William H. Brown & Christopher S. Foote

Transcription

1 William. Brown & Christopher S. Foote Requests for permission to make copies of any part of the work should be mailed to:permissions Department, arcourt Brace & Company, 6277 Sea arbor Drive, rlando, Florida

2 Covalent Bonding & Shapes of Molecules Chapter 1 1-2

3 rganic Chemistry u The study of the compounds of carbon u ver 10 million structures have been identified about 1000 new ones are identified each day! u 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,,, N, and some metals 1-3

4 Electronic Structure of Atoms u Structure of atoms small dense nucleus, diameter m, which contains most of the mass of the atom extranuclear space, diameter m, which contains positivelycharged electrons 1-4

5 Electronic Structure of Atoms u Electrons are confined to regions of space called principle energy levels (shells) each shell can hold 2n 2 electrons (n = 1,2,3,4...) Shell Number of Electrons Shell Can old Relative Energies of Electrons in These Shells higher lower 1-5

6 Electronic Structure of Atoms u Shells are divided into subshells called orbitals, which are designated by the letters s, p, d,... s (one per shell) p (set of three per shell 2 and higher) d (set of five per shell 3 and higher)... Shell rbitals Contained in That Shell 3 3s, 3p x, 3p y, 3p z, plus five 3d orbitals 2 2s, 2p x, 2p y, 2p z 1 1s 1-6

7 Electron Structure of Atoms u Aufbau Principle: orbitals fill from lowest to highest energy u Pauli Exclusion Principle: only two electrons per orbital u und s Rule: for a set of degenerate orbitals, add one electron in each before a second is added in any one Example: Write the ground-state electron configurations for (a) Li (b) (c) Cl 1-7

8 Lewis Structures u Gilbert N. Lewis u Valence shell: the outermost electron shell of an atom u Valence electrons: electrons in the valence shell of an atom; these electrons are used in forming chemical bonds u Lewis structure the symbol of the atom represents the nucleus and all inner shell electrons dots represent valence electrons 1-8

9 u Lewis Structures Table 1.4 gives the Lewis structures for elements 1-18 of the periodic table IA IIA IIIA IVA VA VIA VIIA VIIIA Li Be C N B F e Ne Na Mg Al Si P S Cl Ar 1-9

10 Formation of Chemical Bonds u Atoms bond together so that each atom acquires the electron configuration of the noble gas nearest it in the Periodic Table an atom that gains electrons becomes an anion an atom that loses electrons becomes a cation u Two extremes of bonding ionic bond: a chemical bond resulting from the electrostatic attraction of an anion and a cation covalent bond: a chemical bond resulting from two atoms sharing one or more pairs of electrons 1-10

11 Electronegativity u Electronegativity: a measure of the force of an atom s attraction for the electrons it shares in a chemical bond with another atom u Pauling scale increases left to right in a row increases bottom to top in a column + - Cl

12 Electronegativity Table 1.6 Classification of Chemical Bonds Difference in Electronegativity Between Bonded Atoms less than to 1.9 greater than 1.9 Type of Bond nonpolar covalent polar covalent ionic 1-12

13 Bond Dipoles Table 1.7 Average Bond Dipoles of Selected Covalent Bonds Bond Bond Dipole (D) Bond Bond Dipole (D) Bond Bond Dipole (D) -C -N - -S C-F C-Cl C-Br C-I C- C= C-N C=N

14 Lewis Structures u To write a Lewis structure determine the number of valence electrons determine the arrangement of atoms connect the atoms by single bonds arrange the remaining electrons so that each atom has a complete valence shell show bonding electrons as a single bond (a single line) show nonbonding electrons as a pair of dots 1-14

15 Lewis Structures 2 (8) Water Cl Cl ( 8) ydrogen chloride C N C 4 (8) Methane N 3 (8) Ammonia 1-15

16 Lewis Structures C C C C C 2 4 (12) Ethylene C 2 2 (10) Acetylene C Methanal C C 2 (12) 2 C 3 (24) Carbonic acid 1-16

17 Lewis Structures u In neutral molecules hydrogen has one bond carbon has 4 bonds and no unshared electrons nitrogen has 3 bonds and 1 unshared pair of electrons oxygen has 2 bonds and 2 unshared pairs of electrons halogens have 1 bond and 3 unshared pairs of electrons 1-17

