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Organic Chemistry 1 > Mechanisms 2 > Flashcards

Mechanisms 2 Flashcards

1
Q

Hammond’s Postulate

A

The transition state looks more like the structure it’s closest in energy to.

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2
Q

SN2 Reaction

A

When Does SN2 Happen:

  • Leaving Group Substitution: Methyl > Primary > Secondary (never tertiary)
  • Nucleophile: Strong/ Impatient base or nucleophile
    • Negative sign on X, O, N, or C
    • Small molecule (less steric hindrance) = Zaitseff product
    • Lower down on periodic table is better
    • Halides = SN2 or SN1
  • Polar Aprotic solvent (promotes SN2 and E2)

Regiochemistry: The product’s absolute configuration is the opposite of the reagent’s

Rate Expression: k*[nucleophile]*[electrophile]

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3
Q

SN1 Reaction

A

When Does SN1 Happen:

  • Leaving Group Substitution: Tertiary > Secondary (stable carbocations)
      • Benzyllic & Allylic Carbocations (you can resonance your way into SN1, but steric hindrance keeps SN1 in SN1)
  • Nucleophile: Poor/patient base/nucleophile
    • Neutral nucleophile or negative sign on an X, O, N, or C that can resonate well
    • Small molecule
    • Lower down on periodic table is better (nucleophiles)
    • Halides = SN2 or SN1
  • Polar Protic Solvent (promotes SN1 and E1)

Stereochemistry: Zaitsev (small nucleophile) or Hofmann (large nucleophile)

Reigochemistry: Produces enantiomers/racemic mixture

Rate Expression: k*[electrophile]

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4
Q

E1 Reaction

A

When Does E1 (beta elimination) Happen:

  • Leaving Group: Tertiary and Secondary (stable carbocations)
  • Base: Poor/patient base
    • Neutral base or negative sign on an O, N, or C with resonance (not X)
    • Higher up on periodic table is better (base)
  • Heat promotes elimination reactions

Stereochemistry: Always Zaitsev Product, Trans/E Product

Reaction Rate: k*[electrophile]

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5
Q

E2 Reaction

A

When Will E2 Happen:

  • Leaving Group: Tertiary and Secondary > Primary (not Methyl)
  • Base: Good/impatient base:
    • Negative sign on an O, N, or C (not X)
    • Small base = competition with SN2 & E2 and Zaitseff product
    • Bulky base = E2 and Hoffman product
    • Higher up on periodic table is better (base)
  • Heat promotes elimination reactions
  • Polar Aprotic Solvent (promotes SN2 and E2)

Beta Hydrogen has to be Anti to Leaving Group

Regiochemistry: Zatisev w/small base, Hofmann w/large base

Reaction Rate: k*[electrophile]*[nucleophile]

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6
Q

Reagents: Br2 & light or NBS

A

Free Radical Halogenation:

  • Functionalizes alkanes (typically first step in synthesis)
  • Only occurs on sp3 hybridized carbons

Regiochemistry: Markovnikov Addition or More Stable Radical (benzylic > allylic > tertiary > secondary with resonance)

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7
Q

How to Determine if Good Leaving Group

A

Characteristics:

  • Lower pKa
  • Easier bonds to break
  • Stronger acids

Promote:

  • SN2
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8
Q

How to Determine if Good Nucleophile

A

Characteristics:

  • Larger pKa
  • Releases more energy when bond forms
  • Weak bases
  • Left and Down on Periodic Table (atoms get softer)

Favors:

  • SN2
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9
Q

Very Good Leaving Groups

A

Want EWG:

  • RSO3- (most often R = CF3, tol, or CH3)
  • I-
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10
Q

Good Leaving Groups

A
  • R2O (water, alcohol, or ether)
  • Br-
  • Cl-
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11
Q

Poor Leaving Groups

A
  • F-

All Strong Bases:

  • RO-
  • R2N-
  • R3C-
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12
Q

Very Good Nucleophiles

A
  • RS-
  • NC-
  • I-
  • PR3
  • **R3C-
  • **R2N-
  • **RC≡C-
  • **RO-

​** = Strong bases that will favor elimination with heat

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13
Q

Good Nucleophiles

A
  • Br-
  • R2S
  • NR3
  • Cl-
  • RCO2-
  • N3-
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14
Q

Poor Nucleophiles

A

All Strong Acids:

  • F-
  • HCO3-
  • R2O (water, alcohol, or ether)
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15
Q

Reagents: HX (acid) + Alcohol

A

Halogenation of an Alcohol using HX (X= Cl, Br, or I)

  • Two step mechanism:
    • Create a good leaving group before SN2
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16
Q

Reagents: SOCl2

A

Halogenation of an Alcohol

  • Replaces bad OH leaving group with good Cl leaving group
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17
Q

Reagents: PBr3

A

Halogenation of an Alcohol:

  • Replaces bad OH leaving group with good Br leaving group
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18
Q

