Ethers and Epoxides: ochem2 week 7 part 2

How do you commonly name an ether?

Identify the two carbon chains (R groups) attached to the ether oxygen, name them as substituents, list them in alphabetical order, and add the word "ether" at the end.

Phenyl group

A benzene ring directly bonded to the oxygen in an ether (C6H5-O-R).

Benzyl group

A benzene ring separated from the oxygen by a carbon (C6H5-CH_2-O-R).

Are ethers considered priority groups in IUPAC naming?

No, they are treated as substituents and do not necessitate the lowest possible numbering on their own unless part of a tie-breaker.

How do you identify the parent chain for IUPAC naming of ethers?

For linear structures, it's the longest continuous carbon chain. For rings, the ring itself is the parent chain.

Alkoxy group

One side of the ether oxygen (the shorter R group) combined with the oxygen, named by dropping the "-yl" from the alkyl name and adding "-oxy" (e.g., methoxy, ethoxy).

What are the common conditions for Acid-Catalyzed Ether Synthesis?

Acidic conditions (e.g., H2SO4) with an alcohol.

What is the purpose of the acid in Acid-Catalyzed Ether Synthesis?

The acid protonates the alcohol, converting the hydroxyl group ($-OH$) into a good leaving group (water, -OH_2^+) and making the alcohol a better electrophile.

What mechanism does Acid-Catalyzed Ether Synthesis follow?

SN2 Mechanism.

What type of groups are preferred for the electrophilic carbon in Acid-Catalyzed Ether Synthesis (SN2)?

Primary or methyl groups.

What are the two main steps in Williamson Ether Synthesis?

Step 1: Alkoxide Formation (preparation of nucleophile); Step 2: SN2 Reaction.

Alkoxide

An alcohol with a negative charge on the oxygen (R-O^-), formed by deprotonating an alcohol with a strong base. Alkoxides are powerful nucleophiles.

What type of alkyl halide should be used in Williamson Ether Synthesis?

Primary or methyl alkyl halide.

Why can't secondary or tertiary alkyl halides be used in Williamson Ether Synthesis?

Using secondary or tertiary alkyl halides will predominantly lead to elimination (E2) reactions instead of substitution.

Why are aryl or vinyl halides unsuitable for Williamson Ether Synthesis?

The sp^2 hybridized carbons in these halides prevent the backside attack required for an SN2 mechanism.

How should reactants be chosen for synthesizing asymmetric ethers using Williamson Ether Synthesis?

The more sterically hindered group (secondary/tertiary) should be part of the alcohol component, and the less sterically hindered group (primary/methyl) should be part of the alkyl halide component.

What reagents are used in the modified Williamson Ether Synthesis with silver oxide?

An alcohol and an alkyl halide reacted in the presence of silver oxide (Ag_2O).

What is the modification in Oxymercuration-Demercuration for ether synthesis?

An alcohol (R'OH) is used as the nucleophile in the first step instead of water (H_2O).

What rule does ether synthesis via modified Oxymercuration-Demercuration follow?

Markovnikov's rule.

Why are ethers commonly used as solvents in organic reactions?

Ethers are generally very stable and unreactive functional groups.

Under what conditions can ethers be cleaved?

Under strong acidic conditions, typically using hydrohalic acids (e.g., HBr, HI).

What is the common first step in ether cleavage?

Protonation of the ether oxygen by the strong acid, making the ether a better leaving group.

When does ether cleavage follow an SN2 pathway?

If one of the R groups in the ether is primary or methyl (or sometimes secondary), the halide ion attacks the less sterically hindered carbon after protonation.

What is the result of SN2 cleavage of an ether?

The less substituted side becomes an alkyl halide, and the more substituted side becomes an alcohol.

When does ether cleavage follow an SN1 pathway?

If one of the R groups in the ether is tertiary or can form a stable carbocation (e.g., benzylic), forming a carbocation and an alcohol leaving group after protonation.

What is the result of SN1 cleavage of an ether?

The more substituted side becomes an alkyl halide, and the less substituted side becomes an alcohol.

How can one favor the SN1 pathway over the E1 elimination reaction during ether cleavage?

Run the reactions at cold temperatures.

Epoxides

Cyclic ethers consisting of a three-membered ring containing one oxygen atom and two carbon atoms.

Why are epoxides highly reactive?

Due to significant ring strain, including angle strain (bond angles ≈ 60^\circ) and torsional strain (eclipsing conformation of hydrogens).

What is the industrial method for synthesizing epoxides?

Oxidation of alkenes at high temperatures (e.g., 300^\circ C).

What is the laboratory method for synthesizing epoxides?

Alkenes react with peroxyacids.

Peroxyacid

A compound characterized by a -CO_3H group, containing an extra oxygen atom compared to a carboxylic acid.

What is a common peroxyacid used for epoxide synthesis?

meta-Chloroperoxybenzoic acid (m-CPBA).

What is the stereochemistry of epoxide formation via peroxyacids?

It is a syn addition, meaning the oxygen from the peroxyacid is added to one face of the alkene, making the reaction both stereoselective and stereospecific.

What is an important general reaction of epoxides?

Ring-opening reactions.

Hydrolysis of epoxides

A ring-opening reaction of epoxides that forms anti-diols (two hydroxyl groups on opposite faces of the former alkene carbons).