1
$\begingroup$

The carbon in front of the parenthesis is bonded to 2 CH2 which then bonded to CH3, it cannot bond directly to CH3. I assume the correct way of writing the formula of this molecule will be CH₃CH(CH₂CH₃)₂.

However, the university lecturer says its ok to write CH₃CH(CH₃CH₂)₂ because it is a common knowledge that CH3 cannot bond to both CH2 and C at the same time which indicate that the carbon in front of the parenthesis is bonded to CH2.

Trying to understand the lecturer's explanation, does it mean that in a parenthesis, it doesn't matter whether one person writes the order of bonding forward or backward and it is up to the reader to figure out whether the carbon in front of the parenthesis is bonded to the first or the last carbon in the parenthesis?

I am a non-native English speaker and I hope my question is explained clearly. Tried my best to phase this. Please let me know if any part of the question does not make sense.

$\endgroup$
5
  • 3
    $\begingroup$ The first way in the title is the normal way, the second is an abnormal rendition. Yes the second way can be puzzled out, but why make it hard on the reader? $\endgroup$ – MaxW Feb 9 at 1:39
  • $\begingroup$ Calcium acetate is sometimes written as (CH3COO)2Ca, sometimes Ca(CH3COO)2. Regardless of which way is correct, it is the same compound. $\endgroup$ – Poutnik Feb 9 at 2:46
  • $\begingroup$ @MaxW So the order does not matter, it is just courtesy to write in the correct way according to the order of bonding. Do I understand this correctly? Me being too accurate, answered a test question and said these two are not the same molecule, got mark down for my answer. $\endgroup$ – Dan L Feb 9 at 3:07
  • $\begingroup$ @DanL - It is much more than a courtesy, but your teacher is right they are the same molecule. Frankly I think asking if the two are the same molecule on a test is a stupid question, but just let it go. If your teacher says the earth is flat, then in that classroom the earth is flat. $\endgroup$ – MaxW Feb 9 at 5:39
  • $\begingroup$ @MaxW the earth is flat until one can report him while not getting caught ;) $\endgroup$ – Dan L Feb 9 at 7:31
5
$\begingroup$

In chemistry, as in any "language", there is what must be said to be correct: it is the 1st way your molecule is written.

Your second way of writing it is not wrong but it does not correspond to the globally institutionalized standard. The goal for any language is to be understood no matter where you are. The conventions indicate that it should be written $\ce{CH3-CH-(CH2-CH3)2}$ which does not induce any ambiguity

$\endgroup$
1
  • $\begingroup$ "…globally institutionalized standard"—What standard, exactly? You forgot to add a citation. Similarly, what "conventions" are applicable, exactly, and what is the authoritative source? $\endgroup$ – andselisk Feb 10 at 17:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.