why do some organic chemisty molecules have "penta" within them and not "pent" (like penta-1,3-diene)?
- pisgahchemistLv 75 months ago
Penta vs pent.....
Some may have some "pent-up" anxiety, but allow me to be clear. The Greek prefix for five is "penta-" and is used in the naming of chemical compounds where there is five of something.
We name the compound PCl5 as phosphorous pentachloride where the prefix is "penta-". But if "penta-" is used with a word that begins with a vowel, then the "a" is dropped as in (nonexistent) diphosphorous pentoxide, P2O5 (*).
The bottom line is that the "a" in tetra-, penta-, hexa-, hepta-... is dropped when the prefix is attached to a word beginning with a vowel.
* P2O5 is the empirical formula for the actual compound P4O10. Despite there being no molecules of P2O5, the name diphosphorous pentoxide persists.
- skeptikLv 75 months ago
In Greek, as it is used in Scientific terminology, the prefix is "pent-." As with other such prefixes, it is left like that if the next part of the word begins with a vowel. If the next letter is a consonant, an "-a" is added to make the connecting form of the prefix.
Pent- angle (not "penta- angle")
Penta- gon (not "pent- gon")
In your specific example, if you drop the numbers and hyphens, you get "pentadiene," which is correct. And which interestingly enough actually has TWO numeric prefixes: "penta-" and "di-."
- Anonymous5 months ago
Because thats how its normally used, right?
- 5 months ago
Penta is Koine Greek for five. It's shortened to pent before a vowel - as in pentoxide.