O-Chem Week 2 Notes
O-Chem Week 2 Notes CHEM 2451
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This 10 page Class Notes was uploaded by Shea Claflin on Saturday January 9, 2016. The Class Notes belongs to CHEM 2451 at University of Denver taught by Dr. Peter J. Harrington in Fall 2016. Since its upload, it has received 57 views. For similar materials see Organic Chemistry I in Chemistry at University of Denver.
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Date Created: 01/09/16
Organic Chemistry, Dr. Peter Harrington Notes – Week 2 1-11 CH 4 all angles are 109.5°, shape is tetrahedral The p-orbital is dumbbell shaped, there are 3 p-orbitals in the second shell The p-orbitals are aligned along the x, y, and z axis at 90° angles C electron configuration 1s 2s 2p 2 or 1s 2s 2p 2p 2p 1 0 6 x y z HYBRIDIZATION of valence shell orbitals is used to recreate the valence shell orbital picture in reality 3 The 2sp hybrid orbitals have 25% s character and 75% p character sp orbital (small lobe sometimes not shown) Making Hybrid Orbitals - 4 orbitals go in, 4 orbitals come out - we are not adding/removing energy so new orbitals are intermediate in energy - the 4 new orbitals are all equal in energy VSEPR Theory Bonds are regions of electron density How do you arrange 4 regions of electron density in 3D space to minimize electron-electron repulsion? 3 NH 3 2p ___ ___ ___ 2sp ___ ___ ___ ___ 2s ___ 1s ___ 1s ___ H-H-H bond 107° H O 2p ___ ___ ___ 2sp 3 ___ ___ ___ ___ 2 2s ___ 1s ___ 1s ___ H-O-H bond 105° KEY: non-bonding electron pairs are more diffuse and take up more space, which compresses bond angles BF3 planar, B-F is 120°, Boron is electron deficient BeH 2 H-Be-H bond 180°, linear, Beryllium is electron deficient Bond Length – the distance between the 2 atoms’ centers 1. bond lengths increase as atoms get bigger 2. atomic size increases as you go down a column and as you go right to left across a row 3. bond length decreases as the number of electrons being shared increases Atomic radii measured in picometers = 10 -1meters -10 and angstroms = 10 meters Ethane Ethylene Acetylene C-C 153 picometers C=C 134 picometers C=C 121 picometers Bond Polarity Δ electronegativity < 0.5 non-polar covalent Δ electronegativity > 0.5 polar covalent Δ electronegativity > 1.7 ionic Non-polar Covalent Polar Covalent C-H .35 C-O .89 C-N .49 N-H .84 C-S .03 O-H 1.24 C-Br .41 C-F 1.43 Representation of unequal sharing of electron pair in polar covalent: 1) δ+ less electronegative element - δ more electronegative element 2) with vector arrow, points to more electronegative atom 3) electrostatic potential map: more electronegative elements are redder, and less electronegative elements are bluer total molecular polarity vector 2 polar covalent bonds in water X components: cancel out, they are in the opposite direction Y components: both up, add together Z components: none Overall vector points straight up In carbon dioxide, the arrows cancel out, so there is no polarity 1/13 Electrons as Particles/Electrons as Waves - we started with Lewis dot structures - we transferred a dot from one atom to another to make ionic bond - we shared two dots between atoms to make covalent bond This “electron bookkeeping” will be very useful in this course but… there will be times when an alternative bonding theory explains results that this LCAO (linear combination of atomic orbitals) cannot explain. Molecular Orbital (MO) Theory: Diagram for H 2 Guitar string at rest Pluck the string… now picture two strings as the two H atoms There are two possibilities for joining the waves to make a bond Anti-bonding peak + trough Energy AND or node is region of 0 e density Peak Peak Trough Bonding peak + peak “cylindrically symmetrical” the two bonding electrons (the bond) is cylindrically symmetrical about the axis joining the 2 nuclei – σ – bond (sigma) ALKANES