nucleophilic substitution

[nitelyechos]

i understand the halogen is substituted by the nucleophile (CH₃O-) in most cases but how do i tell which is the exception?

 

problem: R-X + CH₃O- CH₃OR + X^-

 

which haloalkane would not undergo the reaction above?

 

CH₂=CH-CH₂-Br

 

CH₃Cl

 

CH₂=CH-I

 

CH₃CH₂Br

 

CH₃CH₂CH₂Cl

[hypervalent_iodine]

I'm assuming this is a homework question.

 

What do you think the answer is? It will help you to have a look at the substrates in that list and try to work out a product for each one.

[nitelyechos]

as for as i understand in each case the haloalkane would follow the formula

R-X + CH₃O- CH₃OR + X^-

i dont know how any of the haloalkanes in the list would give any products accept CH₃OR + X^-, but i do believe that if one of the haloalkanes is an exception to this formula it would be because of the halogen. the halogen would reform with CH₃O- and give a product like CH₃OX + R or something crazy like that. so am i right that i just need to focus on the halogens? if so my money is on CH₂=CH-I since its the only haloalkane listed with iodide.

[hypervalent_iodine]

In fact, because the atomic radii of iodine is so large, iodide - carbon bonds are more labile and therefore tend to be much more reactive than corresponding chlorides or bromides; substitution not only occurs but occurs at a much faster rates where X = I.

 

I will tell you that it isn't to do with the halide. Have another look at your substrates and draw the reaction product (and the mechanism). Look at the reacting carbon centres and see if you can you pick any substrate in there that looks out of place.

[mississippichem]

I will tell you that it isn't to do with the halide. Have another look at your substrates and draw the reaction product (and the mechanism). Look at the reacting carbon centres and see if you can you pick any substrate in there that looks out of place.

 

To the OP in addition to the above:

 

Let me add that you should find the substrate that has different electronics around the carbon atom attached to the leaving group. I hope this isn't too much of a hint.

 

Remember that traditional S_N1 and S_N2 reactions need to happen at carbons with a certain geometry and a certain electronic structure. If I told you what that electronic structure and geometry were it would give away the answer.