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CHM136 Wizeprep > 7. Nucleophilic Substitution and Elimination Reactions > Flashcards

7. Nucleophilic Substitution and Elimination Reactions Flashcards

1
Q

what are nucleophiles?

A

a molecule that tends to give away its electrons to electrophiles

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2
Q

what are electrophiles?

A

molecules that tend to accept electrons

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3
Q

how are nucleophiles and leaving groups related in terms of basicity?

A

they are conjugate bases

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4
Q

what are leaving groups?

A

parts of a molecule that detach during substitution and elimination reactions

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5
Q

what is the leaving group like if the nucleophile is good?

A

the leaving groups is bad because it is unstable and reactive

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6
Q

what is the leaving group like if the nucleophile is bad?

A

the leaving groups is good because it is stable and unreactive

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7
Q

what must nucleophiles have in terms of electrons and why?

A

a lone pair to attack electrophiles

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8
Q

when does nucleophilicity increase in terms of charge?

A

the more negative charge

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9
Q

why are electrophiles subject to attack by nucleophiles?

A

because they are electron deficient and nucleophiles are attracted to molecules that are more positively charged

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10
Q

what are the most common electrophiles?

A
  1. a carbon at the base of a carbonyl
  2. carbocation
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10
Q

when does nucleophilicity increase in terms of basicity?

A

the more basic

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11
Q

when does nucleophilicity increase in terms of size?

A

the bigger an atom is

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12
Q

what are the different types of electrophiles?

A
  1. carbocations
  2. Lewis acids
  3. molecules with a partial positive charge
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13
Q

what properties must leaving groups have?

A
  1. electronegative halogen
  2. resonance
  3. induction
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13
Q

what are substitution reactions?

A

reactions that replace the leaving group with a nucleophile

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14
Q

what are elimination reactions?

A

reactions where once a base is added to a molecule to take away the protons of a molecule, a leaving group leaves and leaves behind a double bond

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15
Q

what is SN1 reaction?

A

a unimolecular reaction meaning only one species is present in the reaction

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16
Q

what does for instance increasing the concentration of a species in a SN1 reaction do to the rate of the reaction?

A

increases the rate of the reaction

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17
Q

What does SN1 in the SN1 reaction stand for?

A

Substitution, Nucleophilic, Unimolecular

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18
Q

what is the rate equation in SN1 reactions?

A

rate=k[substrate]

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18
Q

what are weak nucleophiles in terms of amount of electrons?

A

atoms with electrons close to having a full octet

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19
Q

what molecules are included in an SN1 reaction?

A

a weak nucleophile and an electrophile with a lot of steric hindrance

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20
Q

what happens if you produce a chiral carbon in an SN1 reaction?

A

you get a race mixture, an equal mixture of two different enantiomers

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20
Q

why do SN1 reactions include a weak nucleophile and an electrophile with a lot of steric hindrance?

A

Electrophiles with a lot of steric hindrance usually have bulky groups making it difficult for the nucleophile to attack. This type of reaction favours slower nucleophilic attacks because the nucleophile is weak

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21
Q

how do you rank the reactivity of molecules in SN1 reactions?

A
  1. rewrite the molecules with the leaving group gone
  2. add a positive charge to the carbon the leaving group left from
  3. rank the molecules, the more substituted the carbocation, the more reactive the molecule is
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22
Q

what is an SN2 reaction?

A

a bimolecular reaction where two species are present in the reaction

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23
Q

what is the reaction rate equation for SN2 reactions?

A

rate=k[substrate][nucleophile]

24
Q

what does increasing the concentration of a species in an SN2 reaction do to the rate of the reaction?

A

increases the reaction rate

25
Q

what molecules are included in SN2 reactions?

A

a strong nucleophile and an electrophile with little steric hindrance

26
Q

how are sodium and potassium related in terms of nucleophiles?

A

they are both strong nucleophiles

27
Q

what are the steps in an SN2 reaction?

A
  1. start off with a nucleophile and a chiral carbon molecule
  2. a nucleophile replaces hydrogen in a chiral carbon molecule as the intermediate step
  3. the halogen leaves as the final step
28
Q

what are the products in SN2 reactions like in terms of stereochemistry?

