1、1. Structure and Bonding,Based on McMurrys Organic Chemistry, 6th edition, Chapter 1,Organic Chemistry,“Organic” until mid 1800s referred to compounds from living sources (mineral sources were“inorganic”) Whler in 1828 showed that urea, an organic compound, could be made from a minerals Today, organ

2、ic compounds are those based on carbon structures and organic chemistry studies their structures and reactions Includes biological molecules, drugs, solvents, dyes Does not include metal salts and materials (inorganic) Does not include materials of large repeating molecules without sequences (polyme

3、rs,1.1 Atomic Structure,Structure of an atom Positively charged nucleus (very dense, protons and neutrons) and smal (10-15 m) Negatively charged electrons are in a cloud (10-10 m) around nucleus Diameter is about 2 10-10 m (200 picometers (pm) the unit angstrom () is 10-10 m = 100 pm,Atomic Number a

4、nd Atomic Mass,The atomic number (Z) is the number of protons in the atoms nucleus The mass number (A) is the number of protons plus neutrons All the atoms of a given element have the same atomic number Isotopes are atoms of the same element that have different numbers of neutrons and therefore diff

5、erent mass numbers The atomic mass (atomic weight) of an element is the weighted average mass in atomic mass units (amu) of an element抯 naturally occurring isotopes,1.2 Atomic Structure: Orbitals,Quantum mechanics: describes electron energies and locations by a wave equation Wave function solution o

6、f wave equation Each Wave function is an orbital, A plot of 2 describes where electron most likely to be Electron cloud has no specific boundary so we show most probable area,Shapes of Atomic Orbitals for Electrons,Four different kinds of orbitals for electrons based on those derived for a hydrogen

7、atom Denoted s, p, d, and f s and p orbitals most important in organic chemistry s orbitals: spherical, nucleus at center p orbitals: dumbbell-shaped, nucleus at middle,Orbitals and Shells,Orbitals are grouped in shells of increasing size and energy Different shells contain different numbers and kin

8、ds of orbitals Each orbital can be occupied by two electrons First shell contains one s orbital, denoted 1s, holds only two electrons Second shell contains one s orbital (2s) and three p orbitals (2p), eight electrons Third shell contains an s orbital,p-Orbitals,In each shell there are three perpend

9、icular p orbitals, px, py, and pz, of equal energy Lobes of a p orbital are separated by region of zero electron density, A node,1.3 Atomic Structure: Electron Configurations,Ground-state electron configuration of an atom lists orbitals occupied by its electrons. Rules: 1. Lowest-energy orbitals fil

10、l first: 1s 2s 2p 3s 3p 4s 3d (Aufbau (“build-up”) principle) 2. Electron spin can have only two orientations, up and down . Only two electrons can occupy an orbital, and they must be of opposite spin (Pauli exclusion principle) to have unique wave equations 3. If two or more empty orbitals of equal

11、 energy are available, electrons occupy each with spins parallel until all orbitals have one electron (Hunds rule).,1.4 Development of Chemical Bonding Theory,Kekul and Couper independently observed that carbon always has four bonds vant Hoff and Le Bel proposed that the four bonds of carbon have sp

12、ecific spatial directions Atoms surround carbon as corners of a Tetrahedron,1.5 The Nature of the Chemical Bond,Atoms form bonds because the compound that results is more stable than the separate atoms Ionic bonds in salts form as a result of electron transfers Organic compounds have covalent bonds

13、from sharing electrons (G. N. Lewis, 1916) Lewis structures shown valence electrons of an atom as dots Hydrogen has one dot, representing its 1s electron Carbon has four dots (2s2 2p2) Stable molecule results at completed shell, octet (eight dots) for main-group atoms (two for hydrogen),Number of Co

14、valent Bonds to an Atom,Atoms with one, two, or three valence electrons form one, two, or three bonds Atoms with four or more valence electrons form as many bonds as they need electrons to fill the s and p levels of their valence shells to reach a stable octet,Valences of Carbon,Carbon has four vale

15、nce electrons (2s2 2p2), forming four bonds (CH4),Valences of Oxygen,Oxygen has six valence electrons (2s2 2p4) but forms two bonds (H2O),Valences of Nitrogen,Nitrogen has five valence electrons (2s2 2p3) but forms only three bonds (NH3),Non-bonding electrons,Valence electrons not used in bonding ar

16、e called nonbonding electrons, or lone-pair electrons Nitrogen atom in ammonia (NH3) Shares six valence electrons in three covalent bonds and remaining two valence electrons are nonbonding lone pair,1.6 Valence Bond Theory,Covalent bond forms when two atoms approach each other closely so that a sing

