Molecular Geometry and Hybrid Orbitals. Molecular Geometry

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1 Molecular Geometry and ybrid Orbitals + -- bond angle 90 o Molecular Geometry Why Should I are bout Molecular Geometry? Molecular geometry (shape) influences... 3 Physical properties: Pentane (bp 36 o ) 2,2-Dimethylpropane (bp 10 o ) hemical properties: ( 3 ) 3 -r + O - ( 3 ) 2 = 2 + r - 3 -r + O - 3 O + r - iological properties: Enzyme specificity

2 Molecular Geometry ow Do We Determine Molecular Shape? Empirically (spectroscopy, etc) Predict via theory Example: O 3 O Lewis structure implies planarity ut other geometries can be conceived So what is the real structure of this molecule? onds vibrating, bending, etc. y geometry we usually mean most common geometry Most common geometry = geometry with lowest energy What ontrols Molecular Geometry? Most common geometry = geometry with lowest energy Lowest energy results from......minimizing electron repulsion (nonbonded interactions; van der Waals repulsions)...resonance, conjugation, and aromaticity ow does the molecule minimize electron repulsion? Moving electron clouds (electron groups) away from each other Electron groups = toms Groups of atoms Lone pairs Valence Shell Electron Pair Repulsion theory (VSEPR)

3 Molecular Geometry Survey est geometry = largest distance between electron groups omposition Geometries Linear Example 2 Linear O O ent O O 3 T-shape Trigonal planar Molecular Geometry Survey est geometry = largest distance between electron groups omposition Geometries Example 4 Square planar Receding back Tetrahedral Projecting out 5 Trigonal bipyramid l l P l l l Explore and verify molecular geometry examples with molecular models

4 ond ngles est geometry = largest distance between electron groups re bond angles always equal? ond angles controlled by magnitude of electron group repulsions Electron groups equal size ll o Electron group repulsions equal N -N- 107 o -- < o Electron group repulsions unequal Which electron groups have greatest repulsion? (Which electron groups are largest?) pproximate size ranking:, F < lone pair, l, r, I < all groups of atoms ond ngles In general... Two electron groups = linear Three electron groups = trigonal planar or distorted trigonal planar Four electron groups = tetrahedron or distorted tetrahedron Warning! tomic geometry may also be influenced by......resonance (covered in hem 30)...conjugation...aromaticity Example: } Not a consideration in hem 30 N Four attachments Trigonal planar due to resonance

5 Molecular Geometry and Orbitals ovalent bonds formed by overlap of orbitals Example: Two 1s orbitals (spheres) overlap to form - bond + ydrogen atoms ydrogen molecule Molecular Geometry and Orbitals What orbitals are used for organic molecules? Example: Methane ( 4 ) (1s) + (2s, 2p x, 2p y, 2p z ) - bonds orthogonal + 1s + 2p x + 2p z wrong -- bond angle! Therefore this orbital combination incorrect Verify with your model kit

6 Molecular Geometry and Orbitals Solution (Linus Pauling, 1931) ombine s, p x, p y, and p z orbitals to form correct # of bonds with correct geometry ombinations = hybrid orbitals ybridization scheme for atom with four electron groups 4 geometry = tetrahedral; need four bonds and four hybrid orbitals Orbital conservation: four hybrid orbitals come from four atomic orbitals s + p x + p y + p z sp 3 + sp 3 + sp 3 + sp 3 sp 3 + 1s - σ bond o Molecular Geometry and Orbitals ybridization scheme for atom with three electron groups Example: arbon = trigonal planar; need three bonds and three hybrid orbitals Three hybrid orbitals come from three atomic orbitals: s + p x + p y + p z sp 2 + sp 2 + sp 2 + p z Mixed Not used Top view without p z Top view with p z Side view 120 o sp 2 p z sp 2 p z 90 o

7 Molecular Geometry and Orbitals ombine orbitals to build sp 2 + 1s sp 2 + sp 2 p z + p z - σ bond - σ bond - π bond Side view - σ bond Top view - π bond π bond not shown - σ bond π bond included Explore and verify with molecular models Molecular Geometry and Orbitals ybridization scheme for atom with two electron groups arbon = linear; need two bonds and two hybrid orbitals Two hybrid orbitals come from two atomic orbitals: Example: s + p x + p y + p z sp + sp + p y + p z Mixed Not used Top view without p z Top view with p z Side view without p y Side view with p y sp p y p z

8 Molecular Geometry and Orbitals ombine orbitals to build sp + 1s sp + sp p y + p y p z + p z - σ bond - σ bond - π bond - π bond Side view p y -p y π bond not shown - σ bond Top view p z -p z π bond not shown - π bond - σ bond Explore and verify with molecular models Molecular Geometry and Orbitals few final notes... Other double bonds (=O, =N) similar orbital arrangements to = Other triple bonds ( N) similar orbital arrangements to, O, N almost always hybridized in organic molecules Four electron groups sp 3 Three electron groups sp 2 Two electron groups sp O O N

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