Synthetic Uses of SN2 Reactions

A

Replace leaving groups with:

  • Any nucleophile
  • OH
  • OR
  • R
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19
Q

Small Nucleophiles

A

Definition:

  • Single atom
  • Methyl or primary not next to a branch point
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20
Q

Large Nucleophiles

A

Definition:

  • Tertiary
  • Secondary
  • Primary next to a branch point (attached directly to a 2° or 3° atom)
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21
Q

Common Polar Protic Solvents

A
  • Water
  • Methanol
  • Ethanol
  • Acetic acid
  • DMF

Favors SN1 (hydrogen bonding stabilizes carbocation in solution)

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22
Q

Common Polar Aprotic Solvents

A
  • Acetone
  • DMSO
  • THF
  • Chloroform
  • Dichloromethane
  • Diethyl ether

Favors SN2 (don’t want hydrogen bonding)

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23
Q

Common Nonpolar Solvents

A
  • Benzene
  • Cyclohexane
  • Liquid carbon dioxide
  • Hexane
  • Carbon tetrachloride
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24
Q

Conjugation

A

Definition:

  • Neighboring p orbitals that are parallet to one another creating a bond-like attraction
  • Favorable
  • Lowers potential energy of molecule
  • Shortens bonds between double bonds because of shared electron density
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25
Hyperconjugation
Definition: * Empty p orbital aligning not perfectly parallel with a sigma bond * Weaker than proper conjugation * Shares electron density and stablizes carbocations (why tertiary is best) Lowers the pKa of most carbocations to zero (easy to pull off hydrogens beta to the carbocation carbon) * Tertiary carbocation is such a strong acid it will react with even a very weak base (why second step in elimination occurs)
26
Zaitsev's Rule
The most substituted alkene is most stable/lowest PE * Elimination = substituted alkene dominates product mixture
27
Reagents: Acid, Heat + Alcohol
Common E1: heating an alcohol in acid: * Eliminates HOH * Produces Zaitsev Product
28
Small Base
Definition: * Single Atom * Methyl or Primary not next to a branch point Directs to Zaitsev Product * Small enough to reach any ß-H with minimal steric hindrance
29
Large Base
Definition: * Tertiary * Secondary * Primary next to a branch point (attached directly to a 2° or 3° atom) Directs to Hofmann Product * Too big and too much steric hindrance to reach crowded hydrogens
30
How to Favor E2 over SN2
When a small base/nucleophile is being used: * Normal conditions will produce both products * Heat will favor elimination (likes strong base) * Removing heat/cold will favor SN2 (likes good nucleophile) * Polar aprotic solvents will favor both Large base will favor E2 because of steric hindrance
31
Nucleophile Only (SN1 or SN2)
Think SUBSTITUTION! Halides: * Cl- * Br- * I- Sulfur Nucleophiles: * HS- * RS- * H2S * RSH
32
Base Only (E2)
Think ELIMINATION! * H- * t-BuO- * ​Steric hindrance makes this better base than nucleophile, favoring E2 over SN2
33
Strong Nucleophile / Strong Base (SN2 or E2)
Think 1 STEP MECHANISM! * HO- * MeO- * EtO- * t-BuO- * Too much steric hindrance to act as a nucleophile, typically strong base
34
Weak Nucleophile / Weak Base (SN1 or E1)
Think 2 STEP MECHANISM! * H2O * MeOH * EtOH
35
E2 is only possible on a substituted cyclohexane ring ONLY when the LG is in an "\_\_\_\_\_\_" position and the beta hydrogens are "\_\_\_\_\_\_" to the LG. This will create the "\_\_\_\_\_\_\_" product.
Axial, anti, E
36
Relative Rates of Radical Halogenation
F2 \>\> Cl2 \> Br2 \>\> I2 * F is so fast it's explosive * Cl selective for secondary \< tertiary = allylic = benzylic * Br selective for secondary \<\< tertiary \<\< allylic \<\< benzylic * I is too slow to be practical * Typically used as an inhibitor to terminate a radical halogenation
37
H-type Preference for Radial Halogenation
Benzyllic \> Allylic \> Tertiary \> Secondary \> Primary \> Methyl * Benzyllic = a H on a benzyllic C / a C next to a benzene ring * Allylic = A H on an allylic C / a C next to a pi bond
38
Reagents: Cl2, hv
Radical Chlorination of an Alkane * Two Products: Markovnikov and Anti-Markovnikov Replacement of alpha-H with Chlorine
39
Reagents: Br2, hv or NBS, hv
Radical Bromination of Alkane: * Markovnikov Replacement of Alpha-H with Bromine * Doesn't interact with double bonds, only replace H's
40
Radical Stability and PE
