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.

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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., 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. 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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