Molecular Structures. Using Molecular Models. Using Molecular Models. Predicting Molecular Shapes: VSEPR. Predicting Molecular Shapes: VSEPR
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1 Molecular Structures Two 2 6 structural isomers: hapter 9: Molecular Structures ethanol dimethyl ether m.p./ b.p./ Molecular shape is important! Small structural changes cause large changes in physical (and chemical) properties Brooks/ole Brooks/ole 2 Using Molecular Models Physical models of 3D-structures: omputer versions: ball and stick space filling Using Molecular Models and-drawn molecules: In the plane of the screen Going back into the screen oming out of the screen 2008 Brooks/ole Brooks/ole 4 1.! e - pairs stay as far apart as possible to minimize repulsions. 2.! The shape of a molecule is governed by the number of bonds and lone pairs present. 3.! Treat a multiple bond like a single bond when determining a shape. Each is a single e-group. 4.! Lone pairs occupy more volume than bonds. Linear Triangular planar Tetrahedral Triangular bipyramidal ctahedral 2008 Brooks/ole Brooks/ole 6 1
2 Basic shapes that minimize repulsions: linear triangular planar tetrahedral triangular bipyramidal If the molecule contains:! only bonding pairs the angles shown are correct.! lone pair/bond mixtures the angles change a little.!! lone pair/lone pair repulsions are largest.!! lone pair/bond pair are intermediate in strength.!! bond/bond interactions are the smallest. octahedral A molecule may be described by its:! electron-pair (e-pair) geometry! molecular geometry These two geometries may be different.! Atoms can be seen, lone pairs are invisible Brooks/ole Brooks/ole 8 2 and 3 e-group central atoms 2 e-groups bond lone pairs pairs 2 0 linear 1 1 linear molecular geometry Linear e-pair geometry bond pairs 3 0 triangular planar 2 1 angular (bent) 1 2 linear Triangular planar e-pair geometry 3 e-groups lone pairs molecular geometry 2008 Brooks/ole 9 2 e-groups: l Be l bonds, 0 lone pairs on Be. Linear. 2 bonds, 0 lone pairs on. (treat double bonds as 1 bond) Linear. Each has 2 e-groups. Each -- unit is linear Brooks/ole 10 3 e-groups: l B l l 120 B has 3 bonds (0 lone pairs). Triangular planar. Each has 3 e-groups. Each is triangular planar. 4 e-groups = tetrahedral e-pair geometry: bond lone pairs pairs 4 0 tetrahedral 3 1 triangular pyramidal 2 2 angular 1 bond, 3 lone pairs? All molecules with only 1 bond are linear! 2008 Brooks/ole Brooks/ole 12 2
3 bonds, 0 lone pairs. All angles = tetrahedral angle VSEPR applies to each atom in a molecule.! Alkanes: each is tetrahedral N 3 bonds, 1 lone pair. Lone-pair/bond > bond/bond repulsion -N- angle is reduced bonds, 2 lone pairs. Two lone pairs -- angle even smaller Brooks/ole Brooks/ole 14 Lactic acid: Tetrahedral Tetrahedral Tetrahedral Tetrahedral Triangular planar Expanded octet atoms: bond lone pairs pairs Shape 5 0 Triangular bipyramidal 4 1 Seesaw 3 2 T-shaped 2 3 Linear 6 0 ctahedral 5 1 Square pyramidal 4 2 Square planar 3 3 T-shaped Remember! lone pairs repel the most.! they get as far apart as possible Brooks/ole Brooks/ole P S l Xe Triangular bipyramidal Seesaw T-shaped Linear P 5 S 4 l 3 Xe Brooks/ole Brooks/ole 18 3
4 Six e-groups = octahedral e-pair geometry 90 S Br Xe 90 ctahedral Square pyramid Square planar Equivalent atoms S 6 Br 5 Xe Brooks/ole Brooks/ole 20 rbitals onsistent with Molecular Shapes rbitals onsistent with Molecular Shapes VB theory: bonds occur when atomic orbitals overlap. 2 (1s) overlaps (1s) (1s) overlaps (2p) ow do atomic orbitals (s, p, d ) produce these shapes? 74 pm 109 pm 2008 Brooks/ole Brooks/ole 22 Valence Bond Theory This works for 2 and, but! Why does Be form compounds?!! Be (1s 2 2s 2 )!! No unpaired e - to share.!! Experiments show: linear Be 2, Bel 2, rbitals onsistent with Molecular Shapes ne s orbital + one p orbital! two sp hybrids.! Why does form 4 bonds at tetrahedral angles?!! (1s 2 2s 2 2p 2 )!! 2p x 1 2p y 1 Two bonds?!! p orbitals are at 90 to each other!! Experiments show: tetrahedral 4, l 4, 2008 Brooks/ole Brooks/ole 24 4
