Chemical_Bonding_-_Lecture_2

Chemical_Bonding_L - Chapter Chapter 8 Chemical Bonding Dipole Moments for Diatomics Molecule Molecule H2 CO HCl NaCl NaF µ(D 0 0.112 1.109 9.001

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Unformatted text preview: Chapter Chapter 8 Chemical Bonding Dipole Moments for Diatomics Molecule Molecule H2 CO HCl NaCl NaF µ(D) 0 0.112 1.109 9.001 8.156 % ionic character 0 2 18 79 88 We now have two measures of ionic character, the electronegativity electronegativity and the dipole moment. dipole Ionic Ionic Character and EN Place Place the following bonds in increasing ionic character (decreasing covalent character): O—H, P—N, As—O, C—H, C—Br, C—P P— As— C— C— C— EN EN of O = 3.5 and of H = 2.1: EN of C = 2.5 and of P = 2.1 : EN of P = 2.1 and of N = 3.0: EN EN of C = 2.5 and of Br = 2.8: EN of As = 2.0 and of O = 3.5: ∆EN = 1.4 ∆EN = 0.4 ∆EN = 0.9 ∆EN = 0.3 ∆EN = 1.5 C—Br < C—P < P—N< O—H < C—H < As—O As— ∆EN: 0.3 < 0.4 < 0.9 < 1.4 < 1.4 < 1.5 0.4 Molecular Molecular Shape and Polarity Molecular Molecular Shape and Polarity Depicting Depicting Molecules and Ions with Lewis Lewis Structures Using Using the Octet Rule The The Octet Rule: When atoms bond, they lose, gain or share electrons to attain a filled outer shell of 8 (or 2) electrons 1) Only the valence electrons appear in a Lewis structure. valence 2) The line joining two atoms represents a pair of electrons shared between two atoms. single bond - two shared electrons, one line double bond - four shared electrons, two lines triple bond - six shared electrons, three lines 3) Dots placed next to an atom represent non-bonding nonelectrons. electrons. Depicting Depicting Molecules and Ions with Lewis Structures 1 2 Example: Formula NF3 EN (F) > EN (N), ∴ N is central atom F FNF Depicting Depicting Molecules and Ions with Lewis Structures F FNF Depicting Depicting Molecules and Ions with Lewis Structures I H. Li II III IV V VI VII VIII He . Be Na . Mg Ca . .. . .C. .N. .O . B. .. . . . . Al . . Si . P. S. . . .. . . . . .. .. F. .. .. Cl . .. .. Ne .. .. Ar .. .. .. .. .. .. .. Depicting Depicting Molecules and Ions with Lewis Structures 3 4 1 x N (Gp 5) = 5 e 3 x F (Gp 7) = 21 e 26 e F FNF 26 e - 6 e = 20 e Depicting Depicting Molecules and Ions with Lewis Structures F FNF Does each atom have full octet? Depicting Depicting Molecules and Ions with Lewis Structures Molecules with more than one central atom Sample Problem 10.2, Methanol, CH4O or CH3OH H can only have one bond, C often has 4 and O 2, can therefore: 1 x C (Gp 4) = 4 e H 1 x O (Gp 6) = 6 e 4 x H (Gp 1) = 4 e HCO 14 e 14 e - 10 e = 4 e remaining CHECK: Octet on O and C, 2 e on each H H H Depicting Depicting Molecules and Ions with Lewis Structures Molecules with multiple bonds Sample Problem, nitrogen gas, N2 2 x N (Gp 5) = 10 e 10 e 10 e - 2 e = 8 e remaining CHECK: No Octet on either N. Change 1 lone pair to a bond CHECK: Octet on only one N. Change 1 lone pair to a bond CHECK: Octet on both N’s. OK N N N N N N The Covalent Bonding Model Relationship Relationship between Bond Order, Length & Energy Bond C C C C C C N N N O O O C C C N N N Order 1 2 3 1 2 3 1 2 3 Length (pm) 143 123 113 154 134 121 146 122 110 Av BE (kJ/mol) 358 745 1070 347 614 839 160 418 945 Exceptions Exceptions to the Octet Rule Incomplete Octet Be Be compounds ⇒ BeH2, BeCl2 BeH2 Be – 2e2H – 2x1e4eH Be H Boron Boron and Al compounds ⇒ BF3, AlCl3, BCl3 BF3 B – 3e3F – 3x7e24eF B F F 3 single bonds (3x2) = 6 9 lone pairs (9x2) = 18 Total = 24 BF3 is stable ⇒ The B central atom has a tendency BF to pick up an unshared e- pair from another ecompound BF3 + NH3 → BF3NH3 H F H F B N H F H F H F B N H F the Bthe B-N bond is an example of a coordinate covalent bond, or a “dative” bond → i.e. a bond in which one of the atoms donates both bonding electrons. electrons. “An exception to the Octet Rule occurs for Expanded valence shells….” Occurs Occurs because of atoms with vacant d-orbitals can dexpand their valence shells to form > 4 bonds (lowers energy), i.e for non-metals from P onwards. nonP is 3s2 3p3 3d0 is 3s If If we treat it just as 3s2 3p3, then must apply Octet rule, 3s ie. ie. P or P - But if we consider it as 3s2 3p3 3d0, then there is room to accommodate extra bonding pairs, ie. Up to 5 bonding pairs P or P or P PCl PCl5 Cl Cl P =5 Cl = 5 x 7 40 valence electrons P Cl Cl Cl 10 valence electrons on P SF6 SF F F F S =6 Cl = 6 x 7 48 valence electrons S F F 12 valence electrons on S F Odd eOdd e- molecules (Free radicals) These These molecules have uneven numbers of electrons ∴ no way that they can form octets. Examples: Examples: NO and NO2. These species have an odd number of electrons. . .. :N O .. .. . .. :O N O .. .. NO2 (present in smog from photochemical NO (present reaction of NO with O3). Radicals Radicals react to form dimeric N2O4 - obeys Octet rule. O N O O +N O O N N O O O O O Resonance Resonance Hybrid Structures When When two or more different Lewis structures are possible for a molecule, they are termed resonance hybrids. The actual The actual electronic structure of such a molecule should be viewed as an average of average the different possible resonance hybrids. Resonance Resonance Hybrid Example: 2Carbonate ion CO3 -2 -2 -2 I II :O: CO :O: : C III :O C O: :O: : :O: :O: :: :O: : :: : : :O: : :: Resonances of SO2 S O O O S O Resonance Hybrid S O O Both bonds are equal Resonance Resonance of NO3─ O N O O O N O O ─ O N O O ─ O ─ Or O O N 1/3 ─ Resonance Hybrid O N 1/3 ─ O 1/3 ─ O Resonance Resonance of CO322─ O C O O O C O O 2─ O C O O 2─ 2/3 ─ O Resonance Hybrid C 2/3 ─ O 2/3 ─ O Formal Formal Charges Formal charge = Number of Number of valence electrons ─ valence electrons in free atom in bonded atom = Number of valence electrons in free atom Number of ─½ bonding electrons ─ Number of nonbonding electrons Example: Carbonate ion FC on C: 4 – 1/2 ( 8) = 0 FC on O*: 6 – 1/2 ( 4) – 4 = 0 FC on O: 6 – 1/2 ( 2) – 6 = – 1 -1 O 0 O * C0 O -1 Sum of formal charges = -2 = charge of the carbonate anion Most Stable Structure Q: Which are the most likely contributors to the Q: most resonance hybrid? A: Resonance structures with: lowest lowest formal charges unlike unlike charges on adjacent atoms a more negative formal charge on more EN more atom Example: cyanate ion -2 0 +1 -1 0 0 0 0 -1 [N C A O ]- [N C B O]- [N C C O ]- A is excluded (largest formal charge) B has -1 on N (less EN than O) C has -1 on O (more EN than N) ∴C most favoured Example: Example: XeO3 (+3) Xe O (-1) O (-1) O O (-1) (-1) Xe O (-1) O (0) (+2) (+1) Xe O (-1) O (0) O O (0) (0) Xe O (0) (0) O (0) F F S F F F F Octahedral 90o 90o Octahedral Octahedral Trigonal Trigonal Bipyramidal 90o 120o Trigonal Trigonal Bipyramidal Tetrahedral Tetrahedral 109.5 o 109.5 o Tetrahedral Tetrahedral Tetrahedral Tetrahedral Trigonal Trigonal Planar 120o 120o Trigonal Trigonal Planar Valence Valence Shell Electron Pair Repulsion Repulsion (VSEPR) Five Five basic molecular shapes based on electron group repulsion. Actual Actual molecular shape depends on whether electron groups are bonding or lone pairs. Bond Bond Angle = angle formed by 2 nuclei with nucleus of central atom. Ideal bond angles shown. shown. Shapes Shapes with two electron groups (Linear (Linear Arrangement) Two Two e groups attached to the central atom, without any lone pairs shape LINEAR shape is LINEAR, bond bond angle of 180˚. E.g. E.g. BeCl2 .. Cl .. Be .. Cl .. C O2 CO O C O Shapes Shapes with three electron groups Three Three e groups around central atom TRIGONAL TRIGONAL PLANAR geometry. eg.1 eg.1 BF3 (e-deficient molecule) molecule) A X3 All the boron family (IIIA) elements have the same geometry, i.e Trigonal Planar .. F B F F 1200 .. .. .. Molecular Molecular shape defined by position of nuclei. If If one e-group is a lone pair e(ie AX2E), the molecule will be BENT be BENT or V-SHAPED. Eg Eg 4. SnCl2, or SO2 AX2E .. S O O Shapes Shapes with four electron groups four four e groups around central atom (AX4 to AX2E2) TETRAHEDRAL electron TETRAHEDRAL electron pair geometry. eg. eg. 1 CH4, 4 bonding electron pairs, pairs, tetrahedral molecular shape, all bond angles 109.5˚ (AX4) H H C H H A X4 eg. 2. Ammonia, NH3, one eg. lone pair, three bonding pairs H H N H AX3E Molecule Molecule shape is a TRIGONAL TRIGONAL PYRAMID (AX (AX3E) H-N-H bond angle is 107.3 ˚ because of repulsion by the lone pair. Ammonium Ammonium ion has all H-N-H Hangles of 109.5˚. Why? eg.3 eg.3 Water, H2O, two lone pairs, two bonding pairs, H H O AX2E2 Molecule Molecule shape is a BENT or VBENT VSHAPED (AX2E2) (AX H-O-H bond angle is 104.5˚ because because of repulsion by the lone