Term
| unequal sharing of electrons gives a bond.... |
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Definition
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Term
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Definition
| the shifting of electrons in a bond in response to the electronegativity of nearby atoms |
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Term
| what are the units of electronegativity? |
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Definition
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Term
| what is the tilting point, so to speak, for when a bond stops being polar covalent and starts just being ionic? |
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Definition
| if the electronegativity difference between the participating atoms is greater than 2, it's ionic. |
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Term
| electrostatic potential map |
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Definition
| shows calculated charge distributions over a molecule; red is electron-rich, blue is electron-poor |
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Term
| polarity of molecules (magnitude and direction of the dipole moment) depends on... |
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Definition
| the vector sum of individual dipoles |
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Term
| what are the units of dipole moments? |
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Definition
| debyes - measure of the charge (in coulombs, Q) at one end of the molecular dipole times the distance between the two charges |
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Term
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Definition
| an electron that participates in resonance - it doesn't actually move, just takes part in a "hybrid" structure |
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Term
| bronsted-lowry base vs. lewis base |
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Definition
bronsted: acid donates H+ in solution, base accepts it
lewis: base donates electron pair, acid accepts electron pair |
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Term
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Definition
| lone pair from a base attacks and acid; the base becomes PROTONATED |
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Term
| what is the equation for the equilibrium constant reaction? |
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Definition
| [product][product]/[reactant][reactant] |
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Term
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Definition
acid equilibrium constant; the larger, the stronger the acid
[H+][A]/[HA]= Ka |
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Term
| a strong acid will have a ______ Ka and a ______ pKa |
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Definition
| a strong acid will have a HIGH Ka and a LOW pKa (-logKa) |
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Term
| to react with an organic compound, the pKa of the opposing acid must be.... |
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Definition
| stronger (more negative, lower) than the pKa of the base |
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Term
| what makes for a strong acid? |
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Definition
| a stable conjugate base; this can be caused by resonance, charge on electronegative ion, H being one away from electronegative ion, or being on a multiple bond |
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Term
| factors that influence acidity: |
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Definition
1. acidity increases with weight of the group H is holding on to - increases towards the RIGHT and DOWN on the periodic table - S-H is a strong acid
2. inductive effect (H on or near an electronegative ion)
3. resonance
4. multiple bonds = stronger acid |
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Term
| three things that are lewis acids |
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Definition
- metal cations
- species with unfilled orbitals (like BF3)
- electrophiles |
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Term
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Definition
| induced dipole-induced dipole intermolecular forces |
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Term
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Definition
| a carbonyl with one side of the C bonded to a nitrogen |
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Term
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Definition
| a carbonyl with one side of the carbon bonded to a chlorine |
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Term
as you add more carbons to an alkane, boiling point goes __________ because....
as you add more substituants to an alkane, boiling point goes __________ because.... |
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Definition
as you add more carbons to an alkane, boiling point goes UP because there are more intermolecular (van der Waals) forces holding them together.
but as you add more substituants, they don't fit together well, so boiling point goes DOWN. |
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Term
| from what factors does ring strain derive in cycloalkenes, and what do larger alkenes do to alleviate it? |
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Definition
| ring strain- loss of planarity, the carbon can't assume a tetrahedral angle; bigger rings can pucker to reduce strain. |
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Term
| how do you tell how much strain a given molecule is under? |
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Definition
| you burn it and measure the heat - the more heat it gives off, the more strain it was under |
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Term
| describe cis and trans decalin |
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Definition
| two fused cyclohexane rings; cis decalin has the two hydrogens at the fuse on the same side; this causes the rings to be held almost at right angles to each other, keeps them from ring-flipping, and is very unstable. Trans-decalin has the fusion hydrogens trans to each other, and the basic shape is more or less planar, and MUCH more stable (although it also doesn't flip) |
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Term
| molecules with n chirality centers have how many stereoisomers? |
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Definition
2n
(ex: 3 chirality centers, nine stereoisomers) |
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Term
| enantiomers that differ in some (but not all!) chirality centers; molecules that differ in ONLY ONE chirality centers; molecules that differ in ALL chirality centers |
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Definition
1. diastereomers
2. epimers
3. enantiomers
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Term
| how does one seperate a racemic mixture? |
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Definition
| by using a chiral reagant |
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Term
| chirality of nitrogen, sulfur, and phosphorus |
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Definition
