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Organic Chemistry II > Ch.15 - Conjugated Systems, Orbital Symmetry, and UV Spectroscopy > Flashcards

Ch.15 - Conjugated Systems, Orbital Symmetry, and UV Spectroscopy Flashcards

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1
Q

How many bonds are conjugated double bonds separated by?

A

one single bond

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2
Q

How many bonds are isolated double bonds separated by?

A

two or more single bonds

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3
Q

Which double bonds are more stable, isolated or conjugated?

A

conjugated

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4
Q

Heats of hydrogenation used to compare what?

A

the relative stabilities of alkenes

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5
Q

More ________ double bonds are usually more stable.

A

substituted

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6
Q

For conjugated double bonds, the heat of hydrogenation is _____ than the sum for the individual double bonds.

A

less

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7
Q

The more stable the compound, the ____ heat released during hydrogenation.

A

less

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8
Q

Which is more stable, a terminal alkyne or a cumulated diene?

A

cumulated diene

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9
Q

Which is more stable, a terminal alkyne of and internal alkyne?

A

terminal alkyne

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10
Q

Which is more stable, an isolated diene or an internal alkyne?

A

internal alkyne

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11
Q

Which is more stable, a conjugated diene or an isolated diene (cis)?

A

isolated diene (cis)

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12
Q

Which is more stable, an isolated diene (trans) or a conjugated diene?

A

isolated diene (trans)

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13
Q

The C2-C3 single bond of Buta-1,3-diene is shorter than ______A.

A

1.54 A

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14
Q

The small amount of overlap across the central C2-C3 bond of a Buta-1,3-diene gives it _______ _______ _______ character.

A

partial double bond

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15
Q

All atoms of buta-1,3-diene are _____ hybridized with overlapping p orbitals.

A

sp2

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16
Q

Each p orbital has ____ lobes.

A

2

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17
Q

How is the wavefunction of a p orbital indicated?

A

by plus (+) and minus (-) signs

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18
Q

What is formed when lobes overlap constructively?

A

a pi bonding (π) MO is formed
(+ and +, or - and -)

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19
Q

What is formed when lobes overlap destructively?

A

a pi antibonding (π*) MO (+ and -)

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20
Q

Which π MO for buta-1,3-diene has the lowest energy, all bonding interactions, electrons are delocalized over 4 nuclei.

A

π1

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21
Q

What are the characteristics of π2 MO for buta-1,3-diene?

A

1) 2 bonding interactions
2) one antibonding interaction
3) higher energy than π1 MO and not as strong

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22
Q

What are the characteristics of π3 MO for buta-1,3-diene?

A

1) 2 antibonding and 1 bonding interaction
2) 2 nodes
3) vacant in the ground state

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23
Q

What are the characteristics of π4 MO for buta-1,3-diene?

A

1) 3 nodes
2) strongly antibonding
3) highest energy MO
4) vacant at ground state

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24
Q

The bonding MOs of both buta-1,3-diene and ethylene are _____ and the antibonding MOs are _______.

A

1) filled
2) empty

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25
Q

Which has a higher energy, buta-1,3-diene or ethylene?

A

ethylene

26
Q

The s-trans conformer is _____ stable than the s-cis.

A

more

27
Q

When the kinetic control is at -80C which product is more favorable?

A

1,2-product

28
Q

When the kinetic control is at 40C which product is more favorable?

A

1,4-product

29
Q

Why does the transition state for the 1,2-addition have a lower Ea at -80C?

A

it is a more stable secondary carbocation

30
Q

Because the kinetics of the reaction determines the product, the reaction is said to be under ______ ______

A

kinetic control

31
Q

The 1,2-addition is still the faster addition at 40C, but what is different than the -80C?

A

the bromide attack is reversible

32
Q

In 40C, the 1,2-product ionizes back to what?

A

allylic cation

33
Q

Why is the 1,4-addition the most stable product at 40C?

A

it has more substiututed double bond

34
Q

Because the thermodynamics of the reaction determines the product, the reaction is said to be under _______ ________.

A

thermodynamic control

35
Q

What is the allylic carbon?

A

the one attached to an sp2 carbon

36
Q

How are allylic cations stabilized?

A

resonance

37
Q

The stability of 1° allylic is about the stability of what?

A

2° carbocation

38
Q

The stability of 2° allylic is about the stability of what?

A

3° carbocation

39
Q

What are the radical stabilities?

A

1° < 2° < 3° < 1° allylic

40
Q

To encourage substitution, use a ____ concentration of reagent with _____,______, or ______ to initiate free radical formation.

A

1) low
2) light
3) heat
4) peroxides

41
Q

During propagation ______ ______ is formed that is stabilized by resonance.

A

allylic radical

42
Q

What 2 things react quickly by the SN2 mechanism?

A

1) allylic halides
2) tosylates

43
Q

The transition state of the SN2 mechanism is stabilized through conjugation with what?

A

the p orbitals of the pi bond

44
Q

The Diels-Alder reaction is a reaction between what?

A

a diene and an electron-deficient alkene (dienophile) which produces a cyclohexane alkene

45
Q

What are the conditions for a diels-alder reaction?

A

change in heat

46
Q

What are the sterochemical requirements of a diels-alder reaction?

A

1) the diene must be in s-cis conformation
2) diene’s C1 and C4 p orbitals must overlap with dienophile’s p orbitals to form new sigma bonds
3) both sigma bonds are on same face of diene (syn stereochemistry)

47
Q

What cannot undergo the Diels-Alder reaction?

A

s-trans dienes

48
Q

What is the endo rule?

A

the stereochemical preference for electron-poor substituents on the dienophile to assume endo positions in a bicyclic diels-alder product

49
Q

The diels-alder reaction is an example of a _______ reaction.

A

pericyclic

50
Q

Woodward and Hoffmann predicted reaction products using their theory of ______ _____ _______ ______.

A

conservation of orbital symmetry

51
Q

HOMO

A

a diene contributes electrons from its highest energy occupied orbital

52
Q

LUMO

A

the dienophile receives electrons in its lowest unoccupied orbital

53
Q

What is the forbidden cycloaddition?

A

[2+2]

54
Q

What is the [2+2] cycloaddition made up of?

A

two ethylenes to form cyclobutene has antibondin overlap of HOMO and LUMO

55
Q

What happens when ethylene goes from the ground state to the excited state?

A

the absorption of correct energy photon will promote an electron to an energy level that was previously unoccupied

56
Q

[2+2] cycloaddition is photochemically ______, but thermally ______.

A

1) allowed
2) forbidden

57
Q

200-400nm photons do what?

A

excite electrons from a π bonding to a π* antibonding orbital

58
Q

A compound that has a longer chain of conjugated double bonds absorbs light at a ______ wavelength.

A

longer

59
Q

what does the spectrometer measure?

A

the intensity of a reference beam through solvent only (Ir) and the intensity of a beam through a solution of the sample (Is)

60
Q

absorbance is the log of the ratio ____/_____

A

Ir / Is

61
Q

what is beer’s law?

A

A = ecl

Organic Chemistry II (8 decks)

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