Lecture 7 Flashcards
How does the substitutant on substituted phenols, protonated anilines, and benzoic acid affect acidity?
Electron-donating substituents destabilize a base (i.e the base of phenol, protonated aniline, and benzoic acid) therefore, decrease the strength of its conjugate acid; electron-withdrawing substituents stabilize a base, which increases the strength of its conjugate acid. Remember: the stronger the acid, the more stable (weaker) its conjugate base.
An example is shown in digital notes.
What are the different ways in which a substitutant can donate and withdraw electrons?
- Electron withdrawal:
- Inductive Electron Withdrawal: If a substituent that is bonded to a benzene ring is more electron withdrawing than a hydrogen, then it will draw the sigm electrons away from the benzene ring more strongly than a hydrogen will. Withdrawal of electrons through a s bond is called inductive electron withdrawal. The +NH3 group is an example of a substituent that withdraws electrons inductively
- Electron Withdrawal by Resonance: If a substituent is attached to a benzene ring by an atom that is doubly or triply bonded to a more electronegative atom, then the electrons of the ring can be delocalized onto the substituent; these substituents are said to withdraw electrons by resonance. Substituents such as C=O, C≡N, SO3H, and NO2 withdraw electrons by resonance. These substituents also withdraw electrons inductively because the atom attached to the benzene ring has a full or partial positive charge and is, therefore, more electronegative than a hydrogen. Example is shown in notes.
- Electron donation:
- Electron Donation by Hyperconjugation: We saw that an alkyl substituent (such as CH3) stabilizes alkenes and carbocations by hyperconjugation—that is, by donating electrons to a p orbital.
- If a substituent has a lone pair on the atom directly attached to a benzene ring, then the lone pair can be delocalized into the ring. These substituents are said to donate electrons by resonance. Substituents such as NH2, OH, OR, and Cl donate electrons by resonance. These substituents also withdraw electrons inductively because the atom attached to the benzene ring is more electronegative than hydrogen. Example shown in notes
Important note
As we said in flashcard 2 atoms such as Cl can donate electrons by resonance but they also withdraw electrons inductively due to differences in electronegativity. Sometimes (such as in the case of Cl) the withdraw of electrons is greater than the donation, so the ring is overall stabilized and thus more acidic. This is not weird as we talked about it before and we should know more from the last lecture about which atoms are activating and which are deactivating.
Why is this concept important to remember?
Okay, now we will start to talk about the main topic of this lecture, which is heterocyclic compounds. We will apply the knowledge learned above to analyze the acidity of a heterocyclic compound depending on the atom incorporated into the ring and the aromaticity (if applicable).
What are Heterocyclic compounds?
Heterocyclic compounds (or heterocycles) are cyclic compounds in which one or more of the atoms of the ring are heteroatoms. In this chapter, we will consider the most prevalent heterocyclic compounds—the ones containing the heteroatom N, O, or S.
What are some important notes about amines before we move forward?
■ The nitrogen in amines is sp3 hybridized with the lone pair residing in an sp3 orbital.
■ Amines invert rapidly at room temperature through a transition state in which the sp3 nitrogen becomes sp2 nitrogen. Shown in digital notes
■ The lone-pair electrons of the nitrogen atom cause amines to react as bases and as nucleophiles. Shown in digital notes
How do we name nitrogen-, oxygen-, and sulfur-containing saturated heterocycles?
A saturated cyclic amine, can be named as a cycloalkane, using the prefix aza to denote the nitrogen atom, For example, azacyclopropane. Saturated heterocycles with oxygen and sulfur heteroatoms are named similarly. The prefix for oxygen is oxa and the prefix for sulfur is thia.
Note that heterocyclic rings are numbered to give the heteroatom the lowest possible number, For example, 2-methylazacyclohexane
What are the relative acidities of ammonium and anilinium ions?
ammonium ions have pKa values of about 10 )nd anilinium ions have pKa values of about 5 (Sections 8.9 and 8.10). The greater acidity of anilinium ions compared with ammonium ions is due to the greater stability of the conjugate bases of anilinium ions as a result of electron delocalization and inductive electron withdrawal.
How does the acidity of heterocyclic amines compare with acyclic amines?
Saturated amine heterocycles containing five or more atoms have physical and chemical properties like those of acyclic amines. This is shown in digital notes.
What are the different aromatic aromatic heterocyclic compounds?
pyridine, pyrrole, furan, and thiophene. The structures are shown in written notes.
What is the electronic structure of pyridine?
Pyridine is an aromatic heterocyclic compound. Each of the six ring atoms of pyridine is sp2 hybridized, which means that each has a p orbital, and the molecule contains three pairs of p electrons. Do not be confused by the lone-pair electrons on the nitrogen—they are not p electrons. Recall that lone-pair electrons are p electrons only if they can be used to form a p bond in the ring of a resonance contributor. Because nitrogen is sp2
hybridized, it has three sp2 orbitals and
a p orbital. The p orbital is used to form the p bond. Two of nitrogen’s sp2 orbitals overlap the sp2 orbitals of adjacent carbons, and its third sp2 orbital (which is perpendicular to nitrogen p orbital) contains the lone pair. The structure is shown in digital notes.