add one C Propane Table 2.1 in the book has the names of alkanes – learn these names All alkanes have the same general formula C n 2n+2 (n=1,2,3…) Different ways to draw Butane: carbons are assumed assumes all C’s have full octet with H’s H H H H H 3-CH 2CH -2H 3 H CH 3CH 2CH -2H 3 H H CH 3H C2 C2 3 H H H Butane C H 4 10 first alkane with constitutional isomers Isobutane b.p. -0.5°C b.p. -11.7°C Butane and isobutene are constitutional isomers, meaning they have the same formula but different “connectivity” They have different physical properties (b.p.) Pentane C5H 12 - 3 constitutional isomers Number of Isomers for various alkanes C6H 14 5 C7H 16 9 C10 22 75 C20 42 366,319 IUPAC System (international union of pure and applied chemistry) 2 4 6 8 1 3 5 7 9 Red dots are branch points NAMING ALKANES 1) find largest carbon chain – length of chain will determine root name (9 C’s = nonane) 2) # the carbons in the chain, L-R or R-L to give first branch point the lowest number 3) Name all substituents attached at branch points Substituent Name C1 methane -CH 3 methyl C2 ethane -CH 2H 3 ethyl C3 propane -CH 2H C2 3 propyl: know isopropyl C4 butane -CH 2H C2 CH2 3 butyl:know isobutyl, sec-butyl, tert- butyl 2 4 6 7 Carbons = heptane 1 3 5 7 Q) What if there are 2 options for numbering carbons? A: Pick the path that includes the most branch points. 6 Carbons = Hexane 2 4 6 Pick this numbering method L-R 1 3 5 5 3 1 Not this one R-L 6 4 2 Q) What if there L-R and R-L give you the same number for the first branch point? A: Pick the path that gives you the lowest number for the second branch point. 4) Link (-) the branch point number to the substituent name 9 C’s nonane 2 4 6 8 2-methyl 5-isopropyl 1 3 5 7 9 5) Arrange the substituent names in alphabetical order and put them in front of the name. Connect numbers and numbers with a comma, and connect letters and numbers with a hyphen. Leave no spaces 5-isopropyl-2-methylnonane 2 4 6 6 C’s hexane 2,3,5 methyl 1 3 5 2,3,5-trimethylhexane 6) Ignore prefixes and sec/tert when arranging in alphabetical order. However, don’t ignore the prefixes iso, neo, and cyclo. # of substituents prefix 2 di 3 tri 4 tetra 5 penta 5 methyl 2 methyl 4 propyl 3 methyl 7 C’s = heptane 2,3,5 trimethyl 4 propyl 2,3,5-trimethyl-4-propylheptane 7) Name substituents with more than 4 carbons using same IUPAC system 1s) find the longest chain starting at the carbon attached to the branch carbon This determines the root, end with suffix yl 2s) number substituent chain starting at carbon attached to branch point carbon 3s) name all subsubstituents attached to the substituent chain 4s) link subsubstituents name with the position number (-) 5s) arrange subsubstituent name alphabetically in front of substituent name 6s) same as step 6 above 7s) isolate the substituent name with parantheses 2 5 1 2 3 9 C’s nonane 2-methyl 5-propyl 1-1-dimethyl 5-(1,1-dimethylpropyl) 2-methyl-5-(1,1-dimethylpropyl)nonane Cycloalkanes Hexane Cyclohexane NOT isomers b.p. 68°C b.p. 81°C C H C H 6 14 6 12 CnH 2n+n C n 2n Cycloalkane Nomenclature 1) Count number of C’s in ring. This determines the root. Table 2.3 in book. Prefix is cyclo. Suffix is ane. CH CH CH 3 2 3 A cyclopentane a cyclobutane Constitutional Isomers C 6 12 C6H 12 Methylcyclopentane ethylcyclobutane 2) Number the ring carbons. If ring has 1 substituent, then that C is at position 1. If ring has two substituents, one is at position 1 and move clockwise or counter clockwise to give the second substituent the lowest number. 1 1 isopropylcyclohexanebutylcyclohexane NO YES NO 6 2 YES 1 1 5 1 3 1 4 4 2 6 3 2 2 3 3 5 1,2dimethylcyclopropane 1,2,4trimethylcyclohexane 6 C's 6 C's 4 C's 3 C's Pick alkane or cycloalkane? Tie goes to cycloalkane more C's, so it is hexylcyloalkane 1cyclopropyl butane 3 C's 4 C's Alkane! 2cyclopropylbutane
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