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chapter10 - 10. Organohalides Based on McMurry's Organic...

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Unformatted text preview: 10. Organohalides Based on McMurry's Organic Chemistry, 7th edition Witches Worse Nightmare! Who can explain this cartoon? 2 DOROTHY "LIQUIDATES" THE WICKED WITCH OF THE WEST Having cornered our friends in the castle, the witch decides the scarecrow should "go first"... ...And Dorothy, in her haste to put out the fire on the scarecrow's arm, throws water on the witch, who promptly melts into nothingness on the floor. The guards gladly give Dorothy the broomstick and our friends head back to the Emerald City. 3 Organic Chemistry Students Worst Nightmare!! Most common NIGHTMARE is a dream about not being prepared, by showing up to school naked. Running to catch a bus. 4 Are you keeping up with the material? Play-Dough 5 Don't Forget to Carry that Organic Textbook to the Library! 6 What Is an Alkyl Halide An organic compound containing at least one carbon- halogen bond (C-X) X (F, Cl, Br, I) replaces H Can contain many C-X bonds Properties and some uses Fire-resistant solvents Refrigerants Pharmaceuticals and precursors General Formula = RX 7 Why this Chapter? Reactions involving organohalides are less frequently encountered than other organic compounds, but reactions such as nucleophilic substitutions/eliminations that they undergo will be encountered Alkyl halide chemistry is model for mechanistically similar but more complex 8 10.1 Naming Alkyl Halides Find longest chain, name it as parent chain (Contains double or triple bond if present) Number from end nearest any substituent (alkyl or halogen) We have already discussed the nomenclature of alkyl halides. You should review this on your own. 9 Naming if Two Halides or Alkyl Are Equally Distant from Ends of Chain Begin at the end nearer the substituent whose name comes first in the alphabet 10 10.2 Structure of Alkyl Halides C-X bond is longer as you go down periodic table C-X bond is weaker as you go down periodic table C-X bond is polarized with slight positive on carbon and slight negative on halogen 11 10.3 Preparing Alkyl Halides from Alkenes: Radical Halogenation Alkyl halide from addition of HCl, HBr, HI to alkenes to give Markovnikov product (see Alkenes chapter) Alkyl dihalide from anti addition of bromine or chlorine 12 10.3 Preparing Alkyl Halides from Alkanes: Radical Halogenation Alkane + Cl2 or Br2, heat or light replaces C-H with C- X but gives mixtures Hard to control Via free radical mechanism It is usually not a good idea to plan a synthesis that uses this method This is messy! How can I maximize the yield of CH3Cl or CCl4? Can you write a mechanism for this reaction? 13 Radical Halogenation of Alkanes If there is more than one type of hydrogen in an alkane, reactions favor replacing the hydrogen at the most highly substituted carbons (not absolute) Be able to write a mechanism for this reaction. Can you explain the distribution of products? 14 Relative Reactivity Based on quantitative analysis of reaction products, relative reactivity is estimated Order parallels stability of radicals Reaction distinction is more selective with bromine than chlorine Know that the order of free radical stability is the same as that for carbocations, i.e. tertiary > secondary > primary > methyl 15 Practice Problems Give an systematic name (IUPAC) for the anesthetic halothane. B r H C l C3 F Give the structure of the C5H12 hydrocarbon that would react with Br2 to give a single monobromination product. Give the structure of the C4H10 hydrocarbon that would react with Br2 to give three monobromination products, two of which are enantiomers. 16 Practice Problems Solution Give an systematic name (IUPAC) for the anesthetic halothane. (2R)2-bromo-2-chloro-1,1,1-trifluoroethane Give the structure of the C5H12 Answer hydrocarbon that would react with Br2 to B r H C l C3 F C3 H C3 H C C3 H C3 H C3C2C2C2B HHHHr 1 ro o u n -b mb ta e H C C3 H B r (2R ro o u n )-2-b mb ta e C3 H H C C2C3 HH B r (2S ro o u n )-2-b mb ta e 17 give a single monobromination product. Give the structure of the C4H10 hydrocarbon that would react with Br2 to give three monobromination products, two of which are enantiomers. Answer C3C2 HH 10.4 Preparing Alkyl Halides from Alkenes: Allylic Bromination N-bromosuccinimide (NBS) selectively brominates allylic positions Requires light for activation A source of dilute bromine atoms Can you explain why bromination of the allyic position is preferred? What kind of mechanism is taking place here? 18 Allylic Stabilization Allyl radical