MCAT Substitution reactions

Question 1

Within a reaction diagram what step forms the rate- limiting step?

Answer 1

The greatest Ea as this step is the slowest and sets the speed of the reaction.

Question 2

Describe nucleophilic substitutions. What are the two types?

Answer 2

The nucleophile attacks electrophile replacing the leaving group.
SN1 and SN2

Question 3

Describe the general mechanism of SN1 reactions?

Answer 3

Leaving group leaves first forming a carbocation. The positive charge can then potentially undergo carbocation rearrangement and then the nucleophile attacks.

Question 4

Describe the reaction order of SN1 reactions?

Answer 4

Reaction of 1 that only depends on the electrophilic substrate and not the nucleophile.

Question 5

What’s the result from an SN1 nucleophilic attack?

Answer 5

Front and back attack causes inversion and retention of configuration resulting in a racemic mixture of enantiomers.

Be careful because only the attacked carbon undergoes rearrangement so if other chiral carbons are not attacked you get a pair of diastereomers instead.

Question 6

Describe the reaction diagram of SN1 reactions

Answer 6

They have two peaks with the highest peak representing the formation of the carbocation thus the rate- limiting step ( and slowest step) is the formation of the carbocation.

Question 7

Solvolysis

Answer 7

When the nucleophile and solvent are the same.

Question 8

Which type of solvent does SN1 reactions use?

Answer 8

Polar protic solvents ( solvents that contains N-H and O-H bonds). They surround the nucleophile and decrease their strength.

Question 9

Which carbocation substrates are best for SN1 reactions?

Answer 9

Goal is more stability ( steric hinderance doesn’t matter in this case)
1 ( worst)- methyl
2- primary
3- secondary
4 ( best)- tertiary

Question 10

What are the best leaving groups?

Answer 10

Bromide, chloride, iodide.

Question 11

Describe the overall mechanism of SN2 reactions?

Answer 11

Simultaneous attack from nucleophile and leaving of the leaving group.

Once mechanistic step = concerted.

Question 12

What’s the result of SN2 reactions?

Answer 12

Backside attack results in an inversion of configuration. This is called the walden inversion.

Inversion only happens at the attacked carbon.

Question 13

Describe the reaction diagram of SN2 reactions?

Answer 13

Contains one peak which is the rate-limiting step and involves the simultaneous bond bond being broken between the leaving group and substrate and bond formation between the nucleophile and substrate.

Reaction doesn’t form any intermediates = reason for just one peak.

Question 14

Describe the rate law of an SN2 reaction?

Answer 14

Contains both the substrate and nucleophile creating an overall reaction rate of 2.

Question 15

What are the reaction conditions required for SN2 reactions?

Answer 15

Sterically unhindered strong nucleophiles.

Requires polar aprotic solvent so they won’t surround the nucleophile and decrease it’s strength.

Question 16

What’s the best substrate for a SN2 reaction?

Answer 16

Less sterically hindered.
Methyl is the best and tertiary is the worst.

Question 17

Describe the appropiate nucleophiles for SN2 and SN1 reactions?

Answer 17

SN2 reactions requires strong sterically unhindered nucleophiles

SN1 reactions requires weak nucleophiles. Steric hinderance is not a factor for it’s nucleophiles.

Question 18

Describe the general mechanism of nucleophilic acyl substitution?

Answer 18

Nucleophile attacks a carbonyl carbon displacing the leaving group.

aka. an addition- elimination reaction.

Question 19

Describe the two mechanistic steps of nucleophilic acyl substitution?

Answer 19

Step 1. Nucelophile attacks carbonyl carbon displacing pi electrons on oxygen forming a tetrahedral intermediate ( because SP2 changes to SP3).

Step 2. pi bond reforms from the electrons on the oxygen causing displacement of the leaving group.

Question 20

Describe the reaction diagram for a nucleophilic acyl substitution reaction?

Answer 20

Contains two peaks with the largest peak being the formation of the tetrahedral intermediate and thus being the rate- limiting step.

Question 21

What is the rate law for nucleophilic acyl substitution?

Answer 21

Second order rate law with both the substrate and nucleophile having a reaction order with respect to one.

