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20 Cards in this Set
- Front
- Back
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What are some basic properties of Aromatic Compounds?
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benzaldehyde - used to describe fragrances from cherries, peaches, and almonds
toluene - balsam benzene - coal distillate, found to cause bone marrow depression naturally occurring = steroidal hormone estrone and morphine synthetic = diazepam (antianxiety) Phenyl derived from pheno ("I bear light") |
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What are some common Ar compounds?
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Benzene-CH3 = Toluene
Benzene-NH2 = Aniline Benzene-OH = Phenol Benzene-C=O-CH3 = Acetophenone Benzene-CHO = Benzaldehyde Benzene-COOH = Benzoic Acid Benzene-CH3-CH3 = ortho-Xylene Benzene-CH=CH = styrene |
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What are properties of Benzene?
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if six p orbitals combine in a cyclic manner; six benzene pi molecular orbitals result
-1,2,3 are bonding lower energy -2,3 have same energy and are degenerate -4,5,6 antibonding high energy -4,5 have same energy and are degenerate -3,4 have nodes, no pi electron density |
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What are characteristics all Ar compounds share?
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1. Benzene is cyclic and conjugated
2. Benzene is unusually stable; 150 kJ more than expected 3. Benzene is planar, sp2, 139 pm 4. Benzene undergoes substitution reactions to retain cyclic conjugation 5. Benzene is a resonance hybrid |
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What are some Heterocycles?
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A Heterocycle is a cyclic compound that contains N, O, or S
Pyridine: -six membered with N -each 5 sp2 Cs has a p orbital perpendicular to plane of ring and each p orbital contains one pi electron -N is sp2, lone pair DOES NOT contribute, therefore can be protonated without destroying Ar -found in alkaloids and NADH Pyrimidine: -2 N in six membered ring -each N is sp2 and each contributes 1 electron to Ar pi system -base found in NA in DNA, uracil, thymine, cytosine Pyrrole: -5 membered heteroyclic found in cytochromes as tetrapyrroles -each C contributes 1 pi electron -sp2 N contributes 2 from its lone pair Imidazole: -5 membered, analog of pyrrole -1 nitrogen contributes 1 and the other contributes 2 -found in histidine |
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What are some polycyclic Ar compounds?
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Napthalene = 2
Anthracene - 3 Benzopyrene = 5 (found in cigarette smoke) Coronene = 7 |
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What are some heteroyclic analogs of Napthalene?
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quinoline, isoquinoline, purine, and indole
quinoline, isoquinoline, and purine (A and G) have pyridine-like N and contribute 1 electron to Ar pi system Indole and purine contain pyrrole like N that contribute 2 pi electrons tryptophan contains indole ring anti-malarial drug quinine contains a quinoline ring |
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What are some defining characteristics of Ar Electrophilic Substitution?
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They are less reactive toward electrophiles than alkenes
use Br2/FeBr3 intermediate carbocation in electrophilic Ar substitution is more stable than a typical carbocation due to resonance but still much less stable than the starting benzene ring instead of adding additional Br carbocation loses H+ otherwise 150 kj/mol would be lost and overall reaction would be very endergonic |
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What happens in Ar Halogenation?
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Benzene + X2/FeX3 --> HaloBenzene
example is Diazepam flourine doesnt react and iodine reacts with CuCl2 Iodine is involved as an electrophile in tyrosine, forms thyroxine |
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What happens in Ar Nitration?
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nitrated with mixture of nitric and sulfuric acids
electrophile is nitronium ion (NO2+) nitronium reacts with benzene to give a carbocation intermediate and a loss of H+ gives the product result is Nitrobenzene does not occur naturally important in laboratory: nitrobenzene + Fe, H3O --> Aniline Aniline is used as a coloring dye |
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What happens in Ar Sulfonation?
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reacts with sulfuric acid
electrophile is HSO3+ or neutral SO3 same reaction as bromination and nitration used as antibiotics for meningities and UTI (sulfanilamide) |
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What happens in Ar Hydroxylation?
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hydroxylation of Ar ring to yield a phenol
difficult and rarely done in laboratory occurs in biological pathways requires O2 and FADH2 critical in synthesis of tyrosine, dopamine, and seratonin |
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What happens in Ar alkylation?
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called Friedel-Crafts reaction
most useful in laboratory ar reactions with alkyl chloride (RCl) in presence of AlCl3 to generate carbocation electrophile occurs in numerous biological pathways: -carbocation is formed by dissociation of an organo diphosphate (ex. is phylloquinone or Vit K) |
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What are limitations of Friedel-Crafts Alkylation?
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1. Only alkyl halides can be used, aryl and vinylic halides carbocations are too high in energy
2. Do not succed on Ar rings that have a strong electron-withdrawing group such as C=O or NH2, ring may not be deactivated 3. Skeletal rearrangement such as a hydride shift or alkyl shift occurs |
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What happens in Ar Acylation?
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acylated by reaction with carboxylic acid chloride (RCOCl) in presence of AlCl3
because of stablization no carbocation rearrangement is necessary |
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What are the Meta-directing Activators and Deactivators and what do they do?
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Activators - ALL Ortho/Para, donate electrons to the ring, lower activation energy, stabilize carbocation, make ring more electron rich
-NHC=OCH3 -CH3 -OCH3 -NH2 -OH Deactivators - withdraw electrons, make ring electron poor, destabliize carbocation, raise activation energy Ortho/Para - Halogens Meta: -C=O (H, OH, CH3, OCH3) -NO2 -NR3 -C=N -SO3H Electron donating or withdraw can have 2 effects: -Inductive (due to electronegativity) -Resonance (due to overlap of p orbital on ring and substituent) |
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What are some of the substituent effects on Electrophilic substitutions?
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Ch3:
-activating, ortho/para -weak donating inductive effect -NO resonance effect OH, NH2: -Activating, ortho/para -weak withdrawing induction -strong donating resonance Halogens: -deactivating, ortho/para -Strong withdrawing induction -weak donating resonace NO2, CN, C=O: -Deactivating, meta -strong withdrawing induction -strong withdrawing resonance |
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What happens in an Ar oxidation reaction?
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strong oxidizing agents such as KMnO4 or Na2Cr2O7
Ar-R --> Ar-CO2H |
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What are properties of side-chain oxidation in Benzene?
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benzylic radicals are stabilized by resonance and thus form more readily than typical alkyl radicals
example is Dopamine hydoxylating to Nor Epi |
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What do Ar rings do to neighbors?
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activates a neighboring benzylic C-H position towards oxidation
activates a neighboring carbonyl group towards reduction ex. Propiophenone is reduced to propylbenzene by catalytic hydrogenation |