is delocalized More stable than typical alkyl radical by 40 kJ/mol (9 kcal/mol Allylic radical is more stable than tertiary alkyl radical 19 Can You Write a Mechanism for This Reaction? Hint: Trace amounts of C6H10 are formed. C 2 C C 2 H + B2 H H H r C2 C C2 B + Hr H H H r B 20 Can You Write a Mechanism for This Reaction? Hint: Trace amounts of C6H10 are formed. S IC IO E ICRA TIO TO H MTR E C N C2 C C2 H + B2 H H H r IN TIO ITIA N B B r r B r B (T eB r b n ismchwe ke th naC o C b n .) r h r-B o d u a r a -H r -C o d lig t h C2 C C2 B + Hr H H H r B P O A A N(Th su o th p p atio step sh u eq al th sto io etricreactio .) R P G TIO e m f e ro ag n s o ld u e ich m n C2 C C2 H H H H C2 C C2 H H H B r B B r r C2 C C2 + H B H H H r C2 C C2 B + B H H H r r C 2 C C 2 B + Hr (S Mo P O A A N H H H r B U f R P G TIO ) C2 C C2 H + B2 H H H r TE M A N R IN TIO A DS FRHA DS O! N O OT N O N 21 Why is it easier to break the C-H bond in the CH3 group during the first propagation step? Why not one of the C-H's on the double bond? A R WF R AIS V W R O O M L M IE C2 C C2 H H H C2 C C2 H H H fre ra ica is re n n sta ilize e d l so a ce b d M LC L RO B LV W OE UA R ITA IE Only when I break the C-H bond in the CH3 group can I resonance stabilize the resulting free radical. Try breaking one of the other CH bonds to verify this in your own mind. C2 C C2 H H H The free radical is distributed among all the p-orbitals. 22 10.5 Stability of the Allyl Radical: Resonance Revisited Three electrons are delocalized over three carbons Spin density surface shows single electron is dispersed 23 Use of Allylic Bromination Allylic bromination with NBS creates an allylic bromide Reaction of an allylic bromide with base produces a conjugated diene, useful in synthesis of complex molecules N-bromosuccinamide This mechanism is more complex than the free radical mechanisms we studied previously. 24 NBS Mechanism RH + NSBr (NBR) RBr + NSH + HBr (trace) + Br2 (trace) INITIATION NSBr + (heat or light) NS Br (N-Br is very week bond) PROPAGATION (two important propagation reactions) NS + H-R NSH + R (allylic radical) NSBr + R NS + RBr -------------------------------------------------------Br + H-R HBr + R (allyl radical) NS + HBr NSH + Br NSBr + Br NS + Br2 Br2 + R RBr + Br --------------------------------------------------Sum RH + NBS RBr + NSH (stociometric reaction) O N B r O N ro o ccin m e(NS -b msu a id B ) TERMINATION 2NS NSSN (not stable, not observed) 2Br Br2 2R R-R (trace?) I am using NSBr as the symbol for NBS in this mechanism. You are not required to know this mechanism, just know what happens when NBS is used. 25 10.6 Preparing Alkyl Halides from Alcohols Reaction of tertiary C-OH with HX is fast and effective Add HCl or HBr gas into ether solution of tertiary alcohol Primary and secondary alcohols react very slowly and often rearrange, so alternative methods are used 26 10.7 Reactions of Alkyl Halides: Grignard Reagents Reaction of RX with Mg in ether or THF Product is RMgX an organometallic compound (alkyl-metal bond) R is alkyl 1, 2, 3, aryl, alkenyl X = Cl, Br, I 27 10.8 Organometallic Coupling Reactions Alkyllithium (RLi) forms from RBr and Li metal RLi reacts with copper iodide to give lithium dialkylcopper (Gilman reagents) Lithium dialkylcopper reagents react with alkyl halides to give alkanes 28 Utility of Organometallic Coupling in Synthesis Coupling of two organometallic molecules produces larger molecules of defined structure Aryl and vinyl organometallics also effective Coupling of lithium dialkylcopper molecules proceeds through trialkylcopper intermediate 29 10.9 Oxidation and Reduction in Organic Chemistry In organic chemistry, we say that oxidation occurs when a carbon or hydrogen that is connected to a carbon atom in a structure is replaced by oxygen, nitrogen, or halogen Not defined as loss of electrons by an atom as in inorganic chemistry Oxidation is a reaction that results in loss of electron density at carbon (as more electronegative atoms replace hydrogen or carbon) Oxidation: break C-H (or C-C) and form C-O, C-N, C-X 30 Reduction Reactions Organic reduction is the opposite of oxidation Results in gain of electron density at carbon (replacement of electronegative atoms by hydrogen or carbon) Reduction: form C-H (or C-C) and break C-O, C-N, C-X second reaction at top of p. 349 here 31 Oxidation Levels Functional groups are associated with specific levels 32 ...
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This note was uploaded on 04/07/2008 for the course CHGN 222 taught by Professor Cowley during the Spring '08 term at Mines.

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