Ethers and epoxides thiols and sulfides

Because chromates are so toxic that it is difficult to find a waste hauler who will take them offyour hands, they are almost never used any more. Instead, people use… The Swern Oxidation. Primary alcohols are oxidized to aldehydes; secondary alcohols are oxidized to ketones.

Amines are the most common organic bases and are often used as nucleophiles in S N 2 reactions,and with carbonyl compounds. Ethers will undergo nucleophilic substitution, but very reluctantly and only with hydrogen halides.

Ethers are unreactive enough to be used commonly as solvents for organic reactions. Epoxides undergo nucleophilic addition reactions. Nucleophiles attack epoxides under both acidicand basic conditions. The nucleophile opens the epoxide ring, forming an alcohol at the carbon alpha tothe point of attack. Under acidic conditions, epoxides react with nucleophiles at the more substituted carbon.

Arene oxides are key intermediates in the carcinogenicity of aromatic compounds in the body, aswell as in the biosynthesis of phenols. Epoxidation reactions using cytochrome P generate arene oxides. Arene oxides rearrange to phenols the biosynthetic route. Arene oxides react with nucleophiles to form addition products the carcinogenic route.

Different ring sizes fit different cations to form inclusion complexes. Thiols, sulfides and sulfonium salts. We have covered nomenclature of thiols and sulfides. Imagine getting on the city bus and having the people next to you get up from their seats and move down to the other end.

Those stories were not uncommon from the people in that lab. Thiols are more acidic than alcohols by an average of about 5 pKa units or so pKa of about 11 for the thiol pictured below. Why might that be? When understanding acidity trends it greatly helps to think about the stability of the conjugate base Recall that acidity will be increased by any factor which stabilizes the conjugate base. In other words, we increase the acidity. Note the greater pKa of the molecule pictured above about The same holds true for selenium, which has an even larger atomic radius.

The weaker basicity of thiolates means that only SN2 reactions occur with alkyl halides. Thiols are more nucleophilic than alcohols, and thiolates are more nucleophilic than alkoxides. Since nucleophilicity is measured by reaction rate , that means that these sulfur nucleophiles tend to react faster with typical electrophiles like alkyl halides than their oxygen-based cousins.

This trend continues as we descend a column of the periodic table, so in general, nucleophilicity increases in this direction as well i. RSe— is even more nucleophilic than RS—. This process resembles the Williamson to a tee.

First, a strong base deprotonates the thiol we use NaH here, but many other bases could also be used. Secondly, we add an alkyl halide, and an SN2 reaction results in formation of S-C and breakage of C-Br with inversion of stereochemistry. Since it comes up so much in exams, the intramolecular version is important to note.

Oxidation In previous posts we saw that primary alcohols are oxidized to aldehydes and secondary alcohols are oxidized to ketones. Thiols can be oxidized to disulfides through treatment with a mild oxidant like iodine I2.

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Their low level of reactivity is one reason why ethers are often used as solvents in chemical reactions, and as protecting groups for reactive —OH group. Ethers oxidize in air to form explosive hydroperoxides and peroxides. Epoxide Ring Opening in Acid In acid: protonate the oxygen, establishing the very good leaving group.

More substituted carbon more positive charge although more sterically hindered is attacked by a weak nucleophile. Very similar to opening of cyclic bromonium ion. Review that subject. Due to resonance, some positive charge is located on this carbon. Inversion occurs at this carbon. Do you see it? Classify the carbons.

S becomes R. Epoxide Ring Opening in Base In base: no protonation to produce good leaving group, no resonance but the ring can open due to the strain if attacked by good nucleophile. Now less sterically hindered carbon is attacked. Formation of ether from alcohols. When you observe this pattern it suggests the use of epichlorohydrin. Both of these bonds will be formed by the incoming nucleophiles.

Alkoxides need be derived from primary, secondary or tertiary alcohols Origin of Reactants What happens if the alkyl halide is not primary? It is an effective octane booster in gasoline, but contaminates ground water if allowed to leak from storage tanks. Further use of MTBE is unlikely. Alkoxymercuration Mechanism of Oxymercuration Reactions of Ethers: Summary of reactions of ethers No reactions of ethers encountered to this point.

Ethers are relatively unreactive. Their low level of reactivity is one reason why ethers are often used as solvents in chemical reactions, and as protecting groups for reactive —OH group. Ethers oxidize in air to form explosive hydroperoxides and peroxides. Epoxide Ring Opening in Acid In acid: protonate the oxygen, establishing the very good leaving group.

More substituted carbon more positive charge although more sterically hindered is attacked by a weak nucleophile. Very similar to opening of cyclic bromonium ion. Review that subject. Due to resonance, some positive charge is located on this carbon. Inversion occurs at this carbon. Do you see it? Classify the carbons.

S becomes R.