VSEPR & HYBRIDIZATION - PowerPoint PPT Presentation

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  2. Two Theories Which Seem To Reinforce Each Other • VSEPR is essentially the theory which helps to explain molecular geometry (shape, and bond angle), because it accounts for the repulsive interaction(s) between the bonded and/or un-bonded electrons of the valence shell. • Hybridization is the theory which replaces G.N. Lewis’s Valence Bond Theory. Hybridization explains the experimental data, indicating equal bond energies and bond lengths involving a central atom, and the common atoms to which it is bonded, despite the fact that the bonding electrons are of different energies (because they are in different sublevels). • Hybridization supports, and is supported by, VSEPR, even though they are two separate (different) theories.

  3. sp3hybridization with carbon, and 4 equal bonds made from two different sublevels of different energies! 2px 2py 2pz sp3sp3sp3sp3 2s 1s E Notice the change in energy. As the s and p orbitals merge, the energy of the new hybridized orbitals is averaged, and equalized. These electrons are not used in bonding and remain unchanged 1s

  4. The volume, shape and energy of the bonding atomic orbitals are changed with sp3 hybridization. Hybridized orbitals. The shaded area is volume where the overlap of the covalent bond occurs. Notice the spatial orientation. Atomic (unhybridized orbitals) Notice that only the 2s and 2p sublevels are diagramed, as the 1s sublevel of C, is not hybridized. http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1%3A_Chapter_1%3A_Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals

  5. sp3 hybrid orbitals point towards the corners of a tetrahedron, thus supporting VSEPR Theory http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1%3A_Chapter_1%3A_Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals

  6. Now, study the aldehyde, methanal, CH2O as an example of sp2 hybridization We only need enough hybrid orbitals to accommodate the groups around the central carbon atom (That would be 3 in this case, the 2 H and the O) Groups can be atoms or un-bonded electrons. Notice that one s and 2 p orbitals are blended, creating 3 bonding orbitals or sp2 hybridization 2px 2py 2pz sp2sp2sp2 There is no need to have this 2pz orbital hybridized. 2p This 2p orbital is used to create a double bond (a pi bond) between the C and the O, thus creating the carbonyl functional group O // H - C - H E 2s Again, these electrons are still not used in bonding and they remain unchanged 1s 1s

  7. sp2 hybridization helps to explain the double bond (pi bond) The carbon-carbon double bond in ethene consists of one sigma bond, formed by the overlap of two sp2 orbitals, and a second bond, [a π (pi) bond]. Pi bonds are formed from the overlap of parallel un-hybridized p orbitals on adjacent atoms. In this case, they are formed by the side-by-side overlap of the two un-hybridized 2pz orbitals from each carbon. Because they are the result of side-by-side overlap (rather then end-to-end overlap like a sigma bond), pi bonds are not free to rotate.  Rotation would break the pi bond.  The presence of the pi bond thus ‘locks’ the six atoms of ethene into the same plane.  In larger alkenes, this phenomenon leads to the production of structural isomers. http://butane.chem.uiuc.edu/pshapley/GenChem2/A6/3.html

  8. When there are only 2 groups surrounding the carbon, as in CO2 sp hybridization occurs … Again, notice that the number of hybridized bonds depends upon the number of bonded atoms Notice that one s and p orbital are blended, creating 2 bonding orbitals or sp hybridization 2px 2py 2pz spsp There is no need to Hybridize these 2 orbitals 2p 2p These 2p orbitals are used to create the pi double bonds between the C and the O atoms O = C = O E 2s Again, these electrons are still not used in bonding and they remain unchanged 1s 1s

  9. So, In Summary …. • sp3 orbitals are • the result of blending one s and 3 p orbitals making the sp3 75% p and 25% s • used to explain the equality of bond length, and strength in molecules such as CH4 (with 4 bonded groups) and leads to tetrahedral molecular shapes, and free rotation around C to C single (sigma) covalent bonds. • sp2hyridized orbitals are • used to explain the existence of the double bond, for, a 2p orbital is left un-hybridized and used to make a pi double bond • used to explain three bonded groups and leads to trigonal planar shapes • used to explain the carbonyl group (with its one double bond) in aldehyde and ketone compounds, when carbon is the central bond. • sp hybridization • explains the linear shape when only two groups are bonded to the central carbon. • leaves two un-hybridized p orbitals capable of producing double bonds (pi bonds) • is 50% s and 50% p • is seen in molecules such as CO2, BeF2, C2H2 • explains multiple double bonds (pi bonds) when carbon is the central atom. http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1%3A_Chapter_1%3A_Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals

  10. To help understand sigma and pi bonds a bit better, try these two videos …Come in with questions, tomorrow! Per usual, Crash Course Chemistry keeps its rapid pace dialogue and imagery going, but each installment has some nice points to make. You might want to stop and replay a section now and then. This 11 minute video has some special points for review and edification: at 3:48 you will get an review at to why water is a polar molecule and at 7:50 there is a really nice visualization of pi bond formation. http://www.youtube.com/watch?v=cPDptc0wUYI • Try this quirky 5 minute video with some very nice points to make about sigma and pi bonds, and hybridization. • Check out the whole video, but take a special look at around 3:58 for a nice little clay model view of pi bond formation. http://www.youtube.com/watch?v=ree49ge4VA4

  11. Citations Based upon ideas / work found at/in: http://www.chem.uiuc.edu/CLCtutorials/104/Hybridization/SeeIt.html Morrison and Boyd: Organic Chemistry Pauling, L: General Chemistry http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__1%3A_Chapter_1%3A_Introduction_to_organic_structure_and_bonding_I/Section_1.5%3A_Valence_bond_theory%3A_sp,_sp2,_and_sp3_hybrid_orbitals http://butane.chem.uiuc.edu/pshapley/GenChem2/A6/3.html https://en.wikipedia.org/wiki/Pi_bond