Constitutional isomers 1 6 6 Two bracelets with the same - - PowerPoint PPT Presentation

Constitutional isomers Two bracelets with the same structural parts assembled differently

1 6 6

Constitutional isomers Two molecules with the same constituent atoms assembled differently

N H H C C H H H H H C N C H H H H H H H

C2H7N

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

.. F Li :F: ..

• Li +

:.. .. F F .. : : .. .. .. F C H H H .. : ..

Bond polarization: a consequence of electronegativity differences Fluorine, F2 Fluoromethane, CH3F Lithium Fluoride, LiF not ∆EN = (4.0 - 4.0) = 0 F-C (4.0 - 2.5) = 1.5 (4.0 - 1.0) = 3.0 nonpolar covalvent polar covalent bond ionic bond bond C-H (2.5 - 2.1) = 0.4 nonpolar covalent bonds

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, ψ1 , ψ2 , ψ3 . . .

• The square of the wavefunction is an atomic
• rbital

Atomic Orbital Shapes and Phases

• 5 classes of orbitals
• s, p, d, f, g

2s orbital

see http://www.d.umn.edu/~pkiprof/ChemWebV2/AOs/index.html for orbital diagrams

2p orbitals 2py 2px 2pz

3d orbitals

Wikipedia’s entry on atomic orbitals describes how the wave-like nature of electrons qualitatively determines the shape of an orbital: http://en.wikipedia.org/wiki/Atomic_orbital

+ =

• r

. . . .

1s e- not between nuclei if populated 1s = antibonding interaction

σ σ*

• =

. .

1s 1s 1s (intersection) e- between nuclei 1s = bonding interaction (exclusion)

H H

energy internuclear distance

104.2 kcal/mol 0.74 Å

+ =

node between nuclei = antibonding interaction

• =

1s 1s 1s (constructive inteference) 1s electron density between nuclei = bonding interaction (destructive interference)

Can this approach be extended to methane, CH4? If the idea of LCAO is H 1s1 + H 1s1 = H2 σ2 H2 σ* Yes, but the mathematics is very challenging! C 1s2 + C 2s2 + C 2px

1 + C 2py 1 + C 2pz + H 1s1 + H 1s1 + H 1s1 + H 1s1 = ?

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

H Be H H B H H Be H . . . H . H:Be:H B H . . . H . . H . H:B:H .. H C H . . . H . . H . H:C:H .. H . H . .. H H C H H H

Using VSEPR to predict molecular geometries Beryllium hydride, BeH2 Borane, BH3 Methane, CH4 180 E 120 E 109.5 E

linear

linear linear linear trigonal planar trigonal planar trigonal planar trigonal planar tetrahedral tetrahedral tetrahedral tetrahedral

π π* C 2p C 2p

energy

. . . .

• r

. . . .

H C H H H N H H H .. O H H .. ..

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 109.5 E Methane 107E Ammonia 105 E Water

distorted tetrahedral distorted tetrahedral distorted tetrahedral distorted tetrahedral

C N H H H O O . . . . . . . . . .

+

• Dipole moments of molecules: net molecular polarity

Draw the Lewis structure for nitromethane, CH3NO2 nitromethane

C N H H H O O . .. . . . . . . .

+

• 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 # 0.4: net

C N H H H O O . .. . . . . . . .

+

• Accounting for the polarization of every bond with ∆EN … 0 gives a more accurate µ but rarely

changes the gross direction and magnitude: net

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