| even though these are tetrahedral, they do NOT form chiral centers because they flip too rapidly. |
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Term
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Definition
| molecule that is achiral but can become chiral by a single alteration |
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Term
| how do you tell if a hydrogen on a prochiral molecule is pro-R or pro-S? |
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Definition
| exchange the hydrogen for something exactly one unit heavier (deuterium!) then find chirality. If the hypothetical chirality is R, the hydrogen is pro-R, and vice versa |
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Term
| what are the four types of reactions and what happens in them? |
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Definition
- additions (usually alkene to alkane - adding something on)
- eliminations (usually alkane to alkene - removing something)
- substitutions (the reactants exchange substituants)
- rearrangements (exactly what it sounds like) |
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Term
| homolytic vs. heterolytic cleavage |
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Definition
homolytic: a bond breaking symmetrically, resulting in radicals
heterolytic: bond breaks, but both electrons go to one fragment |
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Term
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Definition
| the ability of a molecule to have its electrons pushed around; LARGER molecules are more polarizable |
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Term
| a large equilibrium constant means.... |
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Definition
| the reaction is very favored to go to the product side. It does NOT say ANYTHING about how fast the reaction is. |
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Term
| relate Gibbs free energy, the equilibrium constant, enthalpy, and entropy in one equation |
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Definition
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Term
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Definition
| the energy required to break a bond into two radical fragments when in the gas phase at 25C |
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Term
| two reasons for more-highly sub'd carbocation stability |
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Definition
- hyperconjugation (empty p orbital stabilized by properly oriented pi electrons)
- inductive effect - more polarizable groups nearby distribute and stabilize electrons |
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Term
| to display a magnetic field/dipole, an atom must have: |
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Definition
- odd number of protons
- odd number of neutrons
either one - NOT both (then will be even) |
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Term
| quantization and spin states of nuclei |
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Definition
| spin number is quantized and known as "l". In 13C and 1H, l = 1/2. Number of possible spin orientations is 2l+1. Therefore for 13C and 1H, there are two possible spin orientations - parallel and antiparallel. |
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Term
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Definition
| the applied magnetic field to get electrons to polarize |
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Term
| energy required to resonate nucleus in NMR |
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Definition
varies based on the nature of the atom and also on the magnitude of the Bo (larger Bo is, larger energy has to be)
based on Planck's law -> E = hv where h is a constant and v is frequency |
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Term
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Definition
x-axis: strength felt by nucleus
y-axis: old ones measure intensity of signal, we measure time to decay |
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Term
| why are 13C and 1H NMR measured on different scales? |
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Definition
| because of the different energy requirements to bring them into resonance |
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Term
| what is an appropriate solvent for NMR? |
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Definition
| something with deuterium, so it doesn't give off a signal |
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Term
| two factors that influence chemical shift (make the signal go downfield) |
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Definition
1. electronegativity of nearby atoms
2. sp2 hybridization also deshields
- both of these together, like in a carbonyl group, is VERY low field. |
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Term
| what is a problem with doing 13C NMR and how is it addressed? |
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Definition
| low natural abundance causes a lot of background noise, which is solved by running multiple scans |
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Term
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Definition
| are chemically the same and would give the same signals |
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Term
| if replacing a hydrogen would result in a chirality center, the hydrogen is.... |
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Definition
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Term
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Definition
| if you removed one, it would make a chirality center; AND the molecule already has a chirality center! these hydrogens give different signals from each other |
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Term
| how do you find pKa of an acid when given pH and molar concentrations? |
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Definition
| pH = pKa + log [base/acid] |
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Term
| vinylic carbon vs. allylic carbon |
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Definition
vinylic: has a double bond
allylic: one away from a double bonded carbon |
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Term
| why are trans isomers more stable than cis isomers? |
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Definition
| cis isomers have steric strain between the substituent groups |
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Term
| why do alkenes react well in addition reactions? (to become alkanes) |
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Definition
1. their products are lower in energy than their reactants
2. the electrons in the pi bonds are "exposed" and susceptible to reaction
3. alkenes are nucleophiles, so they donate electrons |
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Term
| structure of carbocations |
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Definition
| trigonal planar - sp2 hybridized |
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Term
| two things that contribute to carbocation stability |
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Definition
large EDGs - bigger alkyl groups are more polarizable and donate more electrons to the positively charged carbocation
