Single Covalent Bonds. Guidelines for Writing Lewis Structures. Guidelines for Writing Lewis Structures. Guidelines for Writing Lewis Structures

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1 ovalent Bonding hapter 8: ovalent Bonding Attraction Stable bond Repulsion umber of bonds = umber shared e - pairs Brooks/ole Brooks/ole 2 ovalent Bonding Single ovalent Bonds Lewis structures: dot = 1 e -. Line = 1 pair of e - Potential Energy (KJ/mol) Single bond: one shared pair of e -. Distance between nuclei (pm) 2008 Brooks/ole Brooks/ole 4 Single ovalent Bonds Single ovalent Bonds 2 = 2(7) = 14 valence e -. Share 2 e - to form octets. bonding lone pair bonding lone pair + 2 = 6 + 2(1) = 8 val. e -. Two - bonds. + 3 = 5 + 3(1) = 8 val. e bonds Brooks/ole Brooks/ole 6 1

2 Single ovalent Bonds # of e - shared Group # of to form an octet Example valence e - (8 - A group#) 4A 4 4 in 4 5A 5 3 in 3 6A 6 2 in 2 7A 7 1 in 1. ount the valence e - in the molecule. 2. Draw a skeleton structure. Join atoms with single lines (pairs of e - ). 3. Add e - pairs to form octets (except ). Start with terminal atoms Brooks/ole Brooks/ole 8 Phosphorus trifluoride, P 3 1. P 3 = (7) = 26 valence e - 3. Build octets start with terminal atoms. 3 x (group 7A) P (group 5A) 2. Skeleton (X is central in XY n ). P P 6 e - used in 3 bonds, 20 e - remain (10 pairs) = 26 e Brooks/ole Brooks/ole 10 Phosphate ion, P P 4 3- = 5 + 4(6) + 3 = 32 P (grp 5A) 4 x (grp 6A) charge (-3) 3 e- 2. Skeleton. P is central. P 3. Add e - pairs: P 8 e - used in 4 bonds, 24 e - remain (12 pairs) 32 e - used. Add brackets and overall charge to show this is an ion. P Brooks/ole Brooks/ole 12 2

3 Single ovalent Bonds in ydrocarbons Single ovalent Bonds in ydrocarbons Larger alkanes: Methane: butane 2-methylpropane 2008 Brooks/ole Brooks/ole 14 Single ovalent Bonds in ydrocarbons yclic alkanes: Single ovalent Bonds in ydrocarbons Each alkane -atom can be replaced by: another atom. a functional group. 1-chloro-2-methylpropane l 2-fluorobutane cyclopentanol 2008 Brooks/ole Brooks/ole 16 Multiple ovalent Bonds Too few dots to complete all the octets? onvert lone pairs to shared pairs. 2. Skeleton e - in bonds. Add the other 3 pairs to (outer atom). Each shares 2 e - only has 6 e -. Multiple ovalent Bonds onvert lone pairs to bond pairs. Each shares 2 e - shares 8. shares Brooks/ole Brooks/ole 18 3

4 Multiple ovalent Bonds Multiple ovalent Bonds in ydrocarbons 2. Skeleton 3. 4 e - in bonds. Add 3 pairs to each. 4. onvert lone pairs to bond pairs. ethene 2 4 (ethylene) propene 3 6 (propylene) butene 4 8 (1 isomer shown) (= is a line angle structure: see chapter 3 notes) 2008 Brooks/ole Brooks/ole 20 Multiple ovalent Bonds in ydrocarbons Double Bonds and Isomerism Bond rotation: ethyne (acetylene) - - propyne - -- ree rotation. o rotation around a = bond Brooks/ole Brooks/ole 22 Double Bonds and Isomerism Double Bonds and Isomerism l = l l l l l = = l l l = l l l l l 2008 Brooks/ole Brooks/ole 24 4