Highest to Lowest PE: * H * CH3 * Primary * Secondary * Tertiary * Allylic * Benzylic * X Radicals can be resonance stabilized, but not rearranged * No methyl or hydride shifts, only resonance structures
41
Three Parts of a Radical Chain Reaction
1. Initiation = net generation of radicals * Create a problem 2. Propagation = one radical consumed, one radical produced * Push the problem onto something else 3. Termination = net consumption of radicals * Add terminator/inhibitor to solve the problem * Typically I because it reacts so slowly or radicals with high conjugation structures/high resonance to make them stable
42
Reagents: Br2, cold/dark
Bromonium Ring on Alkene: * Mark Addition * Interacts with double bonds to add 2 Br or Br + OR * Trans Products
43
Reagents: HBr, ROOR
Antimarkovnikov Radical Halogenation of Alkene: * Anti-Mark addition * Interacts with double bond in radical chain reaction
44
Reagents: HBr, cold/dark
Markovnikov Addition of Br to Alkene: * Mark Addition * Interacts with double bond to add 1 Br
45
Synthesis: When starting with an alkane, always "\_\_\_\_\_\_\_\_\_\_"
Start with radical halogenation
46
Reagents: 2 Li°, diethyl ether
Lithium Reagent to make Base: * Only react with alkyl halides to make base * Acidic hydrogens (hydrogens next to a functional group) will neutralize the carbanion before other reactions can occur * Reacts as base when in solution with a hydrogen donator (HOH, HOR) * Doesn't react well with alkyl halides once made into grignard/lithium reagent - likes carbonyls Think of these as salts with a carbanion!
47
Reagents: Mg°, diethyl ether
Grignard Reagent to make Base: * Only react with alkyl halides * Acidic hydrogens (hydrogens next to a functional group) will neutralize the carbanion before other reactions can occur * Reacts as base when in solution with a hydrogen donator (HOH, HOR) * Doesn't react well with alkyl halides once made into grignard/lithium reagent - likes carbonyls Think of these as salts with a carbanion!
48
Reagent: LiAlH4
Reacts as if Hydride Anion (H-): * Strong H- delivery reagent * Reacts with any carbonyl * Reduces aldehydes/ketones to alcohols (followed by acid workup)
49
Reagent: NaBH4
Reacts as if Hydride Anion (H-): * Milder H- delivery reagent * Only reacts with aldehydes and ketones * Reduces aldehydes/ketones to alcohols (followed by acid workup)
50
Reagents: Li°, CuI
Lithium Dialkyl Cuprate Reagents: * Loose half of the original molecules to counterion permanently * Softer and milder than lithium and Grignard reagents = work on alkyl halides once the reagent * Don't react with alkyl halides once the reagent
51
A bulky "\_\_\_\_\_\_\_\_\_" is better than a bulky "\_\_\_\_\_\_\_\_"
nucleophile, electrophile
52
"\_\_\_\_\_\_\_\_\_\_" are better electrophiles than "\_\_\_\_\_\_\_\_\_\_"
Aldehydes, ketones
53
Reagents: Aldehyde, H2O, H3O+ (catalyst)
Hydrate Formation: * Adds H2O to the aldehyde * Protonates C=O * Adds H2O and deprotonates to leave diol
54
Reagents: Aldhyde, HC≡N, -C≡N (catalyst)
Cyanohydrin Formation: * Adds HCN to aldehyde * Protonates existing C=O * Adds C≡N to molecule
55
What base is too patient for nucleophilic addition-elimination?
Any base with Bromide (Br)!
56
Reagents: -Nuc, H-Nuc, Acid Halide
Nucleophilic Acyl Substitution: * Nucelophile attaches to a carbonyl, then reforms the carbonyl by kicking out the LG in a SN1 reaction * Can only occur on sp3 hybridized carbons
57
Reagents: SOCl2, carboxylic acid OR PBr3, carboxylic acid
Acid Halide Preparation from Carboxylic Acids: * Makes acid halide by replacing the OH with Cl or Br
58
Reagents: R'-NH2 (methyl or primary amine), H+ (catalyst), aldehyde/ketone
Imine Formation: * Replaces the C=O with C=N-R' * Produces water * Deprotonates N twice
59
Reagents: R'-NH-R' (secondary amine), H+ (catalyst), aldehyde/ketone
Enamine Formation: * Replaces C-C=O with C=C-NR'2 * Produces water
60
Reagents: 1 molar equivalent R'-OH, H+ (catalyst)
Hemiacetal Formation: * Turns C=O into OH group * Adds OR' group to carbon
61
Reagents: 2nd molar equivalent R'-OH, H+ (catalyst)
Acetal Formation: * Replaces OH group with OR' group * Produces water
62
Reagents: Ethylene glycol, H+ (catalyst), aldehyde/ketone
Protection of a Carbonyl Against Nucleophiles: * Adds OCCO ring onto carbonyl to keep nucleophile from attacking it * Undone with excess water and acid * React more readily with aldehydes than ketones
63
Reagents: 1. NaH (acid), alcohol 2. R'-X (alkyl halide)
Williamson Ether Synthesis: * Turns alcohol into conjugate base, then adds an R' group to produce an ether

Organic Chemistry 1 (2 decks)

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