What are the resonance contributors of pyridine?
Shown in digital notes
What is the electronic structure of pyrrole?
The nitrogen atom of pyrrole is sp2
hybridized. Thus, it has three sp2 orbitals and a p orbital. It uses its three sp2 orbitals to bond to two carbons and one hydrogen. The lone-pair electrons are in the p orbital that overlaps the p orbitals of adjacent carbons. Pyrrole, therefore, has three pairs of p electrons and is aromatic. This is shown in digital notes
What are the resonance contributors of pyrrole and what do they intell
The resonance contributors are shown in digital notes and they intell that the lone-pair electrons of pyrrole form a pi bond in the ring of a resonance contributor; thus, they are pi electrons.
What is the electronic structure of furan and thiophene?
Similar to pyridine and pyrrole, furan and thiophene are aromatic compounds. Both the oxygen in furan and the sulfur in thiophene are sp2 hybridized and have one lone pair in an sp2 orbital. The orbital picture of furan in digital notes (under flashcard 13) shows that the second lone pair is in a p orbital that overlaps the p orbitals of adjacent carbons, forming a pi bond (as shown from the resonance contributors shown in digital notes). Thus, they are pi electrons.
What are other heterocyclic aromatic compounds?
Shown in digital notes, maybe know imidazole by heart, but other than that just look at them long enough to be able to recognize them in the exam and recognize that they are aromatic.
Break and review
Understanding the above heterocyclic aromatic compounds is very important, and memorizing their resonance contributors is even more important and they intell so much information about the compound (such as how they react) as will be seen in the upcoming flashcards
Structure of how I’m going to split this
Okay, I find it best to start with the five-membered rings (Pyrrole, furan, and thiophene) and then go the the six-membered pyridine. With each will discuss their electron density map, acid-base properties, and relevant reactions ;).
What is pyrrolidine?
A nonaromatic heterocyclic amine, structure is shown in the notes (under Flash card 20), it is very important!!
What are the electron density maps of pyrrole and pyrrolidine?
Pyrrole has a dipole moment of 1.80 D and pyrrolidine has a slightly smaller dipole moment of 1.57 D, but as we see from the electrostatic potential maps shown in digital notes, the two dipole moments are in opposite directions. the dipole moment in pyrrolidine is due to inductive electron withdrawal by the nitrogen. Apparently, the ability of pyrrole’s nitrogen to donate electrons into the ring by resonance more than makes up for its inductive electron withdrawal. Note that a better understanding of pyrrole’s electron density map can be made from looking at its resonance contributor and resonance hybrid (flashcard 14)
How do the delocalization energies of the aromatic five-membered heterocyclic compounds compare with benzene and cyclic enolate ion?
we saw that a compound’s delocalization energy increases as the resonance contributors become more stable and more nearly equivalent (3yani similar in structure). The delocalization energies of pyrrole, furan, and thiophene are not as great as the delocalization energies of benzene or the cyclopentadienyl anion, each a compound for which all the resonance contributors are equivalent.
Thiophene, with the least electronegative heteroatom, has the greatest delocalization energy of the three, and furan, with the most electronegative heteroatom, has the smallest delocalization energy. This is what we would expect, because the resonance contributors with a positive charge on the heteroatom are the most stable for the compound with the least electronegative heteroatom and the least stable for the compound with the most electronegative heteroatom*. Recall that the more stable the resonance contributors, the greater the delocalization energy. A summary is shown in digital notes.
- A Connection is made here, where we can assume that the resonance contributors of thiophene are the same as pyrrole and furan and that they have similar electron density maps and properties with differences in extremity (this is not always the case thou in the exam really think about it please)
What are the acid-base properties of pyrrole?
Pyrrole is an extremely weak base because the electrons shown as a lone pair in the structure are part of the pi cloud. In other words, the nitrogen donates the lone pair into the five-membered ring. Therefore, protonating pyrrole destroys its aromaticity. As a result, the conjugate acid of pyrrole is a very strong acid (pKa = -3.8).
Where is pyrrole protonated?
Building up on the previous flashcard, , pyrrole is protonated on a carbon rather than on the nitrogen since the resonance hybrid of pyrrole (flash card 14) shows that there is a partial positive charge on the nitrogen and a partial negative charge on each of the carbons. The carbon it attaches to is C-2 because a proton is an electrophile (as shown in digital notes), and we will later see that electrophiles attach preferentially to the C-2 position of pyrrole.
Why is pyrrole unstable in strongly acidic solutions?
Pyrrole is unstable in strongly acidic solutions because, once protonated, it can readily polymerize as shown in digital notes.