Why do tertiary halogenoalkanes and alcohols react faster than primary and secondary? Watch

Presto
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charco
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(Original post by Presto)




Because the reaction proceeds via a different, faster mechanism.

Check this out - Nucleophilic substitution of haloalkanes (interactive)
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Pigster
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(Original post by Presto)
blah blah blah
I like the OCR A answer (which doesn't have either SN1/SN2 or the inductive effect)... the rate of hydrolysis reactions depends on the strength of the C - X bond; adding alkyl groups to the C atom decreases the strength of the C - X bond. Cute.
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Presto
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(Original post by charco)
Because the reaction proceeds via a different, faster mechanism.

Check this out - Nucleophilic substitution of haloalkanes (interactive)
So both are Sn1? But alcohols reacting with HCl isn't a nucleophilic substituion so why?
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charco
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(Original post by Presto)
So both are Sn1? But alcohols reacting with HCl isn't a nucleophilic substituion so why?
Tetiary alcohols react more easily as the hydroxyl group is more labile as loss leads to the formation of a tertiary carbocation, stabiised be theductive effect of three alkyl groups. It is a reversible reaction. As the concentration of chloride ions increases then they can substitute.
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Presto
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(Original post by charco)
Tetiary alcohols react more easily as the hydroxyl group is more labile as loss leads to the formation of a tertiary carbocation, stabiised be theductive effect of three alkyl groups. It is a reversible reaction. As the concentration of chloride ions increases then they can substitute.
Ok so in tertiary halogenoalkanes the methyl groups push electrons away toward the C-X bond (due electron inductive effect) which makes the C-X bond weaker and makes the release of X- ion easier and it's the same for alcohols, just replace X with OH- right?
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Presto
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(Original post by charco)
Tetiary alcohols react more easily as the hydroxyl group is more labile as loss leads to the formation of a tertiary carbocation, stabiised be theductive effect of three alkyl groups. It is a reversible reaction. As the concentration of chloride ions increases then they can substitute.
I can't process what exactly you're saying, I feel confused. Where does the conc of chloride ions come from?
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PurePaper
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the positive charge can be spread over a greater area meaning that it is more stable.
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Presto
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(Original post by PurePaper)
the positive charge can be spread over a greater area meaning that it is more stable.
No
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Pigster
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(Original post by charco)
Tetiary alcohols react more easily as the hydroxyl group is more labile as loss leads to the formation of a tertiary carbocation, stabiised be theductive effect of three alkyl groups. It is a reversible reaction. As the concentration of chloride ions increases then they can substitute.
You've got yourself confused, there. This is hydrolysis of the bromoalkane, rather than substitution of the alcohol.
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Presto
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(Original post by Pigster)
You've got yourself confused, there. This is hydrolysis of the bromoalkane, rather than substitution of the alcohol.
How is that hydrolysis? Hydrolysis where?
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charco
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(Original post by Pigster)
You've got yourself confused, there. This is hydrolysis of the bromoalkane, rather than substitution of the alcohol.
As I see it they are effectively the same reaction/equilibrium:

ROH + HBr <==> RBr + HOH

Under acidic conditions the reaction shifts to the RHS
Under basic conditions the reaction shifts to the LHS

The same argument should apply when comparing primary and tertiary structures. Any structural units favouring the formation of a carbonium ion (carbocation) should increase the rate of the reaction.

If you look at the ease of reaction with the Lucas test for alcohols, the tertiary alcohol gives an immediate positive test with equimolar HCl/ZnCl2, indicating the formation of RCl, while the secondary alcohols need about five or so minutes. The primary alcohols do not give a positive result at room temperature.
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Presto
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(Original post by charco)
As I see it they are effectively the same reaction/equilibrium:

ROH + HBr <==> RBr + HOH

Under acidic conditions the reaction shifts to the RHS
Under basic conditions the reaction shifts to the LHS
Isn't that nucleophilic substitution? It isn't reversible in the george facer book

(Original post by charco)
The same argument should apply when comparing primary and tertiary structures. Any structural units favouring the formation of a carbonium ion (carbocation) should increase the rate of the reaction.

If you look at the ease of reaction with the Lucas test for alcohols, the tertiary alcohol gives an immediate positive test with equimolar HCl/ZnCl2, indicating the formation of RCl, while the secondary alcohols need about five or so minutes. The primary alcohols do not give a positive result at room temperature.
What's the Lucas test?
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charco
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(Original post by Presto)
Isn't that nucleophilic substitution? It isn't reversible in the george facer book


What's the Lucas test?
That's my point. Effectively, all reactions are reversible under suitable conditions.

Nucleophilic substitution of haloalkanes is a reaction controlled using suitable reagents (NaOH/aq) and conditions that allow the reaction to go to (virtual) completion.
BUT
The reverse reaction, substitution of a halogen into an alcohol also occurs under suitable conditions (ZnCl2/HCl).

The Lucas test
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