pair. For For a given e-arrangement, order eof importance of electron pair repulsions causing deviation from the ideal bond angle is: lone pr-lone pr > lone pr-bonding pr > bonding pr-bonding pr Shapes Shapes with five electron groups TRIGONAL TRIGONAL BIPYRAMIDAL electron pair geometry (AX5 to electron AX2E3). .. F 86.20 Br F .. I .. F .. .. Cl .. .. P I 1800 .. AX3E2 - BrF3 (T-shaped) .. AX2E3 - I3- (Linear) I . . . . Cl AX5 - PCl5 . . .. Cl .. .. Cl .. . . (Trigonal bipyramid) . . . . . . . Cl Shapes Shapes with six electron groups OCTAHEDRAL electron OCTAHEDRAL electron pair geometry (AX6 to AX4E2).. Examples: BrF3, TeF5 , XeOF4 - Examples:SF6, IOF5 Examples: ICl4 , XeF4 - .. F .. .. F .. AX6 .. F S .. F .. .. F .. Sulfur Hexafluoride (12 bonding e’s) Octahedral .. F .. F .. .. F .. .. .. .. F F .. Xe .. .. .. F F .. .. .. .. F I .. .. F .. .. F .. Iodine Pentafluoride (12 e’s, 10 bonding, 1 lp) Square pyramid AX5E Xenon Tetrafluoride (12 e’s, 8 bonding, 2 lp’s) Square planar Odd Odd Shaped Molecules for the Third Millennium Fullerenes Fullerenes Buckminsterfullerene, Buckminsterfullerene, C60, was the first reported in soot in 1985 prepared in 1990. A third form of crystalline carbon? Electron delocalisation. By By atomic insertion, and bonding different groups, a whole new range of materials for lubricants, superconductors, batteries, cancer and AIDS drugs is being developed. See http://www.physik.uni See http://www.physik.unistuttgart.de/ExPhys/2.Phys.Inst./member/k.thier/fullerenes/fulleren e_gallery.html Odd Odd Shaped Molecules for the Third Millennium Nanotubes Nanotubes Discovered Discovered in 1991, these long carbon tubes have fullerene ends. Have Have highly delocalized electrons, are very very conductive, and many times stronger than steel. Suggested Suggested applications include manufacture of nanoscale electronics and wires made by inserting metal atoms inside the tubes. See: See: http://www.rdg.ac.uk/~scsharip/tubes.htm http://www.rdg.ac.uk/~scsharip/tubes.htm http://cnst.rice.edu/pics.html http://cnst.rice.edu/pics.html Molecular Molecular Shape and Reactivity - Enzymes Enzymes Enzymes Protein Protein molecules that catalyse rates of certain chemical reactions on a particular substrate substrate (reactants) (reactants) by factors of 108 - 1011 compared to compared the uncatalysed processes. Enzymes Enzymes have an active site into which a active substrate substrate fits Active Active site must provide a perfect fit for the substrate (“lock and key” concept). Active Active site offers some mild attractive binding for the substrate capture capture substrate, hold hold substrate in right position. Product Product molecule must be able to leave enzyme after reaction. Molecular Shape and Reactivity - Enzymes Example: Example: Alkaline phosphatase Catalyses Catalyses the hydrolysis of phosphate ester molecules R-O-PO3 + H2O R-OH + PO43E at pH 8-10. 8Metalloenzyme Metalloenzyme requires requires Zn2+ or Mg2+ (or both) as cofactors (coenzymes) for catalysis to occur. Molecular structure of E. coli alkaline phosphatase (MW ≈ 75,000 Dalton) Molecular Molecular Shape and the Sense of Smell Nature Nature of odorous molecules Capable Capable of travelling through air-must be a gas, or airfrom a volatile solid or liquid. Must Must have a shape that will fit into an olfactory sensor (receptor sensor (receptor sites) of nerve endings in the nasal passage. passage. The Location of Olfactory Receptors within the Nose Molecular Molecular Shape and the Sense of Smell Seven Seven Primary Odors or Olfactory Receptor Sites CamphorCamphor-like, Musky, Floral, Pepperminty, Etheral, Pepperminty, Etheral, Pungent, Putrid. Molecules Molecules with parts that fit more than one type of receptor receptor site will have a mixture of odors, eg. eg Benzaldehyde Benzaldehyde fits floral, musky and minty receptors, and the combined odor is almond-like. almond- Molecular Shape and the Sense of Smell Different Different molecules that fit the same receptor sites smell the same, eg. for the camphor receptor: ...
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This note was uploaded on 02/07/2011 for the course CHEM 202 taught by Professor Mazenelghoul during the Summer '07 term at American University of Beirut.

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