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Chem 203 > Exam 1 > Flashcards

Exam 1 Flashcards

1
Q

What is a chemical reaction?

A

Electrons moving between orbitals

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2
Q

What does a line represent in a Lewis structure?

A

Valence electrons being stabilized by two nuclei coming together

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3
Q

What is a chemical bond?

A

Attraction of electrons being stabilized by other nucleus

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4
Q

Weak bond BDE

A

40 kcal/mol to break

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5
Q

Strong bond BDE

A

100 kcal/mol to break

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6
Q

What does negative enthalpy mean?

A

Heat is given off

Bonds formed have lower energy than original bonds

We like this

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7
Q

What two factors does covalent bonding rely on?

A

Orbital overlap

Energy match

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8
Q

Schrodinger equation

A

explains how electrons behave

allows us to generate wavefunction

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9
Q

How many nodes does an orbital have?

A 
1s = 0 nodes
2s = 1 node (radial)
2p= 1 node (angular at nucleus)
3s=2 nodes
3p=two nodes
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10
Q

What indicates a p-orbital on a wavefunction graph?

A

A node at the nucleus

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11
Q

Spherical/radial node

A

Occurs when the graph passes 0 and switches from positive to negative, but does not happen at the nucleus

s-orbitals can have spherical nodes

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12
Q

Characteristics of destructive interference

A

Produces an antibonding orbital

There is a node directly between two atoms

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13
Q

Characteristics of constructive interference

A

Produces a bonding orbital

There is stabilization from the two waves coming together

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14
Q

Sigma H2 bond strength

A

104 kcal/mol

Strong bond!

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15
Q

Sigma C-C bond strength

A

85-90 kcal/mol

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16
Q

Sigma bond rotation in alkane energy

A

3kcal/mol

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17
Q

Bond rotation in alkene energy

A

Can’t rotate alkene double bond because of pi-bond

Have to break the bond to rotate

BDE is 65 kcal/mol

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18
Q

Tricks for choosing the most acidic proton

A

Not going to be on the alkene, cause going to go in an orthogonal orbital

Want protons that are next to vinyl carbon

Draw conjugate base and look for delocalization through pi-system

Pi-system will be interrupted by sp3 hybridization

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19
Q

Why does carbon not use s2p hybridization?

A

could only make one orbital

need 2/3 s. Carbon only has 2s orbital to work with and if 2/3 of it are used up it would not be able to make another s2p

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20
Q

Can you have sigma bonds with p-orbitals?

A

Yes, if they arrange end to end

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21
Q

Lewis acid

A

electron pair acceptor

unoccupied orbital

we care about the LUMO

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22
Q

Lewis base

A

electron pair donor

occupied orbital

we care about the HOMO

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23
Q

Why is two OMOs coming together always destabilizing?

A

Because electrons will have to be placed in the antibonding orbital

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24
Q

Can 3 electrons coming together bond?

A

Yes because only 1 would have to be placed in the antibonding orbital

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25
Q

Functionality of bonds?

A

Unusual HOMO and/or LUMO compared to C-C bond or C-H bond of alkanes

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26
Q

Why is a lone pair an unusually high HOMO?

A

It is not getting stabilized by another atom

So this makes it an unusually high HOMO

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27
Q

What can be an unusually high HOMO?

A

Sigma, pi, or lone pairs

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28
Q

What can be an unusually low LUMO?

A

Sigma, pi, or empty atomic orbital

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29
Q

Why does SN2 reactions favor 1º carbons?

A

There is room for the nucleophile to physically access the LUMO

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30
Q

What proves the backside attack of a SN2?

A

chirality

an assymmetric carbon will feature inversion

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31
Q

What are the four main indicators of reactivity?

A
  1. Charge
  2. Differences in polarity
  3. “Surviving” atomic overlap (lone pairs, empty p-orbital)
  4. Poor overlapping atomic orbitals (p-orbitals in pi-bond or poor energy match)
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32
Q

LUMO and HOMO of alcohol depending if an acid or base

A

LUMO: sigma* (low cause of bad energy match)

HOMO: lone pair in sp3 orbital

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33
Q

What happens if there is a bad energy match?

A

There is less stabilization

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34
Q

Can you have only 1 electron in the bonding MO and still be stable?

A

Yes

H2+ is an example of this

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35
Q

Why is a 2p orbital higher in energy than a 2s?

A

2p has a node at the nucleus which prevents electrons from getting as close to the nucleus,

thus minimizing force of attraction and increasing PE

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36
Q

What is a node?

A

Place where phase changes from +/- and passes through 0

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37
Q

What type of nodes do s-orbitals have?

A

radial

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38
Q

What types of nodes do p-orbitals have?

A

Angular

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39
Q

Why do atoms not stabilize by complete constructive inteference?

