Chapter 15

Question 1

What is a cumulated diene?

Answer 1

C=C=C

Question 2

UV-Vis: determines the amount of _______ in a molecule

Answer 2

conjugation

Question 3

Heats of hydrogenation are used to compare the relative _______ of C=C

Answer 3

stabilities
- Lower energy = more stable

Question 4

What are the solvents used in UV-Vis? (2)

Answer 4

1) CH3OH
2) EtOH

Question 5

When part of a double bond, atoms are _______ hybridized with overlapping __ orbitals

Answer 5

sp2
p

Question 6

Each p orbital has two lobes with the wave function indicated by _____ and _____ signs

Answer 6

plus
minus

Question 7

constructive overlap (+ and + or - and -) : _______ MO is formed

Answer 7

pi bonding

Question 8

destructive overlap (+ and - ) : _______ MO is formed

Answer 8

pi anti-bonding

Question 9

What does homo stand for?

Answer 9

highest occupied molecular orbital

Question 10

What does LUMO stand for?

Answer 10

lowest unoccupied molecular orbital

Question 11

HOMO is usually _____ or _____

Answer 11

bonding
NB

Question 12

LUMO is usually _____

Answer 12

anti-bonding

Question 13

Typical excitations occurs from _____ to ______

Answer 13

HOMO to LUMO

Question 14

Conjugation systems have electronic transitions with _______ energies = wavelengths _______ than 200 nm

Answer 14

lower
larger

Question 15

UV-Vis spectroscopy can only detect wavelengths higher than _____ nm

Answer 15

200

Question 16

An increase in the number of pi orbitals leads to a _______ gap between orbitals = _______ energy = _______ wavelength

Answer 16

smaller
lower
longer

Question 17

At ____ conjugated double bonds = visible portion of the spectrum

Answer 17

7

Question 18

What is the formula for Beer’s law?

Answer 18

A = ε c l
- A: absorbance
- ε: molar extinction coef
- c: concentration
- l: path length of cell (1cm)

Question 19

The values of _____ and _____ are read directly from the spectrum. Other values (_____ and _____) could be calculated

Answer 19

λmax and A
c and εmax

Question 20

Rules for UV-Vis orbital diagrams:
1) # of ___ bonds = # of bonding π MO’s
2) There are equal and opposite _____ MO’s
3) Any lone pair on a conjugated carbonyl is placed in a NB orbital at E=_____

Answer 20

1) π
2) anti-bonding
3) 0

Question 21

π —> π* absorptions have _______ ε

Answer 21

higher (5,000 - 30,000)

Question 22

NB —> π* absorptions have _______ ε

Answer 22

lower (<100)

Question 23

Interpreting UV-Vis spectra:
addition of conjugated double bond increase λmax by _______ per double bond

Answer 23

30-40 nm
- on top of base molecule

Question 24

Interpreting UV-Vis spectra: Alkyl groups increase the value of λmax by about _______ per alkyl group

Answer 24

5 nm
- has to be connected to double bond

Question 25

Absorption of UV-visible energy by a molecule results in ________

Answer 25

electronic transitions

Question 26

MO theory for conjugated systems: 1) #MOs (lines we draw) = #AOs (# of _______ with p orbitals) 2) Place MOs _______ (never pair them) and _______ around E=O 3) as the # of nodes increases the energy of the MO _______ - # of nodes = _______ 4) When drawing nodes, place them as _______ as possible

Answer 26

1) atoms 2) singly; symmetrically 3) increases - N-1 4) symmetrically

Question 27

What are the 2 possible planar conformations for butadiene?

Answer 27

1) S-trans (more stable) 2) S-cis

Question 28

The transition state of the Sn2 mechanism is stabilized through _______ with the p orbitals of the pi bond, which lowers the _______ allowing for the reaction to take place faster

Answer 28

conjugation Ea

Question 29

Explain why there is 1,2 and 1,4 addition of HBr to dienes

Answer 29

depending on the temperature, the nucleophile (Br-) will add to the carbon 2 or carbon 4 (resonance allows this to happen)

Question 30

At what temperatures will the 1,2 addition of HBr to dienes occur?

Answer 30

-78°C, 0°C

Question 31

At what temperatures will the 1,4 addition of HBr to dienes occur?

Answer 31

Room temp, 40°C

Question 32

1,2-addition results in the _______ product (@-78°C, 0°C)

Answer 32

kinetic - More stable C+ intermediate

Question 33

1,4-addition results in the _______ product (@Room temp, 40°C)

Answer 33

thermodynamic product - more stable zietsev product

Question 34

The Diels-Alder reaction: important synthetic tool. It involves the [_______] cycloaddition reaction that forms a ____ - member ring with stereochemical control

Answer 34

4+2 6

Question 35

What is the name of the two parts that are combined in a Diels-Alder reaction?

Answer 35

1) diene 2) dienophile

Question 36

Reactivity of a Diels-Alder reaction: to enhance the flow of electrons, _______ are often used on the dienophile, and _______ are used on the diene

Answer 36

EWG (dienophile) EDG (diene)

Question 37

the mechanism of the Diels-Alder reaction is a ________ cyclic movement of 6 electrons

Answer 37

concerted

Question 38

(Diels-Alder RXN) To get proper orbital overlap, the reactants add _______, resulting in the _______ stereochemistry for the products as was found in the starting material

Answer 38

syn same

Question 39

a 2,3 substituted diene spends more time in s-cis than s-trans = leads to _____ faster reaction than unsubstituted diene

Answer 39

10x

Question 40

(Diels-Alder RXN) The _______: when substituents are on the dienophile, two possible products can be theoretically formed, proving that the starting diene was already a ring. However, only the _______ product is formed

Answer 40

Endo Rule endo

Question 41

(Diels-Alder RXN) Unsymmetrical reagents: when both diene and dienophile are asymmetric, the _____ or _____ products are the major product

Answer 41

ortho para - never make meta product!!!

Question 42

(Diels-Alder RXN for diene) 2 Rin = _____ 2 Rout = _____ 1 Rin 1 Rout =_____

Answer 42

cis cis trans

Question 43

The Diels-Alder reaction is an example of a _______ reaction: involves the _______ forming and breaking of bonds within a closed ring of interacting orbitals

Answer 43

pericyclic concerted

Question 44

Pericyclic reactions require _______ or _______ conditions

Answer 44

thermal (heat) photochemical (hv)

Question 45

the two different sets of conditions give different pericyclic products. Woodward and Hoffmann predicted the reaction products using their theory of _______

Answer 45

conservation of orbital symmetry

Question 46

cycloaddition reactions: these reactions can be viewed as electrons flowing form the _______ of one molecule to the _______ of the other reactant

Answer 46

HOMO LUMO

Question 47

ALWAYS use 1 homo and 1 lumo if the ends of the pi system meeting have the _______ mathematical orbital signs, the reactions is _______ using these conditions

Answer 47

same allowed

Question 48

usually in pericyclic reactions you will use the HOMO of the _______ and the LUMO of the _______

Answer 48

nucleophile - diene electrohpile - dienophile

Question 49

In photochemical reactions, the lifetime of excited state molecules is short, resulting in the chance of 2 excited state molecules reacting to be small. therefor only use _______ and _______ where only one is excited

Answer 49

1 HOMO 1 LUMO - this will allow constructive overlap to occur