Can HF be used to cleave ethers? Explain.

Name: Problem 14, Chapter 12: Organic Chemistry
Uploaded: 2021-07-20T17:16:31-08:00
Duration: 4 min 8 s
Channel: Joshua Gibson
Description: Can HF be used to cleave ethers? Explain.

Key Concepts

Acid-Catalyzed Ether Cleavage

This concept involves the process where ethers are protonated by an acid, making the oxygen atom more electrophilic and susceptible to nucleophilic attack, which leads to the breaking (cleavage) of the carbon-oxygen bonds. Such reactions are common in organic synthesis and depend heavily on the strength and nature of the acid used.

Reactivity and Properties of Hydrofluoric Acid

Hydrofluoric acid (HF) is a unique acid that, while capable of protonating ethers due to its acidity, behaves differently compared to stronger acids like HI or HBr. Its weaker acidity and the poor nucleophilicity of the fluoride ion under certain conditions mean that HF does not always efficiently cleave ethers or may require more specific conditions compared to other acids.

Reaction Mechanism for Ether Cleavage (S_N1 vs S_N2)

The mechanism of ether cleavage generally follows either an S_N1 or S_N2 pathway depending on the substrate structure. In an S_N1 mechanism, the protonated ether forms a stable carbocation intermediate, which is then attacked by a nucleophile. In an S_N2 mechanism, the nucleophile directly attacks the protonated ether, leading to a concerted bond cleavage. Understanding these pathways is essential when considering the reactivity of different ethers with various acids.

Acid Strength and Nucleophile Effects

The efficiency of an acid in cleaving ethers is not just determined by its ability to protonate the ether but also by the nature of the nucleophile present after protonation. Strong acids like HI and HBr offer both high protonation capacity and effective halide nucleophiles for substitution reactions, while HF, with its weaker acidity and less effective nucleophile (fluoride), may result in less efficient cleavage of ethers or require alternative reaction conditions.

Transcript

00:01 This question covers the cleavage of ethers by hydrohalyde acids, specifically hf.

00:12 And what we can remember from the text is that other halide acids such as hydrobiotic and hydrobaromic acid, our very strong acids are very capable of cleaving ethers.

00:30 So these guys can cleave ethers.

00:31 To relate the rest of the halide series back to these two, we need to understand what makes these such good ether cleavers.

00:47 And the best way to explain this is we have an ether, this dimethyl ether.

00:56 In order to cleave this ether, we have to have two things happen.

00:59 One, we need to protonate it.

01:00 And then two, there has to be some sort of substitution reaction happening that allows this either to be cleaved into.

01:08 So let's take h .i as our example.

01:11 So if we have h .i.

01:17 If we say, for example, this reaction happens first because hydrolyotic acid is a very strong acid, then we would form an intermediate where we have this iodide ion plus a protonated ether, which because of that formal charge on the oxygen has been activated towards, has been activated towards substitution.

01:48 And the reason why this reaction can happen is because iodide is a very, very large ion.

01:53 You have to think all the way down, you know, if we think of our halides here, we have fluorine at the top, then we have clbr, and then i down here.

02:07 Right.

02:07 I, you know, we know that atomic size increases as we go down the periodic table.

02:14 So iodide is very large.

02:16 Because it's so large, so bulky, that makes it an excellent nucleophile because those electrons have ample access to the substrate they're trying to attack.

02:24 As a result, we would see a reaction like this, wherein iodide would attack the ether, and we would have the formation of methyl iodide.

02:38 And that would be the cleavage of this either...

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