Organic Chemistry Laboratory Summer Portraits: h+p://scien1s1c.tumblr.com

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1 344 rganic hemistry Laboratory Summer 2013 Lecture 5 Introduction to organometallic chemistry July Portraits: h+p://scien1s1c.tumblr.com

2 What is organometallic chemistry? The chemistry of compounds containing a arbon-metal bond Intersection of organic and inorganic chemistry rganic hemistry rganometallic hemistry Inorganic hemistry rganometallic chemistry = organic synthesis enabled by metals

3 Periodic Table Alkali metals s electrons Electronegativity Transition metals d electrons Main group p electrons

4 Periodic Table typical organic compounds Electronegativity rganic

5 Periodic Table common organometallics Alkali metals Electronegativity Main group Transition metals Much more to play with!!

6 rganometallics s-block compounds You are already familiar with some simple organometallic compounds from HEM 343/345 rganolithiums Br hexane + 2 Li + LiBr Li tert-butyl lithium rganomagnesium halides (Grignard reagents) Br + Mg ether MgBr phenylmagnesium bromide Lithium diorganocuprates (Gilman reagents) ether 2 EtLi + ui Et 2 uli + LiI lithium diethylcuprate

7 rganometallics s-block compounds You are already familiar with some simple organometallic compounds from HEM 343/345 Me Me Et Bu + Me Me Li Et Bu H organolithium reagent Me Me new - bond MgBr Grignard reagent + Me Et Me Et H new - bond rganic fragment of reacting as a nucleophile, has carbanion character Reactivity of -atom in a typical organic compound is as an electrophile arbon nucleophile + carbon electrophile = new - bond Why do Grignards and organolithiums react as carbon nucleophiles?

8 harge distribution hlorobenzene l A typical organic compound = posi1vely charged = nega1vely charged l X = 2.55 X l = atom in -l bond is positively charged X = Pauling electronegativity NPA charges, B3LYP/6-31G(d)

9 harge distribution Phenyl lithium Li An organometallic compound = posi1vely charged = nega1vely charged Li X = 2.55 X Li = Reversal of bond polarity -atom of -Li bond is negatively charged X = Pauling electronegativity NPA charges, B3LYP/6-31G(d)

10 arbon-metal bond polarity R E A T I V I T Y -M bond Δ Electronegativity # % ionic character* -K = Na = Li = Mg = Ti = Al = u = B = l = Br = H = Ionic Polar covalent ovalent # Pauling electronegativity, Χ * % ionic character = [(Χ Χ M ) Χ ]

11 Grignard lab Synthesis of benzoic acid Br Mg MgBr 1) 2 H Et 2 2) Hl What you need Dry + clean glassware Drierite column Anhydrous Et 2 Dry ice Bromobenzene Mg bits Small chip of iodine

12 Drierite column A column of drierite sits atop the condenser BLUE = DRY = GD as4 + ol2 PINK = WET = BAD

13 Metal can = anhydrous USE ANHYDRUS ETHER FR GRIGNARD REATIN Wash bottle = Wet D NT USE WET ETHER FR GRIGNARD REATIN

14 DRY IE = SLID 2 LD BURNS HURT T!!

15 Grignard lab Synthesis of benzoic acid Why do we need anhydrous solvent and a drying column? MgBr H H + MgBrH H PhMgBr water pka ~43 pka 15.7 The nucleophilic -atom of the Grignard reagent is strongly basic Water quenches the Grignard reagent as it is formed. Why use diethyl ether specifically? Mg Br Mg Br Et 2 and THF are able to coordinate to Mg Stabilizes compound and enhances solubility

16 Grignard lab Synthesis of benzoic acid Why use bromobenzene specifically? X + X X = F, l, Br BDE = (E Ph radical + E X radical ) - E PhX Why not use aryl iodides? More expensive, less green, side-product issues

17 Structure of Grignard reagents Typically show a Grignard in the form RMgX on paper..but in solution its much more complicated Schlenk equilibrium 2 RMgX R 2 Mg + MgX 2 2 MgBr Mg + MgBr 2 A B A and B cannot be distinguished by reaction product (i.e. their reactivity is exactly the same) Position of equilibrium depends on solvent, temperature, concentration, R-group Grignard reagents are not monomeric across all concentrations Et 2 X Mg Mg X Dimer Et 2 - Solution behavior of Grignard reagents is highly complex - Many reactive species in solution - RMgX works just fine to explain our chemistry

18 Reactivity of Grignard reagents Mg Mg Br atom of Grignard is Nucleophilic -atom of 2 is Electrophilic MgBr 1) 2 H 2) Hl NPA charges, B3LYP/6-31G(d)

19 Reactivity of Grignard reagents Mg Mg Br acceptor orbital -atom of Grignard is Nucleophilic -atom of 2 is Electrophilic MgBr 1) 2 2) Hl H LUM of 2 NB calculation, B3LYP/6-31G(d)

20 Reactivity of Grignard reagents Mg Mg Br MgBr MgBr

21 Nature of the carboxylate anion MgBr Mg d(-) = A LP(1)9 sp 0.65 LP(2)9 s 0 p 1 LP(3)9 s 0 p 1

22 Summary rganometallic chemistry - the chemistry of compounds containing a -M bond - intersection of organic and inorganic chemistry - allows impossible organic reactions to occur rganolithium and Grignard reagents - Polar -M bonds = reactive toward water/oxygen - nucleophilic carbon atom, carbanion character - strongly basic - reactive toward carbonyl groups - used in stoichiometric amounts (i.e. 1:1 or greater) Grignard lab - use dry clean glassware - use dry ether for reaction solvent, regular ether for everything else - tiny chip of I 2 to assist with initiation of reaction - dry ice burns ya! - think about which layer contains crude benzoic acid - think about purification steps

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24 YES!