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Terms in this set (190)

Ring substituents that have lone pairs at the point of ring attachment are ortho/para activators because they have the ability to donate electrons into the ring by resonance.

Electronegative halogens withdraw electrons inductively, and donate electrons by resonance conjugation of their lone‑pair electrons with the π electron system of the ring. The inductive withdrawal is stronger, resulting in ring deactivation, but the lone-pair conjugation leads to ortho/para substitution.

Groups with oxygen or nitrogen (and with lone pairs) attached to the ring inductively withdraw electrons, and they donate electrons by resonance conjugation of their lone‑pair electrons with the π electron system of the ring. In this case, though, the resonance conjugation is favored and the ring is activated. Substitution is favored at the ortho/para positions.

Groups containing π bonds that can conjugate with the π electron system of the ring, such as −CO2H−CO2H and −C≡N−C≡N, remove electrons from the ring through resonance conjugation and thereby deactivate the ring. (Note that the point of attachment for each group, a carbon atom, does not have lone pairs of electrons to donate into the ring.) The deactivation is stronger at the ortho and para positions, so substitution at the meta position is favored.

In the case of phenyltrimethylammonium ion (C6H5N(CH3)+3)C6H5N(CH3)3+), the nitrogen atom lacks lone pairs, so there can be no donation by resonance. Similarly, there are no π bonds in the substituent for electron withdrawal by resonance conjugation. The positive charge strongly withdraws ring electrons by induction, deactivating the ring's reactivity and directing substitution to the meta position.
The substituents on an aromatic ring influence the ring's reactivity towards electrophilic aromatic substitution. Electron donation and withdrawal can occur by either induction or resonance. Induction is limited to one bond's length of influence and is a consequence of the position of that atom on the periodic table. For example, chlorine has a strong inductive effect because it is electronegative; it pulls electrons towards itself within a covalent bond. Resonance occurs when a substituent's lone pair or π bond delocalizes to the ring's conjugated π system.
Substituents that withdraw electrons from the ring deactivate the ring towards electrophilic aromatic substitution. In a deactivating group, the atom directly attached to the ring is also multiply bonded to an electronegative atom.
Substituents that donate electrons into the ring activate the ring towards electrophilic aromatic substitution. Activating groups possess a lone pair of electrons on the atom that is directly attached to the ring. Note that halogens are an exception; their strong electronegativity outweighs their donation by resonance.
Aniline is the strongest activator. Its lone pair of electrons delocalizes into the ring by resonance, making the ring more negative and more reactive toward electrophiles.
Toluene experiences mild electron donation into the ring by the inductive effect of the methyl carbon substituent, so toluene is mildly activated.
Benzene has only hydrogen substituents that neither donate nor withdraw electrons.
Fluorobenzene is capable of donating electrons into the ring by resonance, but it has powerful electronegative induction that deactivates the ring.
Both acetophenone and nitrobenzene deactivate the ring by withdrawing electrons by resonance, but nitrobenzene is most heavily deactivated because it more strongly withdraws by induction and has more resonance contributors.
Complete the generic mechanism for an electrophilic aromatic substitution (EAS) reaction using E as the electrophile, and show how the sigma complex is resonance stabilized. Use curved arrows to show the mechanism and the conversion between resonance structures. Make sure to add any missing charges. Note the use of a generic base (BB) in the last step. Then, label the reaction coordinate diagram for a typical EAS reaction by correctly placing the structures on the diagram.
Step 1: add a curved arrow.SelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseCH
slow−−→→slow
Step 2: add the missing charge, then and add a curved arrow.SelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseCH
sigma complex←−−−−−−−→↔sigma complex
Step 3: add the missing charge, then and add a curved arrow.SelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseCH
⟷⟷
Step 4: add the missing charge, then add curved arrows.SelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseSelectDrawRingsGroupsMoreEraseCHB
fast−−→→fast
Complete the reaction coordinate diagram by placing the starting benzene, transition states, intermediate and product.
Lay the TLC plate on a paper towel because it makes picking it up from the bench top easier.
If you have the slightest doubt about contamination, rinse the pipet, glassware, or both again with distilled water.
Clean up any spills and wash all contaminated surfaces with water.
Wash everything, including pipets, with distilled water.
Place the used TLC spotters and the used TLC plates in the appropriate waste containers provided for this purpose in the hood.

Laboratory methods that should be implemented for this experiment include:
Place a paper towel under the TLC plate to make it easier to pick up from the bench top.
Clean up any spills and wash all contaminated surfaces with water. Potassium permanganate, ninhydrin, and ceric ammonium nitrate solutions are irritants and oxidizers. Potassium permanganate will react with your skin and produce a brown stain. Ninhydrin reacts with your skin and produces a purple stain. These two stains are not harmful and will eventually wear off.
To prevent contamination, you should wash everything, including your pipets, with distilled water. Trace contaminants found naturally in tap water can destroy the accuracy of your analysis. If you have the slightest doubt about contamination, rinse the pipet, glassware, or both with distilled water again.
For disposal in this experiment:
Place the used TLC spotters and the used TLC plates in the appropriate waste containers that are provided in the hood. Dispose of all solutions in the appropriate bottles that are labeled in the hood.
-The β‑pleated sheet is held together by hydrogen bonds between adjacent segments.
-In a β‑pleated sheet, the side chains extend above and below the sheet.
-The secondary level of protein structure refers to the spatial arrangements of short segments of the protein.
Solution

A protein, or polypeptide, is a chain of amino acids folded up into a stable 3‑D structure. Protein structures are described by a series of four levels that increase in complexity from the primary level (amino acid sequence) to the more complex levels: secondary (short segments of helices and sheets), tertiary (overall 3‑D structure), and quaternary (arrangement of subunits).
The secondary level of structure describes the spatial orientation of short segments of amino acids. The two most common secondary structures are the α‑helix and the β‑pleated sheet (often shortened to β-sheet). The α‑helix is a compact coiled structure. The β‑sheet is made up of short strands that are lined up side by side to form an open surface with a pleated appearance.
Both the α‑helix and the β‑sheet are stabilized by hydrogen bonding. Both structures form hydrogen bonds between the amide backbone N−HN−H of one amino acid and the C=OC=O of another amino acid further down the chain. The α‑helix is a compactly wound coil with the side chains protruding from the backbone of the helix. In the β‑sheet, side-by-side segments are closely aligned, and the side chains extend away from both sides of the sheet's surface.
There are two types of bonds that are not directly involved in the stabilization of secondary structure: peptide bonds that join the amino acids, and disulfide bonds that form between some cysteine residues. The disulfide bonds stabilize the third (tertiary) level of protein structure.