Chapter 6: Ionic Reactions (Nucleophilic)

Question 1

What is an Aprotic Solvent?

Answer 1

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

Question 2

What are Carbocations?

Answer 2

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

Question 3

What is a Nucleophile?

Answer 3

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

Question 4

What are Protic Solvents?

Answer 4

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

Question 5

What is Racemization?

Answer 5

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

Question 6

What is Solvolysis?

Answer 6

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

Question 7

What is Steric Hindrance?

Answer 7

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

Question 8

Halides are usually attached to what kind of carbon?

Answer 8

an sp3 hybridized carbon

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

Question 9

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

Answer 9

sp2

Question 10

A Nucleophile is always what?

Answer 10

A nucleophile is always a Lewis Base

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

Question 11

A Nucleophile can either be what?

Answer 11

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

Question 12

When the nucleophile is negative what happens?

Answer 12

the reaction happens right away

Question 13

When the nucleophile is nuetral what happens?

Answer 13

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

Question 14

What is a leaving group?

Answer 14

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

Question 15

What are some good leaving groups?

Answer 15

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

Question 16

What are the Kinetics of SN2 Reactions?

Answer 16

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

Question 17

What are the properties of an SN2 Mechanism?

Answer 17

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

Question 18

SN2 reaction is what type of mechanism

Answer 18

SN2 is a single step mechanism with no intermediates

- presence of a transition step that is very unstable

Question 19

What is the Stereochemistry of SN2 reactions?

Answer 19

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

Question 20

What is the Mechanism for SN1 Reactions?

Answer 20

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

Question 21

What are the 3 steps involved in SN1 reactions?

Answer 21

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

Question 22

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

Answer 22

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

Question 23

What is the Structure of Substrate Effect?

Answer 23

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

Question 24

Concentration and Reactivity of Nucleophile?

Answer 24

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

Question 25

What happens if nucleophiles have the same nucleophilic atom?

Answer 25

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-

Question 26

Solvent Effect (Polar Protic)?

Answer 26

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-

Question 27

What is the order of nucleophilicity in protic solvents?

Answer 27

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

Question 28

Solvent Effect (Polar Aprotic)?

Answer 28

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

Question 29

What is the order of nucleophilicity in aprotic solvents?

Answer 29

F- > Cl- > Br- > I-

Question 30

Leaving Groups Effect?

Answer 30

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

Question 31

The best leaving group among halides are as follows

Answer 31

I- > Br- > Cl- > F-

Question 32

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

Answer 32

F- >> Cl- > Br- > I- | - strong bases are NEVER good leaving groups

Question 33

What are Elimination Reactions?

Answer 33

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

Question 34

What is a Dehydrohalogenation Reaction?

Answer 34

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)

Question 35

What is an Alpha Carbon?

Answer 35

is the carbon that bear the leaving group (Halide)

Question 36

What is a Beta Carbon?

Answer 36

is the carbon bearing the hydrogen atom

Question 37

What is the Mechanism of an E2 Reaction?

Answer 37

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

Question 38

The bigger the molecule, the more....

Answer 38

The bigger the molecule the more it behaves like a base | - methyls don't work in the trend of E2

Question 39

What is an example of a bulky base?

Answer 39

tributoxide (-C-O-)

Question 40

A bulky base induces what type of reaction?

Answer 40

A bulky base always induces an E2 reaction and Hoffmann | - E2 is the preferred reaction over E1 anytime

Question 41

What is the Mechanism of E1 Reactions?

Answer 41

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

Question 42

What is Zaitsev's Rule?

Answer 42

most substituted is more stable than least substituted carbon

Question 43

What is Hoffmann's Rule?

Answer 43

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

Question 44

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

Answer 44

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