Conjugated Pi systems & pericyclic Reactions

Question 1

Dienes

Answer 1

Are compounds that posses two carbon carbon double bonds

Question 2

What are the 3 types of dienes?

Answer 2

1. Cumulated dienes
2. Conjugated Dienes
3. Isolated Dienes

Question 3

Cumulated Dienes (allenes)

Answer 3

The pie bond are adjacent

Question 4

Conjugated dienes

Answer 4

The pie bonds are separated by exactly one sigma bond

Question 5

Isolated Dienes

Answer 5

The pie bonds are separated by two or more sigma bonds

Question 6

__________ contains a continuous system of overlapping p-orbitals (make 4 overlapping p orbitals)

Answer 6

Conjugated Dienes

Question 7

A conjugated bond can also be formed with _________

Answer 7

Carbonyl groups

Question 8

Conjugated diene can be prepared from ___________ via elimination reactions (E2)

Answer 8

Allylic halides & dihalides

Question 9

Sterically hindered bases like _____________ is used to prevent SN2

Answer 9

Potassium Tert-butoxide (t-BuOK)

Question 10

The sigma bond in a double bond is ___________ than a regular sigma bond

Answer 10

Shorter

Question 11

Why is the sigma bond in a double bond shorter than sigma bond in a single bond?

Answer 11

Because in the double bond the sigma bond is sp² so it has more s character (valence theory)

Question 12

What is the molecular orbital theory to explain the why the sigma bon din a double bond is shorter?

Answer 12

Because in the 1st pie bond it experience a double bond connection

Question 13

___________ diene are more stable than isolated dienes

Answer 13

Conjugated

Question 14

At ________ isolated diene & conjugated diene are rapidly interconverting which establish an equilibrium

Answer 14

Room temp

Question 15

The stabilization energy associated with a conjugated diene is _______

Answer 15

15 KJ/mol

Question 16

According to the MO theory the atomic _________ combine to produce molecular orbitals (MO)

Answer 16

P orbitals

Question 17

Electrons in _________ are associated with an individual atom & electrons a molecular orbital are associated with an entire molecule

Answer 17

Atomic orbital

Question 18

1,3-butadiene has ________ overlapping P orbitals

Answer 18

4 overlapping

Question 19

1,3-butadiene has ________ overlapping P orbitals

Answer 19

4 overlapping

Question 20

1,3,5-hexatriene has ________ overlapping p orbitals

Answer 20

6 overlapping

Question 21

There are 3 MOs (molecular orbitals) within the 6 overlapping orbitals in 1,3,5-hexatriene that are highest in energy which are called ___________

Answer 21

HOMO

Question 22

Unoccupied MO are lowest in energy which are called ______

Answer 22

LUMO (lowest unoccupied molecular orbital)

Question 23

Frontier orbitals

Answer 23

The HOMO & LUMO orbitals

Question 24

The _______ contains the highest energy pie electrons which are mostly used in a reaction

Answer 24

HOMO

Question 25

The ________ is the lowest energy MO that is capable of accepting electron density

Answer 25

LUMO

Question 26

What are the two products that are formed with dienes like butadiene with HBr & Br₂?

Answer 26

1,4-adduct & 1,2-adduct

Question 27

When butadiene (or any diene) is treated with HBr (or any reagent) at low temp what product is favored?

Answer 27

1,2-adduct

Question 28

When butadiene (or any diene) is treated with Br₂ (or any reagent) then __________ is favored

Answer 28

1,4-adduct

Question 29

When 1,2-adduct is favored then it is because its _______ controlled

Answer 29

Kinetic

Question 30

When the product 1,4-adduct is formed it is _______ controlled

Answer 30

Thermodynamic

Question 31

What are the 3 groups of pericyclic reactions?

Answer 31

1. Cycloaddition 2. Electrocyclic reaction 3. Sigmatropic rearrangement

Question 32

All 3 pericyclic reaction are a __________ process

Answer 32

Concerted and the transition states are cyclic

Question 33

In a _________ reaction two pie bonds are converted into two sigma bonds resulting in the addition of two reactants to form a ring

Answer 33

Cycloaddition

Question 34

In an __________ reaction one pie bond is converted into a sigma bond which joins the ends of one reactant to form a ring

Answer 34

Electrocyclic

Question 35

In a __________ rearrangement one sigma bond is formed at the expense of another & the pie bond changes its location

Answer 35

Sigmatropic

Question 36

In a _________ reaction two sigma bonds are formed at the same time & a ring is formed (cycloaddition)

