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CHEM312 Final
Conceptual questions
| Question | Answer |
|---|---|
| The two fundamental events in a nucleophilic substitution reaction are... | nucleophilic attack and loss of leaving group |
| How do you determine which acid is stronger? | ARIO Atom= more electronegative and larger are more acidic. Resonance=delocalizing electrons over more atoms. Induction=addition of electronegative atoms makes the compound more acidic. Orbital=sp3. |
| Describe the difference between heterolytic and homolytic reactions? | Heterolytic is a polar reaction; it involves a double headed arrow as the elections always move in pairs. Homolytic is a radical reaction; it involves single headed arrows as electrons move singley to generate ions. |
| Name the four mechanism steps for polar/ionic (heterolytic) reactions | 1. Nucleophilic attack (Nu- makes a new bond to an electrophile+) 2. Loss of leaving group (bond breaks & LG leaves) 3. Rearrangement (driving force is stablitity 1<2<3) 4. Proton transfer (proton is added/removed; acid-base reaction) |
| The LG is always attached to.. | the electrophile/substrate. (also where the Nu attacks) |
| The C bearing the LG in a substitution reaction must be what hybridization? | sp3 aka single bonds (Does not occur on vinyl or aryl systems because the p bonds are electron rich and will repel the Nu) |
| The steps in a stepwise substitution reaction are.. | loss of leaving group THEN Nu attack (SN1) This involves the formation of a carbocation as the intermediate. |
| The steps in a concerted substitution reaction are.. | Nu attack & loss of LG occuring AT THE SAME TIME (SN2) This involves a transition state when the central C is partially bonded to 5 groups. |
| Which substitution reaction (SN1 or SN2) is most likely to occur for a primary, secondary, and tertiary substrate? | Primary: SN2 (Nu attack possible from the back, not sterically hindered) Secondary: SN1 and SN2 both possible Tertiary: SN1 (Nu attack NOT sterically possible-need loss of LG 1st) |
| Describe the expression rate of an SN2 reaction. | k[Nu][R-LG] (Nu and substrate are both involved in the rate determining step-bimolecular) |
| Where does the Nu attack in an SN2 reaction? | the BACK side opposite the leaving group (sterically and electronically this is the only thing that is possible) |
| What is the stereochemistry of SN2 reactions? | Inversion of configuration at the carbon center because the Nu exhibits back side attack on the substrate (LG is a wedge and the substituted Nu is a dash) |
| What needs to happen to an -OH before it can leave? | It needed to be protonated via proton transfer because it is a poor LG |
| Describe the expression rate of an SN1 reaction. | k[L-LG] (loss of LG/formation of the carbocation is the rate determining step-unimolecular) |
| What is the order of carbocation stability? | benzyl > substituted allylic > tertiary > allylic > secondary > primary > vinyl > methyl |
| Breaking bonds is an _______ process and making bonds is an _________ process. | Endothermic; exothermic |
| What is the stereochemistry of an SN1 reaction? | Both possible products are observed because there is an equal probability that the Nu will attack either face. |
| What do the number of humps in an energy diagram represent? | The number of steps in the mechanism. (i.e. can range from 2-5 in an SN1 mechanism depending on if there is proton transfer and carbocation rearrangement) |
| What are the 4 factors that can effect the rate and mechanism of substitution reactions? | LG, Nu, Solvent effects, Carbon center (primary, secondary, or tertiary) |
| Examples of some strong Nu are.. | I-, HS-, RS-, Br-, RO-, HO-, NC-, N3- ..Nucleophilicity increases going down the periodic table (F- < Cl- < Br- < I-). Negatively charged atoms are also more nucleophilic. |
| Examples of some strong LGs are.. | conjugate bases of strong acid; TsO- (negative charge delocalized over 3 oxygens), NH3, I-, H2O, Br- |
| SN2 requires a strong _____ while SN1 requires a strong _____. | Nu; LG |
| Examples of polar protic solvents are.. | ROH, H2O, amines (RNH2).. they have H atoms that can be donated for hydrogen bonding.. used for SN1 (favors dissociation) |
| Examples of polar aprotic solvents are.. | acetone, acetonitrile, DMSO, DMF.. no H atoms available for hydrogen bonding.. used for SN2 (favors nucleophile and aprotic solvents create a polar solvent cage to make the Nu more nucleophilic) |
| The three fundamental events in an elimination reaction are.. | removal of a proton, formation of the C=C p bond, and the breaking of the bond to the LG |
| Elimination reactions which produce constitutional isomers in which one is formed in excess to the other is called ______. | regioselectivity (i.e. when the C=C is located.. usually regioselective towards the more substituted double bond-zaitsev product) |
| An elimination that gives the more substituted alkene is called the ______. | Zaitsev product |
