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45 Cards in this Set
- Front
- Back
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Alkyl halides |
Halogen is bonded to an sp3 carbon alkane |
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Vinyl halide |
Halogen is bonded to an sp2 C alkene |
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Aryl halide |
Halogen is bonded to an sp2 C on a benzene ring |
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Geminal dihalide |
Two halogen atoms bonded to the same C, like gemini twins |
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Vicinal dihalide |
Two halogen atoms bonded to adjacent carbons |
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What affects boiling point? |
1) Greater intermolecular forces, highly bp 2) Greater mass, higher bp 3) Spherical shape, lower bp |
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What occurs in a substitution reaction? |
• Reagent acts as a nucleophile and attacks an electrophilic position • Nucleophile replaces LG |
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Sn2 mechanism 3
Draw the mechanism |
• Employs 2 arrows in 1 step: • No intermediate -> transition state has nucleophile and LG partially attached to molecule • Groups on substrate C get geometrically converted, and usually stereochemically inverted when LG and nucleophile have the same priortity |
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What is the highest energy point of an Sn2 rxn? Why? |
The transition state b/c it has both the LG and nucleophile partially bonded to the substrate, and so it is the most unstable |
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Sn1 mechanism 3
Draw the mechanism |
• Employs 2 arrows in 2 steps • Forms an intermediate carbocation • Forms a racemic mixture |
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What determines the rates of an Sn2 and Sn1 reaction? |
Sn2 = rate is dependent on both the nucleophile and LG, since everything occurs in one step
Sn1 = rate is only dependent on the LG, because the first step is the slow step, and the second step with the nucleophile is so fast |
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Sn2 reactions favored when...why? |
1) Electrophile (the substrate) is a 1º C, or 2º C when all other conditions favor it -> b/c of sterics 2) Nucleophile is strong b/c it speeds up the rxn quick enough for it to occur 3) A good LG -> b/c it can stabilize the charge once it's left 4) Polar aprotic solvent -> protic solvents would slow down the rxn too much b/c the nucleophile would get protonated |
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Sn1 reactions favored when...why? |
1) Electrophile (the substrate) is a 3º C, or 2º C when all other conditions favor it -> b/c the more stable carbocation reacts quicker 2) Nucleophile is weak -> b/c a weak nuc slows down the rxn so much that an Sn2 cannot occur; but the rate of Sn1 is not dependent on the nuc 3) Good LG -> b/c it can stabilize the charge once it's left 4) Polar protic solvent -> this disfavors Sn2 b/c it slows it down so much, but Sn1 rate is not dependent on nuc |
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What is the electrophile? |
• The central carbon atom which looses the LG • Called the substrate |
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What stabilizes a carbocation? |
• The more alkyl groups the more stable the carbocation -> b/c alkyl groups are electron donating towards the positive central C • Resonance from double bonds |
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Nucleophilicity vs. basicity |
Nucleophilicity = measures the rate of attack on an electrophilic carbon
Basicity = measures stability of a charge |
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What makes for a strong or weak nucleophile? (3 each)
Does nucleophilicty reverse in a polar protic solvent? |
Strong: • Negative charge which is wants to donate • Highly polarizable due to larger atomic radii • Donating inductive effect (by a less electronegative group)
Weak: • Resonance • Greater electronegativity • Taking inductive effect (by a more electronegative group) |
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What must be considered for each nucleophile because it can hinder an otherwise good nucleophile?
What affect does this have on basicity? |
Steric hinderance
Many bulky allyl groups
It doesn't have as large as an affect on basicity, so elimination is favored |
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What makes for a good leaving group?
Youtube video? Which is most important factor? |
• Polarizable = stabilizes the transition state • Electronegative = electron withdrawing to polarize substrate • A weak base = a more stable molecule once separate |
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How does the LG affect which substitution reaction will occur? |
A good leaving group doesn't favor either substitution reaction. A bad leaving group means neither will occur |
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Which halide is the best and which is the worst LG? |
I- > Br > Cl >>> F- is the worst b/c it's bond to C is very strong |
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What is solvation? |
The process by which a solvent stabilizes the product, usually a charged species |
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Which solvent favors which reaction? |
Polar aprotic = Sn2 Polar protic = Sn1 or E1 |
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Why do the polar aprotic and protic solvents favor the substitution reactions that they do? 3 |
• A polar protic solvent would protonate the nucleophile, inhibiting it from attacking the substrate. This greatly slows down the rate of the reaction, so that Sn1 occurs before Sn2 can • Sn1's rate is independent of the nucleophile and so the protonation does not affect it • Polar protic solvents have a large dipole moment, and help to stabilize the Sn1's carbocation transition state |
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What role do crown ethers play? 3 |
• They increase of the strength of the nucleophile by salvating the nucleophile's cation • This speeds up the rate of reaction and assists an Sn2 to occur • Fluorine breaks from its cation and becomes fluoride -> a good nucleophile b/c of the negative charge |
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What are the 5 common polar aprotic solvents?
