California State Polytechnic University, Pomona

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1 alifornia State Polytechnic University, Pomona 2-1 Dr. Laurie S. Starkey, rganic hemistry M 314, Wade hapter 2: Structure and Physical Properties of rganic Molecules hapter utline 1) rbitals and Bonding (2-1 to 2-3) 2) ybrid rbitals (2-4) 3) 3-D Sketches (2-5 to 2-7) 4) Physical Properties (2-9 to 2-11) 5) Isomers (2-8) 6) Introduction to Functional Groups (FG) (2-12 to 2-14) 1) rbitals & Bonding (2-1 to 2-3) Atomic rbitals (A) - a region with a high probability of finding electron (e - ) density - defined by mathematical equations called wave functions - mathematical sign of the wave function changes at a "node" - electron density = 0 at any node y y y y x x x x z z z z s p p p Molecular rbitals (M) - formed by overlap of Atomic rbitals (A) to make covalent bonds - TW A's combine to give TW M's (there are TW possible combinations) Example 1 onsider the formation of the sigma bond in 2 by combining two atoms: a b two electrons shared in a σ bond (a σ M) no electron density holding atoms together - "anti"bonding A's (s orbitals) s a s b (same sign) s a - s b (out of phase) s a + s b (in phase) σ* M - antibonding "sigma star" (high E, usually empty) σ M - bonding molecular orbital (favorable overlap, low E) Energy A s a σ* σ s b A antibonding orbital is empty bonding orbital contains two electrons = a sigma bond! PLEASE TE an increase in # of nodes results in an increase in Energy (the orbital with MRE nodes is LESS stable) 2-2

2 Example 2 onsider the formation of a pi bond, by overlapping two p orbitals 2-2 p a p b two A's two possible combinations resulting in two new Ms p a + p b π bond (bonding M) pi bond = cloud of electron density above and below and p a - p b π* "pi star" (antibonding M) Energy A p a π* π p b A Energy verall E levels of M's (n = nonbonding "lone pairs") σ π n π σ antibonding orbitals high E = less stable (usually empty) electrons in these M's are less stable than σ electrons and are more reactive most stable, strongest bond, least reactive FYI: Electronic Transitions (Wade 15-13B, UV-Vis Spectroscopy) π* π ground state pi bond (low E) hν (light E, usually UV light) if... increase # of conjugated pi bonds then... increase resonance stabilization and... decrease E needed for π π* this E gap π* gets smaller if conjugated π π σ π excited state pi bond (high E) lower Energy visible light is absorbed i.e., LR! 2) ybridization (2-4) carbon's As ow are the bonds in methane, 4, formed? carbon's atomic orbitals (As) contain valence electrons s p x p y p z 2p 2s add Energy promote electron 2p 2s But 4 has 4 identical bonds. ow can that be? mixing of As to give new hybrid orbitals type of hybridization (sp, sp 2, sp 3 ) depends on the number of groups around the carbon "regions of electron density"

3 Determining ybridization Example molecule Regions of e density ybridization s p p p Result Geometry (VSEPR) 2-3 practice: assign hybridizations on given molecule 1) complete Lewis structure 2) hybridization is for each atom 3) count "regions" on each atom a "region of electron density" is a lone pair or single bond or double bond or triple bond For the indicated bonds, describe the type of bond and determine which orbitals overlap to form them. 3 bonds 3) 3-D Sketches of Molecules (2-5 to 2-7) note: can rotate about σ bond (many drawings are possible) 2 2 note: AT rotate about π bond (aligned p orbitals)

4 practice: provide 3D sketch of given molecule 1) complete Lewis structure 2) assign atom hybridizations 2-4 3) sketch with maximum number of atoms in the plane of the page try 3-D sketch of allene 2 2 (problem 2-6) ybridization and Resonance (see Wade problem 2-7 and solved problem 2-8) 2 An allylic lone pair must be in a p orbital in order to have resonance delocalization! Atoms do not move in resonance, so hybridization is sp 2 throughout all resonance forms. 4) Physical Properties (2.9 to 2.11) Physical properties, such as water solubility and boiling point (bp) are based on intermolecular forces/attractions. heat Δ (bp) methanol liquid if Types of "nonbonding" interactions A Dipole-Dipole B ydrogen Bonding van der Waal's/London Dispersion methanol gas molecules are strongly attracted to one another, then - requires a lot of energy to separate them from each other - will have a high/low boiling point

