Chapter 19: Amines Flashcards
What is a quaternary amine and what is its charge?
Ammonium. +1
Why does a tertiary amine have a higher boiling point t-butane?
Because it has a molecular dipole, resulting in stronger intermolecular forces, and, therefore, a higher boiling point.
How does the strength of intermolecular forces affect boiling point and melting point?
Stronger intermolecular forces result in a higher BP and MP.
What must a molecule have to participate in hydrogen bonding?
A lone pair and a polarized hydrogen.
How does the boiling point of a primary amine compare to the boiling point of an alcohol?
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).
What is the pKa range of a neutral amine? Can it be deprotonated?
pKa is 33-40. Yes, it can be deprotonated to yield strongly basic reagents (e.g., lithium diisopropyl amide).
What is the pKa of diisopropyl amine?
pKa=35
What solvent do we always use with lithium?
THF
What is a very good strong, bulky base made from a neutral amine?
LDA (lithium diisopropyl amide)
Do higher or lower pKb’s correlate with a stronger base?
Lower
What are the three major factors (in order of importance) that affect amine basicity?
1) Hybridization of the nitrogen
2) Resonance
3) Increasing substitution with alkyl groups
How does the hybridization of the nitrogen affect its basicity (it’s ability to donate electrons or accept a hydrogen)?
The less s character of its molecular orbitals, the more basic the nitrogen. sp3>sp2>sp in basicity
Why does decreasing the s character of a nitrogen increase its basicity?
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).
How does resonance affect the basicity of an amine?
If the lone pair is tied up in resonance, it is not available to act as a base.
Why does increasing the number of alkyl groups on a nitrogen increase its basicity?
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.
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?
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).
What do we need to do to the NR2 group in order to do an elimination with an amine?
Make it a good leaving group.
Why is an NR2 group a poor leaving group?
It is very basic (pKa around 36)
What are two options for turning an NR2 group into a cation? Which is the best option?
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).
What geometry is necessary for a Hofmann Elimination?
Anticoplanar geometry (for E2 reaction)
What molecular geometry is necessary for the intramolecular reaction in the Cope Elimination?
Syn periplanar
What is the major product in the Hofmann and Cope Eliminations? What is the key difference in the Hofmann and Cope Elimination reactions?
a) The Hofmann product
b) The elimination geometry
Why doesn’t a second acylation occur in the Acylation of an Amine reaction?
The amide N is not very basic (because its loan pair is tied up in resonance).
What amines can be used for the formation of a Diazonium Salt?
Any amine (but usually an aryl amine)
