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Organic Chemistry > Chapter 6: Ionic Reactions (Nucleophilic) > Flashcards

Chapter 6: Ionic Reactions (Nucleophilic) Flashcards

1
Q

What is an Aprotic Solvent?

A

Aprotic solvents do not contain protons that are bonded to an electronegative atom
Ex: DMSO known as dimethyl sulfoxide
- favour SN2 reactions

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2
Q

What are Carbocations?

A

Carbocations are positive carbon centres that are extremely unstable and short lived
- stable in the following order 3°>2°>1°

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3
Q

What is a Nucleophile?

A

This is an electron rich specie that will react with an electron poor specie
Ex: OH- is a good nucleophile

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4
Q

What are Protic Solvents?

A

Protic Solvents are solvents that have a hydrogen attached to an electronegative atom
Ex: Water
- favour SN1 reactions

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5
Q

What is Racemization?

A

Racemization is when one converts an optically active compound into a racemic form

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6
Q

What is Solvolysis?

A

Solvolysis is the type of nucleophilic substitution reaction in which the nucleophile is a solvent molecule

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7
Q

What is Steric Hindrance?

A

Steric Hindrance is the prevention or retrogradation of a chemical reaction due to the arrangement of atoms in a molecule

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8
Q

Halides are usually attached to what kind of carbon?

A

an sp3 hybridized carbon

- The C-X bond is polarized because X(Halogen) is very electronegative

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9
Q

Vinylic halides or phenyl halids (Aryl halide) have what kind of hybridized carbons?

A

sp2

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10
Q

A Nucleophile is always what?

A

A nucleophile is always a Lewis Base

- leaving group will take the pair of electrons and leave (Halide Anion)

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11
Q

A Nucleophile can either be what?

A

A nucleophile can either be neutral or negatively charged with unpaired electrons

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12
Q

When the nucleophile is negative what happens?

A

the reaction happens right away

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13
Q

When the nucleophile is nuetral what happens?

A

An alkyloxonium is generated in the first step, which is then converted to a ROH in the second step when excess water removes the proton from the alkyloxonium ion
- The nucleophile is a solvent and occurs predominantly when one is dealing with a neutral nucleophile, a process known as solvolysis

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14
Q

What is a leaving group?

A

A good leaving group is a substituent that can leave and generate a relatively stable and weak basic specie

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15
Q

What are some good leaving groups?

A

Halide ions are good leaving groups because they are weak conjugate bases of a strong acid

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16
Q

What are the Kinetics of SN2 Reactions?

A

A second order reaction is also known as bimolecular reaction
- SN2 stands for substitution nucleophilic and bimolecular reaction

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17
Q

What are the properties of an SN2 Mechanism?

A
  1. Nucleophile approaches carbon centre from the back, thus the side opposite to that of the leaving group
  2. Once the nucleophile begins to form a bond with the carbon atom, the leaving group will leave
  3. Substrate undergoes inversion
  4. Transition State known as a temporary state where the nucleophile and leaving group both make partial bonds to the carbon
  5. Concerted reaction because the bond breaking and forming process occur simultaneously in a single TS
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18
Q

SN2 reaction is what type of mechanism

A

SN2 is a single step mechanism with no intermediates

- presence of a transition step that is very unstable

19
Q

What is the Stereochemistry of SN2 reactions?

A
  1. The nucleophile attacks from the backside of the substrate with respect to the leaving group
  2. This process will lead to an inversion in stereochemistry/configuration
20
Q

What is the Mechanism for SN1 Reactions?

A
  1. SN1 reactions are dependent on the concentration of the substrate, but independent on the concentration of the nucleophile
  2. First Order Rate Reaction and Unimolecular Reaction
  3. The rate determining step is the slow step, which is dependent on the substrate
  4. There are 2 intermediates in SN1
21
Q

What are the 3 steps involved in SN1 reactions?

A
  1. Usually consists of heterolytic cleavage of the carbon halide bond which is an endothermic step
  2. The halide group leaves due to the water is highly ionizable; water reacts quickly with the carbocation to produce an oxonium ion
  3. The third step involves water molecule extracting a proton from the oxonium ion, producing an alcohol as the final product
22
Q

What are some factors that affect whether a reaction will be SN2 or SN1?

A
  • Leaving Group
  • Solvent Effect
  • Structure of Substrate
  • Concentration and reactivity of nucleophile
23
Q

What is the Structure of Substrate Effect?

A

SN2: prefers - methyl>1 ° >2 ° >3 ° (unreactive)
- large bulky groups hinder SN2 reactions and increase energy of activation
SN1: prefers - 3 ° >2 ° >1 ° >methyl
- when a strong acid is used exceptions may occur

24
Q

Concentration and Reactivity of Nucleophile?

