Recall : methane, CH4 – tetrahedralshape (4 equal covalent bonds) Carbon has an electron arrangement 1s22s22p2 There is a contradiction here . . . . . .
Recall: the s orbital shape ONE s orbital in each energy level
Recall: the p orbital shape THREE p orbitals in each energy level (beginning n = 2)
Carbon’s valence shell - 2nd energy level orbitals The 4 orbitals are oriented like so:Problem #1This is not a tetrahedralshape. The 3 p orbitals are at900 to each other and the sorbital is “non-directional”
Carbon ground state electron configuration What is the expected orbital notation of carbon in its ground state?Problem #2? Only 2 electrons appear to be available for bonding
Solution – require 4 unpaired electronsPerhaps carbon “promotes”one of its 2s electrons… …to the empty 2p orbital.
BUT. . . this would mean that three of the bonds in a methanemolecule would be identical,But what about the fourthbond…? Must have a lower bond energy This was not observed by chemists
“hybridization” theory - What if the s orbital is combined with three p orbitals to create four equal hybrid orbitals? “hybridized”These new orbitals have slightly MORE energy thanthe 2s orbital… and slightly LESS energy than the 2p orbitals. … But all 4 hybrid orbitals have equal energy.
Since the 4 hybrid orbitals result from a combination of 1 sand 3 p orbitals, these hybrids are termed sp3 orbitals.
3 3 3 31s 2s 2px 2py 2pz sp sp sp sp y Hybridize x o 109.5 z Summary of sp3 hybridization
Sigma (σ) bonds The bonds formed in methane between carbon and hydrogen atoms are called sigma bonds.As shown in the diagram, a sigma bond is formed by the headon overlap of two orbitals. THE OVERLAP REGION LIESDIRECTLY BETWEEN THE TWO NUCLEI. Nuclei can rotateabout a sigma bond. Video tutorial link to ethane
sp2 hybridizationConsider the electron configuration of boron:When boron bonds with hydrogen, VSEPR predicts atrigonal planar geometry.One s orbital hybridizes with 2p orbitals to form 3 sp2 orbitals
sp2 hybridization also occurs in molecules that contain doublecovalent bonds. For example, ethene C2H4
Each carbon : 3 hybridized sp2 orbitals and 1 p orbitalSigma bond – single bond C-HSigma bond – sp2 orbitals: one half of double bond C=CPi bond – p orbitals: other half of double bond C=C
Pi bondsA pi bond consists of two p orbitals that overlap above andbelow a sigma bond. A double bond always consists of ONEsigma bond and ONE pi bond.The molecule cannot be twisted without breaking the pi bond. Video tutorial ethene
PracticeDraw a diagram to show the orbital overlap in the sigma and pibonds in a molecule of formaldehyde CH2O. State the shape ofthe molecule and the arrangement of negative charge centersaround the central atom.
Formaldehyde 2 Lone Pairs bond Sigma bondtrigonal planar (3 regions of electron density)
sp hybridizationConsider the electron configuration of beryllium:When beryllium bonds with chlorine, VSEPR predicts alinear geometry.One s orbital hybridizes with 1p orbitals to form 2 sp orbitals
sp hybridizationsp hybridization also occurs in molecules that containtriple covalent bonds. For example, acetylene C2H2
In ethyne C2H2 each carbon has 2 sp orbitals and 2 un-hybridizedp orbitals.The single bond between carbon and hydrogen is a sigma bond.The triple bond between the two carbons consists of a sigma bondbetween two sp2 orbitals and two pi bonds between the two sets ofp orbitals.
The two pi bonds are in different planes 900 to each other . Atriple bond always consists of ONE sigma bond and TWO pibonds. The molecule cannot be twisted without breaking thepi bonds.Show video tutorial acetylene