4 C-H s bonds: Td geometry

 

MO Theory using sp3 hybridized orbitals predicts.

 

            - Td nuclear geometry

            -  4 equal length C-H bonds (bonding MO’s equivalent)

 

Advantages of using sp3 hybrid orbitals

 

            1.  better prediction of molecular structure (in some cases)

            2.  sp3 hybrid orbitals overlap better than p orbitals

            3.  no filled nonbonding orbitals are generated in the MO energy diagram (see text)

 

In almost all cases with C, N, and O, some type of hybrid orbital is used to describe the bonding.

 

 

Boron Hydride aka Borane  (BH3)

 

sp2 Hybrid Orbitals

 

 

 

 

sp2 hybrid orbital

 

 

 

same schematic representation     

 

 

even though now ~33% s character and ~66% p character

 

 

 

 

trigonal planar geometry

 

 

 

 

for BH3 (borane), overlap of sp2 hybrids of B with 1s orbitals of H

 

 

Since they come from sp2 hybrid orbitals, B-H s bonds will have 120 H-B-H bond angles and a trigonal planar geometry around B atom.

 

 

C atoms of ethylene use sp2 hybrid orbitals

 

 

 

 

 

 

 

 

 

Beryllium Hydride (BeH2)

 

sp hybridization

sp hybrid

 

same schematic representation     

 

50% s character, 50% p character

 

for BeH2 (beryllium hydride), overlap of sp hybrids of Be with 1s orbitals of H

A similar MO diagram for the s = sp + 1s and s* = sp - 1s orbitals can be drawn

 

The two sp hybrids orbitals have a 180 angle between the orbitals.  The s bonds also have a 180 angle.

 

        

 

C atoms in triple bonds use sp hybrid orbitals.

 

e.g.      acetylene       

 

Summary of Orbitals

 

 

 

 

 

If an atom uses sp3 hybrid orbitals, the atoms are said to be sp3 hybridized.

                         sp2 hybrid                                                    sp2 hybridized

                         sp hybrid                                                     sp hybridized

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

phenylalanine (an amino acid)

 

 

 

 

 

 

 

 

 

glyceraldehyde

(smallest carbohydrate)

 

 

Norlutin

(a steroid used for

oral contraceptives)

 

 

 

 

C-H bond lengths are equal= 1.091 A

C-Cl bond length = 1.781 A

C-H bond length = 1.096

 

 

 

 

 

 

 

 

120 angles

bend bond downward,

p changes to sp3

sp2 orbitals change to sp3

109.5 angles

 

 

orbitals and atoms are only approximately sp, sp2, and sp3, although will generally be labeled specifically as 1 of the 3

 

> or < sp3 hybridization is 1 explanation for deviations from perfect tetrahedral geometry

 

other explanations based on bond lengths, electronegativity, and larger size of lone pair orbitals

 

e.g.  lone pair electrons, larger than bonds, electron repulsion from lone pair pushes bonds to smaller angles

 

 

Reactive Intermediates

 

- highly reactive, unstable, short lived species (nanoseconds - microseconds)

 

- often form during course of a reaction

- part of reaction mechanism

 

carbocation

carbanion

radical

methyl carbocation

methyl carbanion

methyl radical

 

 

 

 

much s character, places    

electrons closer to (+) nucleus

sp2 hybridized

trigonal planar

sp3 hybridized

~Td electronic geometry

pyramidal nuclear geometry

 

~sp2 hybridized

close to trigonal planar

rapidly inverting shallow pyramids

                                                                                                                                                                                               

                       

 

Ethane

 

 

            C atoms : sp3 hybridized

            C-H bonds : sp3 + 1s s bonds

            C-C bond : sp3 + sp3 s bond

            Td nuclear geometry about C atoms

 

Ethane can adopt different conformations.

 

 

conformations (conformers) : different arrangements of atoms that can be converted into one another by rotation about singe bonds

 

 

 

 

 

 

Newman Projections

 

 

dihedral or torsional angle

 

 

 

 

http://www.nap.edu/readingroom/books/biomems/mnewman.html