Ask A Scientist Chemistry Archive

 
name         Amanda
status       student
grade        9-12
location     PA

Question -   Hi, I am in 11th grade and am in general chemistry class.
On carbon bonding quiz involving alkanes, alkenes, and alkynes the
teacher and I disagreed on a certain answer. We were to write down the
correct name of the carbon bond and to identify any double or triple
bonds. Although when it came to C=C-C, I had wrote 1, pentene to show the
double bond. She said this is incorrect, but I STRONGLY disagree and
believe that this can be written either way. I ask another chemistry
teacher and he too said this is correct either 1,pentene, or pentene. The
teacher is deducting points for my answer and for doubting her. Is this
way correct 1, pentene? I GREATLY appreciate if you respond to this.
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The name of a hydrocarbon is composed of 3 parts: 1. A root which
specifies the number of carbon atoms. The first few roots are: "-eth-",
"-prop-", "-but-", "-pent-", "-hex-", etc. 2. A suffix "-ane", "-ene",
"yne" that specifies whether the hydrocarbon contains only single C-C
single, double, or triple bonds respectively. 3. An alphanumeric prefix
that identifies the location and configuration of  any unsaturation,
and/or specifies the location of any branches in the carbon skeleton. For
high molecular weight hydrocarbons this can get involved and there are
specific conventions laid out in any organic chemistry text. There are sub
sets of rules for ambiguous or alternative names.
  For hydrocarbon containing 2 carbon atoms connected by a double bond,
C=C (omitting the hydrogens), the sole name is "ethene". For the hydrocarbon
containing 3 carbon atoms, C=C-C, the sole name is "propene". No other
designation is necessary because C=C-C is identical to C-C=C if all the
substituents (e.g. 'H' atoms) are identical. For the hydrocarbon containing
4 carbon atoms connected by a single double bond (with all other
substituents e.g. 'H' atoms) there are two possible locations for the double
bond: C=C-C-C (this would be 1-butene), and C-C=C-C (this would be
2-butene). In addition, the terminal carbons in 2-butene can be arranged in
several configurations: C2>C=C and
C\C=C/C or C\C=C\C which are the cis- and trans- configurations,
respectively. After this point the number of isomers increases rapidly and
the nomenclature becomes correspondingly intricate to unambiguously name a
given compound. The rules for naming can be found in any organic text or
chemistry handbook.
     Now back to the inquiry: The student incorrectly named the 3-carbon
hydrocarbon, "C=C-C" using the root "pentene". The correct root for this
compound is "propene". No numeric prefix is needed since
"C=C-C" and "C-C=C" are identical compounds. That is why the answer was
marked incorrect -- wrong root for the number of carbon atoms. The compound:
C=C-(C-C-C) is 1-pentene: the root "pent", the suffix "ene" and the prefix
"1" specifying the number of carbon atoms, the presence of a double bond,
and its location respectively. Note here the prefix "1" is necessary.

Vince Calder
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Amanda,

I am sure you know that a compound with 3 carbons would be based on the 
"prop-" root rather than the "pent-" root since "penta-" means five. So I 
am not sure if in your original posting whether you meant to write 
C=C-C-C-C or did indeed want C=C-C and meant to write 1-propene. I will 
address both issues.

When naming compounds we have to adhere to certain agreed upon rules. 
These rules are designed to avoid possible confusion and reduce redundant 
information. In the case of pentene, since we can have C=C-C-C-C, 
C-C=C-C-C, C-C-C=C-C; then we have to avoid confusion by establishing 
where in the straight chain the double bond is. Thus, we would write 
1-pentene, 2-pentene, and 3-pentene respectively. However, in the case of 
C=C-C, there is no possible confusion, because there is only one 
possibility. Whether we draw it out as C=C-C or C-C=C this would still 
have the double bond starting from the first carbon, so although we could 
state 1-propene, the "1" is redundant and superfluous. So we simply write 
propene.

Greg (Roberto Gregorius)
=====================================================
Hi Amanda...
Sorry darling but I think there is a misunderstanding on your part, or 
maybe you just did not pay attention at what you wrote. At all organic 
chemistry there are the prefixes that show how many carbons are there. So 
met=1, et=2, prop=3,but=4, pent=5 and so on.
Now you wrote a chain with 3 carbons and called it pent + ane because you 
have a double bond. Of course if you had wrote a 5 carbon chain with one 
double bond, it would be a pentane 1 or 2 depending upon where the double 
bond was located. Look at what you wrote again.
Thanks for asking NEWTON!
Mabel
(Dr. Mabel Rodrigues)
====================================================
Hi Amanda.   I empathize with your question.

On one hand, we "experts" hate to be in the middle, especially when we are 
not told exactly everything, such as,
what was the "correct" answer expected by the teacher?
What you typed to us was "C=C-C", which is "propene".  (often called 
propylene.)
I am left to assume you meant there were more carbons on the end, which 
you consider to be implied by your story.
If I am to take a stand, I much prefer clear statements to unstated 
implications.

On the other hand, if you have not accidentally crossed your "p" names,
then  yes, C=C-C-C-C is correctly called "1-pentene" or "pentene",
and the former has the advantage of completely specifying the molecule.
("2-pentene" is also a "pentene": "C-C=C-C-C".
And there is no "3-pentene" because it can instead be called "2-pentene", 
counting from the other end.)

I have usually seen a dash or a space rather than a comma, but it is hard 
to believe that punctuation is worth quibbling about.
The expression is unambiguous either way.
Later on, if the journal editors want it a certain way, they will tell you 
before you send your manuscript.

I found the "1-pentene" and "2-pentene" notation used in my old CRC 
Handbook of Chemistry and Physics,
and delineated in the Definitive Rules (IUPAC 1957) section of the same 
book, and used in the current Alfa Aesar chemical catalog.
(Is 1957 the latest standard? )  For 5 carbons it is correct, all right, 
but that might not be the whole issue.

If you have met someone who will not allow you to put forwards all you 
know, seeking to be your best, I sympathize greatly.
I am pretty sure I have felt that pain somewhere before.
Even if you cool your part in the ego-confrontation,
you then find yourself unsupported in learning the things that actually 
are on your learning frontier.
On the other hand, there is some (admittedly depressing) value
to giving answers in exactly the form or extent the teacher has 
specifically requested,
as long as it is not definitively in contradiction to the technical field 
at large.
Perhaps that can be addressed by learning some chemistry in another place,
such as a chemistry club, or AP Chemistry next year, or web surfing, or an 
e-mail list.

Please do not take anything I am saying too personally.
I am trying to guess all possible angles of what you have not quite said,
because I consider it an important subject.

Jim Swenson

PS- if it was 3 C's rather than 5 C's in the test, I would suggest a 
diffident apology to the teacher.
Not so much for challenging her, but rather for making a hasty misreading.
I bet it would smooth things over.
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