A

Strength of the nucleophile is measured with respect to the rate of its SN2 reaction with a known substrate

  • If a nucleophile reacts quickly in an SN2 reaction, then it is a good nucleophile
  • A nucleophile that carries a negative charge is always more reactive than its corresponding conjugate acid
25
Q

What happens if nucleophiles have the same nucleophilic atom?

A

then basicity will determine the nucleophilicity
RO- > HO- > RCO2- > ROH > H2O
- If the groups of nucleophilic atoms are different, ten there is no parallel relationship of basicity to it nucleophilicity
HS- > CN- > I- > OH-

26
Q

Solvent Effect (Polar Protic)?

A

Polar Protic Solvents favour SN1 reactions because it can solvate anions and cations
- Hydrogen bonding impedes a nucleophile, thus, decreasing its reactivity in a substitution reaction
- If a hydrogen bonds to a smaller nucleophilic atom, it leads to a stronger bonding than when the hydrogen is bonded to a large nucleophilic atom in the same group
I- > Br- > Cl- > F-

27
Q

What is the order of nucleophilicity in protic solvents?

A

SH- > CN- > I- > OH- > N3- > Br- > CH3CO2- > Cl- > F- > H2O

28
Q

Solvent Effect (Polar Aprotic)?

A

Aprotic solvents are favourable in SN2 reactions and solvate cations very well
- They do not form hydrogen bonds with anionic nucleophiles; this leads to poor solvation. These naked ions will in turn raise the reactivity of the nucleophile

29
Q

What is the order of nucleophilicity in aprotic solvents?

A

F- > Cl- > Br- > I-

30
Q

Leaving Groups Effect?

A

The leaving group is the group hat will depart from the substrate once the nucleophile attacks the carbon centre
- A good leaving group is one that will generate stable anion or neutral molecule on its own
WEAK BASES ARE GOOD LEAVING GROUPS because they stabilize the negative charge

31
Q

The best leaving group among halides are as follows

A

I- > Br- > Cl- > F-

32
Q

The best leaving groups in terms of basicity, are as follows

A

F-&raquo_space; Cl- > Br- > I-

- strong bases are NEVER good leaving groups

33
Q

What are Elimination Reactions?

A

In an elimination reaction, the species (XY) in this case is eliminated, and an alkene or a double bond product is usually generated

34
Q

What is a Dehydrohalogenation Reaction?

A

A dehydrohalogenation reaction is one where an HXX group is removed from an alkyl halide creating a double bond
- A strong base is required in this process (Sodium ethoxide; EtONa and KOH/EtOH)

35
Q

What is an Alpha Carbon?

A

is the carbon that bear the leaving group (Halide)

36
Q

What is a Beta Carbon?

A

is the carbon bearing the hydrogen atom

37
Q

What is the Mechanism of an E2 Reaction?

A
  1. E2 is a bimolecular reaction, where the rate is dependent on both substrate and the base
    - 1° > 2° > 3°
  2. E2 involves the electron pair of the C-H bond pushing the leaving group away, in the opposite orientation with respect to the base
  3. Prefers Polar Aprotic Solvents
  4. Leaving Group:
    I- > Br- > Cl- > F-
  5. If you have temperature added it will always be elimination and not substitution
38
Q

The bigger the molecule, the more….

A

The bigger the molecule the more it behaves like a base

- methyls don’t work in the trend of E2

39
Q

What is an example of a bulky base?

A

tributoxide (-C-O-)

40
Q

A bulky base induces what type of reaction?

A

A bulky base always induces an E2 reaction and Hoffmann

- E2 is the preferred reaction over E1 anytime

41
Q

What is the Mechanism of E1 Reactions?

A
  1. E1 is a unimolecular reaction, where the rate is dependent on the first step, hence. It is the rate determining step
  2. There are 2 possible products in E1
    - If the solvent molecules act as a nucleophile, then it goes through an SN1
    - If the solvent molecule acts as a base, then it goes through an E1
    E1 WILL ALWAYS HAVE PRODUCTS FROM AN SN1 RXN
  3. Prefers a Polar Protic Solvent
  4. Applies Zaitsev’s rule
42
Q

What is Zaitsev’s Rule?

A

most substituted is more stable than least substituted carbon

43
Q

What is Hoffmann’s Rule?

A

The opposite of Zaitsev’s Rule; states the least substituted carbon is most stable than the more substituted carbon

44
Q

How do you tell which product is the major or minor?

A

If the R groups were inequivalent you choose the one with more R groups
- When you have the same amount of R groups they are both major groups

Organic Chemistry (8 decks)

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