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Organic Chemistry

Substitution/ Elimination reactions

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