Answer 36

Diels-Alder (forms a 4+2 cycloadduct)

Question 37

Can use high temp to get the reverse of a Diels-alder reaction which is called \_\_\_\_\_\_\_\_\_\_\_

Answer 37

Retro Diels-alder reaction (high temp is used to open a ring)

Question 38

\_\_\_\_\_\_\_\_\_ reactions are usually performed at low temp or room temp

Answer 38

Diels- Alder reactions

Question 39

The starting material in a Diel-alder reaction is a ______ & \_\_\_\_\_\_\_

Answer 39

Diene & dienophile

Question 40

When a dienophile doesn't have any substituents the reaction exhibits a ________ & proceeds slowly

Answer 40

Large Activation

Question 41

A diel-alder reaction will proceed more rapidly & higher yield when the dienophile has an electron withdrawing substituent like a \_\_\_\_\_\_\_\_\_\_\_\_

Answer 41

Carbonyl group (ketone, ester, carboxylic acid, CN)

Question 42

When a dienophile is a 1,2-disubstituted alkene the reaction proceeds with stereospecificity which means that a ______ alkene produces a ______ disubstituted ring & a trans alkene produces a trans disubstituted ring

Answer 42

Cis

Question 43

A triple bond can also function as a dienophile where the product is a ring with \_\_\_\_\_\_\_\_\_\_

Answer 43

Two double bonds

Question 44

Cyclopentadiene are permanently locked in a ____ conformation

Answer 44

Cis

Question 45

When cyclopentadiene is used as the starting diene a ___________ is the product

Answer 45

Bridged bicyclic compound

Question 46

\_\_\_\_\_\_ position is syn to the larger bridge & _______ is anti to the larger bridge

Answer 46

Endo, Exo

Question 47

\_\_\_\_\_\_\_\_ cycloadduct is favored most

Answer 47

Endo

Question 48

Why is an endo product favored?

Answer 48

Because the transition state for the formation of the endo product is lower in energy than the transition state for the formation of exo product

Question 49

The Diels-alder reaction occurs when the electron are transferred from the ______ of the diene to the _______ of the dienophile

Answer 49

HOMO, LUMO

Question 50

When the frontier orbitals overlap the phase becomes symmetric which is called \_\_\_\_\_\_\_\_\_\_\_\_\_

Answer 50

Conservation of orbital symmetry

Question 51

Alder reaction have orbital symmetry & its called a \_\_\_\_\_\_\_\_\_\_

Answer 51

Symmetry- allowed process

Question 52

When alder reaction don't have MOs overlap its called what?

Answer 52

Symmetry forbidden

Question 53

For ________ reaction one pie bond is converted into a sigma bond & the rest of the pie bonds change the location & creates a ring

Answer 53

Electrocyclic

Question 54

When heat is used in a electrocyclic reaction than a ____ product is formed & when hv (light) is used then a _____ product is formed

Answer 54

Cis, Trans

Question 55

Under thermal conditions (room temp) for electrocyclic reactions the configuration of the _________ determines the configuration of the products

Answer 55

Reactants

Question 56

The stereochemical outcome for electrocyclic reactions is explained by the HOMO lobes where the lobes rotate in a __________ rotation which leads to a cis product (with trienes)(3 conjugated pie bonds & two vertical nodes)

Answer 56

Disrotatory rotation- where one set of lobes rotated clockwise & the other rotates counterclockwise

Question 57

With 4 pie electrons it has __________ rotation

Answer 57

Conrotatory rotation- where both sets of lobes rotate in the same direction (usually gives trans conformation)

Question 58

Conjugated systems with 4 pie bonds undergo thermal electrocyclic reaction in a ___________ rotation

Answer 58

Conrotatory Rotation

Question 59

Conjugated systems with 6 pie bonds undergo thermal electrocyclic reaction in a ___________ rotation

Answer 59

Disrotatory rotation

Question 60

Sigmatropic rearrangement

Answer 60

Pericyclic reaction where one sigma bond is formed at the expense of another (pie bonds change its location)

Question 61

A [3,3] sigmatropic rearrangement is called what ?

Answer 61

Cope rearrangement

Question 62

Equilibrium for a cope rearrangement favors ____________ alkene

Answer 62

More substituted

Question 63

The oxygen analogue of a cope rearrangement is called what?

Answer 63

Claisen rearrangement

Question 64

The __________ rearrangement is a [3,3] sigmatropic rearrangement & is common for allylic vinylic ethers & allylic aryl ethers

Answer 64

Claisen Rearrangement