Chapter 2 Structure and Properties of Organic Molecules. Advanced Bonding: Review

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1 hapter 2 Structure and Properties of Organic Molecules hemistry 231 Organic hemistry I Fall 2007 Advanced Bonding: Review Atomic Quantum Mechanics cannot explain how molecules like 4 form: Valence Bond Theory (localized) and Molecular Orbital Theory (delocalized) Linear ombination of Atomic Orbitals on different atoms produce molecular orbitals between partners on the same atom give hybrid orbitals which combine with others. onservation of orbitals. Orbitals that are in phase add together: amplitude increases. Orbitals that are out of phase cancel out. Bonding regions vs. antibonding regions Organic Molecules Slide 2-2 1

2 Bonding Region Electrons are between both nuclei, either on or off axis. z axis Organic Molecules Slide 2-3 Sigma Bonding Electron density lies between the nuclei on the z axis (by def). A bond may be formed by s-s, p z -p z, s-p z, or hybridized orbital overlaps along same axis (later). Anything attached to another atom must use one (and only one0 sigma bond per group to do so. The bonding MO is lower in energy than the original atomic orbitals. The antibonding MO is higher in energy than the atomic orbitals.!* AO 2! AO 1 Organic Molecules Slide 2-4 2

3 Bonding Molecular Orbital Two hydrogens, 1s constructive overlap Organic Molecules Slide 2-5 Anti-Bonding Molecular Orbital Two hydrogens, destructive overlap. Antibonding MO s have nodes between nuclei centers Organic Molecules Slide 2-6 3

4 2 : s-s overlap Organic Molecules Slide 2-7 l 2 : p-p overlap onstructive overlap along the same axis forms a sigma bond. Organic Molecules Slide 2-8 4

5 l: s-p overlap What is the predicted shape for the bonding MO and the antibonding MO of the l molecule? Organic Molecules Slide 2-9 Pi (π) Bonding Pi bonds form after sigma bonds; do not form from hybrid orbitals. Sideways overlap of parallel p orbitals; electron density above and below nuclear axis. Organic Molecules Slide

6 Multiple Bonds A double bond (2 pairs of shared electrons) consists of a sigma bond and a pi bond. A triple bond (3 pairs of shared electrons) consists of a sigma bond and two pi bonds. Organic Molecules Slide 2-11 Molecular Shapes Bond angles cannot be explained with simple s and p orbitals. Use VSEPR theory. A A A tetrahedral trigonal planar linear A A trigonal bipyramid (transition state) Organic Molecules Slide

7 sp ybrid Orbitals 2 VSEPR pairs Linear electron pair geometry 180 bond angle 2 atomic orbitals = 2 hybrid orbitals 1 hybrid orbital per group attached to central atom Orbitals not used for hybrid can be used for pi bonding Organic Molecules Slide 2-13 sp 2 ybrid Orbitals 3 VSEPR pairs; Trigonal planar e - pair geometry 120 bond angle 3 atomic orbitals = 3 hybrid orbitals 1 hybrid orbital per group attached to central atom Orbital not used for hybrid can be used for pi bonding Organic Molecules Slide

8 sp 3 ybrid Orbitals 4 VSEPR pairs Tetrahedral e - pair geometry bond angle 4 atomic orbitals = 4 hybrid orbitals 1 hybrid orbital per group attached to central atom NO Orbital available for pi bonding in Period 2 elements Organic Molecules Slide 2-15 Rotation around Bonds Single bonds in acyclic connections freely rotate rotate alled conformations Double bonds cannot rotate unless the bond is broken (restricted rotation). alled configurations Organic Molecules Slide

9 Isomerism Same molecular formula, but different arrangement of atoms: isomers. Two major types: onstitutional (or structural) isomers: differ in their bonding sequence. Stereoisomers: differ only in the arrangement of the atoms in 3D-space. Organic Molecules Slide 2-17 Structural Isomers 3 O 3 and 3 2 O 3 and 3 Organic Molecules Slide

10 Stereoisomers Br 3 Br 3 and Br 3 3 Br is - same side Trans - across is-trans isomers are also called geometric isomers. There must be two different groups on the sp 2 carbon. 3 No cis-trans isomers possible Organic Molecules Slide 2-19 Molecular Dipole Moments Depend on bond polarity (due to electronegativity differences) and bond angles (VSEPR structure). Vector sum of all the bond dipole moments in the molecule. All polar molecules have polar bonds but not vice versa Organic Molecules Slide

