Organic Chemistry Week 1 Notes
Organic Chemistry Week 1 Notes CHEM 2451
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This 6 page Class Notes was uploaded by Shea Claflin on Saturday January 9, 2016. The Class Notes belongs to CHEM 2451 at University of Denver taught by Dr. Peter J. Harrington in Fall 2016. Since its upload, it has received 42 views. For similar materials see Organic Chemistry I in Chemistry at University of Denver.
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Date Created: 01/09/16
Organic Chemistry, Dr. Peter Harrington Notes – Week 1 1-4 Carbon – essential element of o-chem Isotopes of Carbon- Carbon-12, most abundant: 98.9% Carbon-13, has one extra neutron: 1.1% # of protons and neutrons → 13 C # of protons → 6 Focus on the electrons, they are distributed in the space around the nucleus “Aufbau Principle” also can be thought of as the “building up” principle Spherical shells 1 2 3 4 Orbitals s p d f Orbitals: area in space where an electron is likely to be found Pauli Exclusion Principle: each orbital can contain only 2 electrons and they must be paired with opposite spin Hund’s Rule: when filling orbitals with the same energy each orbital gets 1 electron before any get 2 (like putting houses on a monopoly board) Valence shell: shell farthest from nucleus (highest in energy) that contains electrons Valence electrons: electrons located in the valence shell Example of electron configuration: Ne – 1s 2s 2p Octet Rule: elements want to fill their valence shells with 8 electrons Example of ionic compound in which electrons completely transfer from one atom to another: LiF (Lithium Fluoride) 1-6 Electronegativity: ability of an atom in a molecule to attract shared electrons to itself Ranges from 0 (low) to 4 (high) - e For NaCl, an ionic bond New Configuration 2 2 6 1 + 2 2 6 Electronegativity: .93 11Na 1s 2s 2p 3s Na 1s 2s 2p Electronegativity: 3.16 17Cl 1s 2s 2p 3s 3p 5 Cl- 1s 2s 2p 3s 3p 5 Δ = 2.23 These new configurations satisfy the octet rule for both atoms HH or H 2dihydrogen or hydrogen gas) It doesn’t make sense to transfer an electron from one H to the other because they have the same electronegativity 2 1H 1s H H H or electron configuration 1H 1s2 H H H like2He H 2atisfies the duet rule for hydrogen It is much more common in o-chem for atoms to share electrons in covalent bonds CH 4ethane > 90% of natural gas H 2 2 2 Electronegativity: 2.55 6C 1s 2s 2p C H C H Electronegativity: 2.70 1H 1s2 H H Δ = 0.35 Cl2(molecular chlorine or chlorine gas) 2 2 6 2 5 17Cl 1s 2s 2p 3s 3p Cl Cl – Cl NH 3 ammonia gas at room temperature 25°C (boiling point 33°C) 2 2 3 7N 1s 2s 2p 1H 1s 2 C2 4 ethylene (minor component of natural gas) C2 2 acetylene Bonds get stronger as bond order increases single – double – triple Assigning Charges Formal charge (FC): the positive or negative charge an atom “owns” in a molecule Formal charge = # of valence electrons in isolated atom - # of electrons the atom owns in the molecule or Formal charge = # of valence electrons – (nonbonding electrons + ½ bonding electrons) H 2 FCoxygen= 6 – (4 + ½(4)) = 0 FChydrogen 1 – (0 + ½(2)) = 0 H O Hydronium FC = 6 – (2 + ½(3)) = +1 3 oxygen FChydrogen 1 – (0 + ½(2)) = 0 OH Hydroxide FC oxygen 6 – (6 + ½(2)) = -1 FChydrogen 1 – (0 + ½(2)) = 0 We can avoid doing calculations by just counting the number of bonds an atom has. The following atoms have a formal charge of 0 when they have the following number of bonds. 1 4 3 2 1 H C N O F Cl Br I With still bigger molecules… your first “connectivity” option should have the least electronegative elemnt in the middle of the molecule. When drawing CO ca2bon would be place in the middle. 1-8 CO 2 carbon dioxide Electron bookkeeping - + NCS Na sodium thiocyanate 1 N 5 1 C 4 1) Connect the atoms 1 S 6 Anion 1 2) Add one electron (the anion) to N or S Total e = 16 electrons 3) Use more double/triple bonds to satisfy octet rule 4) Both are correct dot structures, together they represent the resonance hybrid for the thiocyanate atom. The charge is spread out over 2 atoms, it is delocalized. Note: is not the same as equilibrium arrows N 3a + sodium azide x 3 + 1 anion = 16 electrons 1) Connect atoms 2) Add 1 electron (anion) 3) Make double/triple bonds 4) Don’t forget formal charges Azide anion C3 6 acetone (nail polish remover) Electron bookkeeping 3 C 12 6 H 6 1 0 6 Anion 1 Total e = 24 electrons - 18 electrons present with give structure - look for atoms without octet satisfied when placing remaining electrons, we could put all 6 electrons on ), but C would not have a full octet. We could put 2 on C, but then O would not be satisfied. There is a double bond between O and C, it has no charges so it is the most stable. Guidelines for Design of Resonance Structures - only non-bonding and double/triple bonds electrons move (don’t move atoms, especially H’s) - all resonance contributers have the number of electrons and same over all charge Rubric for Grading Importance of Resonance Structures 1. structures with all octets satisfied are more important 2. more covalent bonds = more important 3. no positive-negative charge separation = more important 4. negative charge on more electronegative atom = more important Curved Arrow Notation - tracking movement of electrons source to destination - can go forward or backward through structures methane ethylene H H H H C H H C C H H Lewis dot structures tell us nothing about geometry Q) is H angle 90 degrees? Is methane planar? A: NO – 109.5 degreNO – Tetrahedral Q) is H-C-C or H-C-H angle 90 degrees? Is ethane planar? A: NO – 120 degrees YES – flat Revisit atomic orbital geometry: - s orbital: s is for sphere Sphere surrounds nucleus 90% probability you’ll “find” the electron in that sphere