A

Nuclear repulsion

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40
Q

Two reasons why the pi-bond is weaker than most sigma-bonds?

A

1) pi-bond has poor overlap compared to sigma bond

2) pi-bond has electron density above and below atoms. Sigma holds electron density directly between the atoms

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41
Q

Why would atomic orbitals hybridize?

A

To enhance overlap

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42
Q

Nuclear node

A

A node at the nucleus of an atomic orbital

Ex: p-orbitals have nuclear nodes

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43
Q

Antibonding node

A

A node between atoms

Ex: sigma*

44
Q

Alkene pi-bond dissociation energy

A

65 kcal/mol

45
Q

What is the interaction of a LUMO with another LUMO?

A

always inconsequential in regards to energy

46
Q

What causes inversion in a SN2 reaction?

A

the backside attack

47
Q

pKa of alkene next to an oxygen

A

we know the pKa of alkene is ~45. Since the oxygen can probably stabilize the conjugate base a little, expect to be a slightly stronger acide. pka~43

48
Q

When looking at a 3D MO that appears to be sigma bond can we automatically assume it is between two hybridized atomic orbitals?

A

No

If there is no node at the atom, it is probably not hybrizidized and shows maybe an s-orbital interaction

Something to look out for

49
Q

Hyperconjugation

A

a stabilizing effect where nearby sigma bonds help stabilize a carbocation

50
Q

What 3 things can happen to a carbocation?

A

1) SN1
2) E1
3) Hydride shift (then SN1 or E1)

51
Q

What orbitals interact for hyperconjugation in a teritary carbocation?

A

Filled sigma CH and empty 2p orbital

52
Q

What are some things to look for when determining most acidic proton?

A
  1. Allyl proton
  2. Not on an alkene
  3. Able to stabilize on an electroneg. atom through resonance
53
Q

What is the LUMO in an E2?

A

sigma* CH mixed with sigma*CX to create the “true” lumo

54
Q

Allyl proton

A

H that is on a carbon located one position away from the double bond of an alkene

55
Q

Why is steric crowding not an issue to E2 reaction on a tertiary alkyl halide?

A

sigma* CH orbital is still exposed and allows the for the base to attack

56
Q

What is an alkene stabilized by?

A

Filled sigmaCH and empty pi*CC

57
Q

Why is alkene hyperconjugation stabilizing even when using the pi* orbital?

A

The electrons are delocalized

Resonance is overall stabilizing

58
Q

How can you determine if E1 or E2 occurred?

A

1) Stereochemistry: look at configurations of products. E2 will only make 1, while E1 will have 2 configurations (need to use a molecule with two asymmetric carbons)
2) Kinetics: look at the rate law. E1’s RDS is not controlled by the nucleophile. E2’s RDS is reliant on the base

59
Q

True or false: a six membered ring is more stable than an eight membered ring

A

yes

sometimes use hydride shifts to go from 6 membered ring to 8 membered ring

60
Q

4 factors to consider when choosing type of mechanism

A
  1. Substrate
  2. Nucleophile/base
  3. Solvent
  4. Temp
61
Q

What can the substrate tell us about the favored reaction?

A

primary favors SN2 because there is little steric hindrance to backside attack

tertiary favors SN1/E1 because the carbocation can be better stabilized

62
Q

What can the nucleophile/base tell us about the favored reaction?

A

Strong base will favor the E2
Strong nucleophile will favor the SN2

Weak base/nucleophile will favor the E1/Sn1

63
Q

What can the solvent tell us about the favored reaction?

A

Polar aprotic solvent indicates SN2/E2 (DMSO, DMF, acetone)

Polar protic solvent indicates SN1/E1 since hydrogen bonding can stabilize the carbocation/initial leaving group (water, ethanol, ammonia)

64
Q

How can you determine hydrogen bonding?

A

Dipole

Lone pair on oxygen, nitrogen, or flourine that is attached to a hydrogen proton

65
Q

List of good nucleophiles (will favor SN2/E2)

A

NH3

H2O

F- (SN2)

Br- (SN2)

Cl- (SN2)

66
Q

List of polar aprotic solvents (will favor SN2/E2)

A

DMSO, acetone, DMF

*Will not have hydrogen bonding

67
Q

Why is anti- better than syn- for a E2 reaction?

A

Syn is higher in energy so will not be present often

68
Q

What is the nature of the “true lumo” in the E2 reaction?

A

it is pi- in nature

69
Q

Regiochemistry

A

one chemical bond forming is favored over another

deals with different constitutional isomers that could have

70
Q

Stereochemistry

A

deals with configurational differences

71
Q

Sterics

A

size of atoms and how that plays a role

i.e. steric bulk, steric repulsion

72
Q

Electronics

A

where electrons are (what orbitals) and how they move

example of an “electronics argument” is hyperconjugation

73
Q

Why are more substituted alkenes more favored than less substituted alkenes?