18 Formal Charge u Formal charge: the charge on an atom in a molecule or polyatomic ion u To derive formal charge 1. write a correct Lewis structure for the molecule or ion 2. assign each atom all of its unshared (nonbonding) electrons and one-half its shared (bonding) electrons 3. compare this number with the number of valence electrons in the neutral, unbonded atom 1-18

19 Formal Charge number of valence electrons in the neutral, unbonded atom all unshared electrons + one half of all shared electrons If the number assigned to the bonded atom is less than that assigned to the unbonded atom, the atom has a positive formal charge If the number is greater, the atom has a negative formal charge 1-19

20 Formal Charge Example: Draw Lewis structures and show all formal charges for these molecules and ions N C 3 - C 3 N 3 + C 3 - C 3 C 2 - C C 3 2- BF

21 Exceptions to the ctet Rule u Molecules containing atoms of Group IIIA elements, particularly boron and aluminum F B F F Boron trifluoride 6 electrons in the valence shells of boron and aluminum Cl Al Cl Cl Aluminum chloride 1-21

22 Exceptions to the ctet Rule u Molecules and ions that contain an atom with more than eight electrons in its valence shell, including P, which may contain up to 10 electrons in its valence shell Cl Cl Cl C P 3 -P-C 3 --P-- C 3 Trimethylphosphine Cl Cl Phosphorus pentachloride - Phosphoric acid 1-22

23 Exceptions to the ctet Rule u and S, which may contain up to 12 electrons in its valence shell -S- C 3 -S-C 3 ydrogen sulfide Dimethyl sulfoxide --S-- Sulfuric acid 1-23

24 Functional Groups u Functional group: an atom or group of atoms within a molecule that shows a characteristic set of physical and chemical properties u 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 1-24

25 Functional Groups u Alcohols and Ethers -C-C-- An alcohol (Ethanol) -C--C- An ether (Dimethyl ether) 1-25

26 Functional Groups u Aldehydes and ketones C 3 -C- C 3 -C-C 3 An aldehyde A ketone 1-26

27 Functional Groups u Carboxylic acids C 3 -C-- or C 3 C or C 3 C 2 Acetic acid 1-27

28 VSEPR Model 4 regions of e - density (tetrahedral, ) 3 regions of e - density (trigonal planar, 120 ) C C N C N C C 2 regions of e - density (linear, 180 ) C C C 1-28

29 VSEPR Model Example: predict all bond angles for these molecules and ions N 4 + C 3 N 2 C 3 C 3 C=C 2 2 C 3 C 3 - C 3 C C 3 C 2 BF

30 Polar and Nonpolar Molecules u To determine if a molecule is polar, we need to determine if the molecule has polar bonds the arrangement of these bonds in space u Dipole moment ( ): the vector sum of its individual bond dipoles 1-30

31 Polar and Nonpolar Molecules u These molecules have polar bonds, but each has a zero dipole moment C Carbon dioxide = 0 D F F B F Boron trifluoride = 0 D Cl C Cl Cl Cl Carbon tetrachloride = 0 D 1-31

32 Polar and Nonpolar Molecules u These molecules have polar bonds, and a dipole moment greater than zero direction of dipole moment in water Water = 1.85D N Ammonia = 1.47D direction of dipole moment in ammonia 1-32

33 Resonance u For many molecules and ions, no single Lewis structure provides a truly accurate representation - C 3 C and C 3 C - Ethanoate ion (Acetate ion) 1-33

34 Resonance u Linus Pauling s many molecules and ions are best described by writing two or more Lewis structures individual Lewis structures are called contributing structures connect individual contributing structures by a doubleheaded (resonance) arrow the molecule or ion is a hybrid of the various contributing structures 1-34

35 Resonance Examples: Nitrite ion (equivalent contributing structures) Ethanoate ion (equivalent contributing structures) C 3 N C - - N C 3 C

36 Resonance u Curved arrow: a symbol used to show the redistribution of valence electrons u In using curved arrows, there are only two allowed types of electron redistribution: from a bond to an adjacent atom from an atom to an adjacent bond u Electron pushing is a survival skill in organic chemistry. Learn it well! 1-36

37 Resonance u All acceptable contributing structures must 1. have the same number of valence electrons 2. obey the rules of covalent bonding no more than 2 electrons in the valence shell of no more than 8 electrons in valence shell of 2nd period elements 3rd period elements may have up to 12 electrons in their valence shells 3. differ only in distribution of valence electrons 4. have the same number of paired and unpaired electrons 1-37