| An elimination that gives the less substituted alkene is called the _____. | Hoffman (anti-zaitsev) product |
| Describe the stereoselectivity of elimination reactions. | Usually favor more stable trans-product over cis-product |
| What is the process and expression rate of an E2 reaction? | rate=k[B][R-LG]. It is a concerted process, so the removal of a proton (H+) by the base, loss of LG and formation of C=C occur simultaneously. |
| Describe the stereochemistry needed for an E2 reaction to occur. | H-C and C-LG bonds can NOT be in the same plane (one needs to be a dash and the pther wedge), the must be antiperiplanar (think newman projections), LG also needs to be axial) |
| E2 pathway usually occurs with a strong _____ and a poor ____. | base; LG (also a R-LG that would not lead to a stable carbocation for E1 to occur) |
| Big Bulky Bases (sterically hindered bases) will favor the _______ product. | Hofmann (less substituted) |
| What does it mean if something is stereospecific? | There is only one stereoisomer product produced (could be due to only 1 H in the beta position). |
| What is the process and expression rate of an E1 reaction? | rate= [k][R-LG]. It is a stepwise process; loss of LG to generate a carbocation intermediate THEN loss of proton (H+) from the carbocation to form the p-bond |
| E1 pathway usually occurs with a good ____ and a weak ____. | LG; Nu (also with a stable carbocation) |
| Dehydration (loss of water) is an _____ process and dehydrohalogenation (loss of Hx) is an ____ process. | E1 (need strong acid); E2 (need strong base) |
| In electrophilic addition, the C=C acts as the _________. | nucleophile |
| Syn addition of two new sigma bonds across C=C is a ________ process. | concerted (both groups come together, so they are added to the same side- both wedges or both dashes) |
| Anti addition of two new sigma bonds across C=C is a _________ process. | stepwise (groups are added on opposite sides- one dash and one wedge) |
| What is the regioselectivity in electrophilic addition if the two atoms that are added are different? | The Nu is always added to the more electrophilic carbon. The electrophile is added first to the less substituted position in order to make the more stable carbocation. |
| Regioselectivity is predicted by Markovnikov's rule, which is defined as... | the addition of the H atom to the C with the most Hs already present (the less substituted carbon) so that the halogen is placed at the more substituted C |
| Hydroboration/Oxidation of alkenes is stereoselective because.. | only 2 of the 4 stereoisomers are formed because it is a concerted process (aka syn addition) |
| Concerted electrophilic addition is ALWAYS _______. | stereoselective (syn addition) |
| Why is BH2 placed at the less substituted C in hydroboration/oxidation reactions? | steric and electronic considerations |
| Why are terminal alkynes more acidic than other hydrocarbons? | It has increased s character (sp-> closer to the nucleus so more stable) However, the C-H bond is not strongly acidic and a strong base (NH2-) is needed to deprotonate it. |
| What does tautomerization mean? | To transform from an enol to a ketone or aldehyde |
| What will always be the product from ozonolysis on an alkyne? | 2 carboxylic acids |
| A radical is a species that contains a(n) | unpaired electron |
| Why are alkyl groupd weakly electron donating in radicals? | due to hyperconjugation and inductive effects |
| Radicals are excellent nucleophiles or electrophiles? | electrophiles |
| Unlike carbocations, radicals do not undergo _______. | rearrangement |
| What are the three steps to radical substitution? | 1. Initiation (form radical). 2. Propagation (transfer radical to a different species). 3. Termination (combine radicals). |
| What are the two components that dictate the selectivity of radical halogenations? | reactivity of R-H (tertiary is the most reactive) and reactivity of X radical (i.e. Br is less reactive than Cl, but tends to be more selective in choosing a weaker R-H) |
| What do the Reactivity factors in predicting halogenation products tell us about Br and Cl | Bromination is 1640 times more likely to occur at the tertiary position over the primary while Chlorination is only 5.2 times more likely. High regioselectivity of bromination at secondary radical while chlorination is at primary. |
| How does the equation for predicting the selectivity of radicals work? | [H1(R1)]/[H1(R1)+H2(R2)] Consider all the possible products and look at the different kinds of H atoms present on the original substrate. |
| What is the hybridization of radicals? | sp2 (lone electron is in the p orbital) |
| What are allylic radicals more stable? | Similar to allylic carbocations, the p system of a double bond can also stabilize an adjacent radical through resonance. |
| The oxygen in an alcohol compound acts as a Nu or electrophile? | Nu |
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amandatucker10
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