5 A Dipole-Dipole - attraction between polar molecules (consider geometry! Is l 4 polar?) a polar molecule: 2-5 al verall trend: bp B ydrogen Bonding - strongest known dipole due to on or polarity bp both are extremely polar bonds, can cause -bond formation hydrogen-bonding in water: δ+ δ- δ+ hydrogen-bonding in DA base pairs: thymine (T) adenine (A) bp Van der Waal's/London Dispersion Forces - induced (temporary) dipoles in nonpolar molecules temporary attraction because of uneven distribution of electrons - the greater the surface area, the greater the VDW/Dispersion forces (think "Velcro") - the higher the MW, the higher the bp (if all polarity is equal) bp > 300

6 bp 36 straight-chain vs. branched bp 10 to predict boiling points 1) -bonding ( or ) 2) polar vs. nonpolar 3) MW, bp 4) branching (least important!) Water Solubility (2-11) - "like dissolves like" (see Figures 2-26 to 2-29) - water is polar and can form hydrogen bonds (-bonds) miscible with water - polar acetone - -bond acceptor 5) Isomerism (2-8) Isomers are different compounds that have the same molecular formula. onstitutional (Structural) Isomers: same formula, different connectivity Stereoisomers: same formula AD same connectivity, but different spatial arrangement (3D)

7 alifornia State Polytechnic University, Pomona rganic hemistry, M 314, Dr. Laurie S. Starkey 2-7 hapter 2 Summary (Wade textbook) I. Atomic rbitals (A's) combine to give Molecular rbitals (M's) (2-1 to 2-3) A) Bonding M's (σ, π) contain electrons in covalent bonds B) Antibonding M's (σ, π ) are usually empty, can contain excited electrons ) Relative energies, stabilities of M's II. ybrid rbitals (2-4) A) sp 3 hybridization i) 4 regions of electron density ii) tetrahedral geometry B) sp 2 hybridization i) 3 regions of electron density ii) trigonal planar geometry iii) contains an unhybridized p orbital ) sp hybridization i) 2 regions of electron density ii) linear geometry iii) contains two unhybridized p orbitals III. 3-D sketches (2-5 to 2-7) A) determine hybridization to learn geometry about each atom B) draw aligned p orbitals to show π bonds ) apply sp 2 hybridization for atoms involved in resonance IV. Physical Properties (2-9 to 2-11) A) onbonding (intermolecular) Interactions affect bp, mp i) dipole-dipole for polar molecules (δ+, δ-) ii) hydrogen bonding for molecules containing, or F (STRG dipole) iii) van der Waal's (London dispersion) temporary dipole moments a) explains why bp varies by MW (higher MW, higher bp) b) straight vs. branched molecules (greater surface area, higher bp) B) mp increases for molecules that can pack tighter (more spherical, higher mp) ) water solubility increases with polarity, hydrogen-bonding V. Isomerism (2-8) A) structural (constitutional): same molecular formula, different connectivity B) cis-trans (stereoisomers): structures vary only by orientation in space VI. Intro to Functional Groups (FG) ( ) Suggested Textbook Problems: see syllabus

8 2-8 hybrid orbitals (sp, sp2, sp3) contain lone pairs or are used to form σ bonds p orbitals are used to form π bonds, may contain lone pairs (for resonance) or may be empty

9 M 314 Functional Group Example Abbreviation ame alkane 4 R methane alkyl halide l RX or Rl chloromethane (methyl chloride) alkene 2 2 R 2 R 2 ethene (ethylene) alkyne RR ethyne (acetylene) M 315 M 316 alcohol ether amine aldehyde ketone carboxylic acid acid chloride (acyl halide) RR or R 2 RR or R 2 methanol (methyl alcohol) methoxymethane (dimethyl ether) methanamine (methyl amine) 2-propanone (acetone) ethanoic acid (acetic acid) ethanoyl chloride (acetyl chloride) ester methyl ethanoate R 2 R (methyl acetate) amide RR 2 ethanamide 2 (acetamide) anhydride nitrile aromatic 2 l R R 3 R R 2 Rl R 2 R or (R) 2 R Ar ethanal (acetaldehyde) ethanoic anhydride (acetic anhydride) ethanenitrile (acetonitrile) benzene