hyperconjugation - empty p orbital interacts with properly oriented H-C sigma bonds |
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Term
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Definition
| the physical structure of a transition state most closely resembles its most stable neighbor (energetically) - explains why more substituted carbocations form faster - TS is lower in energy and so moves faster! |
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Term
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Definition
x: effective strength felt by nucleus
y: intensity of signal |
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Term
| why do 1H and 13C have different scales? |
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Definition
| because of the different rf values needed to bring them into resonance |
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Term
| what is an appropriate solvent for NMR? |
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Definition
| one which doesn't give off a signal, like DCCl3, which has a deuterium |
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Term
| how do you measure chemical shift in ppm? |
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Definition
| distance from TMS signal (Hz) / instrument frequency (Hz) |
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Term
| how does hybridization influence chemical shift? |
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Definition
| sp3 is more upfield than sp2 (sp2 is less shielded) |
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Term
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Definition
Distortion Enhancement by Polarization Transfer - does three scans:
1. broadband decoupled is a normal 13C showing all carbon shifts
2. DEPT-90 shows CH only
3. DEPT-135 shows CH and CH3 as positive and CH2 as negative |
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Term
| three kinds of polymerization; when is each most likely to happen? |
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Definition
- radical polymerization (default)
- acid-catalyzed (when there is an EDG to stabilize the carbocation)
- base-catalyzed (when there is an EWG to stabilize the carbanion) |
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Term
| three stereospecific polymerizations |
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Definition
- isotacting means everything is on the same side
- syndiotactic means alternatic sides
- atactic means random placement |
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Term
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Definition
| enables you to control polymerization (getting isotactic or syndiotactic) by using TiCl3 and Al-Cldiethyl; exploiting that surface AlCl3 has only 5 Cl attached |
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Term
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Definition
| takes an alcohol and uses SN2 to replace with halide, reversing stereochem; then us an ox agent to replace with OH again but DONT use SN2, and you'll get the opposite stereochemistry you had before! |
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Term
| what are three things reaction rate is based on? |
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Definition
- relevant concentrations
- temperature
- ΔG‡ |
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Term
| what does rate law depend on? |
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Definition
| mechanism - therefore it can only be determined experimentally |
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Term
when ranking SN2 substrates, which is LESS reactive - a primary halide whose carbon is attached to a quartenary carbon, or a secondary carbon?
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Definition
| the primary carbon attached to a tertiary carbon is less reactive even than a secondary carbon! |
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Term
| what are two substrates on which SN2 will NEVER occur? |
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Definition
| aryle halide or vinylic halide - Nu: physically can't get in from behind |
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Term
| what makes for a good leaving group? |
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Definition
| species that can best stable negative charge (larger halogens, say) |
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Term
| what does a polar protic solvent do to an SN2 reaction? |
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Definition
| forms a solvation shell around Nu:, stabilizing it and increating AE |
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Term
| how do the four factors of interest in SN2 reactions affect the reaction in terms of mechanics/energy? |
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Definition
1. using a protic solvent will lower the energy of the Nu:, increasing AE, so we use an aprotic one to raise the Nu: energy
2. using a highly substituted substrate requires more energy to overcome the steric strain, raising the TS and the AE
3. using a bad leaving group requires more energy to get rid of it, raising TS and AE
4. using a very charged nucleophile - high energy of Nu: means TS is easier to achieve/AE is lower |
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Term
| why do you need a polar solvent for SN2? |
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Definition
| polar solvents will dissolve salts but won't for a solvation shell to stabilize Nu: |
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Term
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Definition
| in elimination reactions, the more substituted alkene product will form (more substituted double bond) |
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Term
| how could you tell that an elimination was E2 or E1 without looking at any of the conditions? |
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Definition
| Deuterium isotope effect - in E2, having a D instead of an H will slow the reaction, because E2 is a one step reaction and therefore breaking the stronger C-D bond is part of the rate-limiting step. In E1 there are two steps, and breaking the D-C bond isn't the rate limiting one, so it won't slow the reaction at all |
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Term
| what are two reasons why E2 happens in antiperiplanar geometry rather than synperiplanar? |
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Definition
1. synperiplanar has tons of steric strain so it's very high in energy
2. in synperiplanar the leaving group will repel the Nu: |
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Term
| why must the E2 reaction happen with periplanar geometry? |
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Definition
| to allow the sp3 orbitals to overlap (need this to form a double bond in a single step) |
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Term
| what does mass spec measure, exactly? |
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Definition
| mass/charge ratio; practically, charge is usually 1 so gives mass |
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Term
| what do you get when you hit an organic molecule with an electron in mass spec? |
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Definition
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