Constitutional isomers. Two bracelets with the same structural parts assembled differently

Transcription

1 Constitutional isomers Two bracelets with the same structural parts assembled differently

2 Constitutional isomers C C N C C N C 2 7 N Two molecules with the same constituent atoms assembled differently

3 Rules for writing the Lewis structure of a molecule: 1) MF, constitution, and net charge must be known 2) Draw the dot structures of all the atoms of the MF 3) Determine the total number of valence electrons 4) Connect the atoms by 2 electron covalent bonds 5) Maximize the number of covalent bonds (multiple bonds between atoms are OK) 6) Determine formal and net charge 7) Replace bonded dot pairs with line bonds if desired * * * * * 8) If 2 or more structures (resonance structures) can be drawn, the more representative has a) all octets filled b) minimal formal charge c) has formal charge with the least separation d) has formal charge assigned by relative electronegativities

4 Bond polarization: a consequence of electronegativity differences Fluorine, F 2 Fluoromethane, C 3 F Lithium Fluoride, LiF : F F: : F C - :F: Li + not : F Li EN = (40-40) = 0 F-C (40-25) = 15 (40-10) = 30 nonpolar covalvent polar covalent bond ionic bond bond C- (25-21) = 04 nonpolar covalent bonds

5 Atomic Orbitals Quantum mechanics provides a mathematical description of electron energies and locations It s called a wave equation The solution of the wave equation gives a series of wavefunctions, ψ 1, ψ 2, ψ 3 The square of the wavefunction is an atomic orbital

6 Atomic Orbital Shapes and Phases 5 classes of orbitals s, p, d, f, g 2s orbital see for orbital diagrams

7 2p orbitals 2p y 2p x 2p z

8 3d orbitals

9 Wikipedia s entry on atomic orbitals describes how the wave-like nature of electrons qualitatively determines the shape of an orbital:

10 - = e - not between nuclei if populated energy 1s 1s (exclusion) = antibonding interaction σ* 1042 kcal/mol 1s 1s σ 074 Å internuclear distance + = or 1s 1s (intersection) e - between nuclei = bonding interaction

11 - = 1s 1s (destructive interference) node between nuclei = antibonding interaction + = 1s 1s (constructive inteference) electron density between nuclei = bonding interaction

12 If the idea of LCAO is 2 σ * 1s 1 + 1s 1 = 2 σ 2 Can this approach be extended to methane, C 4? Yes, but the mathematics is very challenging! C 1s 2 + C 2s 2 + C 2p x 1 + C 2p y 1 + C 2p z + 1s 1 + 1s 1 + 1s 1 + 1s 1 =?

13 So, to model how atomic orbitals overlap to provide valance bonds, we will choose to combine only 2 orbitals at a time invent hybrid atomic orbitals to reconcile LCAO with VSEPR

14 Using VSEPR to predict molecular geometries Beryllium hydride, Be 2 Borane, B 3 Methane, C 4 Be :Be: B :B: C :C: 180 E Be 120 E 1095 E B C linear trigonal planar tetrahedral

15 π* energy C 2p C 2p π or

16 VSEPR geometries can be distorted is the electrostatic interactions between paris of valence elctrons are not identical: lp - lp interactions > lp - bonded pair interactions > bonded pair - bonded pair interactions 1095 E C Methane N 107E Ammonia O 105 E Water distorted tetrahedral

17 Dipole moments of molecules: net molecular polarity Draw the Lewis structure for nitromethane, C 3 NO 2 C O N O + nitromethane -

18 Dipole moments of molecules: net molecular polarity quantitatively, µ = Q r visually, dipole moment can be described as a vector with magnitude and direction that is the sum of the bond polarity vectors of a molecule Usually, it s OK to ignore the contributions of formally nonpolar covalent bonds that have EN # 04: O C N + O - net

19 Accounting for the polarization of every bond with EN 0 gives a more accurate µ but rarely changes the gross direction and magnitude: O net C N + O -

20 Dipole moments of molecules: net molecular polarity *+ *- electrostatic potential maps use color to indicate charge distributions