Chemical Bonding and Molecular Structure (Chapter 9) Ionic vs. covalent bonding Molecular orbitals and the covalent bond (Ch. 10) Valence electron Lewis dot structures octet vs. non-octet resonance structures formal charges VSEPR - predicting shapes of molecules Bond properties electronegativity polarity, bond order, bond strength 20 Oct 97 Bonding and structure (2)

Rules for making Lewis dot structures 1. Count no. of valence electrons (- don’t forget to include the charge on molecular ions!) — 2 for # of PAIRS 2. Place a bond pair (BP) between connected atoms 3. Complete octets by using rest of e- as lone pairs (LP) 4. For atoms with <8 e-, make multiple bonds to complete octets 5. Assign formal charges : fc = Z - (#BP/2) - (#LP) Indicate equivalent (RESONANCE) structures 6. Structures with smaller formal charges are preferred - consider non-octet alternatives (esp. for 3rd, 4th row) OCTET RULE: #Bond Pairs + #Lone Pairs = 4 (except for H and atoms of 3rd and higher periods) #lone pairs at central atom in AXn = {(#e-) - 8*n}/2 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Sulfur Dioxide, SO2 Rules 1-3  O—S —O These equivalent structures are called: + — RESONANCE STRUCTURES. The proper Lewis structure is a HYBRID of the two. Each atom has OCTET . . . . . BUT there is a +1 and -1 formal charge 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) SO2 (2) Alternate Lewis structure for SO2 uses 2 double bonds Sulfur does not obey OCTET rule BUT the formal charge = 0 O = S = O This is better structure than O=S+-O- since it reduces formal charge (rule 6). 3rd row S atom can have 5 or 6 electron pairs NB: # of central atom lone pairs = (3*6 -8*2)/2 = 1 in both O=S+-O- and O=S=O structures 20 Oct 97 Bonding and structure (2)

Thiocyanate ion, (SCN)- Which of three possible resonance structures is most important? A. S=C=N B. S=C - N C. S-C N -0.52 -0.32 -0.16 Calculated partial charges ANSWER: C > A > B 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) MOLECULAR GEOMETRY Molecule adopts the shape that minimizes the electron pair repulsions. VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs. 6_VSEPR.mov 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) No. of e- Pairs Around Central Atom CAChe image Example Geometry 180o linear 2 F—Be—F F 120o planar trigonal B 3 H tetrahedral 109o C 4 20 Oct 97 Bonding and structure (2)

Structure Determination by VSEPR Ammonia, NH3 There are 4 electron pairs at the corners of a tetrahedron. H N lone pair of electrons in tetrahedral position The ELECTRON PAIR GEOMETRY is tetrahedral. 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) VSEPR - ammonia Ammonia, NH3 H N lone pair of electrons in tetrahedral position Although the electron pair geometry is tetrahedral . . . . . . the MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL. 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) AXnEm notation a good way to distinguish between electron pair and molecular geometries is the AXnEm notation where: A - atom whose local geometry is of interest (typically the CENTRAL ATOM) Xn - n atoms bonded to A Em - m lone pair electrons at A NH3 is AX3E system  pyramidal (NB this notation not used by Kotz) 20 Oct 97 Bonding and structure (2)

1. Draw electron dot structure VSEPR - water Water, H2O H - O - H •• 1. Draw electron dot structure 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. The electron pair geometry is TETRAHEDRAL. 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) VSEPR - water (2) H - O - H •• Although the electron pair geometry is TETRAHEDRAL . . . . . . the molecular geometry is bent. H2O - AX2E2 system - angular geometry 20 Oct 97 Bonding and structure (2)

1. Draw electron dot structure Formaldehyde, CH2O VSEPR - formaldehyde • C H O 1. Draw electron dot structure 2. Count BP’s and LP’s: At Carbon there are 4 BP but . . . 3. These are distributed in ONLY 3 regions. Double bond electron pairs are in same region. There are 3 regions of electron density Electron repulsion places them at the corners of a planar triangle. • C H O Both the electron pair geometry and the molecular geometry are PLANAR TRIGONAL  120o bond angles. H2CO at the C atom is an AX3 species 20 Oct 97 Bonding and structure (2)

