Covalent Bonding What is covalent bonding? Covalent Bonds: overlap of orbitals σ-bond π-bond Molecular Orbitals Hybrid Orbital Formation Shapes of Hybrid Orbitals Hybrid orbitals and Multiple Bonds resonance structures Mary J. Bojan Chem 110 1
Molecular Orbitals Lewis structures: accounting for bonding and lone-pair electrons (where are the electrons?) VSEPR: Electron-pair structure, spatial distribution of electrons (3D) How are bonds made? We know electron distribution in atoms: atomic orbitals: (s, p, d ) What is the electron distribution in molecules? Two models: Valence Bond Theory Valence orbitals on one atom overlap with valence orbitals on another atom: this overlap of orbitals is a covalent bond. Molecular Orbital Theory Covered in Chem 112 Mary J. Bojan Chem 110 2
Covalent Bonding H + H H 2 H (1s) H (1s) H 2 molecule Covalent bonding = overlap of atomic orbitals to form a new orbital Mary J. Bojan Chem 110 3
Why does a covalent bond form? H + H H 2 Two forces operating: balance of forces a bond length (0.74 Å for H 2 ) Mary J. Bojan Chem 110 4
σ-bond results from electron density is Bond Types s-s Examples: s-p pp π-bond results from electron density is Two p-orbitals Mary J. Bojan Chem 110 5
Imagine how Bonding in CH 4 might occur Carbon ground-state: (1s 2 )2s 2 2p 2 1s of H 2p of C Using only unpaired subshell electrons: Expect: The molecule would not have an octet on carbon. Mary J. Bojan Chem 110 6
Propose the following process for bonding in CH 4 : 1. Promote electrons on C 2. hybridization Four atomic orbitals mix to form four hybrid orbitals 2s + 3 2p 4 sp 3 Mary J. Bojan Chem 110 7
Propose the following process for bonding in CH 4 : 3. Bond formation: Form 4 C H bonds by overlapping each hybrid sp 3 orbital with an 1s orbital of hydrogen. σ-bond formation The new bonds are 109 o apart. One of the four bonds formed by overlap of an sp 3 orbital with a hydrogen 1s orbital NOTE: start with four atomic orbitals s p x p y p z end up with four hybrid orbitals 4 sp 3 Hybrid orbitals: combinations of atomic orbitals (on one atom). better for bonding (more directed) Mary J. Bojan Chem 110 8
sp 3 Hybrid Orbitals 1 x s + 3 x p = 4 x sp 3 Four atomic orbitals mix to form four hybrid orbitals The notation means that each hybrid is composed of 1/4 s and 3/4 p orbitals. Mary J. Bojan Chem 110 9
sp and sp 2 Hybrid Orbitals Two atomic orbitals mix to form two hybrid orbitals 1 x s + 1 x p 2 x sp 1 x s + 2 x p 3 x sp 2 Three atomic orbitals mix to form three hybrid orbitals Mary J. Bojan Chem 110 10
Summary Problem: Can t use atomic orbitals to describe bonding in molecules Solution: make molecular orbitals by mixing atomic orbitals (call them hybrid orbitals) Two atomic orbitals mix to form two hybrid orbitals 1 x s + 1 x p 2 x sp Three atomic orbitals mix to form three hybrid orbitals 1 x s + 2 x p 3 x sp 2 Four atomic orbitals mix to form four hybrid orbitals 1 x s + 3 x p 4 x sp 3 Five atomic orbitals mix to form five hybrid orbitals 1 x s + 3 x p +1 x d 5 x sp 3 d Six atomic orbitals mix to form six hybrid orbitals 1 x s + 3 x p +2 x d 6 x sp 3 d 2 Each hybrid orbital can accommodate 1 pair of electrons. Use VSEPR to determine shape of hybrid orbitals: the electron pairs will get as far from each other as possible. Mary J. Bojan Chem 110 11
Summary of hybridization types The hybridization scheme can be deduced from the electron-pair geometry of the molecule. Number of electron pairs Atomic orbitals used Hybrid type formed Electron-pair geometry Examples 2 s, p two sp linear BeF 2, HgCl 2 3 s, p, p three sp 2 trigonal planar BF 3, SO 3, CO 3 2 4 s, p, p, p four sp 3 tetrahedral CH 4, NH 3, H 2 O, NH 4 + 5 s, p, p, p, d five sp 3 d trigonal bipyramidal PF 5, SF 4, BrF 3 6 s, p, p, p, d, d six sp 3 d 2 octahedral SF 6, ClF 5, XeF 4, PF 6 Mary J. Bojan Chem 110 12
We can use the concepts of hybrid orbitals to explain bonding situations like multiple bonds and resonance. Multiple Bonds ethylene: shape about C: hybrid orbitals on C bond angles Only need three hybrid orbitals (3 electron domains) Approach hybridization of C with the end in mind One s and two p atomic orbitals combine to form 3 H H C C H H Mary J. Bojan Chem 110 13
Multiple Bonds One C C and two C H bonds (on each carbon) are H C C H formed using sp 2 orbitals on carbons. ( σ- bonds) H H Note: there is an electron in the p-orbital These p-orbitals can overlap, sideways: π-bond Mary J. Bojan Chem 110 14
Orbital Theory of Bonding explains: Why double bonds occur frequently with C, N, and O but not with larger molecules Why rotation about double bond does not occur Mary J. Bojan Chem 110 15
Orbital Theory of Bonding explains resonance e- pair geometry: hybrid orbitals on N and O are N and O have singly occupied p-orbitals Recall: resonance is a BLENDING of the resonance structures REAL structure is a blend of the three Lewis structures. Mary J. Bojan Chem 110 16
All of the p-orbitals overlap at the same time: this is called delocalized π-bonding. Difference between localized and delocalized π bonding Molecules with resonance structures have delocalized π bonding Delocalized bonding brings added stability to a molecule. Mary J. Bojan Chem 110 17
Demonstration of stability due to resonance Same reaction: hydrocarbon + Br 2 Colorless red ALKANE Heptane + Br 2 ALKENE (and ALKYNES) 2-pentene + Br 2 AROMATIC toluene + Br 2 CH 3 Mary J. Bojan Chem 110 18
Stability of aromatic hydrocarbons Alkene + Br 2 reacts readily Aromatic + Br 2 : no reaction π bonds of alkenes are very reactive toward addition. (π bonds in alkynes even more so.) π-bonds in benzene are NOT reactive due to the extra stability of delocalized π system Mary J. Bojan Chem 110 19
Summary of Covalent bonding 1. Draw Lewis Structure 2. Use VSEPR to determine shape Ø e- pair geometry Ø molecular geometry 3. What hybrid orbitals are involved in bonding? Determined by electron pair geometry. (Know the shapes of the hybrid orbitals.) 4. Is the molecule polar? Determined by molecular geometry. Remember: Ø Each single bond = covalent bond = σ bond Ø A covalent bond forms when orbitals overlap. Ø σ-bond: head-on overlap Ø π-bond: sidewise overlap Mary J. Bojan Chem 110 20
Determine the hybrid orbitals on the nitrogen in angle 1 and the C in angle 2. Angle #1 Angle #2 A. sp 3 sp 2 B. sp sp 2 C. sp 2 sp 3 D. sp 2 sp E. sp 3 sp 3 Mary J. Bojan Chem 110 21