38 Resonance u Examples of ions and a molecule best represented as resonance hybrids carbonate ion C 3 2- acetate ion acetone nitrate ion C 3 C 2 - C 3 CC 3 N

39 Resonance u Preference 1: structures with filled valence shells contribute more than those with unfilled valence shells C C C3 C Greater contribution; both carbon and oxygen have complete valence shells Lesser contribution; carbon has only 6 electrons in its valence shell 1-39

40 Resonance u Preference 2: structures with a greater number of covalent bonds contribute more than those with fewer covalent bonds C C C3 C Greater contribution (8 covalent bonds) Lesser contribution (7 covalent bonds) 1-40

41 Resonance u Preference 3: structures with separation of unlike charges contribute less than those with no charge separation C 3 -C-C 3 Greater contribution (no separation of unlike charges) - C 3 -C-C 3 Lesser contribution (separation of unlike charges) 1-41

42 Resonance u Preference 4: structures that carry a negative charge on the more electronegative atom contribute more than those with the negative charge on the less electronegative atom + C 3 C C 3 Lesser contribution C 3 C C 3 Greater contribution + C 3 C C 3 Can be ignored 1-42

43 Quantum or Wave Mechanics u A. Einstein: E=hv (energy is quantized) light has particle properties u L. debroglie: wave / particle duality u E. Schrödinger wave equation 2 is the probability of finding an electron in a given region of space 95% probability region is called an atomic orbital 1-43

44 Molecular rbital Theory u Electrons in atoms exist in atomic orbitals u Electrons in molecules exist in molecular orbitals (Ms) u Using Schrödinger equation, we can calculate the shapes and energies of Ms 1-44

45 Molecular rbital Theory u Rules: combination of n atomic orbitals gives n M Ms are arranged in order of increasing energy Ms fill by same rules as for atomic orbitals: Aufbau principle: fill beginning with LUM Pauli exclusion principle: no more than 2 e - in a M und s rule: filling of degenerate orbitals 1-45

46 Molecular rbital Theory u Terminology ground state = lowest energy excited state = NT lowest energy = sigma bonding M * = sigma antibonding M = pi bonding M * = pi antibonding M 1-46

47 ybrid rbitals u Shapes of Atomic rbitals (s,p,d,f...) u Wave function may be positive or negative or zero at a nodal surface an s orbital u ybridization of orbitals (L. Pauling) the combination of two or more atomic orbitals to form a new set of atomic orbitals, called hybrid atomic orbitals z y x z y a p z orbital x 1-47

48 ybrid rbitals u The Problem: 2s and 2p atomic orbitals would give bond angles of approximately 90 instead we observe approximately 109.5,120, and 180 u A Solution hybridization of atomic orbitals 2nd row elements use sp 3, sp 2, and sp hybrid orbitals for bonding 1-48

49 ybrid rbitals u We deal with three types of hybrid atomic orbitals sp 3 sp 2 sp (1 s orbital + 3 p orbitals) (1 s orbital + 2 p orbitals) (1 s orbital + 1 p orbital) u verlap of hybrid atomic orbitals can form two types of bonds, depending on the geometry of the overlap bonds are formed by direct overlap bonds are formed by parallel overlay 1-49

50 sp 3 ybrid rbitals each sp 3 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 sp 3 hybrid orbitals are directed toward the corners of a regular tetrahedron at angles of

51 sp 2 ybrid rbitals each sp 2 hybrid orbital has two lobes of unequal size unhybridized 2p orbital the three sp 2 hybrid orbitals are directed toward the corners of an equilateral triangle at angles of 120 sp 2 hybrid orbitals the unhybridized 2p orbital is perpendicular to the plane of the sp 2 hybrid orbitals 1-51

52 sp ybrid rbitals each sp 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 z unhybridized 2p orbitals lie on the y and z axes y x the two sp hybrid orbitals lie in a line at an angle of 180 sp hybrid orbitals the two unhybridized 2p orbitals are perpendicular to each other and to the line through the two sp-hybrid orbitals 1-52

53 ybrid rbitals ybridization Types of Bonds to Carbon Example Name sp 3 four sigma bonds -C-C- Ethane sp 2 sp three sigma bonds and one pi bond two sigma bonds and two pi bonds C C -C C- Ethylene Acetylene 1-53

54 End Chapter