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Chem 203 > Exam 3 > Flashcards

Exam 3 Flashcards

1
Q

How would stereochemistry of products of Diels-Alder change if the reaction was stepwise?

A

Would see more configurations

2^n configurations at each stereocenter

*We don’t see this because Diels-Alder is not stepwise

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2
Q

Where is a dashed constituent on the six-membered ring located on the diene?

A

Outside

when attacks from the top

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3
Q

Where is a wedge/up constituent on the six-membered ring located on the diene?

A

Inside

when attacks from the top

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4
Q

How can you tell if a Diels-Alder reaction has occurred?

A

Look at the number of configurational isomers

If see less than 2^n, then we know at some point the steps were concerted which indicate a Diels-Alder

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5
Q

Are endo constituents wedges or dashes?

A

Dashes

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6
Q

Are exo constituents wedges or dashes?

A

wedges

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7
Q

Is cyclohexane more stable than benzene? Why or why not?

A

Yes

Benzene’s pi-bonds make it less stable than cyclohexane which is made entirely of unreactive sigma bonds

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8
Q

How much more stable is cyclohexane compared to benzene?

A

49.8 kcal/mol

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9
Q

Why reaction is rearomatization of electrophilic aromatic substitution similar to?

A

E1 reaction

Form carbocation, than kick off proton/leaving group

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10
Q

How much aromatic stabilization does benzene have? Compared to what?

A

Benzene is 36 kcal/mol more stable than the hypothetical 1,3,5 cyclohexatriene (bonds not delocalized)

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11
Q

How many nodes does psi-6 of benzene have?

A

3 nodes

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12
Q

How many nodes dow psi-3 of benzene have?

A

1 node

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13
Q

Why is benzene not capable of a Diel’s Alder reaction with its self?

A

Both molecules would lose their aromaticity

This is highly unfavored

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14
Q

Why does Friedel-Craft alkylation often result in over-alkylation?

A

You are adding an alkyl group to the aromatic ring

This alkyl group then activates by making a better nucleophile

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15
Q

How can one experimentally stop over-alkylation of Frield-Craft?

A

Add a large excess of aromatic rings, so it is unlikely for the electrophile to interact with the aromatic ring

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16
Q

Activating group

A

An activating group is often an electron donating group that makes a reaction more likely to occur

For example: EDG makes electrophilic alkylation more likely to occur since making aromatic ring a better nucleophile

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17
Q

Why are halogens O/P-directing yet deactivating?

A

Halogens have a lone pair that can act as an EDG and be in conjugation with aromatic system, BUT due to electrogativity and strong inductive effect they have some EWG properties

This makes halogens have an overall slower rate of electrophilic addition compared to unsubstituted benzene

This makes them an O/P-director and also a deactivator

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18
Q

What is a good approach to explaining why something is a good O/P or meta director?

A

Draw the opposite and explain why this is so bad (normally due to close positive charges)

Ex: if something is o/p, draw meta intermediate and explain why this sucks

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19
Q

Rules for drawing poly-ene conjugated MOs:

A

1) Orbitals alternate in symmetry with respect to the pseudo mirror plane passing through the midpoint of the chain
2) Lowest energy MO has 0 nodes; the number of nodes increases by one going to the next highest level (rings are different-can have degenerate orbitals)
3) Highest energy MO has a node between every adjacent orbital
4) In chains with an odd number of atoms, where the central atom lies in the mirror plane, antisymmetric MOs must have 0 contribution from the central atom. There is a nonbonding level to which alternate p-orbitals make 0 contribution

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20
Q

What are the conditions for kinetic control?

A

low temperature

shorter reaction times

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21
Q

What are the conditions for thermodynamic control?

A

high temperature

longer reaction times

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22
Q

Under kinetic control is the reaction at equilibrium?

A

No since the reaction is not allowed to reverse

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23
Q

Under thermodynamic control is the reaction at equilibrium?

A

Yes

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24
Q

Why does reversibility matter for kinetic versus thermodyanmic control?

A

Whatever product forms the fastest will be the product that is “stuck” since the reaction cannot reverse to make the slower, but more stable, product

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25
Q

Why do high temperatures matter for thermodynamic control?

A

High temperatures allow the products to have enough energy to overcome higher barriers for the reverse reaction to occur

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26
Q

Why will the thermodyanmic product start to build up at equilibrium?

A

the energy barrier for the reverse reaction to occur away from the thermodyanmic product is higher (since product is more stable) and makes less thermodyanmic product reverse compared to the kinetic product

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27
Q

Why would we not originally expect endo product to be favored in Diels-Alder? What makes it favored?

