Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 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)