halogenoalkane nucleophilic substitution with OH- ?

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thebrahmabull
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  1. I have a few questions: halogenoalkane nucleophilic substitution with OH- 1.Why do primary halogenoalkanes follow sn2 mechanism and not sn1?

    2. Why does sn1 occur in tertiary halogenoalkane and not sn2?

    3. What mechanism occurs in secondary halogenoalkanes and why?

    Thanks a lot. I am studying for edexcel and i couldnt find the exact info anywhere( perhaps because i didnot scan every page)
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Workangel_24
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(Original post by thebrahmabull)
  1. I have a few questions: halogenoalkane nucleophilic substitution with OH- 1.Why do primary halogenoalkanes follow sn2 mechanism and not sn1?

    2. Why does sn1 occur in tertiary halogenoalkane and not sn2?

    3. What mechanism occurs in secondary halogenoalkanes and why?

    Thanks a lot. I am studying for edexcel and i couldnt find the exact info anywhere( perhaps because i didnot scan every page)
Hopefully this will help you!

http://www.chemguide.co.uk/mechanism...ub/whatis.html
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♥Samantha♥
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1) primary haloalkanes could not go down Sn1 as
- the primary carbocation would be too unstable to be formed. The carbocation is electron deficient but there are not enough carbons for a positive inductive effect to stabilise the charge on the cation
- therefore the first step has a very high activation energy as the formation of the carbocation is very energetically unfavourable. So no matter how long you wait the halogenoalkane won't ionise in the first place.
- finally steric hindrance in primary halogenoalkanes is less, which allows the formation of the transition state.

2) tertiary haloalkanes go through Sn2 for basically all the opposite reasons.
- due to the large alkyl groups attached to the main carbon, steric hindrance is great. The alkyl groups 'block' the OH from attacking the back of the carbon, preventing the formation of the transition state.
- the tertiary carbocation is relatively stable, as the surrounding alkyl groups have a positive inductive effect on the positive carbon, donating their electron density to stabilise the charge. Therefore the first ionisation step has a relatively low activation energy (though the step is still slow, as it is still not very favourable).

3) secondary halogenoalkanes can go by either Sn1 or Sn2, depending on the halogenoalkane. However Sn1 will occur very, very slowly.
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alow
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(Original post by ♥Samantha♥)
1) primary haloalkanes could not go down Sn1 as
- the primary carbocation would be too unstable to be formed. The carbocation is electron deficient but there are not enough carbons for a positive inductive effect to stabilise the charge on the cation
- therefore the first step has a very high activation energy as the formation of the carbocation is very energetically unfavourable. So no matter how long you wait the halogenoalkane won't ionise in the first place.
- finally steric hindrance in primary halogenoalkanes is less, which allows the formation of the transition state.

2) tertiary haloalkanes go through Sn2 for basically all the opposite reasons.
- due to the large alkyl groups attached to the main carbon, steric hindrance is great. The alkyl groups 'block' the OH from attacking the back of the carbon, preventing the formation of the transition state.
- the tertiary carbocation is relatively stable, as the surrounding alkyl groups have a positive inductive effect on the positive carbon, donating their electron density to stabilise the charge. Therefore the first ionisation step has a relatively low activation energy (though the step is still slow, as it is still not very favourable).

3) secondary halogenoalkanes can go by either Sn1 or Sn2, depending on the halogenoalkane. However Sn1 will occur very, very slowly.
You forgot about solvent effects.
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