Pi Bonds
Organic Chemistry 1 and 2
Function/Purpose:
- Two atoms forming one bond - broken in half - that locks the compound in a particular shape
- Roughly a 120 deg separation from each bond
- What make the Pi bond limited compared to rotation from sigma bond is the two havles of the bond forming
Fundamnetals:
- H2C=CH2 - Ethylene used as example
Inorganic Chemistry Perspective:
Basics: Ground State C
- How many valence electrons does H and C have respectively?
- Writing out the Electron configuration for each atom:
- H = 1s1 = 1 valence electrons
- C = 1s2 2s2 2p2 = 4 valence electrons
- So if H bonds with C twice, how many valence electrons are left?
- Then how is it possible for each H2C to form a bond to fulfill their required octet?
- Applying the Electron configuratoin for each bond:
- H's 1s1 will bond with one of the valence electrons of C (one of the 2s2 electrons)
- C's are left with two valence electrons (2p2) but they still need an octet
- Solution: Forming a Double Bond
- (C-H)X2 means that C currently shares with H so it needs 2 more electrons to form an octet
- C atom normally has 4 valence electrons + 2 valence electrons from two different H atoms
- Meaning 6/8 electrons fulfilled
Organic Chemistry Perspective:
Refinement of Double Bond: Focus On Shape, VSEPR, and Hybridization
- Initial Watch Before Reading
- What does sp2 mean?
- It seemed easy with just taking valence electrons and electron
configuration into account but unfortunately its a little complicated
- Remeber that a double bond is a Pi AND Simga bond combined (Pi = bond #1 and Sigma = bond #2 --> "Double Bond")
- Therefore a simple two sigma bond perspective noted above is too simple
- Technically, there are 3 sigma bonds formed by the C atom:
- 2 from two separate H-s
- 1 from C bonding with the other C
- So HOW do we get a Pi Bond?
- Hybridization! The distinct shape is dictated by the creation of sp2 orbitals
- sp2 is a combination of C's 2s and 2p electrons
- Ground State = 2s2 2p2 = 4 valence electrons
- Valence Bond = sp2 X3 (1 electron each) and one 2p electron
- So HOW is it hybridized?
- Hybridization Watch Me Before Reading
- S orbitals are directionless as a sphere
- P orbitals are broken into X, Y and Z cooridnates
- It looks like an one of the game pieces from an old childhood game Jacks
- S orbitals have no direction and P orbitals are about
90 degrees from each other AND the desired shape is roughly 120 from
only three bonds
- We want the product to look like a Jack but with only THREE bonds on the horizontal and leaving the the TWO vertical bonds intact
- 2Px and 2Py hybridize with 2s to create three hybrid orbitals with mostly constructive + wave functions
- Taken together it looks like the Mercedes Symbol and the Pz orbital is comming at and away from the image
- Physics Aside:
- REMEBER THAT electron orbitals can have a positive and negative spin (they mathematicaly vibrate from + to -)
- S orbitals can only have one wave function or the other
- P orbitals have both wave functions but in different planes (X, Y, Z)
- So with the Shapes in Order -
mind you Px, Py, Pz become relatively arbitrary because the Pz would
really become like a Py axis as if originally, the molecules bonded by
to the top or below each other
- It can really start to mess with you visually as try to rearrangesomething in your head
- At any case, one of the hybrids bonds with another hybrind to form a sigma bond C-C
- The remaining hybrids form bonds with two different H atoms
- The most important bond forms to seal the structure
- The sigma bond initially formed allows free rotation but like magnets when it gets to a certain position, it locks
- WHen the Pi electron (remeber it has both + and - wave function) constructively lines up, it forms the Pi bond
Concepts:
- Waves and Harmonics - PHYSICS!
- Covalent Bond - Types of Interactions - CHEM
- Valence Electrons - CHEM
- Octet rule - CHEM
- Molecular orbital theory - CHEM
- Orbital Hybridization - CHEM
- Quantum mechanics - PHYS
- Electron configuration - CHEM
Terminology:
Additional Links:
