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Organic Chemistry 1 > Mechanisms 2 > Flashcards

Mechanisms 2 Flashcards

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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
Q

Hyperconjugation

A

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)
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26
Q

Zaitsev’s Rule

A

The most substituted alkene is most stable/lowest PE

  • Elimination = substituted alkene dominates product mixture
27
Q

Reagents: Acid, Heat + Alcohol

A

Common E1: heating an alcohol in acid:

  • Eliminates HOH
  • Produces Zaitsev Product
28
Q

Small Base

A

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
Q

Large Base

A

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
Q

How to Favor E2 over SN2

A

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
Q

Nucleophile Only (SN1 or SN2)

A

Think SUBSTITUTION!

Halides:

  • Cl-
  • Br-
  • I-

Sulfur Nucleophiles:

  • HS-
  • RS-
  • H2S
  • RSH
32
Q

Base Only (E2)

A

Think ELIMINATION!

  • H-
  • t-BuO-
    • ​Steric hindrance makes this better base than nucleophile, favoring E2 over SN2
33
Q

Strong Nucleophile / Strong Base (SN2 or E2)

A

Think 1 STEP MECHANISM!

  • HO-
  • MeO-
  • EtO-
  • t-BuO-
    • Too much steric hindrance to act as a nucleophile, typically strong base
34
Q

Weak Nucleophile / Weak Base (SN1 or E1)

A

Think 2 STEP MECHANISM!

  • H2O
  • MeOH
  • EtOH
35
Q

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.

A

Axial, anti, E

36
Q

Relative Rates of Radical Halogenation

A

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
Q

H-type Preference for Radial Halogenation

A

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
Q

Reagents: Cl2, hv

A

Radical Chlorination of an Alkane

  • Two Products: Markovnikov and Anti-Markovnikov Replacement of alpha-H with Chlorine
39
Q

Reagents: Br2, hv or NBS, hv

A

Radical Bromination of Alkane:

  • Markovnikov Replacement of Alpha-H with Bromine
  • Doesn’t interact with double bonds, only replace H’s
40
Q

Radical Stability and PE

A

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
Q

Three Parts of a Radical Chain Reaction

A
  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
Q

Reagents: Br2, cold/dark

A

Bromonium Ring on Alkene:

  • Mark Addition
  • Interacts with double bonds to add 2 Br or Br + OR
  • Trans Products
43
Q

Reagents: HBr, ROOR

A

Antimarkovnikov Radical Halogenation of Alkene:

  • Anti-Mark addition
  • Interacts with double bond in radical chain reaction
44
Q

Reagents: HBr, cold/dark

A

Markovnikov Addition of Br to Alkene:

  • Mark Addition
  • Interacts with double bond to add 1 Br
45
Q

Synthesis: When starting with an alkane, always “__________”

A

Start with radical halogenation

46
Q

Reagents: 2 Li°, diethyl ether

A

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
Q

Reagents: Mg°, diethyl ether

A

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
Q

Reagent: LiAlH4

A

Reacts as if Hydride Anion (H-):

  • Strong H- delivery reagent
  • Reacts with any carbonyl
  • Reduces aldehydes/ketones to alcohols (followed by acid workup)
49
Q

Reagent: NaBH4

A

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
Q

Reagents: Li°, CuI

A

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
Q

A bulky “_________” is better than a bulky “________”

A

nucleophile, electrophile

52
Q

“__________” are better electrophiles than “__________”

A

Aldehydes, ketones

53
Q

Reagents: Aldehyde, H2O, H3O+ (catalyst)

A

Hydrate Formation:

  • Adds H2O to the aldehyde
  • Protonates C=O
  • Adds H2O and deprotonates to leave diol
54
Q

Reagents: Aldhyde, HC≡N, -C≡N (catalyst)

A

Cyanohydrin Formation:

  • Adds HCN to aldehyde
  • Protonates existing C=O
  • Adds C≡N to molecule
55
Q

What base is too patient for nucleophilic addition-elimination?

A

Any base with Bromide (Br)!

56
Q

Reagents: -Nuc, H-Nuc, Acid Halide

A

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
Q

Reagents: SOCl2, carboxylic acid OR PBr3, carboxylic acid

A

Acid Halide Preparation from Carboxylic Acids:

  • Makes acid halide by replacing the OH with Cl or Br
58
Q

Reagents: R’-NH2 (methyl or primary amine), H+ (catalyst), aldehyde/ketone

A

Imine Formation:

  • Replaces the C=O with C=N-R’
  • Produces water
  • Deprotonates N twice
59
Q

Reagents: R’-NH-R’ (secondary amine), H+ (catalyst), aldehyde/ketone

A

Enamine Formation:

  • Replaces C-C=O with C=C-NR’2
  • Produces water
60
Q

Reagents: 1 molar equivalent R’-OH, H+ (catalyst)

A

Hemiacetal Formation:

  • Turns C=O into OH group
  • Adds OR’ group to carbon
61
Q

Reagents: 2nd molar equivalent R’-OH, H+ (catalyst)

A

Acetal Formation:

  • Replaces OH group with OR’ group
  • Produces water
62
Q

Reagents: Ethylene glycol, H+ (catalyst), aldehyde/ketone

A

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
Q

Reagents:

  1. NaH (acid), alcohol
  2. R’-X (alkyl halide)
A

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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