Transcript PowerPoint Presentation - Valence Bond Theory Structure

Organic Chemistry - 246A
Homework DUE Friday, 5 Sept
Problems in McMurry
1.24; 1.28; 1.31; 1.45; 1.46; 1.47
=> (1.48—1.52 BONUS Problems)
Valence Bond Theory
Structure & Bonding (Chapter 1, pp 1—28)
Know e— Configuration
& Valence of H, C, N, O
F, Si, P, S, Cl, Br and I
Atomic Structure
(Review if Necessary)
• Positively charged nucleus (very dense, protons and
neutrons) and small (~10—15 m or 10—5 Å)
• Negatively charged electrons are in a much larger cloud
(10-10 m) around nucleus
• Diameter is about 2  10—10 m [200 picometers (pm),
or about 2 Å [1 angstrom (Å) is 10—10 m = 100 pm]
Atomic Number & Atomic Mass
(Review if Necessary)
• The atomic number (Z) is the number of protons in the
atom's nucleus (gives number of e— in neutral atom)
• The mass number (A) is the number of protons plus
neutrons
• All the atoms of a given element have the same atomic
number (Z), but may differ in mass (A)
• Isotopes are atoms of the same element that have different
numbers of neutrons and therefore different mass numbers
• The atomic mass (atomic weight) of an element is the
weighted average mass in atomic mass units (amu) of an
element’s naturally occurring isotopes
Electronic Configuration
(Review if Necessary)
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Orbitals are grouped in Shells of increasing size and energy
Each orbital can be occupied by 2 e—
1st shell contains only one s orbital, denoted 1s, holds 2 e—
2nd shell contains one s orbital (2s) and 3 p orbitals (2p), 8 e—
3rd shell contains an s orbital (3s), 3 p orbitals (3p), and 5 d orbitals
(3d), 18 e—
s Orbitals Are Spherical
• Each s orbital has
spherical symmetry
• A test charge will
“see” the same charge
density regardless of
the direction it
approaches the atom
p Orbitals Are Bi-Directional
• In each shell there are
three perpendicular p
orbitals, px, py, and pz
of equal energy
• Lobes of p orbital
separated by region of
zero electron density, a
node
• The 3 p orbitals
together actually
provide a spherical
distribution of electron
density
d Orbitals Have Two Planes of
Symmetry
• d Orbitals have 4 lobes
• The 5 d orbitals when
taken together, also
produce a spherical e—
distribution
• All of these orbitals (s,
p, d, and even f) were
described by French
mathematicians in
studies of “flooded
planets” in the 1700’s
Hybridization
Carbon is Tetrahedral
• In 1858 Kekulé and Couper independently observed that
carbon always has four bonds
• In 1874 van't Hoff and Le Bel proposed that the four bonds
of carbon have specific spatial directions
• van't Hoff suggested that the four atoms surround carbon
as corners of a tetrahedron
Note that a dashed line
indicates a bond is behind
the page
Note that a wedge indicates a
bond is coming forward
Covalent Bonding in Carbon
• Atoms bond because the resulting compound is
more stable than the separate atoms
• Organic compounds have covalent bonds from
sharing electrons (G. N. Lewis, 1916)
• Lewis structures show valence e— as dots
– H has one dot (1 valence e— )
– C has four dots (4 valence e— )
• A pair of e— (:) can form a covalent bond
• Stable molecule results at completed shell, or octet
(8 dots) for main-group atoms (2 for hydrogen)
Lewis Structures Focus on
e— Pairs and Octets
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H (1s1) forms 1 bond [1 bonding pair]
O (2s2 2p4) forms 2 bonds [2 bonding pairs and 2 lone pairs]
N (2s2 2p3) forms 3 bonds [3 bonding pairs and 1 lone pair]
C (2s2 2p2) forms 4 bonds [4 bonding pairs]
Non-Bonding
—
e
(Lone Pairs)
• The N in NH3 (ammonia) has 5 valence e—
• The 3 H atoms contribute another 3 valence e—
• In the molecule, 6 valence e— make 3 covalent bonds, and
the remaining 2 valence e— are a nonbonding or lone pair
Valence Bond Theory (VBT)
• A covalent bond forms
when two atoms approach
each other closely so that
an e— from each atom can
form a bonding pair
• The e— are paired in the
overlapping orbitals and
are attracted to nuclei of
both atoms
• A bond that is formed
from 2 hydrogen atoms
creates a bond with
cylindrical symmetry
running through the H—H
bond axis, and is denoted
a sigma bond (s-bond)
Bond Energy & Bond Length
Bond Lengths & Angles
• In CH4 sp3 orbitals on C overlap with 1s orbitals
on 4 H atom to form four identical C-H bonds
• Each C–H bond has a length of 110 pm (1.10 Å)
• Each H—C—H bond angle is 109.5°
• Very similar lengths & angles will be found in all
alkanes
Bonding pair represented
as a straight line.
The Structure of Ethane
• In H3C—CH3 the 2 C’s bond to each other by s overlap of
an sp3 orbital from each C
• The 3 sp3 orbitals on each C overlap with H 1s orbitals to
form six C—H s bonds
• Bond angles are still ~109.5° and C—H bonds are still
1.10Å (110 pm), but the C—C s-bond is longer than the
C—H s-bond at 1.54Å (1.54 pm)
• The C—H s-bonds are stronger (420 kJ) than the C—C
s-bonds (376 kJ)