17、ly occupied orbital on one atom overlaps a singly occupied orbital on the other atom Electrons are paired in the overlapping orbitals and are attracted to nuclei of both atoms HH bond results from the overlap of two singly occupied hydrogen 1s orbitals H-H bond is cylindrically symmetrical, sigma ()

18、 bond,Bond Energy,Reaction 2 H H2 releases 436 kJ/mol Product has 436 kJ/mol less energy than two atoms: HH has bond strength of 436 kJ/mol. (1 kJ = 0.2390 kcal; 1 kcal = 4.184 kJ),Bond Length,Distance between nuclei that leads to maximum stability If too close, they repel because both are positivel

19、y charged If too far apart, bonding is weak,1.7 Hybridization: sp3 Orbitals and theStructure of Methane,Carbon has 4 valence electrons (2s2 2p2) In CH4, all CH bonds are identical (tetrahedral) sp3 hybrid orbitals: s orbital and three p orbitals combine to form four equivalent, unsymmetrical, tetrah

20、edral orbitals (sppp = sp3), Pauling (1931),Tetrahedral Structure of Methane,sp3 orbitals on C overlap with 1s orbitals on 4 H atom to form four identical C-H bonds Each CH bond has a strength of 438 kJ/mol and length of 110 pm Bond angle: each HCH is 109.5, the tetrahedral angle.,1.8 Hybridization:

21、 sp3 Orbitals and the Structure of Ethane,Two Cs bond to each other by overlap of an sp3 orbital from each Three sp3 orbitals on each C overlap with H 1s orbitals to form six CH bonds CH bond strength in ethane 420 kJ/mol CC bond is 154 pm long and strength is 376 kJ/mol All bond angles of ethane ar

22、e tetrahedral,1.9 Hybridization: sp2 Orbitals and the Structure of Ethylene,sp2 hybrid orbitals: 2s orbital combines with two 2p orbitals, giving 3 orbitals (spp = sp2) sp2 orbitals are in a plane with 120 angles Remaining p orbital is perpendicular to the plane,Bonds From sp2 Hybrid Orbitals,Two sp

23、2-hybridized orbitals overlap to form a bond p orbitals overlap side-to-side to formation a pi () bond sp2sp2 bond and 2p2p bond result in sharing four electrons and formation of C-C double bond Electrons in the bond are centered between nuclei Electrons in the bond occupy regions are on either side

24、 of a line between nuclei,Structure of Ethylene,H atoms form bonds with four sp2 orbitals HCH and HCC bond angles of about 120 CC double bond in ethylene shorter and stronger than single bond in ethane Ethylene C=C bond length 133 pm (CC 154 pm),1.10 Hybridization: sp Orbitals and the Structure of A

25、cetylene,C-C a triple bond sharing six electrons Carbon 2s orbital hybridizes with a single p orbital iving two sp hybrids two p orbitals remain unchanged sp orbitals are linear, 180 apart on x-axis Two p orbitals are perpendicular on the y-axis and the z-axis,Orbitals of Acetylene,Two sp hybrid orb

26、itals from each C form spsp bond pz orbitals from each C form a pzpz bond by sideways overlap and py orbitals overlap similarly,Bonding in Acetylene,Sharing of six electrons forms C C Two sp orbitals form bonds with hydrogens,1.11 Hybridization of Nitrogen and Oxygen,Elements other than C can have h

27、ybridized orbitals HNH bond angle in ammonia (NH3) 107.3 Ns orbitals (sppp) hybridize to form four sp3 orbitals One sp3 orbital is occupied by two nonbonding electrons, and three sp3 orbitals have one electron each, forming bonds to H,Hybridization of Oxygen in Water,The oxygen atom is sp3-hybridize

28、d Oxygen has six valence-shell electrons but forms only two covalent bonds, leaving two lone pairs The HOH bond angle is 104.5,1.12 Molecular Orbital Theory,A molecular orbital (MO): where electrons are most likely to be found (specific energy and general shape) in a molecule Additive combination (b

29、onding) MO is lower in energy Subtractive combination (antibonding) forms MO is higher,Molecular Orbitals in Ethylene,The bonding MO is from combining p orbital lobes with the same algebraic sign The antibonding MO is from combining lobes with opposite signs Only bonding MO is occupied,Summary,Organ

30、ic chemistry chemistry of carbon compounds Atom: positively charged nucleus surrounded by negatively charged electrons Electronic structure of an atom described by wave equation Electrons occupy orbitals around the nucleus. Different orbitals have different energy levels and different shapes s orbitals are sphe