A

Would not expect it to be favored due to steric crowding

Endo product is favored due to secondary orbital stabilization

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28
Q

3 types of polycyclic molecules:

A

1) spiro
2) fused
3) bridged

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29
Q

Spiro polycyclic molecule

A

connected at a single carbon

looks like an “X” between the two rings

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30
Q

Fused polycyclic molecule

A

two rings share a “wall”

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31
Q

Decalin

A

fusion of two cyclohexanes

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32
Q

What is the main HOMO/LUMO reaction of Diels-Alder?

A

Homo: psi-2 (diene)
Lumo: pi*cc (dienophile)

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33
Q

What is the secondary HOMO/LUMO reaction of Diels-Alder?

A

Homo: pi cc (dienophile)
Lumo: psi-3 (diene)

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34
Q

Why is the retro Diels-Alder reaction possible at high temperatures?

A

Retro Diels-Alder reaction features an increase in entropy

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35
Q

What makes a molecule aromatic?

A

1) Cyclic
2) Fully conjugated (every atom needs a p-orbital)
3) Planar
4) Huckel’s rule

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36
Q

Why does a ring have to be planar in order to be aromatic?

A

Allows p-orbitals to have full overlap

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37
Q

Huckel’s rule

A

Must have 4n+2 pi-electrons

38
Q

Acylium cation

A

Generated as the electrophile in Friel-Crafts Acylation

Carbon double bonded to an O and single bonded to another group

39
Q

Acyl group

A

contains a double bond oxygen

40
Q

Why do electron donating groups speed up electrophilic aromatic substitutions?

A

They make the ring a better nucleophile

Also help to stabilize the carbocation that is created

41
Q

Why do electron withdrawing groups speed up nucleophilic aromatic substitutions?

A

They make the the ring a better electrophile

Also help to stabilize the anion that is created

42
Q

What controls conjugate addition either 1,2 or 1,4?

A

Strength of the nucleophile

43
Q

What does a strong nucleophile in conjugate addition produce?

A

The 1,2 kinetic product

This is because the reaction is not reversible because the nucleophile is too strong to kick off

44
Q

What does a weaker nucleophile in conjugate addition produce?

A

the 1,4 thermodyanmic product

This is because the reaction is able to be reversed and produce the 1,4 product

45
Q

What is the cutoff for a “strong base” in conjugate addition?

A

pKa of around 25

Anything above is too strong to make the reaction reversible

46
Q

How can you tell if an electron withdrawing group will be able to stabilize a negative charge during nucleophilic aromatic substitution?

A

look at where the negative charge currently is and know that it will be able to be placed on alternating carbons

SO if there is an electron withdrawing group at one of the alternating carbons, the EWG will be able to stabilize the neg charge

47
Q

What controls the rate of a reaction?

A

The stability of the intermediate

More stable intermediate means a faster rate

48
Q

Enol

A

an organic compound containing a -OH group bonded to an sp2 carbon

C=C(OH)

49
Q

Enolate

A

anion formed when a hydrogen is removed from the alpha carbon

50
Q

alpha carbon

A

carbon closest to a functional group (often a carbonyl for our purposes)

51
Q

What is the proximity effect in the 1,2-addition of conjugated dienes?

A

After initial protonation, the nucleophile will attack the 1,2 carbocation

This is due to the 1,2 carbocation being closer than the 1,4

Kinetic reason (NOT an electronic reason due to greater stability of 1,2 cation)

52
Q

Inverse demand Diels Alder

A

The minor reaction that can occur between the diene LUMO and the dienophile HOMO

53
Q

Why are cyclic dienes excellent Diels Alder reagents?

A

They are already in the reactive s-cis conformation

54
Q

Why do esters have a higher pKa than ketones?

A

Esters have a strong EDG group that will add ED into the system with an already negative charge

Ketones just have a methyl group that will add comparably less ED into the neg. system because just uses hyperconjugation, not full on resonance

55
Q

Why is the Sn1 mechanism not optimal for nucleophilic aromatic substitution?

A

Sn1 would create a cation on an alkene after the leaving group departs

This “phenyl carbocation” is highly unstable because the empty orbital is sp2. This is a waste of s-character that wants to have electrons

56
Q

How to tell if Diels-Alder product has enantiomer?

A

If the dienophile is cis-, then both groups will be either axial or equatorial. In this case, whether attack top or bottom results in IDENTICAL molecules

If the dienophile is trans-, then one group will be axial and the other group will be equatorial. In this case, attacking top or bottom will result in DIFFERENT molecules

This rule does NOT apply if there are other assymetric carbons. Then will see different configurational relationships

57
Q

How to synthesize C-C and C-H bonds?

