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11 Cards in this Set
- Front
- Back
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What are properties of Alky halides?
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haloalkanes
saturated sp3 hybridized carbon nature: -chloromethane is released in large amounts by ocean kelp, as well as by forest fires and volcanoes industrial: -antesthetics, refrigerants, pesticides, and solvents medicine: -epibatidine more potent than morphine for blocking pain |
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What are some way to prepare alkyl halides?
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1. HX and X2 with alkenes in electrophilic addition:
-HI, HBr, or HCl give markovnikov -bromine and chlorine undergo anti addition through a halonium intermediate 2. Alkyl halides from alcohols -works best with tertiary alcohols -treat alcohol with HCl, HBr, or HI -HCl or HBr gas with cold ether -primary or secondary are converted by thionyl chloride (SOCl2) or PBr3, less acidic |
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How was Nucleophilic Substitution discovered?
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(+) and (-) malic acid occurred via an inversion in configuration at the chirality center
each step involves the substitution of one nucleophile for another (Cl or OH) X is a good leaving group (often a halide) |
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What are Sn2 reactions?
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bulky substrate makes bond difficult due to steric effects
order of preference: tertiary < neopentyl < secondary < primary < methyl vinylic halides and aryl halides are unreactive towards Sn2 some nucleophiles that work with Sn2: H2O, CH3CO2, NH3, Cl, HO, CH3O, I, CN, HS Properties of Nucleophilicity: -parallels basicity -increases going down column of periodic table -negatively charged more reactive than neutral best leaving groups are those that best stabilize negative charge in the transition state; weak bases like Cl and toyslate make good leaving groups but OH (causes solvation) and NH2 (strong bases) make poor leaving groups alkyl fluroides, alcohols, ethers, and amines do not undergo Sn2, for alcohol OH must be converted to a better leaving group unsolvated anions have the greatest nucleophilicity: CH3OH < H2O < DMSO < DMF < CH3CN |
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What are Sn1 reactions?
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reactivity:
Methyl < Primary < Secondary < Tertiary rate-limiting step is slowest the more stable the carbocation intermediate the faster the reaction allylic and benzylic cations favor reaction primary allylic and primary benzylic carbocations are as staale as sceondary alkyl carbocations secondary allylic and benzylic carbocations are about as stable as a tertiary alkyl carbocation reactivity: H2O < Cl < Br < I = TosO nucleophile has NO role in Sn1 reaction rate stabilizing the intermediate carbocation should increase rate of Sn1 according to Hammond much more reactive in polar solvents |
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How are Sn1 and Sn2 known in biological chemistry?
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pathway for thousands of terpenes
substrate is often an organo diphosphate leaving group is pyrophosphate ion PPi pyrophophate is "biological equivalent" of a halogen Sn1: fragrant alcohol found in roses and used in perfumes 2 Sn1 reactions occur with pyrophosphate as the leaving group both times Sn2: -involved in all methylations -CH3 donar is SAM contains a positively charged Sulfur -SAM is biological equivalent to CH3Cl -Epi to Nor Epi with SAM |
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What is Zaitsev's rule?
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in the elimination of HX from an alkyl halide, the more higly substituted alkene product predominates
Elimination takes place by E1, E2, and E1cB (biological) E1: C-X bond breaks first to give carbocation intermediate, followed by base removal of a proton to yield alkene E2: -C-H and C-X bonds break simultaneously, no intermediates -most common in laboratory, treated with strong base E1cB: C-H bond breaks first, giving a carbanion that loses X- to form alkene |
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What are properties of the E2 reaction?
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1. E2 shows second order kinetics, both base and alkyl halide take part in rate-limiting step
2. Deuterium isotope effect -Carbon-protium bond is weaker than Carbon-deuterium bond -rate of elimination is faster for Carbon-protium 3. E2 reactions occur with periplanar geometry -either anti (staggered, low energy) or syn periplanar (elclipsed, higher energy) sp3 of C-H and C-X must overlap occurs most easily in anti periplanar Meso compounds give E alkene, no Z anti periplanar is important in cyclohexane rings, hydrogen have to be trans diaxial for E2 to occur |
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What are properties of the E1 reaction?
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E1 and Sn1 normally occur together when alkyl halide is treated with a protic solvent and a nonbasic nucleophile
mixtures of the two are usually obtained because E1 substrates are also the best Sn1 substrates 1. first order 2. no deuterium isotope effect 3. no geometric requirement |
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What are properties of E1cB reaction?
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carbanion intermediate
common in substrates that have a poor leaving group such as OH, poor leaving group disfavors E1 and E2 Beta hydroxyl carbonyl compounds commonly converted to conjugated unsaturated carbonyl compounds (2 away from carbonyl group) ex is biosynthesis of fats |
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What happens in Primary, Secondary, and Tertiary Alkyl halides?
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Primary:
-Sn2 substitution occurs of good nucleophile used -E2 occurs if strong base -E1cB can occur of leaving group is 2 away from carbonyl but if halide is too good probably be E2 Secondary: -Sn2 if a weakly basic nucleophile used in polar aprotic solvent -E2 if strong base -E1cB 2 carbons away from carbonyl -Sn1 and E1 if weakly basic nucleophile and Protic solvent Tertiary: -E2 with strong base -Sn1 and E1 in neutral conditions, such as in pure ethanol or water -E1cB 2 away from carbonyl |