A

a change in position of the original stereochemistry

29
Q

when is stereochemistry considered in substitution reactions?

A

when a chiral product is formed

30
Q

what needs to happen to alcohol as a leaving group and why?

A

alcohol needs to be converted into a different functional group because it is a terrible leaving group

31
Q

what are the products in SN1 reactions like in terms of stereochemistry?

A

race mixture, two enantiomers

32
Q

why are alcohols a bad leaving group? (3)

A
  1. alcohols are strong bases so they are less likely to stabilize after they leave
  2. the oxygen in the alcohol may cause it to be too large to leave
  3. alcohols are stabilizing solvents so they may be less likely to leave the overall molecule because it is stabilizing it
33
Q

what are four ways to convert alcohol into a better leaving group?

A
  1. convert to water through protonation
  2. convert into a tosyl through tosylation
  3. convert to a methyl through methylation
  4. convert to an alkyl halide through
34
Q

how does an alcohol group turn into an alkyl halide?

A

instead of the side chain attaching to the alcohol it attaches to a halide

35
Q

describe protonation to make alcohol a better-leaving group.

A

a hydrogen is added to the alcohol to turn it into water

36
Q

what type of solvent is used in SN2 reactions and why?

A

polar aprotic solvent because fewer protons allow the strong nucleophile to swap places with the leaving group more easily

37
Q

what type of solvent is used in SN1 reactions and why?

A

polar protic solvent because more protons allow the leaving group to leave more easily

38
Q

what is an E1 reaction?

A

An unimolecular reaction where one species is present in the reaction

39
Q

what does increasing the concentration of a species in an E1 reaction do to the rate of the reaction?

A

increases the rate of the reaction

40
Q

what molecules are included in E1 reactions?

A

a weak base and an electrophile with a lot of steric hindrance

41
Q

what does E1 in the E1 reaction stand for?

A

Elimination, Unimolecular

42
Q

what is the rate equation of E1 reactions?

A

rate=k[substrate]

42
Q

what are the steps of an E1 reaction?

A
  1. start off with a molecule that has a halide
  2. the halide is removed leaves behind a positive charge of carbon
  3. an extra bond goes to where the carbocation is and the positive charge disappears
43
Q

what is an E2 reaction?

A

a bimolecular reaction where two species are present

44
Q

what does increasing the concentration of a species in an E2 reaction do to the rate of the reaction?

A

increase the rate of the reaction

45
Q

what is the reaction rate equation of an E2 reaction?

A

rate=k[substrate][base]

46
Q

what molecules are included in an E2 reaction?

A

a strong base and an electrophile with little steric hindrance

46
Q

what is the planar relationship between the hydrogen being deprotonated and the leaving group?

A

anitiperiplanar

47
Q

what is a vicinal dihalide?

A

a molecule with two halogens on opposite sides of a bond

47
Q

what are the steps of an E2 reaction?

A
  1. start off with a molecule that has a halide
  2. the oxygen from the hydroxyl group removes hydrogen from a carbon
  3. the carbon with the removed hydrogen turns into a double bond
  4. the leaving group comes off
  5. the end product is the molecules with a double bond plus the halogen and water
48
Q

what is regioselectivity?

A

preferential elimination at one site over another

49
Q

what is a germinal dihalide?

A

molecules with two halogens on the same side of a double bond

50
Q

what are the properties of elimination reactions?

A
  1. regioselectivity
  2. stereoselectivity
51
Q

what is stereoselectivity?

A

preferential formation of one isomer over the other

52
Q

what happens when a non-bulky base is used in elimination reactions?

A

a Zaitef product is formed, when a halogen turns into a double bond at the most substituted carbon in a trans manner to reduce steric strain

53
Q

what happens when a non bulky base is used in elimination reactions?

A

a Hoffman is formed when a halogen turns into double bonds and goes to the least substituted carbon

54
Q

what type of reaction do alcohols favour?

A

substitution

55
Q

what type of reaction do alkoxide favour?

A

elimination

56
Q

what type of reaction do more substituted alkyl halides favour?

A

elimination

57
Q

what type of reaction do molecules with a bulky base favour?

A

elimination

58
Q

what type of reaction with a strong acid, heat and a weak nucleophile favour?

A

elimination reaction

CHM136 Wizeprep (11 decks)

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