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Organic Chemistry
Substitution/ Elimination reactions
| Question | Answer |
|---|---|
| What does an SN2 reaction do to its stereochemistry with a chiral center? | Inversion of configuration (Nucleophile attacks from opposite side of leaving group) |
| What does the rate of a SN2 reaction depend on? | rate=k[substrate][nucleophile] |
| What is the order of reactivity of different classes of substrates in SN2 reactions? | Methyl> primary> secondary> tertiary (does not react due to steric hindrance) |
| What does the rate of a SN1 reaction depend on? | rate=k[substrate] |
| What is the order of reactivity of different classes of substrates in SN1 reactions? | tertiary> secondary> primary> Methyl (does not react because of lack of substitution on electrophilic C) |
| What structures of substrates accelerate and decelerate a SN2 reaction? | Beta branching slows SN2 An adjacent pi bond accelerates SN2 |
| What makes a reactive leaving group? | X (leaving group) mus be electronegative or polarizable atom X- should be a weak stable base (conjugate base of a strong acid) |
| For which reaction mechanism is nucleophilicity most important and how does nucleophilicity change across the periodic table? | Most important for SN2 nucleophiles stronger as move backwards across rows of PT and down columns. Same true for basicity. Strong base is a strong nucleophile. |
| What are good, moderate, and poor nucleophiles? | Poor: (worst)CH3COOH, H2O, ROH (best) Moderate: (worst)CH3COO-, F-, Cl-, CH3SH, NH3(best) Good: (worst) N3-, Br-, OH-, OR-, NR3, CN-, I-, SH- (best) |
| What kind of solvent favors the SN1 reaction? | Polar protic solvent least polar CH3COOH (HOAc), CH3OH(MeOH), H2O, CF3COOH(TFA)most polar |
| What kind of solvent favors the SN2 reaction? | Polar aprotic solvent least polar CH3CH2OCH2CH3(ether), THF (4C ring with O in ring), CH2Cl2, CH3COCH3(acetone), DMF (NR2CH=O), CH3CN, DMSO(CH3S=OCH3) |
| How can hydroxide (a bad LG) be converted to a good LG? | 1. Use a strong acid to convert -OH to H2O 2. Convert alcohol to a tosylate (sulfanate)using TsCl and pyridine |
| When does intramolecular cyclization occur? | When there are two potential LGs, one being worse than the other (usually O or N) and cyclization is entropically favored. (acid-base rxns occur faster than substitution) |
| When does a 1,2-hydride shift occur? | When there is potential for the + charge to be on a more substituted C. |
| When does a 1,2-alkyl shift occur? | When a more substituted C adjacent to the C+ can stabilize the carbocation by accepting th +charge and losing an alkyl group. |
| How does a pi bond in a substrate structure affect the structure of the product? | A pi bond provides resonance stabilization of the carbocation. The pi bond should be on the most substituted carbons to have the most stable product structure. |
| What does E1 stand for and which substitution reaction does it compete with? | Elimination unimolecular competes with SN1 |
| What does E2 stand for and which substitution reaction does it compete with? | elimination bimolecular competes with SN2 bcs the nucleophile can act as a base |
| What does E1CB stand for and which substitution reaction does it compete with? | elimination unimolecular conjugate base not usually compete with substitution |
| What is the Zaitsev product rule? | The most substituted alkene will form as a product because it is most stable |
| What stabilizes a pi bond? | Increasing substitution of the C's in the pi bond stabilizes |
| What does the rate of the E2 mechanism depend on? | rate=k[substrate][base] |
| What type of product does the E2 mechanism favor? | Favors Zaitzev product with antiperiplanar transition |
| What does the rate of the E1 mechanism depend on? | rate=k[substrate] |
| What kind of product is preferred in the E1 mechanism | Favors Zaitsev product SN1 usually favored over E1 but both types of products will form unless use dehydrating conditions (strong acid) |
