bonding in benzene

describe the bonding in benzne, include the model used for the arrangement of electrons.

is this a correct statement- "The C-H bonds are sigma bonds, formed by overlap of atomic orbitals on carbon and 1s on hydrogen atoms."

am finding this pi/sigma stuff hard to understand

thanks

Reply 1

Natalie Lane

9

right, the sigma bonding is the basic single bond you've always heard about, it's a simple overlapping between the orbitals involved in the boding and forms a directional single bond. The pi bond is what makes it a double bond, this is when p-orbitals kind of merge so that the area qhere the technically unbonded electron in the p-orbital is larger and covers more than one nucleus.
I think that benzene is the where the carbons form a circle and they all have a single (sigma) bond with eachother as well as one large pi-bond shared among all of those in the ring

try this site: http://ibchem.com/IB/ibc/bonding/bon_htm/14.3.htm

Reply 2

Ben.S.

11

Jack Schitt

describe the bonding in benzne, include the model used for the arrangement of electrons.

is this a correct statement- "The C-H bonds are sigma bonds, formed by overlap of atomic orbitals on carbon and 1s on hydrogen atoms."

am finding this pi/sigma stuff hard to understand

thanks

OK - this is the 'organic' way to think about it (not the more rigorous symmetry and secular equations approach, although the results are similar):

Each carbon atom essentially has four orbitals involved in bonding - the 2s, 2px, 2py and 2pz (the 1s is too low in energy/contracted to interact with anything). The 2s and two of the 2p orbitals are said to interact to give three sp^2 hybrid atomic orbitals - which are geometrically distinct from each of the 2s and 2p orbitals. Each carbon's set of sp^2 orbitals are in the plane of the ring (xy) and separated by 120 degrees. These orbitals overlap to form the 'sigma framework' - each carbon bonds to one hydrogen (hydrogen 1s overlapping with one of the carbon sp^2 HAOs) and two other carbons (sp^2 overlap). This gives the usual two-centre-two-electron bonding scheme. Because only 2 of the p orbitals were used in the hybridisation scheme, each carbon still retains a pz orbital. These can overlap side-on - forming a 'pi system' (sigma = end-on, pi = side-on). Each carbon contributes one electron to the system - so that gives six electrons in total. Each MO can only accommodate two, so that fills the three most strongly bonding MOs.

Ben

bonding in benzene

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