11 Effect of Lone Pairs Lone pairs of electrons contribute strongly to the dipole moment. Organic Molecules Slide 2-21 Intermolecular Forces Strength of attractions between individual molecules influence m.p., b.p., and solubility, esp. for solids and liquids. lassification depends on structure. Dipole-dipole interactions London dispersions ydrogen bonding Organic Molecules Slide

12 Dipole-Dipole Forces Between polar molecules; requires fixed and permanent dipole. Partial positive end of one molecule aligns with partial negative end of another molecule. Lower energy than repulsions, so net force is attractive. Larger dipoles cause higher boiling points and higher heats of vaporization. Organic Molecules Slide 2-23 Dipole-Dipole Organic Molecules Slide

13 London Dispersions Occurs for all molecules; most important IMF between nonpolar molecules Temporary dipole-dipole interactions caused by instantaneous, but fleeting, molecular dipoles due to electron cloud distortions. Larger atoms are more polarizable, as are larger molecules. Branching lowers b.p. because of decreased surface contact between molecules. Organic Molecules Slide 2-25 Dispersions Organic Molecules Slide

14 ydrogen Bonding Not a formal bond; rather, a strong dipole-dipole attraction. For this to occur, one of the organic molecules undergoing -bonding must have N- or O- while the other must have an N or O. The hydrogen attached to an O or N on one molecule is strongly attracted to the lone pair of electrons belonging to an O or N on the other molecule. O- more polar than N-, so stronger hydrogen bonding. Organic Molecules Slide 2-27 Bonds Organic Molecules Slide

15 Boiling Points and Intermolecular Forces 3 2 O ethanol, b.p. = 78 3 O 3 dimethyl ether, b.p. = N N N 3 trimethylamine, b.p. 3.5 ethylmethylamine, b.p. 37 propylamine, b.p O 3 2 N 2 ethanol, b.p. = 78 ethyl amine, b.p. = 17 Organic Molecules Slide 2-29 Solubility Like dissolves like. Polar solutes dissolve in polar solvents. Nonpolar solutes dissolve in nonpolar solvents. Molecules with similar intermolecular forces will mix freely. oeger s Rules Organic Molecules Slide

16 Ionic Solute with Polar Solvent ydration releases energy. Entropy increases. Organic Molecules Slide 2-31 Ionic Solute with Nonpolar Solvent => Organic Molecules Slide

17 Nonpolar Solute with Nonpolar Solvent => Organic Molecules Slide 2-33 Nonpolar Solute with Polar Solvent => Organic Molecules Slide

18 lasses of ompounds lassification based on functional group. Three general classes ydrocarbons ompounds containing oxygen ompounds containing nitrogen. Organic Molecules Slide 2-35 lassification of ompounds In addition to classifying compounds according to functional group, we also use a broader scheme to describe/discuss compounds and atoms in reaction schemes: lassification by hybridization: the hybridization of the atom at which reaction occurs is quite often important to know. lasses are sp 3, sp 2, and sp and are used for all atoms. lassification by carbon attachment: this classification is only used for sp 3 carbons; any atom or functional group will by default carry that carbon s classification. lasses are primary (1 ), secondary (2 ), tertiary (3 ), and quaternary (4 ). 1 o carbon O 1 o hydrogen 1 o alcohol 2 o carbon Br 2 o hydrogen 2 o halide 3 o carbon 3 o hydrogen Organic Molecules Slide

19 ydrocarbons Alkane: single bonds, sp 3 carbons ycloalkane: sp 3 carbons form a ring Alkene: double bond, sp 2 carbons ycloalkene: double bond in ring Alkyne: triple bond, sp carbons; not usually found in a ring Aromatic: simple: contains a benzene ring; more complicated: follow uckel s rules (later) Into this category we also include the hydrocarbon derivatives such as the nitro- and the halogen- containing compounds that do not contain other functional groups. Organic Molecules Slide

20 hapter 2 omework 23, 24, 28, 29, 35, 37, 39, 40, 42, 44 Organic Molecules Slide