 2 bonds, so you count the atom at the end twice. It as if you are counting each bond separately. $\endgroup$ – LDC3 Apr 17 '14 at 5:45
  • $\begingroup$ Mass priority traces back to Cahn-Ingold-Prelong (CIP) notation rules for assigning chirality and olefin configuration, etc. CIP is self-consistent nomenclature not geometry. L-amino acids alanine and cysteine have opposite CIP chiral labels. $\endgroup$ – Uncle Al Apr 17 '14 at 14:45
  • $\begingroup$ How does this rule of mass work for purine (en.wikipedia.org/wiki/Purine)? The number 1 atom (nitrogen) has bonded mass of 36 (3 x carbon) whereas the carbon between 2 nitrogen atoms has bonded mass of 43 (3 x nitrogen + hydrogen). $\endgroup$ – charlesdarwin Jun 4 '18 at 18:31

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