why do some organic chemisty molecules have "penta" within them and not "pent" (like penta-1,3-diene)?

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  • 5 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.

  • 5 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.

    Like this:

    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-."

  • Anonymous
    5 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.

    • skeptik
      Lv 7
      5 months agoReport

      I think it was the specific example that prompted the question. As the follow-up comment under this answer would seem to indicate.

      >> "So if I write it this way is it still correct or not?"

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