5 sp ybrid rbitals Be compounds (Be 2, Be 2 ): Energy, E 2p 2p 2p 2s Isolated Be atom Promotion 2p 2p 2p 2s rbital hybridization Two unhybridized p orbitals Two sp hybrid orbitals on Be in Be 2 sp 2 ybrid rbitals B forms three sp 2 hybrid orbitals:!! ne s orbital mixes with two p orbitals.!! ne p orbital is unmixed. Each sp hybrid (180 apart) holds one e -. Two equivalent covalent bonds form Brooks/ole Brooks/ole 26 sp 2 ybrid rbitals B compounds (B 3, B 3 ): Energy, E 2p 2p 2p 2s Isolated B atom Promotion 2p 2p 2p Each sp 2 hybrid (120 apart) holds one e -. Three equivalent covalent bonds form. 2s rbital hybridization ne unhybridized and vacant p orbital Three sp 2 hybrid orbitals of B in B 3 sp 3 ybrid rbitals forms four sp 3 hybrid orbitals:!! ne s orbital mixes with three p orbitals.!! All p orbitals are mixed. In, each sp 3 hybrid (109.5 apart) holds one e -. our equivalent covalent bonds form Brooks/ole Brooks/ole 28 sp 3 ybrid rbitals N and compounds (N 3, 2 ) have more e - : sp 3 ybrid rbitals ctet rule molecules have tetrahedral e-pair shape.! sp 3 hybridized ( 4, N 3, 2, 2 S, P 3, ) " bond 2008 Brooks/ole Brooks/ole 30 5
6 ybridization in Expanded ctets Summary: Mixed ybrids (#) Remaining Geometry s+p sp (2) p+p Linear s+p+p sp 2 (3) p Triangular planar s+p+p+p sp 3 (4) Tetrahedral d orbitals can also form hybrids: ybridization in Molecules with Multiple Bonds A carbon atom can have a:! tetrahedral center ( 4, 3, 2 6 ) = sp 3! triangular-planar center ( 2, 2 4 ) = sp 2 Mixed ybrids (#) Remaining Geometry s+p+p+p+d sp 3 d (5) d+d+d+d Triangular bipyramid s+p+p+p+d+d sp 3 d 2 (6) d+d+d ctahedral 2008 Brooks/ole Brooks/ole 32 ybridization in Molecules with Multiple Bonds ybridization in Molecules with Multiple Bonds (sp 2 ) + (sp 2 ) overlap (" bond): ormaldehyde is similar: Unhybridized p orbitals each contain one e -. " bond overlap 2008 Brooks/ole Brooks/ole 34 ybridization in Molecules with Multiple Bonds ybridization in Molecules with Multiple Bonds A third type of center is seen:!! linear center ( 2 2, acetylene) = sp hybridized " bond: (sp) + (sp) overlap: overlap 2008 Brooks/ole Brooks/ole 36 6
7 ybridization in Molecules with Multiple Bonds # bonds prevent bond rotation: = =! $- 2$+ $- the arrow points to $-, the + shows $+ Non-rotating double bonds allow cis-trans isomerism to occur.! The dipoles cancel because of 2 s shape.! the bond dipoles have equal size but point in opposite directions Brooks/ole Brooks/ole 38 Molecule µ (D) l 1.07 Br 0.79 I S l l l l Brooks/ole Brooks/ole 40! Polar molecules: bond dipoles do not cancel! Water is polar: + Net dipole bserved dipole, µ = 1.85 D! 2008 Brooks/ole Brooks/ole 42 7
8 No net dipole + Net dipole 4 is non polar 3 is polar 2008 Brooks/ole Brooks/ole 44 Noncovalent Interactions Molecules are sticky and attract each other. P 5 + P 3 l 2 P 4 l + P 3 l Brooks/ole Brooks/ole 46 London orces $+ $- $+ $-! Strength (0.05 % 40 kj/mol): Small molecule = few e - = weak attraction. Large molecule = many e - = stronger attraction.! The only force between nonpolar molecules Brooks/ole 47 London orces Atom Molecule # of e - bp ( ) e 2 &269 Ne 10 &246 Ar 18 &186 Kr 36 & &188 l 2 34 &34 Br I & & & More e - = larger attraction = greater stickiness = higher b.p Brooks/ole 48 8
9 Dipole-Dipole Attractions Polar molecules attract each other. Strength = 5 % 25 kj/mol. Dipole-Dipole Attractions nonpolar # of e - bp ( ) polar # of e- bp ( ) Si 4 18 &112 P 3 18 &88 Ge 4 36 &90 As 3 36 &62 Br Il With equal number of e - (and same shape): dipole/dipole > London 2008 Brooks/ole Brooks/ole 50 ydrogen Bonds An especially large dipole-dipole attraction.!! 10 % 40 kj/mol!! ccurs when bonds directly to, or N, & N are small with large electronegativities.!! results in large $+ and $- values. ydrogen Bonds on one molecule interacts with on another molecule. -bonds are usually drawn as dotted lines Brooks/ole Brooks/ole 52 ydrogen Bonds Noncovalent orces in Living ells Phospholipids form lipid bilayers: Water is a liquid at room T (not a gas). Polar end = hydrophilic (water loving). Nonpolar end = hydrophobic (water hating) Brooks/ole Brooks/ole 54 9
10 Biomolecules: DNA and Molecular Structure Biomolecules: DNA and Molecular Structure In DNA there are 4 possible bases adenine (A), thymine (T), guanine (G), or cytosine () 2008 Brooks/ole Brooks/ole 56 Biomolecules: DNA and Molecular Structure Biomolecules: DNA and Molecular Structure omplementary base pairs: 2008 Brooks/ole Brooks/ole 58 10
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