CHM 234| Pericyclic Reaction (Cont.)
CHM 234| Pericyclic Reaction (Cont.) CHM 234
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This 7 page Class Notes was uploaded by Alvin Notetaker on Sunday March 20, 2016. The Class Notes belongs to CHM 234 at Arizona State University taught by Pillai in Fall 2015. Since its upload, it has received 17 views. For similar materials see General Organic Chemistry II in Chemistry at Arizona State University.
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Date Created: 03/20/16
Pericyclic Reactions Predicting the product when both reagents are unsymmetrically substituted D Donating group W Withdrawing group Formation of 1,4 Product Example: Formation of 1,2 Product Example: The formation of 1,3 Product will not form. If a Diels–Alder reaction creates a product with a chiral center, than it will product a racemic mixture. Retrosynthetic Analysis of the Diels Alder Reaction MO Description of Cycloadditions If there is a conversation of orbital symmetry, then it is symmetryallowed . When the orbital doesn’t overlap, it is symmetryforbidden. This is a [2+2] cycloaddition. Here, an electron in the HOMO will move to LUMO in the other. When [2+2] cycloaddition is react with heat, the orbital will create symmetryforbidden, so light is the only way a [2+2] cycloaddition can react. Electrocyclic Reactions Thermal Photochemical (light) Four π electrons Conrotatory Disrotatory Six π electrons Disrotatory Conrotatory Disrotatory New sigma orbital will rotate opposite of each other to form a sigma bond. Conrotatory New sigma orbital will rotate the same way of each other to form a sigma bond. Six π electrons Thermal & Photochemical Four π electrons Thermal & Photochemical Sigmatropic Rearrangements [3,3] Sigmatropic rearrangement Shows where the new sigma bond forms (3 position) and where the old sigma bond breaks (1 position). The Cope Rearrangement A [3,3] sigmatropic rearrangement where all six of the cyclic system is carbon . The Claisen Rearrangement A [3,3] sigmatropic rearrangement where it is observed to have vinylic ethers . Example: [1,5] sigmatropic rearrangement Example: UVVis Spectroscopy The necessary energy , excited electron from HOMO to LUMO, is either the UV or visible region of the spectrum. UVVis spectroscopy gives structural information about molecules. A = log I0 I More conjugation gives a smaller HOMO to LUMO gap. Chromophore Absorbing UVVIS light (The conjugated chain). Auxochromes Group of atoms attached to a chromophore. Chapter 18 Aromatic Compounds Aromatic includes benzenes and benzene derivatives. Nomenclature of Benzene Derivatives Monosubstituted Derivatives of Benzene For some compound, the name comes from the parent name (benzene). Other names: If the substituent chain has a longer carbon chain, then it will become the parent chain. 1phenylheptane Disubstituted Derivatives of Benzenes Based on the position of two substituents, it will be either ortho, meta, para. Ortho = adjacent Meta = separated by one carbon Para = Opposite of one another 1,2dibromobenzene orthodibromobenzene Common name for dimethyl benzene is x ylene metaxylene 1,3dimethylbenzene Polysubstituted Derivatives of Benzenes ● Identify and name the parent ● Identify and name the substituents ● Assign a locant to each substituents ● Arrange the substituents alphabetically Parent chain Name Phenol 4bromo2methylphenol Toluene 5bromo2hydroxytoluene Benzene 4bromo1hydroxy2methylbenzene Stability of Benzene Benzene is stable because its bonding orbitals are filled with electrons. Cyclobutadiene and Cyclooctatetraene stability Both are unstable. There MO are not filled with electrons like benzene. Cyclobutadiene react with itself at 78°C to make cyclooctatetraene. Hückel’s Rule A cyclic compound must have odd number of pi electrons pairs to be aromatic. Aromatic Compounds ● Cyclic ● Planer ● Every ring atom must have a p orbital ● Odd number of pair of pi electrons MO Theory and Frost Circles The unpaired electron makes cyclobutadiene very reactive. The unpaired electrons makes this a ntiaromatic.
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