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Organic II Test I

Test I

Ether's Formula R-O-R' R is Alkyl or Aryl
Structure of Ether Can be thought as an analogue of water
Uses Unreactive, Strongly Polar (lone pairs on O), Solvents for Organic Reactions
Ethers form complexes with molecules that have vacant orbitals enabling unstable molecules to be used as reagents. Example: Hydroboration uses BH3THF
Crown Ethers Macrocyclic Ethers, which help to solvate metal cations and allow inorganic salts to dissolve in organic solvents. 18-Crown 6 for Potassium
Nomenclature of Ethers Common names of ethersadd the suffix ether afternaming the groups on either side of the oxygen. Iupac names ethers by taking the more complex alkyl group as the root name and naming the remaining part as an alkoxy group.
Cyclic Ethers Naming a heterocyclic compound depends on the ring size and number of oxygens
Epoxides Three membered rings named using the term epoxy as a substituent. Cis refers to the substituents, not the expoxide which must be cis/syn. Epoxides have considerable ring strain.
Oxetans Four membered rings with one oxygen. Have ring strain but less than epoxides.
Furans Five membered rings with one oxygen and two double bonds. Furan is an aromatic molecule.
Pyrans These are six membered rings with one oxygen and two double bonds
Dioxanes Six membered rings with two oxygens
Williams Synthesis .
Alkoxymercuration - Demercuration .
Bimolecular Dehydration of Alcohols .
Reaction of Ethers Typically ethers are stable and chemically inert, although they can undergo two types of reactions (cleavage, oxidation)
Cleavage Ethers are cleaved by H-Br and H-I generating the corresponding alkyl halids
Ethers are bases, but in acidic conditions. Acidic conditions leads to protonation of the ether oxygen, which can then undergo substitution reactions. The alcohol produced reacts to generate a second molecule of alkyl halide.
Phenyl ethers Slightly different, and cleave to give alkyl halides and phenols. Reaction stops at the phenol stage since the sp2 carbon of the C-OH bond does not allow the required Sn1 or Sn2 reactions to generate the second molecule of aryl halide
Oxidation of Ethers Ethers may also auto-oxidize if left in the presence of oxygen for extended periods of time. (Dangerous) The peroxides and hydroperoxides are unstable and explosive.
Epoxide Reactions Unlike straight chain ethers epoxides are very reactive (ring strain), and are useful intermediates because of their versatility.
Synthesis of an Epoxide Recall the alkene and Peroxyacid --> epoxide and carboxylic acid. MCPBA is one of the most common epoxidising reagents. Epoxidation better for electron rich double bond
Synthesis of Epoxide from Halohydrins When halohydrins are treated with base, an intramolecular cyclisation occurs, and epoxides are formed.
Halohydrin formation produced from alkenes by reaction with halogens in presence of water
Acid Catalyzed Ring Opening Epoxides react to release their consideral (25 kcal/mol) strain energy. Recall that the acidic hydrolysis of epoxides gives anti diols.
Acid Catalyzed Ring Opening Continued Overall Transformation alkene to anti 1,2-diol can be achieved in one step by reaction with aqueous peroxyacids.
Epoxide opening with alcohol and acidic catalyst Epoxides can be ring opened by alcohols with acidic catalysis to generate alkoxy alcohols with anti Steriochemistry
Hydrohalic Acids Epoxides react with H-X to produce halohydrins, which react further with H-X to generate 1,2 dihalides. (It is easier, however, to just add X2 to an alkene.
Base Catalyzed Ring Opening Normal ethers do not undergo nucleophilic substitution or eliminations because the alkoxide anion. That is why acid catalysis is required.
Epoxide Base Catalyzed Ring Opening Epoxides are different. The release of strain when an epoxide is opened more than compensates for the poor leaving group ability and so epoxides with open under nucleophilic conditions. Epoxide has lower Ea than corresponding straight chain ether.
Reaction of Hydroxide (or alkoxide) With a symmetric epoxide generates anti diols identical to those produced under acidic conditions.
Orientation of Ring opening epoxides Unsymmetrical Epoxides give products with Different regiochemistry with basic opening compared to acidic opening.
Basic Conditioned orientation of ring opening of epoxides Under basic conditions that alkoxides simply attacks the least sterically hindered carbon in an Sn2 displacement
Acidic Conditioned orientation of Ring opening of epoxides Under Acidic conditions, the alcohol seems to attack the more hindered carbon, but it is more complicated. The protonated epoxide has resonance. Structure II is major contributor. Nucleophile attacks the carbon with greatest positive partial charge.
Organometallic Reagents Grignard and organolithium reagents also attack epoxides at the least hindered carbon to generate alcohols (after acidic workup)
Created by: 540599337