epoxide opening and ether cleavage for all you orgo experts, destroyer is wrong?

[americanpierg]

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Saw a thread like this, but there was only one reply. Looking for more people to comment on this.

Destroyer says acidic opening of an epoxide always leads to the more substituted product, while my textbook says it shows both SN2 and SN1 characteristics, leading to a more substituted product when a tertiary carbon is present and less substituted product when a primary and secondary are present.

Does acidic opening of an expoxide always occur on the more substituted side like destoryer says?


Question number two...

Ethers are unreactive unless treated with a strong acid. Does the cleavage always occur to produce the more substituted product too? Book says an ether with only a primary and secondary alkyl group will undergo SN2 attck and produce the less substituted (less hindered) product, while if there is a tertiary, benzyllic, or allylic group, it will undergo SN1 (or E1 at low temp which is weird cuz Elimination is usually high temps...some sort of kenetic control?) and produce the more substituted product. (bascially same explanation for epoxide but this time for ethers)

 

[UCB05]

Saw a thread like this, but there was only one reply. Looking for more people to comment on this.

Destroyer says acidic opening of an epoxide always leads to the more substituted product, while my textbook says it shows both SN2 and SN1 characteristics, leading to a more substituted product when a tertiary carbon is present and less substituted product when a primary and secondary are present.

Does acidic opening of an expoxide always occur on the more substituted side like destoryer says?

In general you should follow the destroyer guideline. Acid-catalyzed epoxide opening favors the more substituted product because protonation of the oxygen creates a good -OH+ leaving group. The ring opens in Sn1 manner with the more substituted carbocation which then is attacked by a nucleophile.

Base-catalyzed opening favors the less substituted product because the base attacks the less-substituted carbon in a Sn2 manner to open the ring.

 

[americanpierg]

In general you should follow the destroyer guideline. Acid-catalyzed epoxide opening favors the more substituted product because protonation of the oxygen creates a good -OH+ leaving group. The ring opens in Sn1 manner with the more substituted carbocation which then is attacked by a nucleophile.

Base-catalyzed opening favors the less substituted product because the base attacks the less-substituted carbon in a Sn2 manner to open the ring.

The OH group never leaves though? The reasoning behind it would be more than I would need to know anyways so ill just memorize it and move on.

Question number two...

Ethers are unreactive unless treated with a strong acid. Does the cleavage always occur to produce the more substituted product too? Book says an ether with only a primary and secondary alkyl group will undergo SN2 attck and produce the less substituted (less hindered) product, while if there is a tertiary, benzyllic, or allylic group, it will undergo SN1 (or E1 at low temp which is weird cuz Elimination is usually high temps...some sort of kenetic control?) and produce the more substituted product. (bascially same explanation for epoxide but this time for ethers)

 

[akeurogh]

The OH group never leaves though? The reasoning behind it would be more than I would need to know anyways so ill just memorize it and move on.

Question number two...

Ethers are unreactive unless treated with a strong acid. Does the cleavage always occur to produce the more substituted product? Book says an ether with only a primary and secondary alkyl group will undergo SN2 attck and produce the less substituted (less hindered) product, while if there is a tertiary, benzyllic, or allylic group, it will undergo SN1 (or E1 at low temp which is weird cuz Elimination is usually high temps...) and produce the more substituted product. (bascially same explanation for epoxide but this time for ethers)

your book sounds like its making things a little bit complicated.

 

[americanpierg]

your book sounds like its making things a little bit complicated.

its mcmurry 6th ed, its really a great book because it explains everything very well and a very easy read.

just noticed the destroyer discrepency and went back to book to check if i had read and memorized it wrong the first time through, seems the book was just wrong.

Acidic epoxide opening was wrong, but now im wondering if acidic ether opening was wrong too or not.

 

[UCB05]

The OH group never leaves though? The reasoning behind it would be more than I would need to know anyways so ill just memorize it and move on.

Question number two...

Ethers are unreactive unless treated with a strong acid. Does the cleavage always occur to produce the more substituted product? Book says an ether with only a primary and secondary alkyl group will undergo SN2 attck and produce the less substituted (less hindered) product, while if there is a tertiary, benzyllic, or allylic group, it will undergo SN1 (or E1 at low temp which is weird cuz Elimination is usually high temps...) and produce the more substituted product. (bascially same explanation for epoxide but this time for ethers)

The -OH+ group leaves off one side of the epoxide, yes, but stays attached to the molecule on the other epoxide ring carbon. This is a little bit different from ethers because the epoxide, being a 3-molecule ring, is already under significant ring strain and so is more amenable to the Sn1 character reaction which opens up the ring before nucelophilic attack. I could imagine an ether, being free of any sort of bond strain, might require a little push by a nucleophile to get the bond breaking reaction going, hence more Sn2 character (which favors attack at the less substituted carbon). Your book still implies that if the intermediate formed while the alkoxide leaves is stable, i.e. tertiary, benzyllic or allylic, the reaction proceeds by Sn1 mechanism.

Even so, imo this is a lot deeper than you'll need to know for the DAT

 

[americanpierg]

The -OH+ group leaves off one side of the epoxide, yes, but stays attached to the molecule on the other epoxide ring carbon. This is a little bit different from ethers because the epoxide, being a 3-molecule ring, is already under significant ring strain and so is more amenable to the Sn1 character reaction which opens up the ring before nucelophilic attack. I could imagine an ether, being free of any sort of bond strain, might require a little push by a nucleophile to get the bond breaking reaction going, hence more Sn2 character (which favors attack at the less substituted carbon). Your book still implies that if the intermediate formed while the alkoxide leaves is stable, i.e. tertiary, benzyllic or allylic, the reaction proceeds by Sn1 mechanism.

Even so, imo this is a lot deeper than you'll need to know for the DAT

thanks

 

[iDreamofDent]

Simple explanation: for the acid catalyzed opening the proton adds to the epoxide O and then the ring opens to form the most stable carbocation (most substituted) which is then where the substrate adds
Base catalyzed: the base abstracts the most acidic proton, which is the proton on the least substituted carbon (due to the inductive effects of alkyl groups decreasing acidity) the substrate then adds here

So you have Acid=most sub...base=least sub

 

[ugination87]

yup. acid is most substituted side, base is least substituted side