Polar Reactions and Alkenes: Nucleophilic Addition and Energy Changes - Prof. Richard Hubb, Study notes of Organic Chemistry

Download Polar Reactions and Alkenes: Nucleophilic Addition and Energy Changes - Prof. Richard Hubb and more Study notes Organic Chemistry in PDF only on Docsity! Alkenes are nucleophilic!!! Reactions can be carried out between an electron rich double bond and electron poor electrophiles H-Br: * Stray Arid Cp® Sge> * Posefal K © donoe k® 'S & poor , Hrerctre cb 76 on <lectrogliic 2 Reaction Mechanism: Rule 3: Electrophile can be either positively charged or neutral Ss iP A pe ots " @aa:* koe Nose --> —~~ + %e Rule 4: The octet rule must be followed. No second row atom can be left with 10 electrons. If 2 electrons attack a carbon with 8 valence electrons, then 2 electrons must leave as a result. 7) 2On Ou Cecbon les Se's & mush lee lrwe3) Specific Examples: “O° “oO ADL + &e + O6r &= oN? - @ Hyd + OU-c -— Kan —¢} + Sg 62) " trgramoleuder” to: Equilibria and Energy Changes: aA +66 = ec+ JD key = Lemsnets | . leY [xy? ee Lees] Ea Te Fr 1S D1, tre ceccton procends th He Cisht Were ZI) the renetiron PPceds he He Leppe Example: [crac 6] Keq = ———— TS 27 sce” . Cremer [h8e} Preteed$ eight AG*(standard free energy) is the AG of a reaction carried out under standard conditions se late 2+ 298K, AG? and k,, are mathematically related: x Aw = “eT Ia Key —AC?, keg ec ax R= B 316 S/Uemi) T = Temperature (K) e = 2.718 In Keq = natural log of Kegq 10 How do we use these equations? Example: Ethylene + HBr has ak.g= 25x10" Calculate AG° at 298 K: ln Key = [n (7.506?) = Ig. Nee = -QT lake = - ( Sus Tugba (20949 8.15 Ac s ~4y, $00 T/mo) AG= -4u.g Kol Exersenzc Bvocable (lets bh rg) 11 The free-energy change is made up of two terms: Ae - Ak’ -— TAS vw uu Ents tenn He Saget (BominenE) ‘inpy tom | me=en, + BBE OS chy be | Aw = -44.8 leap.) AK** 84. As*= -owa kta Ts 296k 12 Example: A — Rte C Eliminator) Ss° ss pote {i — 2] A+Q > © CAlbIm> Asis asdve fz2—\J Pihylene + HBr yields a_a_— AS* = !32 Fa) 2 G Summary of all we have discussed: TableS.2 95385 Be prepared to do calculations! 15 Describing a reaction: Bond dissociation energy (D): [AE] Aerwunt s& rey required th break « sive, bond to Prine twee redtcet tases Cges—phese +b zstcd See Table 6.3 for an extensive data list Methane C-H bond [D = 438.4 kJ/mol] 438.4 KJ/mol absorbed to break a C-H bond 438.4 KJ/mol released to form a C-H bond 16 17 Calculating ∆Ho   Monochlorination of methane Reactant Bonds Broken: C­H ∆ = 438 kJ/mol Cl­Cl ∆ = 243 kJ/mol Total ∆ = 681 kJ/mol Product Bonds Formed: C­Cl ∆ = 351 kJ/mol H­Cl ∆ = 432 kJ/mol Total ∆ = 783 kJ/mol ∆Ho= 681 kJ/mol ­ 783 kJ/mol = ­102 kJ/mol Only an approximation Does not account for entropy or solvation (solvent interaction) First Step of the Ethylene + HBr Reaction: fL Trensithen Shi Choe + Sg, (intermediates) AG, é Wc=cn, + Hor (Fenetnts) Renn Prisress ———> As reactants approach each other (collide), the energy goes up (electron cloud repulsion) As they approach, the C-H bond begins to form 20 Transition State: Represents the highest energy structure between the reactants and the intermediate (or products) ¥& + Unstable/Cannot be isolated In this example the transition state is made up of: x c=c Pochealt, broken Ga bond) Hr CAH Cond partrally formed Activation Energy: The energy difference between the reactants and the transition state WT Determines how rapid, the reachon ctewy ab< Shr temp W Leese Note ~ stor reactor KK Small NOT = ~ Fish reccton 