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Chem 203 > Exam 2 > Flashcards

Exam 2 Flashcards

1
Q

Gibbs free energy of alkene reactions

A

Enthalpy favors alkene to alkane reactions

At higher temperatures, entropy favors the reverse (alkene to alkane) reactions

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2
Q

Electrophilic addition to alkenes overall mechanism

A

H of acid attacks less substituted carbon

Carbocation forms

Initial leaving group attacks the carbocation

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3
Q

regioselectivity of electrophilic addition to alkenes

A

H goes to less substituted carbon

Oftentimes, a halogen goes to the more substituted carbon

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4
Q

stereoselectivity of electrophilic addition to alkenes

A

there isn’t any

the halogen can attack the carbocation from either top or bottom, resulting in 2 enantiomers being formed

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5
Q

What does hydration of an alkene require?

A

Acid catalyst

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6
Q

Why does hydration require an acid catalyst?

A

Protonate H2O, so that OH- is not the initial leaving group

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7
Q

Is hydration reversible?

A

Yes through E1 reactions

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8
Q

E1 reaction

A

leaving group leaves and carbocation is formed

then nucleophile deprotonates H from other carbon and alkene is formed

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9
Q

Hydration overall mechanism

A

Acid protonates H2O

H3O+ deprotonates alkene and forms carbocation

H2O attacks carbocation

H2O deprotonates alkane to make neutral

Neutral alkane and H3O+ are formed

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10
Q

Regioselectivity of hydration

A

The -OH group will be added to the more substituted carbon due to more substituted carbon having the cation that H2O attacks

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11
Q

Markovinkov alcohol

A

-OH group is added to more substituted carbon

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12
Q

When are markovinkov alcohols seen?

A

Hydration

Halogenation with a nucleophilic solvent

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13
Q

Stereoselectivity of hydration

A

there isn’t any

H2O can attack the carbocation from either top or bottom

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14
Q

How does an acid catalyst lower the LUMO of H2O?

A

Makes the LUMO sigma*O+ H

The + charge on oxygen means that there is a greater Zeff and the orbital’s energy will be lowered, making a lower LUMO

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15
Q

Solvent of halogenation

A

Nucleophilic or non-nucleophilic solvent matters

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16
Q

Regioselectivity of halogenation?

A

None seen with a non-nucleophilic solvent

Nucleophilic solvent will only attack the more substituted carbon since halonium cation intermediate creates a slight positive charge on the more substituted carbon

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17
Q

Halonium cation

A

intermediate in halogenation where there is a positively charged halogen intermediate

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18
Q

Stereoselectivity of halogenation?

A

Nucleophile will only attack anti- in second step since this is where the sigma* lobe is exposed

Similar to a SN2 reaction

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19
Q

What does anti- attack in halogenation produce?

A

Only the trans- configuation

Syn would result in the cis- configuration (Meso) which we don’t see

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20
Q

LUMO of halogenation

A

Sigma* Br-Br or Sigma* Cl-Cl

Great LUMOs because of:

1) low starting position on MO diagram
2) poor overlap of bulky orbitals

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21
Q

Peroxycarboxlyic acid LUMO

A

O-O is a low lumo due to initial low starting energy

22
Q

Is expoxidation concerted? What indicates this?

A

Yes expoxidation is concerted

This is indicated by only seeing cis- (meso) configuration or only seeing trans- configurations, not both, in products

(* cis or trans depends on E or Z starting configuration)

Essentially the concerted mechanism “locks” the original configuration in place

23
Q

E configuration alkene products of expoxidation

A

2 Trans enantiomers

24
Q

Z configuration alkene products of expoxidation

A

1 meso cis- configuration

25
Q

Regiochemistry of expoxidation in base

A

Base will attack the less substituted side of the intermediate since it attacks in a SN2-like nature

26
Q

Regiochemistry of expoxidation in acid

A

Acid will create a cation intermediate

This positive charge will allow the nucleophile to attack the more substituted (and more positive) carbon

27
Q

Configurational diastereomers

A

can only happen with 2 or more asymmetric carbons

configurations are not the same or mirror images

ex: RS and RR or E and Z

28
Q

Configurational enantiomers

A

can happen with only 1 or more asymmetric carbons

carbons that are completely symmetric

ex: RRR to SSS

29
Q

Example of Conformational enantiomers

A

rotating around a sigma bond to make gauche on the other-side

30
Q

Meso compound checklist

A
  1. Has at least 2 stereocenters
  2. There is an internal mirror plane
  3. Ensure tht each half is infact a mirror image by doing R/S
  4. Each stereocenter must have the same connectivity
31
Q

Can Meso compounds be chiral?

A

No

Always achiral due to internal plane of symmetry

32
Q

Can meso compounds have an enantiomer?

A

No since it is achiral

33
Q

Why does going from a chair to boat conformation not change cis to trans?

A

A conformational rotation around a sigma bond does not break a bond to make a configurational change

34
Q

Why is an acid catalyst needed in dehydration through an E1 mechanism?

A

-OH is a bad initial leaving group so it needs to be acid catalyzed

35
Q

Halohydrin

A

halogen and hydroxyl (OH) group bonded to adjacent carbon atoms

36
Q

Regioselectivity of hydroboration

A

The partial positive charge on the transition intermediate makes the more-substituted charge gain a hydrogen proton

37
Q

Stereoselectivity of hydroboration/oxidation

A

SYN mechanism since hydroboration is concerted

38
Q

triakyl borane

A

the final product of hydroboration since the reaction will include 3 alkenes for every 1 borane

39
Q

HOMO and LUMO of the key rearrangement of oxidation of trialkyl borane

A

HOMO: sigma B(-) - C

LUMO: sigma* o-o

40
Q

What are the reactants of oxidation of an alkyl borane?

A

Alkyl borane

Hydrogen peroxide

Hydroxide (a strong base to deprotonate H2O2)

41
Q

What are the products of oxidation of an alkyl borane?

A

3 alcohols and B(OH)3

42
Q

What is the stereochemical relationship between cis and trans?

A

Configurational diastereomers

43
Q

Which direction does the o-o bond break in percarboxylic acid?

A

Percarboxylic acid breaks in the direction that places a negative charge on the carboxylic acid and not on the hydroxide

This is favored by pKas

44
Q

Why do we use percarboxylic acid?

A

the low o-o lumo

45
Q

Why do we use percarboxylic acid?

A

the low o-o lumo

46
Q

Difference between epoxidation in acid and base?

A

there is regioselective difference

in acid, the oxygen is protonated which creates a partial positive charge on the more substituted carbon and nucleophile will attack the more substituted carbon

in base, the nucleophile attacks in SN2-like manner, so will attack the less substituted carbon

47
Q

Nonbonding

A

a term that describes the energy of an orbital

if a molecular orbital is nonbonding, it has the same energy as the starting atomic orbitals

48
Q

Conjugation

A

it is the linking together of p orbitals beyond a simple pi-bond

leads to the delocalization of electrons

49
Q

What is true of the configuration and conformation of meso compounds?

A

They are configurationally and conformationally identical

50
Q

What to check when molecules seem like configurational enantiomers?

A

Check that they are not meso compounds

Meso compounds have the R/S - S/R configuration that also appears in configurational enantiomers which can be confusing

51
Q

In the initial step of halogenation how should the halogen be oriented to show the 3 arrow step?

A

Vertically over the alkene

Chem 203 (7 decks)

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