Upload
dodieu
View
218
Download
0
Embed Size (px)
Citation preview
Amines and Heterocyclic Compounds
• Organic deriva7ves of ammonia, NH3 • Nitrogen atom with a lone pair of electrons, making amines
both basic and nucleophilic • Occur in plants and animals
Amines – Organic Nitrogen Compounds
• A heterocycle is a cyclic compound that contains atoms of two or more elements in its ring, usually C along with N, O, or S
Heterocyclic Amines
• Amines and carbonyl compounds are the most abundant and have rich chemistry
• In addi7on to proteins and nucleic acids, a majority of pharmaceu7cal agents contain amine func7onal groups
Why this Chapter?
• Bonding to N is similar to that in ammonia – N is sp3-‐hybridized – C–N–C bond angles are close to 109° tetrahedral value
Proper7es of Amines
• An amine with three different subs7tuents on nitrogen is chiral (in principle but not in prac7ce) : the lone pair of electrons is the fourth subs7tuent
• Most amines that have 3 different subs7tuents on N cannot be resolved because the molecules interconvert by pyramidal inversion
Chirality Is Possible (But Not Observed)
• Amines with fewer than five carbons are water-‐soluble
• Primary and secondary amines form hydrogen bonds, increasing their boiling points
Amines Form H-‐Bonds
• The lone pair of electrons on nitrogen makes amines basic and nucleophilic
• They react with acids to form acid–base salts and they react with electrophiles
Basicity of Amines
• Amines are stronger bases than alcohols, ethers, or water
• Amines establish an equilibrium with water in which the amine becomes protonated and hydroxide is produced
• The most convenient way to measure the basicity of an amine (RNH2) is to look at the acidity of the corresponding ammonium ion (RNH3
+) • High pKa → weaker acid
and stronger conjugate base.
Rela7ve Basicity
• Amides (RCONH2) in general are not proton acceptors except in very strong acid
• The C=O group is strongly electron-‐withdrawing, making the N a very weak base
• Addi7on of a proton occurs on O but this destroys the double bond character of C=O as a requirement of stabiliza7on by N=
Amides
• The N lone-‐pair electrons in arylamines are delocalized by interac7on with the aroma7c ring π electron system and are less able to accept H+ than are alkylamines
Basicity of Arylamines
• Can be more basic or less basic than aniline • Electron-‐dona7ng subs7tuents (such as –CH3,
–NH2, –OCH3) increase the basicity of the corresponding arylamine
• Electron-‐withdrawing subs7tuents (such as –Cl, –NO2, –CN) decrease arylamine basicity
Subs7tuted Arylamines
Subs7tuted Arylamines (Cont’d)
Henderson-‐Hasselbalch Equa7on:
• In what form do amines exist at physiological pH inside cells?
Biological Amines and the Henderson-‐Hasselbalch Equa7on
pH = p Ka+ log
A−⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦
so logA−⎡⎣ ⎤⎦HA⎡⎣ ⎤⎦
= pH − p Ka
Saturated Heterocyclic Compounds
Saturated amine heterocycles containing five or more atoms have physical and chemical proper7es typical of acyclic amines.
pKa Values
Amines are the most common organic bases.
Five-‐Membered Ring Aroma7c Heterocyclic Compounds
• If the nitrogen atom occurs as part of a ring, the compound is designated as being heterocyclic
• Each ring system has its own parent name
Common Names of Heterocyclic Amines
Polycyclic Heterocycles
• Pyrole is an amine and a conjugated diene, however its chemical proper7es are not consistent with either of these structural features
Pyrole and Imidazole
• Electrophilic subs7tu7on reac7ons occur at C2 because it is posi7on next to the N
• A more stable intermediate ca7on having 3 resonance forms • At C3, only 2 resonance forms
Chemistry of Pyrole
The Resonance Contributors for Pyrrole
Pyrrole is an extremely weak base because its lone pair is needed for its aroma7city.
Dipole Moments
The dipoles are in different direc7ons.
Delocaliza7on Energy
Delocaliza7on energy increases as the resonance contributors become more stable and more nearly equivalent.
Electrophilic Aroma7c Subs7tu7on Reac7ons
Why at the 2-‐Posi7on?
If the 2-‐Posi7on Is Unavailable, Subs7tu7on will Occur at the 3-‐Posi7on
Rela7ve Reac7vites
Five-‐Membered Ring Aroma7c Heterocycles are More Reac7ve Than Benzene
Protona7on Occurs at the 2-‐Posi7on
a proton is an electrophile so, like other electrophiles, it goes to the 2-position
Pyrrole Polymerizes in Acid
Pyrrole is more acidic than pyrrolidine because its conjugated base is stabilized by electron delocaliza7on.
pKa Values
pKa Values
Pyridine is Aroma7c
pKa Values
The pyridinium ion is a stronger acid than a typical ammonium ion.
Which is More Reac7ve in an Electrophilic Aroma7c Subs7tu7on Reac7on?
Pyridine Can React as a Nucleophile
The Resonance Contributors for Pyridine
Electrophilic Subs7tu7on Occurs at the 3-‐Posi7on
Rela7ve Reac7vity
Pyridine is Less Reac7ve Than Benzene in Electrophilic Aroma7c Subs7tu7on Reac7ons
Pyridine Also Undergoes Nucleophilic Aroma7c Subs7tu7on Reac7ons
the mechanism
Nucleophilic Subs7tu7on Occurs at the 2-‐Posi7on and the 4-‐Posi7on
Nucleophilic Aroma7c Subs7tu7on Reac7ons
In nucleophilic aroma7c subs7tu7on reac7ons, the ring has a leaving group that can be replaced by a nucleophile.
Reac7ons of the Side Chain
Subs7tuted pyridines undergo the same side-‐chain reac7ons that subs7tuted benzenes undergo.
Forming Diazonium Ions
The keto form is more stable than the enol form.
The Methyl Group at the 2-‐Posi7on or 4-‐Posi7on is “Acidic”
Some Reac7ons That Result from the Acidity of the Methyl Group
Heterocyclic Amino Acids
Imidazole
The Nitrogens are Equivalent in Protonated Imidazole and in the Imidazole Anion
An7histamines
Purine and Pyrimidine
The Porphyrin Ring System