Molecular Structures. Chapter 9 Molecular Structures. Using Molecular Models. Using Molecular Models. C 2 H 6 O structural isomers: .. H C C O..

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1 John W. Moore onrad L. Stanitski Peter. Jurs hapter 9 Molecular Structures Stephen. oster Mississippi State University Molecular Structures 2 6 structural isomers: ethanol dimethyl ether m.p./ b.p./ Molecular shape is important! Small structural changes cause large property changes. 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 The Valence Shell Electron Pair Repulsion model is used to predict shapes. Key ideas: 1. e - stay as far apart as possible. Repulsions are minimized. 2. Molecule shape is governed by the number of bond and lone present. 3. Treat multiple bonds like single bonds. Each is a single e - group. 4. Lone occupy more volume than bonds. Linear Triangular planar Tetrahedral Triangular bipyramidal ctahedral 1

2 Shapes that minimize repulsions: If a molecule contains: bonding only these angles are correct: linear triangular planar tetrahedral triangular bipyramidal octahedral Bonds and lone determine shape. Use the notation AX n E m m lone on n atoms bonded to central atom A central atom A These angle change (a little) if any X is replaced by a lone pair: lone pair/lone pair repulsions are largest. lone pair/bond pair are intermediate in strength. bond/bond interactions are the smallest. Molecules may be described by their: electron-pair (e - pair) geometry molecular geometry (molecular shape) These geometries may be different. Atoms can be seen, lone are invisible. AX n E m : 2 e - group central atoms (m + n = 2) bond Linear e - pair geometry 2 e - groups lone 2 0 AX 2 E 0 linear 1 1 AX 1 E 1 linear molecular geometry AX 2 E 0 examples: l Be l Linear. 2 bonds, 0 lone on. (treat double bonds as 1 bond) Linear. Each -- unit is linear. AX n E m : 3 e - group central atoms (m + n = 3) bond 3 e - groups lone 3 0 AX 3 E 0 triangular planar 2 1 AX 2 E 1 angular (bent) 1 2 AX 1 E 2 linear molecular geometry Triangular planar e - pair geometry 2

3 AX 3 E 0 examples: l B l l 120 Triangular planar. AX n E m : 4 e - group central atoms (m + n = 4) Tetrahedral e - pair molecular geometry bond 4 e - groups lone 4 0 AX 4 E 0 tetrahedral 3 1 AX 3 E 1 triangular pyramidal geometry Each is AX 3 E 0 = triangular planar. AX 1 E 3? All molecules with only 1 bond are linear! 2 2 AX 2 E 2 angular (bent) AX 4 E 0 All angles = tetrahedral angle. VSEPR applies to each atom in a molecule. Alkanes: each is tetrahedral. N AX 3 E 1 Lone-pair/bond > bond/bond repulsion: -N- angle is reduced. AX 2 E 2 Two lone : -- angle is even smaller. Lactic acid: Tetrahedral Tetrahedral Tetrahedral Tetrahedral Triangular planar Expanded ctets entral atoms with five or six e - : Bond Lone 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 lone repel the most. they get as far apart as possible. 3

4 Expanded ctets Expanded ctets AX n E m : m + n = 5 Triangular bipyramidal e - pair geometry. P S l Xe Triangular bipyramidal Seesaw T-shaped Linear The atoms are non-equivalent. Green atoms are axial; blue atoms are equatorial. Expanded ctets Expanded ctets AX n E m : m + n = 6 ctahedral e - pair geometry: All atoms are equivalent in AX 6 E 0 S Br l l I l l ctahedral Square pyramid Square planar rbitals onsistent with Molecular Shapes Lewis dot + VSEPR predict molecular shapes, but ow do atomic orbitals (s, p ) lead to these shapes? Valence bond theory: bonds occur when partiallyoccupied atomic orbitals overlap. 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, 2 (1s) overlaps (1s) 74 pm (1s) overlaps (2p) 109 pm form 4 bonds at tetrahedral angles? (1s 2 2s 2 2p 2 ). 2p x1 2p y1 Two bonds? p orbitals are at 90 to each other Experiments show: tetrahedral 4, l 4, 4

5 rbitals onsistent with Molecular Shapes Atomic orbitals (As) can be hybridized (mixed). Sets of identical hybrid orbitals form identical bonds. Number of hybrids formed = number of As mixed. sp ybrid rbitals Be compounds (Be 2, Be 2 ): ne s orbital + one p orbital two sp hybrids. Each sp hybrid (180 apart) holds one e -. Two equivalent covalent bonds form. sp 2 ybrid rbitals B forms three sp 2 hybrid orbitals: ne s orbital mixes with two p orbitals. ne p orbital remains unmixed. sp 2 ybrid rbitals B compounds (B 3, B 3 ): Each sp 2 hybrid (120 apart) holds one e -. Three equivalent covalent bonds form. sp 3 ybrid rbitals forms four sp 3 hybrid orbitals: ne s orbital mixes with three p orbitals. All p orbitals are mixed. sp 3 ybrid rbitals N and compounds (N 3, 2 ) have more e - : In, each sp 3 hybrid (109.5 apart) holds one e -. our equivalent covalent bonds form. 5