A

Need a carbon nucleophile and a carbon electrophile to join together

58
Q

Examples of carbon nucleophiles

A

CH3Li, diene, alkyne anion

59
Q

Examples of carbon electrophiles

A

Carbonyl derivatives, alkyl halides, some aromatic rings, carbocation, nitriles

60
Q

Why are organometallics good nucleophiles?

A

Have a high HOMO because the metal is higher in energy than the carbon (almost like a negative formal charge on the carbon)

61
Q

Synthesis of Grignard

A

oxidative insertion

magnesium loses two electrons

62
Q

Grignard

A

H3C - Mg - Br

general form:
R- Mg- X

63
Q

Alkyllithium

A

general form:

R - Li

64
Q

Synthesis of alkyllithium

A

2Li react with alkylhalide

Makes alkyllithium and Li-X

65
Q

First step of synthesis of alkyllithium

A

Donates a single electron to the sigma*C-X orbital

66
Q

Why can organometallics not be in ROH or water solutions?

A

organometallics are extremely basic (due to their high homo) and will deprotonate the solvent instead of carrying out wanted reaction

67
Q

What solvent do we often use with organometalics?

A

Ethers

68
Q

Ketones/aldehydes with organometallics

A

Organometallic homo attacks the carbonyl

quench with hydrogen proton to generate an alcohol

69
Q

Epoxides with organometallics

A

Organometallic nucleophile attacks less substituted carbon attached to epoxide

Quench with H+ to generate an alcohol

70
Q

Where does the nucleophile attack an aromatic ring?

A

at a carbon that has a good leaving group and where the resulting negative charge can be stabilized by an EWG

71
Q

What is the homo of the enolate?

A

psi-2

72
Q

What is the homo of the enol?

A

psi-2

73
Q

What do EDGs do?

A

Increase nucleophilicity

74
Q

In keto-enol tautomerism which side is favored at equilibrium?

A

the aldehyde is favored at equilibrium

the bonds in the aldehyde are overall stronger

75
Q

Differences in keto-enol tautomerism if in acid or base?

A

In acid, you will protonate the oxygen first

In base, you will protonate the oxygen last

76
Q

Is there “directing” in nucleophilic aromatic substitution like with electrophilic aromatic substitution?

A

No

Nucleophile will attack the carbon with the favorable leaving group

77
Q

What does halogenation require?

A

Lewis acid catalyst to help protonate and make sigma bonds easier to break

78
Q

What lewis acid catalysts are used in halogenation?

A

AlBr3

FeBr3

79
Q

What is generally the first step of electrophilic aromatic substitution?

A

the creation of a good electrophile

often something with a positive charge on it

80
Q

What does Friedel-Crafts alkylation require?

A

Lewis acid catalyst to help protonate and make sigma bonds easier to break

81
Q

When do you have to watch for rearrangements in Friedel-Crafts alkylation?

A

During the generation of the electrophile

If halogen is attached to ethyl group (and greater) sometimes will rearrange to stabilize the positive charge

82
Q

Starting products of Friel-Crafts acylation?

A

Aromatic ring, ketone with a halogen, lewis acid catalyst ,water

83
Q

Why is water needed in Friel-Crafts acylation?

A

need to remove the AlCl3 when it reattaches to the ketone in the final step

84
Q

What decides the regiochemistry of electrophilic aromatic substitution?

A

the groups attached to the aromatic ring

meta and o/p directors

85
Q

What is the second step in nucleophilic aromatic substitution after the Nu is attached?

A

the negative charge comes “back down” to kick off the leaving group

86
Q

heteroaromatic molecule

A

an aromatic molecule that also includes like an oxygen/nitrogen in the ring

example: furan

87
Q

What are ways to generate the acylium ion?

A

acid anhydride and lewis acid catalyst

ketone with halide and lewis acid catalyst (Friedel-Crafts acylation)

88
Q

What makes a good lewis acid catalyst?

A

an empty p-orbital

89
Q

In conjugate addition of unsaturated carbonyls what are the thermodyanmic/kinetic products?

A

1,4 is the thermodynamic product

1,2 is the kinetic product

90
Q

What reaction is the most similar to nucleophilic aromatic substitution?

A

nucleophilic acyl substitution

91
Q

What is nucleophilic acyl subsitution?

A

when a nucleophile attacks a carbonyl group and a leaving group is kicked out

92
Q

What is the relationship between end and exo products?

A

Configurational diastereomers

Chem 203 (7 decks)

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