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

Question 1

How many bonds are conjugated double bonds separated by?

Answer 1

one single bond

Question 2

How many bonds are isolated double bonds separated by?

Answer 2

two or more single bonds

Question 3

Which double bonds are more stable, isolated or conjugated?

Answer 3

conjugated

Question 4

Heats of hydrogenation used to compare what?

Answer 4

the relative stabilities of alkenes

Question 5

More ________ double bonds are usually more stable.

Answer 5

substituted

Question 6

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

Answer 6

less

Question 7

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

Answer 7

less

Question 8

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

Answer 8

cumulated diene

Question 9

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

Answer 9

terminal alkyne

Question 10

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

Answer 10

internal alkyne

Question 11

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

Answer 11

isolated diene (cis)

Question 12

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

Answer 12

isolated diene (trans)

Question 13

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

Answer 13

1.54 A

Question 14

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

Answer 14

partial double bond

Question 15

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

Answer 15

sp2

Question 16

Each p orbital has ____ lobes.

Answer 16

2

Question 17

How is the wavefunction of a p orbital indicated?

Answer 17

by plus (+) and minus (-) signs

Question 18

What is formed when lobes overlap constructively?

Answer 18

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

Question 19

What is formed when lobes overlap destructively?

Answer 19

a pi antibonding (π*) MO (+ and -)

Question 20

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

Answer 20

π1

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

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

Answer 24

1) filled
2) empty

Question 25

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

Answer 25

ethylene

Question 26

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

Answer 26

more

Question 27

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

Answer 27

1,2-product

Question 28

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

Answer 28

1,4-product

Question 29

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

Answer 29

it is a more stable secondary carbocation

Question 30

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

Answer 30

kinetic control

Question 31

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

Answer 31

the bromide attack is reversible

Question 32

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

Answer 32

allylic cation

Question 33

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

Answer 33

it has more substiututed double bond

Question 34

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

Answer 34

thermodynamic control

Question 35

What is the allylic carbon?

Answer 35

the one attached to an sp2 carbon

Question 36

How are allylic cations stabilized?

Answer 36

resonance

Question 37

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

Answer 37

2° carbocation

Question 38

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

Answer 38

3° carbocation

Question 39

What are the radical stabilities?

Answer 39

1° < 2° < 3° < 1° allylic

Question 40

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

Answer 40

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

Question 41

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

Answer 41

allylic radical

Question 42

What 2 things react quickly by the SN2 mechanism?

Answer 42

1) allylic halides 2) tosylates

Question 43

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

Answer 43

the p orbitals of the pi bond

Question 44

The Diels-Alder reaction is a reaction between what?

Answer 44

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

Question 45

What are the conditions for a diels-alder reaction?

Answer 45

change in heat

Question 46

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

Answer 46

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)

Question 47

What cannot undergo the Diels-Alder reaction?

Answer 47

s-trans dienes

Question 48

What is the endo rule?

Answer 48

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

Question 49

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

Answer 49

pericyclic

Question 50

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

Answer 50

conservation of orbital symmetry

Question 51

HOMO

Answer 51

a diene contributes electrons from its highest energy occupied orbital

Question 52

LUMO

Answer 52

the dienophile receives electrons in its lowest unoccupied orbital

Question 53

What is the forbidden cycloaddition?

Answer 53

[2+2]

Question 54

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

Answer 54

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

Question 55

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

Answer 55

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

Question 56

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

Answer 56

1) allowed 2) forbidden

Question 57

200-400nm photons do what?

Answer 57

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

Question 58

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

Answer 58

longer

Question 59

what does the spectrometer measure?

Answer 59

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

Question 60

absorbance is the log of the ratio ____/_____

Answer 60

Ir / Is

Question 61

what is beer's law?

Answer 61

A = ecl