A

They can have more hyperconjugation between sigma ch and pi*cc

74
Q

Why are elimination reactions favored at higher temperatures?

A

At higher temperatures, entropy is more positive, so elimination reactions favored since they feature an increase in energy

75
Q

Why does SN1 result in mixture of both conformations?

A

Nucleophile can attack either lobe of the empty 2p in the carbocation

76
Q

What is required for diastereomers?

A

2 asymmetric carbons

Diastereomers: same R/S configuration

Enantiomers: different R/S configuration

77
Q

Which reactions are regioselective?

A

E1 and E2 reactions are regioselective

78
Q

Why is cyclopropane a significantly stronger acid than propane?

A

Look at the stability of the conjugate base

Cyclopropane has weird hybrids between the C-C due to steric strain. They have lots of p-character in order to bend

This makes the orbital that holds the H proton have lots of s-character

Therefore, it is a more stable orbital for the lone pair to be in the greater s-character orbital of the cyclopropane versus the normal propane

79
Q

Banana bond

A

sigma poor overlap caused by weird hybrid with less s-character

have to stretch bonds in order to account for steric strain in bizarre constitutions

80
Q

pKa of alkane

A

50

81
Q

pKa of alkene

A

45

82
Q

pKa of alkyne

A

25

83
Q

What accounts for the differences in pKas between alkane to alekene to alkyne?

A

As the s-character of the orbital increases, the stability of placing a lone pair in that orbital increases, and therefore alkyne have lower pKas than alkanes

84
Q

Why is O+sp3 lower in energy than Osp3?

A

There is a greater Zeff

Same number of protons, pulling at less electrons

85
Q

Do enantiomers produce different configurations in E2 reaction?

A

No

RR to SS will still have the same alkene configuration

86
Q

How do you produce different configurations in E2 reactions?

A

Have to just rotate one group

RR to RS for example will have different alkene configuration

87
Q

E configuration

A

higher priorities on opposite sides

88
Q

Z configuration

A

higher priorities on same sides

89
Q

Working definition of hyperconjugation

A

Resonance with sigma bonds

90
Q

Can you have hyperconjugation with the empty orbital being a sigma*?

A

Yes

91
Q

Why does adding an electronegative species help with hyperconjugation?

A

Once you have an electronegative species with a lone pair, like oxygen, now you just have straight up resonance

The lone pair can delocalize

This is favored to hyperconjugation resonance

92
Q

What does Et-OH indicate?

A

SN1 or E1

Polar protic

93
Q

What does a big bulky base indicate?

A

E2

E2 will trump SN2 when there is a bulky base

Example: -OH

94
Q

What does a good nucleophile but weak base indicate?

A

SN2

example: Br- or halogens

95
Q

When is it okay to have a carbocation on a primary carbon?

A

When there is additional stabilization besides hyperconjugation

Examples: allylic carbocation, benzyl carbocation, carbon cation on cyclopentane

96
Q

Why is the sigma CC unusually high in cyclopentane?

A

It is greater in p-character because it needs to form weird sigma bonds

this greater p-orbital raises the energy of the sigma CC

97
Q

Bicarbonate

A

COOOH -

98
Q

Chloroform

A

HCCl3

99
Q

Why is the bond angle of CF3 lower than CH3?

A

In C-F sigma bonds, electron density is pulled towards the F since highly electronegative. This decreases the amount of s-character that C has to use in the C-F bonds. This lack of s-character decreases the bond angle

100
Q

What happens to bond angles as you lower % s-character?

A

they decrease

101
Q

Why does an amine in ring not undergo ready inversion?

A

The steric strain of the ring makes inversion difficult

102
Q

Why does 1,2-difluoroethane prefer to be gauche to anti?

A

Electronics argument

Gauche allows for the electrons to have hyperconjugation between the sigma* CF and sigma CH which stabilizes

103
Q

When making the most stable alkane which groups should be anti?

A

The largest STERIC groups

May not be the top priority groups

104
Q

What is the LUMO of the E1 deprotonation step?

A

The “true lumo” is a mix between the sigma*CH and the empty 2p orbital

105
Q

Why are cyclopropanes good at making carbocations?

A

The sigma “banana bonds” in cyclopropane have significant p-character due to steric strain

This p-character has a better energy match with the empty 2p orbital in a carbocation and is able to stabilize it

106
Q

Quick tips for IUPAC naming

A

use smaller groups if possible (Ex: 2 methyl instead of isopropyl)

If naming a cyclohexanOL make #1 the -OH group (suffix goes first in ring)

If you have two substituents (even the same) on one carbon, list the number twice

Chem 203 (7 decks)

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