Chapter 19: Amines

Question 1

What is a quaternary amine and what is its charge?

Answer 1

Ammonium. +1

Question 2

Why does a tertiary amine have a higher boiling point t-butane?

Answer 2

Because it has a molecular dipole, resulting in stronger intermolecular forces, and, therefore, a higher boiling point.

Question 3

How does the strength of intermolecular forces affect boiling point and melting point?

Answer 3

Stronger intermolecular forces result in a higher BP and MP.

Question 4

What must a molecule have to participate in hydrogen bonding?

Answer 4

A lone pair and a polarized hydrogen.

Question 5

How does the boiling point of a primary amine compare to the boiling point of an alcohol?

Answer 5

The amine can hydrogen bond, and its boiling point is around 50 degrees Celcius. However, the alcohol is a better hydrogen bonder (because it has a greater dipole moment), so it’s boiling point is closer to the BP of water (about 97-98 degrees Celsius).

Question 6

What is the pKa range of a neutral amine? Can it be deprotonated?

Answer 6

pKa is 33-40. Yes, it can be deprotonated to yield strongly basic reagents (e.g., lithium diisopropyl amide).

Question 7

What is the pKa of diisopropyl amine?

Answer 7

pKa=35

Question 8

What solvent do we always use with lithium?

Answer 8

THF

Question 9

What is a very good strong, bulky base made from a neutral amine?

Answer 9

LDA (lithium diisopropyl amide)

Question 10

Do higher or lower pKb’s correlate with a stronger base?

Answer 10

Lower

Question 11

What are the three major factors (in order of importance) that affect amine basicity?

Answer 11

1) Hybridization of the nitrogen
2) Resonance
3) Increasing substitution with alkyl groups

Question 12

How does the hybridization of the nitrogen affect its basicity (it’s ability to donate electrons or accept a hydrogen)?

Answer 12

The less s character of its molecular orbitals, the more basic the nitrogen. sp3>sp2>sp in basicity

Question 13

Why does decreasing the s character of a nitrogen increase its basicity?

Answer 13

1) It puts the lone pair further away from the nitrogen (easier to donate electrons).
2) Less s character means the lone pair is in a higher energy orbital. This leads to a greater delta G of reaction (i.e., the reaction is more favorable when the lone pair is in a higher energy orbital).

Question 14

How does resonance affect the basicity of an amine?

Answer 14

If the lone pair is tied up in resonance, it is not available to act as a base.

Question 15

Why does increasing the number of alkyl groups on a nitrogen increase its basicity?

Answer 15

1) Inductively donating alkyl groups will increase the ability of the nitrogen to donate a lone pair.
2) Inductively donating groups stabilize the conjugate acid.

Question 16

What must be true of the alkyl halide electrophile in an SN2 reaction with a primary neutral amine in order for the reaction to go all the way to the ammonium ion?

Answer 16

1) Must have minimal sterics (primary halide with no beta carbon substitution).
2) Must have a good leaving group (I>Br>Cl; best electrophile is H3CI).

Question 17

What do we need to do to the NR2 group in order to do an elimination with an amine?

Answer 17

Make it a good leaving group.

Question 18

Why is an NR2 group a poor leaving group?

Answer 18

It is very basic (pKa around 36)

Question 19

What are two options for turning an NR2 group into a cation? Which is the best option?

Answer 19

1) Protonation (not a good option because it will lose protons easily)
2) Do an SN2 reaction using the amine as a nucleophile (this is best).

Question 20

What geometry is necessary for a Hofmann Elimination?

Answer 20

Anticoplanar geometry (for E2 reaction)

Question 21

What molecular geometry is necessary for the intramolecular reaction in the Cope Elimination?

Answer 21

Syn periplanar

Question 22

What is the major product in the Hofmann and Cope Eliminations? What is the key difference in the Hofmann and Cope Elimination reactions?

Answer 22

a) The Hofmann product

b) The elimination geometry

Question 23

Why doesn’t a second acylation occur in the Acylation of an Amine reaction?

Answer 23

The amide N is not very basic (because its loan pair is tied up in resonance).

Question 24

What amines can be used for the formation of a Diazonium Salt?

Answer 24

Any amine (but usually an aryl amine)

Question 25

What reagent is highly reactive and very efficient for making methyl ethers? Why is the downfall?

Answer 25

Diazomethane; it is very dangerous

Question 26

Are neutral amines good acids?

Answer 26

No; their pKa’s are between 33 and 40

Question 27

Do neutral amines usually act as acids or bases?

Answer 27

bases

Question 28

What are the 3 steps of the Hofmann Elimination?

Answer 28

1) Make the amine a good LG.
2) Change the counterion.
3) Elimination

Question 29

What are the 2 steps of the Cope Elimination?

Answer 29

1) Make the amine a good LG.

2) Elimination

Question 30

What is the product of an Acylation of an Amine reaction?

Answer 30

Amide

Question 31

What kind of carbonyl reagent is needed in the Acylation of an Amine reaction?

Answer 31

An acid halide

Question 32

What are the necessary reagents in an Acylation of an Amine reaction?

Answer 32

An amine with at least one hydrogen, an acid halide, and either triethyl amine or pyradine to neutralize the acid

Question 33

Which amines can be used in a Diazonium Salt Formation reaction?

Answer 33

Any amine works

Question 34

What are the reagents in a Diazonium Salt Formation reaction?

Answer 34

Amine, NaNO2, and HCl

Question 35

What are four ways of synthesizing an amine?

Answer 35

1) via Alkylation
2) Gabriel Amine Synthesis
3) via Reductions
4) via a Rearrangement

Question 36

When synthesizing a primary amine from NH3 and an alkyl halide, how do you keep from over-alkylating the NH3?

Answer 36

Use excess NH3

Question 37

What are two reasons for using excess NH3 in the synthesis of an amine via alkylation?

Answer 37

1) To avoid over-alkylation.

2) Excess NH3 neutralizes any acids formed.