5 Double Bonds and Isomerism Bond Properties: Bond Length Atom size and bond type are important: 3 = 3 3 = Brooks/ole Brooks/ole 26 Bond Properties: Bond Length Bond Properties: Bond Enthalpies 2008 Brooks/ole Brooks/ole 28 Bond Properties: Bond Enthalpies Estimate Δ for the following reaction from bond E. Bond Properties: Bond Polarity Bonding pairs are not always equally shared. 4 (g) (g) 2 (g) (g) Δ = {sum of bonds broken} {sum of bonds formed} = {4 D D - } {2 D D - } = {4(416) + 2(498)} {2(803) + 4(467)} = 814 kj (experimental value is 802 kj) δ+ δ- has stronger e - attraction. Polar bond Brooks/ole Brooks/ole 30 5

6 Linus Pauling developed the first scale Based on bond energies. Pauling s scale: = 4.0 (arbitrary). Unitless. increasing electronegativity 2008 Brooks/ole Brooks/ole 32 Differences, ΔE, determine bond polarity: ΔE = 0.0 ΔE = 0.7 ΔE = 1.9 ΔE = 3.0 or the following bond pairs, indicate the δ+ and δ- atoms and choose the more polar bond: l and Br ; Si Br and Br increasing electronegativity covalent bond polar covalent bonds increasingly ionic increasingly covalent ionic bond ΔE ( and Br) > ΔE ( and l) Br is more polar than l. (Electronegativities: Br = 2.8, l = 3.0, = Brooks/ole Brooks/ole 34 or the following bond pairs, indicate the δ+ and δ- atoms and choose the more polar bond: l and Br ; Si Br and Br. ormal harge Used to study charge distribution in a molecule. increasing electronegativity Largest ΔE between Si and Br SiBr is more polar than Si. (Electronegativities: Si = 1.8, = 2.5, Br = ormal charge of each atom = (# of valence e - ) (e - on the atom). ote: sum of formal charges = molecular charge 2008 Brooks/ole Brooks/ole 36 6

7 ormal harge [ ] Valence e Lone pair e ½ shared e ormal harge ormal harge If there is choice between Lewis structures: Smaller formal charges are favored. egative formal charges should be on the most E atoms Like charges should not be on adjacent atoms ormal charges: heck: Σ (formal charges) = ion charge = Brooks/ole Brooks/ole 38 ormal harge Which l 2 - structure is preferred? Lewis Structures and Resonance zone has 2 equivalent structures: l l ormal charges: Both: obey the octet rule have the same number and types of bonds have the same formal charges Experiments show that the bonds are identical Brooks/ole Brooks/ole 40 Lewis Structures and Resonance Lewis Structures and Resonance Resonance in 3 2- Resonance structures differ only in e - pair positions. Atom positions must not change. Experiment: All three bonds = 129 pm Typical bond lengths - =143 pm; = = 122 pm Brooks/ole Brooks/ole 42 7

8 Exceptions to the ctet Rule Be and B form e - deficient compounds: ewer than Eight Valence Electrons ften very reactive. Be B 2 + 2(1) = 4 valence e (7) = 24 valence e - B + B 2008 Brooks/ole Brooks/ole 44 dd umber of Valence Electrons Some stable molecules have an odd number of e -. More Than Eight Valence Electrons (6) = 17 valence e - Very reactive. Most stable molecules have paired e Brooks/ole Brooks/ole 46 More Than Eight Valence Electrons l (7) = 28 val. e - Make octets on. 24 e - used, 4 remain. [bonds (3 x 2); Lone pairs (3 x 6)] Add 2 lone pairs to l the 3 rd period element. l Aromatic ompounds Typically have strong, often pleasant, odors. ontain a benzene or benzene-like ring Its - bonds have equal lengths: or A solid ring shows resonance in aromatics 2008 Brooks/ole Brooks/ole 48 8

9 Aromatic ompounds ther aromatic examples: Aromatic ompounds aming benzene rings: ortho meta para = benzaldehyde toluene naphthalene o-xylene m-xylene 3 3 p-xylene m.p./ b.p./ Brooks/ole Brooks/ole 50 Aromatic ompounds Systematic names: ,2 dimethylbenzene l l 3 1,3 dimethylbenzene l l 3 1,4 dimethylbenzene l l 1,2,3 trichlorobenzene l 1,2,4 trichlorobenzene l l 1,3,5 trichlorobenzene 2008 Brooks/ole 51 9