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Organic Chemistry > Ch. 8 – Alkyl Halides & Elimination Reactions > Flashcards

Ch. 8 – Alkyl Halides & Elimination Reactions Flashcards

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1
Q

What is an elimination reaction?

A

loss of starting materials to form a pi bond

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2
Q

What substance do alkyl halides undergo elimination reactions with?

A

bronsted-lowry bases

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3
Q

What is dehydrohalogenation?

A

removal of H and X; beta elimination because it involves the loss of elements from two adjacent atoms

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4
Q

What is the alpha carbon?

A

carbon bonded to X

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5
Q

What is the beta carbon and can there be more than one?

A

carbon bonded to H right next to alpha carbon; yes

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6
Q

What three steps occur for general elimination mechanism?

A

1) base removes proton from beta carbon and forms H-B+
2) electron pair in beta C-H forms new pi bond between alpha and beta
3) new pi bond boots off electron pair in C-X bond which goes with leaving group

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7
Q

What are the four most common dehydrohalogenation bases?

A 
(alkoxides)
Na+/ -OH sodium hydroxide
K+ / -OH potassium hydroxide
Na + / -OCH3 sodium methoxide
Na+ / -OCH2Ch3 sodium ethoxide
K+ / -O(CH3)3 potassium tert-butoxide
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8
Q

T/F: Having more than one beta carbon will lead to multiple products that are constitutional isomers.

A

true

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9
Q

What are alkenes?

A

products of elimination, C=C

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10
Q

What are important factors about alkenes (geom, hybridization)?

A

sp2 hybridized, trigonal planar (120º)

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11
Q

What does double bond consist of? What types of overlap cause these bond types?

A

one sigma and one pi bond

  • sigma caused by end-end of sp2 overlap (lies in plane)
  • pi caused by side-side of 2p overlap (lies perpendicular and has restricted rotation
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12
Q

What are mono/di/tri/tetrasubstituted alkenes?

A

number of carbon atoms bonded to carbons of double bonds

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13
Q

Does a double bond have free or restricted rotation? What does this cause?

A

restricted rotation, stereoisomers (cis and trans)

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14
Q

When are diastereomers possible?

A

whenever two groups on each end of C=C double bond are different

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15
Q

Are cis or trans alkenes more stable?

A

trans because groups are further apart and there are reduced steric interactions

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16
Q

How does the number of R groups bonded to C=C affect stability?

A

more R groups = more stable

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17
Q

Why do R groups increase stability?

A

they are sp3 hybridized and donate electron density to lower % s character sp2 carbon (inductive effect)

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18
Q

Do sp2 or sp3 more readily accept electron density?

A

sp2

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19
Q

What are the two types of elimination mechanisms?

A

E2 and E1

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20
Q

What is an E2 mechanism?

A

one step (concerted), second order, bimolecular

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21
Q

What is rate equation for E2 mech?

A

rate = k[R-X][base]

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22
Q

What are general steps for E2 mechanism?

A

1) base removes proton from beta carbon
2) C-H bond breaks and electron pair forms pi bond
3) leaving group breaks off and takes second electron pair

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23
Q

What are the characteristics of the energy diagram for E2 mech?

A
  • one barrier (hump)
  • transition state has 4 partial bonds
  • entropy favors products (more molecules of product)
  • products lower energy
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24
Q

What factors affect rate of E2 mech?

A

alkyl halide: more substituted = faster rate (opposite of Sn2)
strong base = faster rxn
better leaving group = faster rxn
polar aprotic solvent = faster rxn

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25
Q

T/F: When there are two or more beta carbons, equal amounts of product are made.

A

false, one product predominates

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26
Q

What is the more major product in elimination reactions?

A

major product = more stable product (more subs on double bond)

27
Q

What is the Zaitsev rule?

A

major product in beta elimination has more substituted double bond

28
Q

What does regioselective mean?

A

when a reaction yields more of one constitutional isomer (E2 is regioselective)

29
Q

Why does Zaitsev rule occur?

A

double bond is partially formed in transition state so increasing stability of double bond lowers Es for reaction and rate increases

30
Q

What product is major when stereoisomers are formed?