109o because both the C and O VSEPR - Bond Angles Methanol, CH3OH H •• Define bond angles 1 and 2 Angle 1 = H-C-H = ? Angle 2 = H-O-C = ? Answer: H—C—O—H •• Angle 1 H Angle 2 109o because both the C and O atoms are surrounded by 4 electron pairs. 6_CH3OH.mov AXnEm designation ? at C at O AX4 = tetrahedral AX2E2 = bent 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) VSEPR - bond angles (2) Acetonitrile, CH3CN H 1 H—C—C 2 •• N Define bond angles 1 and 2 Angle 1 = ? 109o Angle 2 = ? 180o Why ? : The CH3 carbon is surrounded by 4 bond charges The CN carbon is surrounded by 2 bond charges AXnEm designation ? at CH3 carbon at CN carbon AX4 = tetrahedral AX2 = linear 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) What about: STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE ? PF5 BF3 SF4 20 Oct 97 Bonding and structure (2)

Geometry for non-octet species also obey VSEPR rules Consider boron trifluoride, BF3 The B atom is surrounded by only 3 electron pairs. Bond angles are 120o Molecular Geometry is planar trigonal BF3 is an AX3 species 20 Oct 97 Bonding and structure (2)

Compounds with 5 or 6 Pairs Around the Central Atom 6_VSEPR.mov F Trigonal bipyramid 120° 90° P 5 electron pairs AX5 system F Octahedron 90° S 6 electron pairs AX6 system 20 Oct 97 Bonding and structure (2)

Sulfur Tetrafluoride, SF4 • •• S Number of valence e- = 34 No. of S lone pairs = {17 - 4 b.p. - 3x4 l.p.(F)} = 1 lone pair on S There are 5 (BP + LP) e- pairs around the S THEREFORE: electron pair geometry ? F • S F • S = trigonal bipyramid OR AX4E system. Molecular geometry ? 20 Oct 97 Bonding and structure (2)

Sulfur Tetrafluoride, SF4 (2) 90° axial F • S 120° equatorial Lone pair is in the equatorial position because it requires more room than a bond pair. Molecular geometry of SF4 is “see-saw” Q: What is molecular geometry of SO2 ? 20 Oct 97 Bonding and structure (2)

Bonding with Hybrid Atomic Orbitals - Carbon prefers to make 4 bonds as in CH4 But atomic carbon has an s2p2 configuration Why can it make more than 2 bonds ? 6_CH4.mov 4 C atom orbitals hybridize to form four equivalent sp3 hybrid atomic orbitals. 20 Oct 97 Bonding and structure (2)

Orbital Hybridization BONDS SHAPE HYBRID REMAIN e.g. s2p2  2 linear {2 x sp & 2 p’s} C2H2 3 trigonal {3 x sp2 & 1 p} C2H4 planar 4 tetrahedral {4 xsp3 } CH4 20 Oct 97 Bonding and structure (2)

Multiple Bonds s and p Bonding in C2H4 ­ ­¯ 2p 2s ­ 3 sp2 hybrid orbitals p orbital C atom orbitals are COMBINED (= re-hybridized) to form orbitals better suited for BONDING The extra p orbital electron on each C atom overlaps the p orbital on the neighboring atom to form the p bond. The 3 sp2 hybrid orbitals are used to make the C-C and two C-H  bonds H C sp2 120° 6_C2H4-sg.mov 6_C2H4-pi.mov 6_C2H4.mov 20 Oct 97 Bonding and structure (2)

Consequences of Multiple Bonding Restricted rotation around C=C bond in 1-butene = CH2=CH-CH2-CH3. See Butene.Map in ENER_MAP in CAChe models. 27 233 E (kJ/mol) -180 0 180 C-C=C angle (o) P. 475 - Photo-rotation about double bonds lets us see !! 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Properties What is the effect of bonding and structure on molecular properties ? - bond order - bond length - bond strength - bond polarity - MOLECULAR polarity Buckyball in HIV-protease, see page 107 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Order the number of bonds between a pair of atoms. H C N triple, BO = 3 1 s and 2 p CH2CHCN Acrylonitrile single BO = 1 1 s double, BO = 2 1 s and 1 p 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Order (2) Fractional bond orders occur in molecules with resonance structures. Consider NO2- Bond order = Total # of e - pairs used for a type of bond of bonds of that type Bond order in NO2- = 3 (e - pairs in N-O bonds) 2 (N - O bonds) N-O bond order in NO2- = 1.5 20 Oct 97 Bonding and structure (2)