Question 22

What is aromatic substitution and what are the two types?

Answer 22

Aromatic substitution is when a nucleophile displaces a LG on the ring.

Two types are nucleophilic aromatic substitution and electrophilic aromatic substitution.

Question 23

Describe electrophilic aromatic substitution v. nucleophilic aromatic substitution?

Answer 23

Electrophilic aromatic substitution - substitution reaction in which the aromatic ring acts as a nucleophile and attack an electrophile which displaces the LG.

Nucleophilic aromatic substitution - substitution reaction in which the aromatic ring is the electrophile which is attacked by the nucleophile which displaces the LG on the ring.

Question 24

Substitution v. Addition reactions

Answer 24

Substitution reactions are reactions in which a nucleophile replaces a LG.

Addition reactions are reactions in which two groups are added across a double bond to crease two new sigma bonds.

Question 25

Describe the general mechanism of addition reactions?

Answer 25

Nucleophile attacks electrophilic carbon and the hydrogen is attached to the electronegative oxygen or nitrogen.

Question 26

What happens when the nucleophile attack the carbonyl carbon in addition reactions?

Answer 26

If the nucleophilic attack results in a chiral carbon you get a pair of enantiomers or diasteromers ( determining if other chiral carbons are present or not).

Question 27

Describe the mechanism of electrophilic addition?

Answer 27

Pi bond acts as the nucleophile attacking the electrophile. This creates a major and minor product.

They major product is when the electrophile is added to the carbon with most alkyl groups and the carbocation is formed on the carbon with less alkyl groups ( this is where the hydrogen is added)

The minor product is when the electrophile is added to carbon with least alkyl groups and the carbocation is formed in the carbon with most alkyl groups ( this is where the hydrogen is added).

Question 28

Nucleophilic addition v. Electrophilic addition reactions

Answer 28

Nucleophilic addition is where the pi bond is the electrophile.

Electrophilic addition is where the pi bond acts as the nucleophile.

Question 29

Elimination reactions

Answer 29

Reactions in which a leaving group leaves resulting in the formation of a pi bond.

Broken down into E1 and E2.

Question 30

Describe E1 reactions

Answer 30

Leaving group leaves first forming a carbocation ( can undergo a methyl or hydride shift).

Nucleophile attacks hydrogen on adjacent carbon turn the C-H electrons into the new pi bond.

Question 31

Describe the reaction diagram of an E1 reaction.

Answer 31

Has two peaks with a carbocation intermediate. It has the highest peak leading up to it and so it’s the rate- limiting step.

Question 32

Zaitsev’s rule

Answer 32

Because more than one adjacent carbon can have protons the nucleophile attacks the carbon that results in the pi bond forming on the side with most alkyl groups.

We get major and minor products because of the different carbons the nucleophile can attack resulting in major product ( follows zeitsevs rule) and minor product ( pi bond forms on least substituted side).

Question 33

E1 dehydration of an alcohol

Answer 33

Acid- catalyzed reactions in which the hydroxyl group is protonated transforming it into a oxonium ion which is a good LG.

Once the LG de- attaches and water is formed it acts as a nucleophile and attacks hydrogen of neighboring carbon forming the double bond.

Question 34

Describe E2 reactions

Answer 34

Nucleophile attacks hydrogen which breaks C-H bond and forming a new pi bond as the LG leaves in one concerted mechanism.

For this to happen the hydrogen and leaving group must be in an anti- stereochemical position!!!!

Question 35

Describe the reaction mechanism of E2 reactions

Answer 35

Has a single peak as there is no transition state because of the nature of the concerted mechanism.

Question 36

What is the result of E2 reactions with bulky bases v. less bulky bases?

Answer 36

Bulky bases create a kinetic product ( high energy, least stable) in which double bond is on side with least alkyl groups.

Less bulky groups create thermodynamic (less energy, more stable) products in which the double bond is on side with most alkyl groups

Question 37

What’s the thermodynamic v. Kinetic product of elimination reactions?

Answer 37

Zetseivs product is the thermodynamic product formed from less sterically hindered bases.

Hoffman product is the kinetic product formed from more sterically hindered bases.