Many p-block elements which are the first in their respective groups show $pπ-pπ$ multiple bonding. Why doesn't carbon do so?


closed as unclear what you're asking by ron, Jannis Andreska, Loong, Klaus-Dieter Warzecha, ringo May 10 '15 at 19:03

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • 4
    $\begingroup$ What's wrong with graphite, diamond, fullerenes, and several other carbon allotropes? $\endgroup$ – Nicolau Saker Neto May 10 '15 at 17:03
  • 3
    $\begingroup$ ...or all of organic chemistry. $\endgroup$ – ron May 10 '15 at 17:05
  • $\begingroup$ but those are allotropes of carbon. $\endgroup$ – Anoneemus May 10 '15 at 17:06
  • $\begingroup$ @shashikant So? ... They are carbons bonded to other carbons! BTW look at this : {en.wikipedia.org/wiki/Diatomic_carbon} $\endgroup$ – NeilRoy May 10 '15 at 17:11
  • $\begingroup$ @shashikant I believe you are talking about carbon-carbon π bonding. Is it? $\endgroup$ – Rajat Jain May 10 '15 at 17:16

Okay, I believe that you mean to ask why a $C_2$ molecule does not exist? Well, it does exist. So, when we use molecular orbital theory to calculate the bond order of $C_2$, which is

$$B.O.=\frac{\mathrm{(No.~of~electrons~in~bonding~orbitals) - (No.~of~electrons~in~ antibonding~orbitals)}}{2}$$

We have 4 electrons in bonding and 0 in antibonding for $C_2$So putting in formula-$B.O.=(4-0)/2=2$As the bond order is 2, the molecule cannot exist.

But when a more rigorous approach to MOT is applied, it shows that both the bonds present in the molecule must be π bonds and no sigma bonds must be present. This according to me is not possible.

While Wikipedia also confirms that there is a speculation regarding the structure. It says -

A recent paper by S. Shaik reports that a quadruple bond exists in $C_2$

And hence though the actual molecule has been found, its chemistry is still being studied.

  • 1
    $\begingroup$ Actually, the electrons not in the bonding orbitals are in non-bonding orbitals, not anti-bonding orbitals. $\endgroup$ – LDC3 May 10 '15 at 17:27
  • 2
    $\begingroup$ $\ce{C2}$ does exist, see here $\endgroup$ – ron May 10 '15 at 17:27
  • 1
    $\begingroup$ @Rajat Jain You should edit it again, now it's self- contradictory $\endgroup$ – Mithoron May 10 '15 at 19:09
  • $\begingroup$ In support of ron's comment, we actually have already a question about it on the network: chemistry.stackexchange.com/q/594/4945 $\endgroup$ – Martin - マーチン May 22 '15 at 9:16
  • $\begingroup$ Diatomic carbon does not have a quadruple bond but a double bond. It only exists in the gas phase. $\endgroup$ – Ali Caglayan Jun 10 '15 at 14:57

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