Alkyl Halides - Lecture Slides | Organic Chemistry I | CHEM 2010, Study notes of Organic Chemistry

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Alkyl Halides Based on McMurry’s Organic Chemistry, 6th edition 2 What Is an Alkyl Halide?  An organic compound containing at least one halogen attached to an sp3 hybridized carbon  X (F, Cl, Br, I) replaces H  Can contain many C-X bonds  Properties and some uses  Fire-resistant solvents  Refrigerants  Pharmaceuticals and precursors CH; Br Br CH, CH:CHOH.CHCHCH,CH, BCA PHCH ACE 4 65 4 CH, CH, 4-dimethvlhevtane 2-Bromo-4.5-dimethvlheptane Bs rom m0-2.4 Cl Cl CH;CHCHCHCH,CH; 1 2 3 |4 5 CH; 2,3-Dichloro-4-methylhexane {©2004 Thomson - Brooks/Gole 6 Naming with Multiple Halides  If more than one of the same kind of halogen is present, use prefix di, tri, tetra  If there are several different halogens, number them and list them in alphabetical order 7 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 Properties of the Halomethanes TABLE 10,1 A Comparison of the Halomethanes Bond length Bond strength Dipole moment Halomethane (pm) (k)/mol) (kcal/mol) (D) CH3F 139 452 108 1.85 CH;Cl 178 351 84 1.87 CH3Br 193 293 70 1.81 CHI 214 234 56 1.62 ©2004 Thomson - Brooks/Cole 10 xX Electrophilic ol <— carbon 7~ ©2004 Thomson - Brooks/Cole 11 12 10.3 Preparing Alkyl Halides  Alkyl halide is formed by the addition of HCl, HBr, HI to alkenes to give the Markovnikov product (see Alkenes chapter)  Alkyl geminal dihalide from anti addition of bromine or chlorine Initiation step Propagation steps (a repeating cycle) Termination steps Overall reaction ©2004 Thomson - Brooks/Cole A H,C—H H—Cl - ~ nen Cl- H;C- + Step 2_ oP H,C—Cl Ccl—Cl H,;C: + -‘CH3; —~> H;C—CHs Cl- + -CH3 ——~ Cl—CHg, Cl: + -Cl — Cl—Cl CH, + Cl, —> CH;Cl + HCl, 16 10.4 Radical Halogenation of Alkanes  If there is more than one type of hydrogen in an alkane, the reaction favors replacing the hydrogen at the most highly substituted carbons: 3o>2o>1o  The number of available hydrogens is a factor  Methyl hydrogens, for example, often outnumber 2o or 3o hydrogens. Chlorination is unselective: Dichloro-, h ; CH,CH,CH,CH, + Cl, ——> CH,CH,CH,CH,Cl + CH,CH,CHCH, + ‘ichloro-, Butane ©2004 Thomson - Brooks/Cole CH, CH,CHCH, + Cl, 2-Methylpropane © Thomson - Brooks Cole 1-Chlorobutane tetrachloro-, 2-Chlorobutane and so on CH, hv | —. CHLCCH, + Cl 2-Chloro-2- methylpropane 30:70 CH, CH,CHCH,Cl 1-Chloro-2- methylpropane 35:65 Dichloro-, trichloro-, tetrachloro-, and so on 17  4 RCH3 ReCHe R3CH Ree Cl Energy Ro HE! Ri Cl. Reaction progress —————> © 2004 Thomson/Brooks Cale 20 21 Bromination is much more selective than chlorination: 35% 65% i " Bee — ot + HX CH, CH; 2-Methylpropane AH° = —50 kJ for X = Cl AH® = +13 kJ for X = Br 22 H H — Br —=> HBr + ©2004 Thomson - Brooks/Cole + HBr Allylic radical N—Br —— Bro + Brg H Br N—H Br: 25 Allylic Stabilization at Allylic H 360 kJ/mol (87 kcal/mol) Alkyl S y! 400 kJ/mol (96 kcal/mol) sit XL Vinylic 445 kJ/mol (106 kcal/mol) ©2004 Thomson - Brooks/Cole | i 1 i t | Cc H—C R—C: R—C R—C Cc Som | | | | ‘gin | H H H R | /\ Vinylic << Methyl < Primary < Secondary < Tertiary < Allylic ay (2) (3Y) CS a No Sissel. ome Ce stable stable 26 27 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 Allylic Radicals as Reaction Intermediates: CH,CH,CH,CH,CH,CH,CH — CH, 1-Octene | NBS, CCl, a ie a. s H,CH,CH,CH,CH,CHCH CH, <—> CH,CH,CH,CH,CH,CH =CHCH, Br | CH,CH,CH,CH,CH,CHCH = CH, + CH,CH,CH,CH,CH,CH =CHCH,Br 3-Bromo-1l-octene (17%) 1-Bromo-2-octene (83%) (53:47 trans:cis) ©2004 Thomson - Brooks/Cole 31 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 Practice Problem 10.1: Products? HH <—a H,C H,C —»b —a A H H «—B B me = H,C HC H,C ie “ Ch ~ O° ” oe H Br 3-Bromo-4,4-dimethyl- 3-Bromo-6,6-dimethyl- cyclohexene cyclohexene 3-Bromo-5,5-dimethyl- cyclohexene ©2004 Thomson - Brooks/Cole 32 Problem 10.6: Explain? CH, CH,Br NBS CCl, Major product ©2004 Thomson - Brooks/Cole Problem 10.6: mechanism oe OF ‘ more stable alkene 37 10.7 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 SOCl, “Pyridine + SO, + HCl O O Benzoin Desy] chloride (86%) OH Br 3 CH;CH,CHCH;, —;,~ ae 3 CH;CH,CHCH; + HPO; 2-Butanol 2-Bromobutane (86%) ©2004 Thomson - Brooks/Cole 40 Problem 10.23: Products? H,C OH HBr 9 ae 9 Ether OH NBS PBry (c) CO) ca, * CY iter” 7 (b) CH,CH,CH,CH,OH SOCl, ——_—- ? 41 Prob. 10.8: Synthesize from alcohols? (a) " (b) Ps a CH:COH, CH,CHCH,CHCH, CH, (c) OH (d) OH “ BrCH,CH,CH,CH,CHCH, CoH CECH a CH; ©2004 Thomson - Brooks/Cole 2 Grignard Carbon as Nucleophile: b+ I Mgl | Mg " | _-— Basic and nucleophilic WC es Ether we ~ H7/ ~y Hd ~y H H Iodomethane Methylmagnesium iodide ©2004 Thomson - Brooks/Cole 45 46 Reactions of Grignard Reagents  Many useful reactions  RMgX behaves as R- (adds to C=O)  RMgX + H3O+ yields R-H 47 10.9 Organometallic Coupling Reactions  Alkyllithium (RLi) forms from RBr and Li metal  RLi reacts with copper iodide to give lithium dialkylcopper (Gilman reagents) Henry Gilman (1893-1986) Problem 10.23 (continued): () CH,CH,CHBrCH, = 2> a? —=> B? Li Cul (f) CH,CH,CH,CH,Br ——— A? —— 8B? Pentane (g) CH,CH,CH,CH,Br + (CHs),CuLi set ? 50 51 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 52 10.10 Oxidation and Reduction in Organic Chemistry  In organic chemistry, 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 easily recognizable 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: loss of H, and/or gain of O, N, X C=C +Br, \ H H Ethylene H H \ / C=C +H, / \ H H Ethylene H H \ i C=C +HBr / \ H H Ethylene ©2004 Thomson - Brooks/Cole —. Br Br i f “~~ H ul Wt H H \, df ep ON H H Wt Ethane H Br \, df qo} H H We Bromoethane Oxidation: Two new bonds formed between carbon and a more electronegative element Reduction: Two new bonds formed between carbon and a less electronegative element Neither oxidation nor reduction: One new C-H bond and one new C-—Br bond formed 55 56 Oxidation Levels  Functional groups are associated with specific oxidation levels 57 Practice Problem 10.2: Rank in order of increasing oxidation level Problem 10.42: Identify reagents Cyr a yom b opm 61 Problem 10.42 a = BH3/THF followed by H2O2/OH-1 (hydroboration/oxidation) b = PBr3/ether c = (CH3)2CuLi/ether