21 Typical Organic Reactions: UYo- fro KS poo} Act WK Leo 4T rl cxn bles glee shoe bets room +enp. 4 > Fo KA)mol Xn times place atc Lister temp Crore & needed) The transition state is the pivitol point. From there, the reaction can proceed in either direction: *« Back to the reactants x Forward to the products/intermediates AG* is the energy difference between the reactants and the products 22 25 Reaction Intermediate: A chemical species that exists only transiently during a multistep  reaction Differs from: Each step has its own unique: Reactants Transition state Products Activation Energy Transition state Energy change Full Energy Diagram for Ethylene + HBr: Cha, Be Re«chon Frosress — How many steps? a Transition states? & Intermediates? | 26 Quick Overview of Enzyme Catalyzed Reaction: Uncatalyzed ee Enzyme catalyzed Reaction progress —————> (©2007 Thomsen Higher Edueation 27 Organohalogen compounds: [C, H, X] X = Halogen Halogens are considered to be equivalent to hydrogens when calculating degrees of unsaturation Example: Cy Hy ac —> “Cy hy” vs. [eu He | Hip - Hg = Ih, fi Organooxygen compounds: [C, H, O] Oxygens are not counted. They are ignored when calculating degrees of unsaturation Example: Cgh,O —) Cry Hg Geme no Chense) My-Hy > Hy = 2h -f2{ 30 Organonitrogen compounds: [C, H, N] Subtract the number of nitrogens from the total number of hydrogens when calculating degrees of unsaturation Example: Cs Han ———+5 "Cs hg™ Kin-hg * 2h, = [2] “—~o“™ ~ lol's lo ns 31 Summary - Degree of Unsaturation * Compare the number of hydrogens for the saturated formula and the molecular formula in question *x Add the number of halogens to the number of hydrogens * Do not count the oxygens in your comparison *x Subtract the number of nitrogens from the number of hydrogens 32 Common Names: Alkenes TABLE 6.1 Common Names of Some AHeerres’ A/Kens Compound Systematic name Common name H,C=CH, Ethene Ethylene CH,;CH=CH, Propene Propylene CH; CH;C =CH, 2-Methylpropene Isobutylene CH H,C—=C— CH=CH, 2-Methyl-1,3-butadiene Isoprene CH,CH = CHCH=CH, 1,3-Pentadiene Piperylene “Both common and systematic names are recognized by IUPAC for these compounds. ‘©2004 Thomson - Brooks/Cole Alkene Substituents: koe kia Wc=ch—ey methylene group vinyl group allyl group 35 36 Electronic Structure of Alkenes Carbon atoms in a double bond are: Three equivalent orbitals at 120o bond angles (planar) The fourth orbital is an atomic p orbital Two sp2 orbitals combine to form a sigma bond Sideways, additive interaction between two p orbitals  creates a pi bond An occupied pi orbital (bonding) prevents rotation around  the sigma bond Rotation is prevented by a large rotational energy barrier Rotation of pi bond 1s prohibitively expensive (energy): a bond Broken 7 bond after rotation (p orbitals are parallel) (p orbitals are perpendicular) (©2004 Thomsor/Brooks Cole Bond must Ge bwleen in order to Pt amend He S bot. Envy, bere DS He Gd Sreykh 37 Cis/Trans Naming Examples: cis te ~dime, | ~Y-eu 40 The E,Z Designation: Sequence Rules New Example: Maem oo ew fg , o™ HK be Py u [7Tve distnet Lompounas | *K Nahe cmpds is “7s or “trans The cis/trans designation only works with alkenes... Fr Hut Lee twe tdetel soups (Fen cnet ving) Corben 41 Compare where the higher priority group on each carbon is with respect to the C=C bond and then use the appropriate prefix: E: Entgegen — Opposite sides Z:. Zussamen - “dosetur™ on He seme side © low : @- --- cae LD 2 ON : lows E Doub and © lor Low ZS Doubl. Gord 42