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Organic Chemistry > Conjugated Pi systems & pericyclic Reactions > Flashcards

Conjugated Pi systems & pericyclic Reactions Flashcards

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1
Q

Dienes

A

Are compounds that posses two carbon carbon double bonds

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2
Q

What are the 3 types of dienes?

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

Cumulated Dienes (allenes)

A

The pie bond are adjacent

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4
Q

Conjugated dienes

A

The pie bonds are separated by exactly one sigma bond

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5
Q

Isolated Dienes

A

The pie bonds are separated by two or more sigma bonds

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6
Q

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

A

Conjugated Dienes

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7
Q

A conjugated bond can also be formed with _________

A

Carbonyl groups

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8
Q

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

A

Allylic halides & dihalides

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9
Q

Sterically hindered bases like _____________ is used to prevent SN2

A

Potassium Tert-butoxide (t-BuOK)

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10
Q

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

A

Shorter

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11
Q

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

A

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

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12
Q

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

A

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

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13
Q

___________ diene are more stable than isolated dienes

A

Conjugated

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14
Q

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

A

Room temp

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15
Q

The stabilization energy associated with a conjugated diene is _______

A

15 KJ/mol

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16
Q

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

A

P orbitals

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17
Q

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

A

Atomic orbital

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18
Q

1,3-butadiene has ________ overlapping P orbitals

A

4 overlapping

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19
Q

1,3-butadiene has ________ overlapping P orbitals

A

4 overlapping

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20
Q

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

A

6 overlapping

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21
Q

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

A

HOMO

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22
Q

Unoccupied MO are lowest in energy which are called ______

A

LUMO (lowest unoccupied molecular orbital)

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23
Q

Frontier orbitals

A

The HOMO & LUMO orbitals

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24
Q

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

A

HOMO

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25
Q

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

A

LUMO

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26
Q

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

A

1,4-adduct & 1,2-adduct

27
Q

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

A

1,2-adduct

28
Q

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

A

1,4-adduct

29
Q

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

A

Kinetic

30
Q

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

A

Thermodynamic

31
Q

What are the 3 groups of pericyclic reactions?

A
  1. Cycloaddition
  2. Electrocyclic reaction
  3. Sigmatropic rearrangement
32
Q

All 3 pericyclic reaction are a __________ process

A

Concerted and the transition states are cyclic

33
Q

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

A

Cycloaddition

34
Q

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

A

Electrocyclic

35
Q

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

A

Sigmatropic

36
Q

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

A

Diels-Alder (forms a 4+2 cycloadduct)

37
Q

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

A

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

38
Q

_________ reactions are usually performed at low temp or room temp

A

Diels- Alder reactions

39
Q

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

A

Diene & dienophile

40
Q

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

A

Large Activation

41
Q

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

A

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

42
Q

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

A

Cis

43
Q

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

A

Two double bonds

44
Q

Cyclopentadiene are permanently locked in a ____ conformation

A

Cis

45
Q

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

A

Bridged bicyclic compound

46
Q

______ position is syn to the larger bridge & _______ is anti to the larger bridge

A

Endo, Exo

47
Q

________ cycloadduct is favored most

A

Endo

48
Q

Why is an endo product favored?

A

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

49
Q

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

A

HOMO, LUMO

50
Q

When the frontier orbitals overlap the phase becomes symmetric which is called _____________

A

Conservation of orbital symmetry

51
Q

Alder reaction have orbital symmetry & its called a __________

A

Symmetry- allowed process

52
Q

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

A

Symmetry forbidden

53
Q

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

A

Electrocyclic

54
Q

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

A

Cis, Trans

55
Q

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

A

Reactants

56
Q

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)

A

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

57
Q

With 4 pie electrons it has __________ rotation

A

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

58
Q

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

A

Conrotatory Rotation

59
Q

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

A

Disrotatory rotation

60
Q

Sigmatropic rearrangement

A

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

61
Q

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

A

Cope rearrangement

62
Q

Equilibrium for a cope rearrangement favors ____________ alkene

A

More substituted

63
Q

The oxygen analogue of a cope rearrangement is called what?

A

Claisen rearrangement

64
Q

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

A

Claisen Rearrangement

Organic Chemistry (47 decks)

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