Nucleophilic Substitution

Question 1

Electron poor and tend to have a slight positive or a positive charge (or have open orbitals). Tend to accept electrons to form covalent bonds

Answer 1

Electrophile

Question 2

True or False: Pi bonds can be nucleophiles

Answer 2

True

Question 3

How do the definitions of a lewis acid or base come into play with nucs/electrs?

Answer 3

Lewis acids accept electrons (like an electrophile) Lewis bases donate electrons (like nucleophiles)

Question 4

Which of the following would be the strongest nucelophile: Hydroxide ion Water Hydronium ion

Answer 4

The hydroxide ion because it has a negative charge so it has electrons to donate

Question 5

An unknown ion has the electron configuration 1s2 and you have observed that it has 3 protons. Do you predict this element to be electrophilic or nucleophilic?

Answer 5

It should be electrophilic because if it has 3 protons that means it is Lithium. Lithium should have a 2S2 configuration. If it has a 1S2 that means it is positively charges and will act as an electrophile.

Question 6

True or False: OH is a good leaving group

Answer 6

False, OH2 is a good leaving group so we need to protonate OH in order to turn it into a good LG

Question 7

Are halogens good or bad LG’s?

Answer 7

Good LG’s

Question 8

What is the rate limiting step of SN1?

Answer 8

Formation of a carbocation. This is why the rate is only dependent on the first reactant turning into a carbocat.

Question 9

What is the most likely substrate you will encounter in SN2 reactions?

Answer 9

Alkyl halide

Question 10

Is this SN1 or SN2?

Answer 10

SN2

Question 11

Which would be the best LG?

Cl-

OH-

CH3

Answer 11

Cl-

Question 12

True or false: An SN1 reaction proceeds in at least two steps, while an SN2 reaction proceeds in just one step.

Answer 12

This statement is true. An SN1 reaction proceeds through the formation of a carbocation first, then a nucleophilic attack. It is for this reason categorized as stepwise reaction mechanism. SN2 reactions, by contrast, happen in a single step that is driven by a nucleophile attacking the electrophilic site of the substrate, and these reactions are therefore categorized as concerted reactions.

Question 13

Sodium bromide and tert-butanol are reacted in an SN1 reaction to form an alkyl halide. This reaction can be sped up by which of the following steps?

Answer 13

The carbocation intermediate must be formed, and this is the rate-limiting step of SN1 reactions.So increasing the concentration of tert-butanol would do it.

Question 14

An SN2 reaction must be of at least which reaction order kinetics?

Answer 14

Second order reactions depend on the concentration of two species to the first power. This is the minimal reaction order for any SN2 reaction, as the reaction rate must depend on at least the concentration of the nucleophile and the concentration of the substrate.

Question 15

Which of the following compounds would most readily serve as the substrate for an SN1 reaction?

• Primary alcohol in a basic envrionemnt
• Primary alcohol in an acidic environment
• tertiary alcohol in an acidic environement
• tertiary alcohol in a basic environment

Answer 15

Tertiary alcohol in an acidic environement. Of the options presented, a tertiary alcohol would most readily form the carbocation intermediate that is required for an SN1 reaction to take place.

Question 16

What is the major predictive factor of whether something will be SN1 or SN2?

Answer 16

Substitution predicts the stability of the carbocation intermediate for SN1 reactions. Also, polar protic solvents (highly electronegative and H-bond forming) favor SN1

Question 17

True or False: tertiary carbocations are less stable than secondary

Answer 17

False, it is more.

Question 18

True or False: Primary alkyl halides do not undergo SN1 to a meaningful extent

Answer 18

True, they dont form stable carbocation intermediates

Question 19

Why do polar protic solvents promote SN1?

Answer 19

1. The slight negative charge (like the oxygen on water) will stabilize the carbocation
2. the slight positive charge (like the hydrogens on water) will interact with alkyls that are separated to keep them from going back to its orginal molecule

Question 20

True or False: SN2 reactions are more likely to occur at tertiary carbons fo rthe stability of the carbocation

Answer 20

False, they almost never occur at tertiary carbons due to steric hindrance

Question 21

What kind of solvent promotes SN2?

Answer 21

Polar aprotic like acetone and DMSO. (Polar protic solvent coudl weaken nucleophile which we dont want for SN2)

Question 22

True or false: Ethanol, ethanoic acid, and propanol are all solvents which are conducive to SN1 reactions.

Answer 22

This statement is true. All the named structures are polar protic solvents. Polar protic solvents are most conducive to SN1 reactions (note that strong acids in the absence of good nucleophiles may lead to elimination reactions, although these are NOT directly tested on the MCAT).

Question 23

What is DMSO?

Answer 23

POLAR APROTIC

dimethylsulfoxide (DMSO). Note that the two terminal methyls are not likely to dissociate protons readily, making this a polar aprotic solvent conducive to SN2 reactions.

Question 24

Would cyanided make a good nucleophile?

Answer 24

Cyanide anion is a very strong nucleophile, and strong nucleophiles drive SN2 reactions.

Question 25

What product type would this form:

Answer 25

This rxn would form an imine

An amine as a nucleophile, an acidic environment, and a ketone for a substrate are all conducive to the formation of imines.

Question 26

When will you most likely see the formation of a tetrahedral intermediate?

Answer 26

A tetrahedral intermediate is formed whenever a nucleophilic attack on a double bonded carbon occurs (commonly a carbonyl carbon). Formation of a new bond pushes the electrons from the pi bond onto the leaving group, and a carbon with four sigma bonds remains. Intermediates, while unstable, exist longer than transition states and can sometimes be isolated.

Other situations in which a trigonal planar molecular geometry undergoes nucleophilic attack are possible, but attacks on a carbonyl carbon are by far the most common scenario on the MCAT for the formation of tetrahedral intermediates.

Question 27

What is the most likely substitution mechanism to procede in this reaction?

Answer 27

The given reagents are most likely to undergo an SN1 reaction, as propanol may act as both the nucleophile and as a polar protic solvent. SN1 reactions proceed through carbocation intermediates, which conform to a trigonal planar geometry. Addition of the nucleophile subsequently may lead to the formation of both enantiomers.

Product would have the new functional group replacing the iodine, not the fluorine

Question 28

The reaction between ethyl bromide and sodium cyanide in acetone is most likely to be….

Answer 28

A strong nucleophile, a polar aprotic solvent and a primary alkyl halide with a good leaving group for a substrate all suggest an SN2 mechanism.

Question 29

Phosphorylation of certain residues on amino acids can induce rapid conformational change in proteins. The process involves the formation of a phosphoester bond, in a process somewhat similar to Fischer esterification in that the alcohol functional group acts as a nucleophile. Knowing this, which of the alpha amino acids do you expect to commonly find phosphorylated?

Answer 29

Serine, threonine, tyrosine (your -OH containing groups)

Question 30

True or False: The stability of carbanions is opposite that of carbocations.

Answer 30

True