Know their structures? |
1) Acetonitrile 2) Acetone 3) DMF 4) DMSO 5) DME |
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Walden inversion |
An inversion of configuration via an Sn2 rxn |
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In an Sn1 rxn, is there usually more retention or inversion of configuration? |
Usually more inversion in the racemic mixture |
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What occurs in a elimination reaction? |
• Reagent acts as a base and removes a beta proton • Nucleophile removes LG, and a double bond is formed |
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What effect does heat have on a rxn? |
It favors the elimination rxn over the substitution |
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What is an α carbon and a β carbon? |
α carbon = C attached to LG β carbon = C attached to α C |
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E2 mechanism How many arrows in how many steps? Intermediate/transition state? What happens? (3) |
• Employs 3 arrows in 1 step: • No intermediate -> transition state has base, LG and H partially attached to molecule 1) Base takes a β proton from the β positioned C 2) This rxn causes the expulsion of the LG at the α position 3) A double bond then forms from the α C to the β carbon |
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E1 mechanism How many arrows in how many steps? Intermediate/transition state? What happens? (3) |
• Employs 2 arrows in 2 steps: • Forms a carbocation intermediate 1) LG leaves on its own accord, forming a carbocation intermediate (same as Sn1 rxn) 2) Carbocation looses a β proton to the base 3) A double bond then forms from the charged α C to the β carbon |
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Stereoselective vs. stereospecific When does each occur? During which rxn does each occur? How many products do they produce? Which are these products? |
Stereoselective: • Occurs when β C has 2 H's attached to it • Occurs during both the E1 and E2 rxns • Produces more than 1 product: the Zaitsev cis or trans and Hoffman cis or trans
Stereospecific - anticoplanar: • Occurs when β C has only 1 H attached to it • Occurs only during the E2 rxn • Produces only 1 product: either cis or trans |
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When both a cis and trans product are possible, which is preferred? |
The trans product b/c it minimizes overlap and is more stable |
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Does rearrangement occur in Sn1, Sn2, E1, E2, all, some, rxns?
Why? |
Rearrangement (methyl or hydride shifts) occur in Sn1 and E1 rxns only
Never in E2 or Sn2 b/c there is no carbocation |
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Which rxns have the same conditions? What is the result of this? |
E1 and Sn1
A mixture of products is obtained, although one will be favored over the other. Depends on whether: 1) The solvent is a weak nucleophile or weak base 2) There is heat involved - E1 |
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What is the chemical formula of ethanol? Is it a nucleophile or base? Which reaction occurs? |
CH3--CH2--OH
It is both a nucleophile and a base! Both E1 and Sn1 occur |
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What are 3 strong bases that are NOT nucleophiles?
Which reaction(s) do they favor? |
t-butoxide DMAP DBU
They favor E2 |
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What is anticoplanar? For which reaction does the molecule need to be anticoplanar? Why must this occur? |
Anticoplanar = when the LG and proton to be removed are anti (180º) from to another
Must be for an E2 elimination to occur
The atoms must aligned like this b/c it minimizes any steric hinderance and allows a pi bond to form in the transition state |
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Which elimination reaction has no geometrical requirements in order for it to occur? |
E1
E2 must be anticoplanar |
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When is the Hoffman product favored over the Zaitsev product? Why? |
When the base is bulky
It is easier for the bulky base to abstract the least hindered H+, and so it gives the less substituted alkene as the major product |
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What is a prerequisite for an anticoplanar conformation on a cyclohexane? |
• An anticoplanar conformation can only be achieved when both the H and LG occupy axial positions on a chair conformation • The chair must flip to this position in order for the reaction to take place |
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What happens when there is no nucleophile or base, but an acid?
Which reaction(s) is this? |
1) The acid gives up a H to the solution to form a base 2) The lone pairs of the LG attract the H, but this gives the LG a positive charge (ex. OH becomes H2O+ 3) The LG breaks off and leaves all on its own in order to take its electron from its C bond and neutralize the positive charge 4) This forms a carbocation as the alpha C is now positively charged
Acid as a reagent = indirectly incites LG to leave = Sn1 or E1 rxn |
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What happens when LG is a poor leaving group? |
It must be protonated, which gives it a positve charge and incites it to leave |
