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CH223 > CH223 Part 1: Alkenes & Dienes (Conceptual) > Flashcards

CH223 Part 1: Alkenes & Dienes (Conceptual) Flashcards

1
Q

Rules for Alkene Nomenclature (5)

A
  1. look for longest continuous chain
  2. indicate position of double bond starting at end
  3. add substituent and their positions in alphabetical order
  4. indicate geometry of double bond
  5. alcohols, amines, amides, aldehydes, esters, and carboxylic acids have priority
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2
Q

What functional groups have naming priority over double bonds?

A

alcohols, amines, amides, aldehydes, esters, and carboxylic acids

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3
Q

How is a double bond formed energetically?

A

2 electrons in a sigma bond (sp2 hybridized overlap) and 2 electrons in pi bond (p orbital overlap)

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4
Q

Why is rotation around a double bond so difficult?

A

involves breaking the pi bond

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5
Q

Bonds between alkyl groups and alkene carbons are polarized toward the ___ hybridized atom

A

sp2

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6
Q

Why do alkenes have small dipol moments?

A

% of s character in an sp2 hybridized orbital is greater

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7
Q

The greater the degree of substitution on the alkene, the _____

A

greater its stability

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8
Q

What are trans substituted alkenes more stable than cis-substituted alkenes?

A

less steric hinderance

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9
Q

What is the exception to the trans/cis stability rule?

A

the cis conformation of medium-sized cyclic alkenes is more table than the trans conformation

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10
Q

Where does conjugation occur?

A

in uninterrupted chains of p orbitals (alternate double and single bonds)

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11
Q

Where is the allylic carbon?

A

beta to beta to the end of the double bond

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12
Q

In non symmetrically substituted allylic species, are the resonance structures equivalent?

A

No, increased substitution at positive allylic carbon leads to greater stability

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13
Q

Allylic halides react ___ than their nonallylic counterparts in both SN1 and SN2 reactions

A

faster

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14
Q

Why are SN1 reactions with allylic halides so fast?

A

the carbocations are more stable because of resonance

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15
Q

Why are SN2 reactions with allylic halides so fast?

A

the CH2Cl group is less sterically crowded when attached to the sp2 carbon of the allylic halide vs the sp3 carbon of the alkyl halides

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16
Q

What are free radicals?

A

species that contain unpaired electrons

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17
Q

Homolytic cleavage

A

breaking covalent bond so that each retains one electron

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18
Q

Heterolytic cleavage

A

breaking covalent bond so that one atom retains both

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19
Q

What does free radical halogenation occur with allylic radicals but not alkyl radicals

A

there is a greater stability of allylic radicals because of resonance

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20
Q

When is free-radical substitution favored?

A

at high temperatures (300-500° C)

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21
Q

Are double bonds or single bonds more acidic?

A

double bonds because the deprotonated form makes an allyl anion that is relatively stable

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22
Q

Are conjugated or isolated dienes more stable?

A

conjugated dienes

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23
Q

What is the definition of delocalization energy?

A

the increased stability due to conjugation

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24
Q

Why are conjugated dienes more stable?

A

increase in the delocalization of their pi electrons

25
Q

Are cis or trans conjugated dienes more stable?

A

trans due to steric hinderance

26
Q

When is carbon delocalization in conjugated dienes most effective?

A

when all four carbons lie in the same plane

27
Q

What are allenes?

A

double bond right next to another double bond

28
Q

Why is conjugation not possible n an allene?

A

allenes are nonplanar as the 2p orbitals of the central carbon are perpendicular

29
Q

What is the regioselectivitty of electrophilic addition to conjugated dienes governed by?

A

the stability of the resulting carbocation

30
Q

Where does protonation of a conjugated diene always occur?

A

at the end of the diene unit because an allylic carbocation results

31
Q

What is the 1,2-addition product?

A

the addition of a proton to carbon 1 and and Br to carbon 2 in a conjugated diene

32
Q

What is the 1,4-addition product?

A

the addition of a proton to carbon 1 and a bromide to carbon 4 in a conjugated diene

33
Q

At lower temperatures, what conjugated diene addition is favored?

A

the 1,4-addition (faster); said to be under kinetic control

34
Q

At room temperature, what conjugated diene addition is favored?

A

the 1,2-addition (more stable); said to be under thermodynamic control

35
Q

What is a pericylic reaction?

A

a change in the bonding relationships that takes place as a continuous, concerted reorganization of electrons

36
Q

What does concerted mean?

A

there is only one transition state and NO intermediates involved

37
Q

What is a cycloaddition reaction?

A

the union of two smaller, independent pi systems to form a cyclic molecule

38
Q

During a cycloaddition reaction, what bonds are made and which are destroyed?

A

sigma bonds are made and pi bonds are formed

39
Q

What are Diels-Alder reactions?

A

[4+2] cycloadditons

40
Q

The product of a Diels-Alder reaction always contains ___ than the reactants.

A

one more ring

41
Q

What must a diene be able to do in order to a cycloaddition to occur?

A

must be able to adopt a s-cis conformation

42
Q

What is an s-cis conformation?

A

when both alkenes are on the same side of the sigma bond

43
Q

Can dienes locked in an s-trans conformation react in a Diels-Alder?

A

No

44
Q

Can dienes locked in a s-cis conformation react in a Diels-Alder

A

Yes! These are incredible reactive

45
Q

In a Diels-Alder, which reagents are the nuclophile?

A

the diene

46
Q

In a Diels-Alder, which reagants are the electrophile?

A

the dienophile

47
Q

What activates a diene?

A

adding EDGs makes the diene more electron rich and thus more nucleophilic

48
Q

What activates an electrophile?

A

adding EWGs makes the dienopphile more electron poor and thus more nucleophilic

49
Q

What are some common EWGs?

A

C=O, CN, NO2, F, CF3

50
Q

What are some common EDGs?

A

OCHs, CH3, NR2

51
Q

Why is resonance important in Diels-Alder reactions?

A

determines orientation of diene and dienophile

52
Q

What else can be used as a dienophile besides an alkene?

A

alkynes (leaves a double bond in the cyclic product)

53
Q

What about the conformation is retained in a Diels-Alder?

A

cis or trans conformation

54
Q

How do you indicate trans conformation in a cyclic Diels-Alder product?

A

one pointing up (wedged) and one pointing down (dashed)

55
Q

How do you indicate cis conformation in a cyclic Diels-Alder product?

A

one pointing up (wedged) and other pointing up too (wedged)

56
Q

What is the endo product of a Diels-Alder?

A

when both are pointing down (aka away from bridge)

57
Q

What is the exo product of a Diels-Alder?

A

when both are pointing up (aka towards the bridge)

58
Q

Is the endo product or exo product more major?

A

the endo product

59
Q

Can intramolecular Diels-Alder reactions occur?

A

Yes, these are possible when both the diene and dienophile are located on the same molecule

CH223 (5 decks)

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