Why is 1,1-dimethylbenzene impossible?

1,664

enter image description here

One of the determining features of benzene is a delocalised 'cloud' of electrons, which overall counts for 3 double bonds. In other words, Each carbon atom in the ring has a single bond to one neighbour, and a double bond to the other - this leaves one remaining bonding orbital - so only one more substituent can bond to each carbon on the ring. Adding a second substituent to a single carbon on the ring would break one of the C=C double bonds in the benzene ring in order to limit the number of bonds to the central carbon to 4.

This would be a very unfavourable reaction because benzene is an aromatic molecule, meaning that it is more stable than its hypothetical non-aromatic analogue (a cyclohexatriene) such that almost all of benzene's reactions will lead to a product which retains the aromaticity.

Once the double bond is broken, the ring is no longer aromatic, so is no longer called a benzene ring, instead it would be called a 1,3-diene. As you can see from the image below, adding two substituents to one carbon atom in the ring is possible, but the product would be less stable than the reactant.

Share:
1,664

Related videos on Youtube

Max Li
Author by

Max Li

Updated on October 23, 2022

Comments

  • Max Li
    Max Li less than a minute

    It seems possible, as each carbon needs four bonds, so the first carbon could be bonded to two adjacent carbons and two methyl groups.

    • orthocresol
      orthocresol about 6 years
      The carbons in the benzene ring are connected by alternating double and single bonds.
    • Ivan Neretin
      Ivan Neretin about 6 years
      @MaxLi You are thinking of it the wrong way. Each methyl is a substituent, which means that it substitutes something (at least nominally). That is, your methyl is there instead of something (usually hydrogen). Are there two hydrogens on that carbon in benzene? Why or why not?
    • porphyrin
      porphyrin about 6 years
      or put another way if you want to place two methyl groups onto one carbon atom then you would loose one double bond completely and leave the adjacent carbon with one spare electron and one H atom. In this way you would destroy the aromaticity.
    • jerepierre
      jerepierre about 6 years
      Have you drawn the structure of this compound?
  • Mithoron
    Mithoron about 6 years
    It would be radical nor diene until hydrogen atom would be added
  • Jack
    Jack about 6 years
    @Mithoron my radical chemistry is pretty rusty, but I doubt you would be able to isolate the radical species due to it being incredibly unstable. To be honest I'm not sure which reagents you could use in order to obtain the diene from benzene in the first place.