| What conditions favor SN2 mechanism? | Strong Nu, unhindered substrate, aprotic solvent, lower temperatures |
| What conditions favor E2 mechanism? | strong base (OH-, OR-, NH2-, H-), bulky base, hindered substrates, higher temperatures |
| What conditions favor SN1/E1 mechanism? | Neutral/weak base, substrates must form stable C+(tertiary, secondary, allylic, benzylic), usually forms both products |
| What mechanism would a primary substrate react by? | SN2 unless there is beta branching or a bulky base (hindered substrate or Nu) then E2 |
| What mechanism would a secondary substrate react by? | SN2 if: good Nu, aprotic solvent, lower temps E2 if: strong/bulky base, higher temps SN1/E1 if: polar protic solvent with neutral Nu |
| What mechanism would a tertiary substrate react by? | E2 if: any strong base SN2: never!!! SN1/E1 if: polar protic solvent, neutral Nu |
| What are some exceptions to Zaitsev Rule? | Hofmann elimination (least substituted alkene forms) if: E2 mechanism uses a poor leaving group, there is a bulky base with a sterically hindered substrate (tertiary or secondary), if elimination can yield a conjugated pi system |
| When does a reaction use the E1CB mechanism? | There must be a very acidic H beta to the leaving group, typically occurs with an electron withdrawing group such as a carbonyl adjacent to beta hydrogens. Works with a poor leaving group. |
| How can you make an alcohol using substitution? | 1. R-X+ -OH--->ROH (SN2) 2. R-X+ H2O--->ROH (SN1) 3. R-X+ CH3COO- -->CH3COOR+ -OH--->ROH (SN2) |
| How can you make an ester using substitution? | R-X+ RCOO- ---> RCOOR (SN2) |
| How can you make an ether using substitution? | 1. R-X+ OR(alkoxide)--->R'OR (SN2) (Williamson ether synthesis: make alkoxide first: 2ROH+ 2M^0--->2M+-OR+ H2) 2. R'-X+ HOR--->R'OR (SN1) 3. 2ROH+ catalytic H2SO4--->R'OR |
| How can you make an alkyl halide using substitution? | 1. ROH+ H-X--->R-X 2. R-Cl (or R-Br)+ NaI+ acetone--->R-X 3. ROH+ TsCl & pyridine--->R-OTs+ acetone--->R-X 4. ROH+ (PI3 or PBr3 or PCl3)--->R-X 5. ROH+ SOCl2--->R-X 2-5 usually with primary, secondary substrates through SN2 |
| How can you make an amine using substitution? | Gabriel synthesis: use phthalimide in reaction as toned down version of NH2. 1.KOH to ionize 2.R-X (SN2 to attach N to R group) 3.-OH+ H2O (to separate R-N from the rest of phthalimide molecule) |
| How can you make phosphorous and sulfur molecules using substitution? | P&S both good Nu, poor bases so E2 not problem 1.sulfur: R-X+ -SCH3 (DMSO solvent)--->R-SCH3 2.phosphorous: PPh3+ R-X (ether solvent)--->PPh3R+ |
| How can carbon nucleophiles be used to make nitriles with substitution? | R-LG+ Na(+)(-)CN +(DMF solvent)--->R-CN |
| How can carbon nucleophiles be used to make alkynes with substitution? | 1.H-CC-R+ NH2-(lNH3 solvent)--->-CC-R+ NH3 2.Ph-CC-H+ R-X--->Ph-CC-R |
| How can you make hydrocarbons using substitution? | R-X+ H- --->R-H+ X- tone down H- by using LiAlH4 (use ether as solvent bcs very reactive) or NaBH4 (use methanol as solvent) |
| How does epoxide ring opening occur using substitution? | 1.Basic conditions: epoxide ring+ NaOCH3 & methanol--->open ring with OH and CH3O 2.Acidic conditions: epoxide ring+ HOCH3 & TsOH---> ring with H on O and CH3OH on most substituted C on ring--->open ring |
| How can elimination be used to make alkenes? | 1.Dehydration: CR2HCR2OH+ strong acid & heat--->CR2CR2+ H20 (E1 tertiary 50Celcius, secondary 150Celcius, primary >200Celcius E2) 2.Dehydrohalogenation: CH2XCH3+ strong base(B-)--->CR2CR2+ BH+X- (E2 any substrate) |
| How can elimination be used to make alkynes? | Double loss of HX (2X adjacent): RCHX-CHXR+ 2B---->RCCR+2HB+2X- 1.Internal alkyne: 2eq. strong base CH3CHBr-CHBrCH3+ 2KOH& heat--->CH3CCCH3+2KBr+2H2O 2.Terminal alkynes: 3eq. strong base phCHClCHCl+3NaNH2+NH3 solvent+HCl(second step)--->PhCCH |
Created by:
jebeard
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