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1/19/2011 John E. McMurry http://www.cengage.com/chemistry/mcmurry Chapter 1 Structure and Bonding Richard Morrison • University of Georgia, Athens Scientific Revolution Scientific revolution leads to: • Safer and more effective medicines • Cures for genetic diseases • Increased life span • Improved quality of life Scientific advances in medicine and biology require an understanding of organic chemistry Organic Chemistry Organic and Biological Chemistry in Modern Medicine • Coronary artery disease (CAD) is directly correlated with blood cholesterol levels • 75% of blood cholesterol (1000 mg each day) is biosynthesized • Drugs known as Statins reduce the risk of CAD by lowering blood cholesterol levels • 3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) to mevalonate conversion • Crucial step in biosynthesis of cholesterol • Atorvastatin (Lipitor) inactivates enzyme that catalyzes HMG-CoA to vevalonate conversion 1 1/19/2011 Organic Chemistry Carbon Organic chemistry • All organic compounds contain the element carbon • • • • • • 4A element Shares four electrons Forms four strong covalent bonds Bonds to other carbons to create chains and rings Not all carbon compounds are derived from living organisms Over 99% of 37 million known compounds contain carbon Common Elements in Carbon Compounds 2 1/19/2011 1.1 Atomic Structure: The Nucleus Structure of atom • • Nucleus • Positively charged • Made up of protons (positively charged) and neutrons (neutral) • Small (10-14 to 10-15 m in diameter) • Contains essentially all the mass of the atom Electron cloud • Negatively charged electrons in cloud around nucleus • Atomic diameter is about 2 Angstroms (Å) -10 m = 100 pm ; 1 pm = 10-12 m • 1 Å = 10 • Atomic diameter in SI units is 2 10-10 m (200 picometers (pm)) Atomic Structure: The Nucleus Atomic number (Z) • Number of protons in the atom's nucleus • All atoms of a given element have the same atomic number Mass number (A) • Number of protons plus neutrons in the atom’s nucleus Isotopes • Atoms of the same element (same Z) that have different Numbers of neutrons and therefore different mass numbers (different A) Atomic mass (atomic weight) • Weighted average mass in atomic mass units (amu) of an element’s naturally occurring isotopes 1.2 Atomic Structure: Orbitals Quantum Mechanical Model • Behavior of a specific electron in an atom described by mathematical expression called a wave equation • Wave equation is similar to mathematical expression used to describe motion of waves in fluids • Solution of wave equation is called a wave function • Wave function is an orbital • Orbital denoted by Greek letter psi, • Plot of 2 describes volume of space around nucleus that the electron is most likely to occupy • Electron cloud has no sharp boundary 3 1/19/2011 Atomic Structure: Orbitals Four different kinds of orbitals for electrons • Denoted s, p, d, and f • s and p orbitals most important in organic and biological chemistry • s orbitals • spherical, nucleus at center • p orbitals • dumbbell-shaped, nucleus at middle • d orbitals • four cloverleaf-shaped and one dumbbell-doughnut Atomic Structure: Orbitals Orbitals are occupied by zero, one, or two electrons and are grouped in electron shells of increasing size and energy Electron Shell • • • • A group of an atom’s electrons with the same principal quantum number First shell contains one s orbital, denoted 1s, which holds only two electrons Second shell contains four orbitals, one s orbital (2s) and three p orbitals (2p), which hold a total of eight electrons Third shell contains nine orbitals, one s orbital (3s), three p orbitals (3p), and five d orbitals (3d), which hold a total of 18 electrons Atomic Structure: Orbitals p Orbital • • • • In each shell, beginning with the second, there are three mutually perpendicular p orbitals, px, py, and pz, of equal energy Lobes of p orbitals are separated by region of zero electron density, a node Each lobe has a different algebraic sign, + and -, represented by different colors Algebraic signs are not charges 4 1/19/2011 1.3 Atomic Structure: Electron Configuration Ground-state electron configuration • Most stable, lowest-energy electron configuration of an atom Three rules: 1. 2. Aufbau principle • Lowest-energy orbitals fill first: 1s 2s 2p 3s 3p 4s 3d Pauli Exclusion Principle • Electron spin can have only two orientations, up and down • Only two electrons can occupy an orbital, and they must be of opposite spin to have unique wave equations Atomic Structure: Electron Configuration Hund's rule 3. • If two or more empty orbitals of equal energy are available, electrons occupy each orbital with parallel spins until all orbitals have one electron 1.4 Development of Chemical Bonding Theory • • • • • 1858 - August Kekulé and Archibald Couper independently proposed that carbon is tetravalent (always forms four bonds) Emil Erlenmeyer proposed a carbon-carbon triple bond for acetylene Alexander Crum Brown proposed a carbon-carbon double bond 1865 - Kekulé suggested that carbon chains can double back to form rings of atoms 1874 - Jacobus van’t Hoff and Joseph Le Bel proposed four atoms to which carbon is bonded sit at the corner of a regular tetrahedron 5 1/19/2011 