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) 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 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

Bonding pair represented as a straight line. 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)