6 sp 3 ybrid rbitals ctet rule molecules have tetrahedral e - pair shape. sp 3 hybridized ( 4, N 3, 2, 2 S, P 3, ) ead-to-head bond = a sigma bond (σ bond). There are: 4 σ bonds in 4 σ bond 3 σ bonds in N 3 2 σ bonds in 2 ybridization 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: 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 ybridization in Molecules with Multiple Bonds ybridization in Molecules with Multiple Bonds arbon atoms form: tetrahedral centers ( 4, 3, 2 6 ) = sp 3 triangular-planar centers ( 2, 2 4 ) = sp 2 The double bond in ethene is composed of: a σ bond head-to-head overlap of sp 2 on each atom. a π bond sideways overlap of p As on the atoms. (sp 2 ) + (sp 2 ) overlap (σ bond): Unhybridized p orbitals each contain one e -. σ bond overlap Sideways overlap forms one π bond the lobes above and below the plane together equal 1 bond ybridization in Molecules with Multiple Bonds ormaldehyde is similar: ybridization in Molecules with Multiple Bonds also forms linear centers: 2 2 (acetylene) = sp hybridized The triple bond is: one σ bond two π bonds sp hybridization leaves two unmixed p orbitals on each. 6

7 ybridization in Molecules with Multiple Bonds ybridization in Molecules with Multiple Bonds σ bond: (sp) + (sp) overlap: π bonds prevent bond rotation: Two p orbitals on each contain a single e -. overlap Molecule - bonding - rotation ethane ( 3 3 ) σ yes ethene ( 2 = 2 ) σ + π no ethyne ( ) σ + π + π no Two π bonds above and below overlaps are 1 bond. front and back overlaps are a second bond. Non-rotating double bonds allow cis-trans isomerism to occur. Most bonds are polar (e.g. -) is δ-, is δ+ (EN = 3.5, EN = 2.5) But many molecules are nonpolar (e.g. 2 ). = = δ- 2δ+ δ- arrow points to δ-, the + shows δ+ The dipoles cancel because of 2 s shape. have equal size but point in opposite directions. Water is polar (bond dipoles do not cancel) + Net dipole Dipole, μ = 1.85 D Dipole moment (μ) is a measure of molecule polarity: Units: coulomb meter (m) Debye (D) weakly polar highly polar nonpolar (μ=0) Molecule μ (D) l 1.07 Br 0.79 I l l l l 4 0 A molecule is nonpolar if it is: AX n E 0 and all X are identical. 2 AX 2 E 0 linear 4 AX 4 E 0 tetrahedral l 4 AX 4 E 0 tetrahedral P 5 AX 5 E 0 triangular bipyramidal divisible into nonpolar AX n E 0 shapes Pl 3 2 triangular planar (Pl 3 ) + linear (P 2 ) Xe 4 linear (Xe 2 ) + linear (Xe 2 ) 7

8 AX n E m molecules are polar if they don t divide into nonpolar shapes, and: m 0: 2 AX 2 E 2 bent polar N 3 AX 3 E 1 pyramidal polar No net dipole 4 is non polar 3 is polar + Net dipole The X in AX n E 0 differ: 2 l 2 AX 4 E 0 tetrahedral polar P 4 l AX 5 E 0 triangular bipyramidal polar ow polar? It depends on the number, type, and geometry of the polar bonds. Pl 5 Non polar AX 5 E 0 ; identical X + Pl 4 Polar AX 5 E 0 X differ Pl 3 2 P 3 l 2 Non polar AX 5 E 0 and X differ. BUT divisible into nonpolar shapes: linear + triangular planar Polar AX 5 E 0 and X differ. Doesn t divide into nonpolar shapes Noncovalent Interactions Molecules attract each other. Intermolecular forces: also called noncovalent interactions. are small (compared to bonding forces). do not include ionic or metallic-bonding forces. Three types: London forces. dipole-dipole attraction. hydrogen bonding. London orces Also called dispersion forces. Random e - motion produces a temporary dipole in one molecule which induces a dipole in another. London orces Noble Gas alogen ydrocarbon # of e - bp ( ) # of e - bp ( ) # of e - bp ( ) e Ne l Ar Br Kr I Strength ( kj/mol): Small molecule = few e - = weak attraction. Large molecule = many e - = stronger attraction. ccur between all atoms and molecules. The only force between nonpolar molecules. More e - = larger attraction = higher b.p. 8

9 Dipole-Dipole Attractions Polar molecules attract each other. Strength: 5 25 kj/mol. Dipole-Dipole Attractions Nonpolar Molecules Polar Molecules # of e - bp ( ) # of e - bp ( ) Si P Ge As Br Il Relative importance of dipole/dipole and London is hard to predict: Dipole London bp ( ) I small (0.38 D) large (54 e - ) 36 l large (1.07 D) small (18 e - ) 85 stickier ydrogen Bonding An especially large dipole-dipole attraction kj/mol. ccurs when bonds directly to, or N. ydrogen Bonding on one molecule interacts with on another molecule., & N are small with large electronegativities. results in large δ+ and δ- values. -bonds are usually drawn as dotted lines. ydrogen Bonding Water is a liquid at room T (not a gas). 9