Development of Chemical Bonding Theory Atoms bond because the compound that results is more stable and lower in energy than the separate atoms • • Energy is released from the chemical system when a bond forms Energy is consumed by the system when a bond breaks Valence shell • Outer most electron shell of an atom • Eight electrons in valence shell (an electron octet) impart special stability to noble-gas elements in 8A • Ne (2 + 8); Ar (2 + 8 + 8); Kr (2 + 8 + 18 + 8) • Chemistry of main group elements governed by their tendency to take on electron configuration of the nearest noble gas Development of Chemical Bonding Theory Ionic compounds • • • • Some elements achieve an octet configuration by gaining or losing electrons Ions form when an electron is gained or lost from a neutral atom Ions are charged because they have different numbers of protons and electrons Ions are held together by an electrostatic attraction, like in Na+ Cl-, forming an ionic bond Covalent compounds • Covalent Bond • Molecule • Carbon achieves an octet configuration by sharing electrons • • Bond formed by sharing electrons between atoms Neutral collection of atoms held together by covalent bonds Development of Chemical Bonding Theory Lewis structures (electron-dot structures) • • Dot representations of covalent bonds in molecules Valence shell electrons of an atom are represented as dots Kekulé structures (line-bond structures) • Two-electron covalent bond is represented by a line 6 1/19/2011 Development of Chemical Bonding Theory Number of covalent bonds depends on how many additional valence electrons needed to reach noble-gas configuration • • H (1s) needs one more electron to attain (1s2) and forms one bond N (2s22p3) needs three more electrons to attain (2s22p6) and forms three bonds Lone-pair electrons • Valence-shell electron pairs not used for bonding Worked Example 1.1 Predicting the Number of Bonds to Atoms in a Molecule How many hydrogen atoms does phosphorus bond to in phosphine, PH?? 1.5 The Nature of Chemical Bonds: Valence Bond Theory Valence bond theory • Bonding theory that describes a covalent bond as resulting from the overlap of two atomic orbitals • Electrons are paired in overlapping orbitals and are attracted to nuclei of both atoms, thus bonding the two atoms together • • • H–H bond results from the overlap of two singly occupied hydrogen 1s orbitals H-H bond is cylindrically symmetrical Bonds formed by head-on overlap of two atomic orbitals along a line drawn between the nuclei are sigma ( ) bonds 7 1/19/2011 The Nature of Chemical Bonds: Valence Bond Theory Bond strength • H2 molecule has 436 kJ/mol less energy than the starting 2 H• atoms, the product is more stable than the reactant and the H-H bond has a strength of 436 kJ/mol • Conversely, the bond dissociation energy of H2 is 436 kJ/mol because it requires 436 kJ/mol of energy to break the H2 bond The Nature of Chemical Bonds: Valence Bond Theory There is an optimum distance between nuclei that leads to maximum stability called the bond length Bond length • The distance between nuclei at the minimum energy point • Because a covalent bond is dynamic, like a spring, the characteristic bond length is the equilibrium distance between the nuclei of two atoms that are bonded to each other 1.6 sp3 Hybrid Orbitals and the Structure of Methane Carbon has four valence electrons (2s22p2) that form four bonds Methane CH4 • All four carbon-hydrogen bonds in methane are identical and are spatially oriented toward the corners of a regular tetrahedron sp3 hybrid orbitals • • A hybrid orbital derived from the combination of an s atomic orbital with three p atomic orbitals Linus Pauling (1931) showed mathematically how s orbitals and p orbitals on an atom can combine, hybridize, to form four equivalent atomic orbitals with tetrahedral orientation called sp3 hybrids 8 1/19/2011 sp3 Hybrid Orbitals and the Structure of Methane Two lobes of a p orbital have different algebraic signs (+ and -) in corresponding wave functions • When a p orbital hybridizes with an s orbital • Positive p lobe adds to s orbital generating larger lobe of sp3 hybrid • Negative p lobe subtracts from s orbital generating smaller lobe of sp3 hybrid • Resulting asymmetrical sp3 hybrid is strongly oriented in one direction • Larger lobe of sp3 hybrid can overlap more effectively with an orbital from another atom forming much stronger bonds than s or p orbitals sp3 Hybrid Orbitals and the Structure of Methane When each of the four identical sp3 hybrid orbitals of carbon overlaps with the 1s orbital of a hydrogen atom, four identical C-H bonds are formed and methane results • Each C-H bond in methane has a strength of 436 kJ/mol and a length of 109 pm • The four C-H bonds of methane have a specific geometry • The angle formed between two adjacent bonds describes a characteristic bond angle 1.7 sp3 Hybrid Orbitals and the Structure of Ethane Orbital hybridization accounts for the bonding together of carbon atoms into chains and rings Ethane C2H6 • Tetrahedral • Bond angles are near 109.5º • Carbon-carbon single bond • Formed by overlap of sp3 hybrids from each carbon • The remaining sp3 hybrids of each carbon overlap with 1s orbitals of three hydrogen atoms to form six carbonhydrogen bonds 9