Ethers, Epoxides & Sulfides: Lecture 1 - Structure, Nomenclature & Properties, Study notes of Organic Chemistry

Download Ethers, Epoxides & Sulfides: Lecture 1 - Structure, Nomenclature & Properties and more Study notes Organic Chemistry in PDF only on Docsity! Chemistry 318/310M Professor Sessler Lecture 34 (Dr. Ian Wasser, Guest Lecturer) Ethers, Epoxides and Sulfides: Lecture 1 Today: New Material, Chapter 11 (A Short Lecture…) Friday (11/19/04): Review Exam III Answer Key Monday (11/22/04): New Material, finish Ch. 11 Wednesday (11/24/04): No Class, Happy Thanksgiving Monday (11/29/04): Professor Sessler Returns Physical Properties of Ethers Ethers are polar molecules where the oxygen has a partial negative charge, and the carbons have partial positive charges. This leads to dipole interactions in the liquid, although these are weak. Ethers cannot act as hydrogen bond donors and are less soluble in water than alcohols. However they can accept hydrogen bonds which makes them more soluble in water than hydrocarbons. Due to their weak dipole dipole intercations ethers have lower boiling points than the alcohols of corresponding molecular weight. They have boiling points much closer to hydrocarbons of similar molecular weight. Compare with alcohols: Preparation of Ethers The most common ether preparation is the Williamson ether synthesis. It is a general method for ether synthesis involving the SN2 reaction between an alkyl halide and an alkoxide anion. In planning a Williamson ether synthesis it is crucial to select conditions and reactants that maximize nucleophilic substitution and minimizes β- elimination. Br + +Na-O CH3 O + Na+Br- SN2 QuickTime™ and a Sorenson Video decompressor are needed to see this picture. Ether Synthesis Problems O-K+ CH3CH2Br+ OCH2CH3 KBr+ OCH2CH3 OCH3 O-K+ CH3I+ KI+ OCH3 Acid-Catalyzed Dehydration of Alcohols Some of the commercially available ethers are synthesized on industrial scale using high temperatures and strong acids. The alcohol group is activated by the acid and by an SN2 attack of a hydroxyl group it is displaced to give an ether. Dehydration is a competitive process, but it requires higher heat. Therefore dehydration can be reduced by controlling experimental conditions. H3C H2 C OH 2 H2SO4 140oC H3C H2 C O H2 C CH3 + H2O mech: H3C H2 C OH H OSO3H H3C H2 C OH H H3C H2 C OH H3C H2 C O H2 C CH3 H H2O + H3C H2 C O H2 C CH3 + H2O H Acid-Catalyzed Addition of Alcohols to Alkenes Under the right conditions alcohols can be added into alkenes to give ethers. Limits are alkenes that can form stable carbocations and primary and methyl alcohols. Example: t-butyl methyl ether + CH3OH H+ C CH3 O CH3H3C CH3 2-methoxy-2-methylpropane t-butyl methyl ether + HO CH3 H H HO CH3 O CH3 H HO CH3 HO CH3 H +O CH3 H mech: Reactions of Ethers Ethers are resistant to chemical reaction, like hydrocarbons. They are stable to potassium dichromate or permanganate. Very strong bases are unreactive towards ethers, and except for 3o alkyl ethers they are not reactive toward weak acids at moderate temperatures. For these reasons, among others, ethers make good solvents. However strong acids, such as HX, can cleave ethers. Ether cleavage requires a strong acid and a good nucleophile. Therefore, concentrated HI and HBr are the acids of choice for this reaction. H3C H2 C C H2 H2 C O H2 C C H2 H2 C CH3 HBr2+ heat Br H2 C C H2 H2 C CH32 + H2O mech: O H OH2 O H + H2O O H Br HO + Br HOH OH2 HO H Br Br + H2O Oxidation of Ethers - Formation of Hydroperoxides One of the hazards of working with low-molecular weight ethers (other than their high flammability) is their reactivity with oxygen. Anhydrous ethers react with O2 at the C-H adjacent to the ether oxygen to give explosive hydroperoxides. O + O2 O H OHO A hydroperoxide Hydroperoxidation proceeds by a radical chain mechanism. If the C-H next to the oxygen is secondary, rates of peroxidation are increased due to the stability of the 2o radical intermediate. All these criteria are met for the alkylation of the acetylide ion when the alcohol is protected as a tert-butyl ether. HC OH 1. H2SO4 HC O 1. Na+NH2- 2. CH3CH2Br C O H3O+/H2O C OH + The protecting group ether is added with acid, stabile under the basic alkylating conditons and removed with weak acid to regenerate the alcohol. Another type of ether protecting group is the silyl ether, which is removed by acid or (F-) fluoride ion. OH + Cl Si CH3 CH3 CH3 O Si CH3 CH3 CH3 O Si CH3 CH3 CH3 + F- OH F Si CH3 CH3 CH3+ Enjoy Your Afternoon….. (Class is over early!) Since there was an Exam Last Night,