Bond Order and Bond Length Bond order is related to two important bond properties: (a) bond strength as given by DE 745 kJ 414 kJ 123 pm 110 pm Formaldehye (b) Bond length - the distance between the nuclei of two bonded atoms. 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Length - depends on size of bonded atoms: Molecule R(H-X) H- F 104 pm H- Cl 131 pm H- I 165 pm - depends on bond order. Molecule R(C-O) CH3C- OH 141 pm O=C=O 132 pm C O 119 pm 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Strength Bond Dissociation energy (DE) - energy required to break a bond in gas phase. See Table 9.5 BOND STRENGTH (kJ/mol) LENGTH (pm) H—H 436 74 C—C 347 154 C=C 611 134 CºC 837 121 NºN 946 110 The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond. 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Strength (2) Bond Order Length Strength HO—OH 1 149 pm 210 kJ/mol O=O 2 121 498 kJ/mol 1.5 128 ? 303 kJ/mol O3 (g)  3 O(g) HOW TO CALCULATE ? Hrxn = {3xHf(O) - Hf(O3)} = {3x249.2 - 142.7} = 605 kJ/mol 2 O-O bonds in O3  DE (O3) = 605/2 = 302.5 kJ/mol 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Polarity HCl is POLAR because it has a positive end and a negative end (partly ionic). Polarity arises because Cl has a greater share of the bonding electrons than H. Calculated charge by CAChe: H (red) is +ve (+0.20 e-) Cl (yellow) is -ve (-0.20 e-). (See PARTCHRG folder in MODELS.) 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Bond Polarity (2) Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond. BOND ENERGY “pure” bond 339 kJ/mol calculated real bond 432 kJ/mol measured Difference 92 kJ/mol. This difference is the contribution of IONIC bonding It is proportional to the difference in ELECTRONEGATIVITY, c. 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Electronegativity, c c is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling (1901-94) Nobel prizes: Chemistry (54), Peace (63) See p. 425; 008vd3.mov (CD) 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Electronegativity, c Figure 9.7 F has maximum c. Atom with lowest c is the center atom in most molecules. Relative values of c determines BOND POLARITY (and point of attack on a molecule). 20 Oct 97 Bonding and structure (2)

Therefore OH is more polar than OF Bond Polarity Which bond is more polar ? (has larger bond DIPOLE) O—H O—F c H 2.1 O F 3.5 4.0 c(A) - c(B) 3.5 - 2.1 Dc 1.4 3.5 - 4.0 0.5 (O-H) > (O-F) Therefore OH is more polar than OF Also note that polarity is “reversed.” 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Molecular Polarity Molecules—such as HCl and H2O— can be POLAR (or dipolar). They have a DIPOLE MOMENT. Polar molecules turn to align their dipole with an electric field. 20 Oct 97 Bonding and structure (2)

Predicting molecular polarity A molecule will be polar ONLY if a) it contains polar bonds AND b) the molecule is NOT “symmetric” Symmetric molecules 20 Oct 97 Bonding and structure (2)

Molecular Polarity: H2O Water is polar because: a) O-H bond is polar b) water is non-symmetric The dipole associated with polar H2O is the basis for absorption of microwaves used in cooking with a microwave oven 20 Oct 97 Bonding and structure (2)

Bonding and structure (2) Carbon Dioxide CO2 is NOT polar even though the CO bonds are polar. Because CO2 is symmetrical the BOND polarity cancels -0.73 +1.46 -0.73 The positive C atom is why water attaches to CO2 CO2 + H2O  H2CO3 20 Oct 97 Bonding and structure (2)

NON-symmetric molecules Molecular Polarity in NON-symmetric molecules B—F bonds are polar molecule is symmetric B—F, B—H bonds polar molecule is NOT symmetric Atom Chg.  B +ve 2.0 H +ve 2.1 F -ve 4.0 B +ve F -ve BF3 is NOT polar HBF2 is polar 20 Oct 97 Bonding and structure (2)

Fluorine-substituted Ethylene: C2H2F2 C—F bonds are MUCH more polar than C—H bonds. (C-F) = 1.5, (C-H) = 0.4 CIS isomer both C—F bonds on same side  molecule is POLAR. TRANS isomer both C—F bonds on opposite side  molecule is NOT POLAR. 20 Oct 97 Bonding and structure (2)

Chemical Bonding and Molecular Structure (Chapter 9) Ionic vs. covalent bonding Molecular orbitals and the covalent bond (Ch. 10) Valence electron Lewis dot structures octet vs. non-octet resonance structures formal charges VSEPR - predicting shapes of molecules Bond properties electronegativity polarity, bond order, bond strength 20 Oct 97 Bonding and structure (2)