Organic chemistry (2)

Organic Chemistry William H Brown Christopher S Foote Brent L Iverson 1-1 Covalent Bonding & Shapes of Molecules Chapter 1-2 Organic Chemistry The study of the compounds of carbon Over 10 million compounds have been identified • about 1000 new ones are identified each day! C is a small atom • it forms single, double, and triple bonds • it is intermediate in electronegativity (2.5) • it forms strong bonds with C, H, O, N, and some metals 1-3 Schematic View of an Atom • a small dense nucleus, diameter 10-14 - 10-15 m, which contains positively charged protons and most of the mass of the atom • an extranuclear space, diameter 10-10 m, which contains negatively charged electrons 1-4 Electron Configuration of Atoms Electrons are confined to regions of space called principle energy levels (shells) • each shell can hold 2n2 electrons (n = 1,2,3,4 ) N u mb er of Relative En ergies Electrons S hell of Electrons S hell Can Hold in Thes e Shells h igh er 32 18 low er 1-5 Electron Configuration of Atoms Shells are divided into subshells called orbitals, which are designated by the letters s, p, d, f, • s (one per shell) • p (set of three per shell and higher) • d (set of five per shell and higher) S hell O rb itals Contain ed in Th at S hell 3s , 3p x , 3p y , 3p z, p lu s five 3d orbitals 2s , 2p x , 2p y , 2p z 1s 1-6 Electron Configuration of Atoms Aufbau Principle: • orbitals fill in order of increasing energy from lowest energy to highest energy Pauli Exclusion Principle: • only two electrons can occupy an orbital and their spins must be paired Hund’s Rule: • when orbitals of equal energy are available but there are not enough electrons to fill all of them, one electron is added to each orbital before a second electron is added to any one of them 1-7 Electron Configuration of Atoms The pairing of electron spins 1-8 Electron Configuration of Atoms Table 1.3 The Ground-State Electron Configuration of Elements 1-18 1-9 Lewis Dot Structures Gilbert N Lewis Valence shell: • the outermost occupied electron shell of an atom Valence electrons: • electrons in the valence shell of an atom; these electrons are used to form chemical bonds and in chemical reactions Lewis dot structure: • the symbol of an element represents the nucleus and all inner shell electrons • dots represent valence electrons 1-10 Molecular Orbitals pi bonding and antibonding MOs 1-60 Molecular Orbitals • computed pi bonding and antibonding MOs for ethylene 1-61 Molecular Orbitals • computed pi bonding and antibonding orbitals for formaldehyde 1-62 Hybrid Orbitals The Problem: • bonding by 2s and 2p atomic orbitals would give bond angles of approximately 90° • instead we observe bond angles of approximately 109.5°, 120°, and 180° A Solution • hybridization of atomic orbitals • 2nd row elements use sp3, sp2, and sp hybrid orbitals for bonding 1-63 Hybrid Orbitals Hybridization of orbitals (L Pauling) • the combination of two or more atomic orbitals forms a new set of atomic orbitals, called hybrid orbitals We deal with three types of hybrid orbitals sp3 (one s orbital + three p orbitals) sp2 (one s orbital + two p orbitals) sp (one s orbital + one p orbital) Overlap of hybrid orbitals can form two types of bonds depending on the geometry of overlap σ bonds are formed by “direct” overlap π bonds are formed by “parallel” overlap 1-64 sp3 Hybrid Orbitals • each sp3 hybrid orbital has two lobes of unequal size • the sign of the wave function is positive in one lobe, negative in the other, and zero at the nucleus • the four sp3 hybrid orbitals are directed toward the corners of a regular tetrahedron at angles of 109.5° 1-65 sp3 Hybrid Orbitals • orbital overlap pictures of methane, ammonia, and water 1-66 sp2 Hybrid Orbitals • the axes of the three sp2 hybrid orbitals lie in a plane and are directed toward the corners of an equilateral triangle • the unhybridized 2p orbital lies perpendicular to the plane of the three hybrid orbitals 1-67 Bonding in Ethylene 1-68 Bonding in Formaldehyde 1-69 sp Hybrid Orbitals • two lobes of unequal size at an angle of 180° • the unhybridized 2p orbitals are perpendicular to each other and to the line created by the axes of the two sp hybrid orbitals 1-70 Bonding in Acetylene, C2H2 1-71 Hybrid Orbitals Group s Orb ital Predicted Bond ed Hyb rid Bond to Carb on ization A ngles Types of Bond s to Carbon Examp le N ame HH sp 109.5° sigma b on ds H-C-C-H Ethan e HH 2 sp sp 120° 180° sigma b on ds and p i bond sigma b on ds and p i bonds H H C H C Eth ylen e H H-C C-H Acetylene 1-72 Bond Lengths and Bond Strengths Orb ital Overlap Bond Length (pm) Bon d Strength [k J (k cal)/mol] N ame Formula Bond Ethan e HH H-C-C-H C-C sp -sp 153.2 376 (90) C-H sp -1s 111.4 422 (101) H C-C sp 2-sp 2, 2p-2p 133.9 727 (174) H C-H sp 2-1s 110.0 464(111) C-C C-H sp-sp, tw o 2p -2p 121.2 109.0 966 (231) 556 (133) HH H Ethylene C C H Acetylene H-C C-H sp-1s 1-73 Covalent Bonds & Shapes of Molecules End Chapter 1-74 [...]... shows a characteristic set of physical and chemical properties Functional groups are important for three reason; they are 1 the units by which we divide organic compounds into classes 2 the sites of characteristic chemical reactions 3 the basis for naming organic compounds 1-26 Alcohols contain an -OH (hydroxyl) hydroxyl group : -C-O-H : Fu nctional group H H H-C-C-O-H H H Ethan ol (an alcohol) Ethanol