A

more stable stereoisomer (trans)

31
Q

What does stereoselective mean?

A

when reaction produces more of one stereoisomer when two or more are possible

32
Q

What is E1 mechanim?

A

two steps, first order, unimolecular

33
Q

What is rate equation for E1 mechanism?

A

rate = k[alkyl halide]

34
Q

Which bond breaks first in E1 mech?

A

bond to leaving group breaks before pi bond is formed

35
Q

What are steps of E1 mech?

A

1) heterolysis of C-X bond forms carbocation (RDS)

2) base removes proton from carbon adjacent to carbocation and electron pair in C-H bon forms new pi bond

36
Q

What are characteristics of energy diagram for E1?

A
  • two humps

- step one is higher hump (RDS)

37
Q

How do elimination reactions produce alkynes?

A

two consecutive elimination reactions form two pi bons

38
Q

What are the two starting materials that can be used to form an alkyne?

A

vicinal dihalide or geminal dihalide

39
Q

What is a vicinal dihalide?

A

has two X atoms on adjacent carbon atoms

40
Q

What is a geminal dihalide?

A

has two X atoms on same carbon atoms

41
Q

What is the intermediate product formed when forming alkynes?

A

vinyl halide

42
Q

What type of bases are needed to form an alkyne?

A

VERY strong bases (usually amides -NH2) because it is harder to remove H and X from sp2 hybridized carbons

43
Q

T/F: Sn1 and E1 have exact same first step

A

true, both produce carbocation

44
Q

How do Sn1 and E1 differ in step two?

A

Sn1: nuc attacks carbocation to form substitution product
E1: base removes a proton and forms pi bond

45
Q

T/F: the conditions for E1 to occur and the same as for Sn1 to occur

A

True (3º alkyl halide, weak nucleophile/base, polar protic solvent)

46
Q

What happens when E1 or Sn1 reaction could occur?

A

forms mixture

47
Q

How many atoms does the transition state of E2 reaction have in its plane?

A

4 (co planar C-x and C-H)

48
Q

What are the two conformations for the E2 transition state?

A

syn periplanar and anti periplanar

49
Q

What is syn and anti periplanar?

A

syn periplanar is when H and X are on the same side

anti periplanar is when H and X are on opposite sides

50
Q

What is needed to E2 reaction, syn or anti periplanar geometry?

A

anti periplanar because it is staggered and therefore lower in energy which allows electron rich base and leaving group to be further away from eachother

51
Q

For halocycloalkanes, how must geometry occur for E2 reaction to take place?

A

must be anti periplanar and have trans diaxial position for X and H

52
Q

What is trans diaxial geometry?

A

C-X and C-H are both in anxial position of ring

53
Q

Are both of the cis conformation rings reactive for E2? How many difference products can cis form?

A

no, only X axial conformation is reactive

can form two different products because it has two beta hydrogens in axial position (minor or major depending on alkene substitution)

54
Q

Are both trans ring conformations reactive for E2? How many different products can trans ring form?

A

only trans with axial halogen is reactive

can only form one product because there is only one axial beta hydrogen

55
Q

Should stereochemistry be drawn for alkene products?

A

alkenes and subs should be drawn in plane because they are trigonal planar

56
Q

What is the most important factor in determining E1 or E2?

A

strength of base; E2 = strong base; E1 = weak base

57
Q

Can E1 reactions occur for methyl or 1º?

A

No, but E2 can

58
Q

What is the only reaction that methyl can undergo?

A

Sn2

59
Q

What reactions can 1º undergo?

A

Sn2, E2

60
Q

What reactions can 2º undergo?

A

Sn2, E2, Sn1, E1

61
Q

What reactions can 3º undergo?

A

E2, Sn1, E1

62
Q

Do 1º benzylic or allylic compounds undergo E2?

A

no, only Sn2 or Sn1 (or E1 if there is resonance)

63
Q

Do 3º benzylic or allylic compounds undergo Sn2?

A

no, only E2, Sn1 and E1

64
Q

When do E1 reactions go for 1º allylic and benzylic ompounds?

A

not for benzylic but if there is resonance in allylic

Organic Chemistry (26 decks)

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