[FutureDoc]

I'm having a little trouble deciding the reactivity of the following halides:

1-chlorobutane
1-bromobutane
bromocyclopentane
bromocyclohexane
sec-butyl chloride
sec-butyl bromide
tert-butyl chloride

I know the reactivity order for SN1 is usually 3* > 2* > 1* > methyl, and the reverse is true for SN2, but how do I decide whether the chlorides or bromides will react faster? Also, what about the bromocycloepentane compared to the bromocyclohexane? Both are secondary, so I don't know how to determine which one will react faster under each condition.

[macman104]

For chloride over bromide, which one is a more stable leaving group?  Especially for SN2, which one is more polarizable and can stabilize the transition state better?

For the cyclopentane, vs cyclohexane, I personally feel that is a crappy comparison.  My inclination would be to say that there is little difference, unless I'm missing something.

[FutureDoc]

For chloride over bromide, which one is a more stable leaving group?  Especially for SN2, which one is more polarizable and can stabilize the transition state better?

OK, I re-read my textbook, and I see where bromide is a better leaving group than chloride, because its less basic. Because its a better leaving group, the compounds with bromide will react faster than those with chloride.

I've got one more question on the topic. We did an experiment in lab using these halides and had them undergo SN1 and SN2 reactions, but if no precipitate was present after 5 minutes, we heated the halide. Why/how does heating help SN1 and SN2 reactions proceed?

[macman104]

OK, I re-read my textbook, and I see where bromide is a better leaving group than chloride, because its less basic. Because its a better leaving group, the compounds with bromide will react faster than those with chloride.

Right!  Also, it is larger and more polarizable, so especially for SN2, it can stabilize the transition state better as well!

I've got one more question on the topic. We did an experiment in lab using these halides and had them undergo SN1 and SN2 reactions, but if no precipitate was present after 5 minutes, we heated the halide. Why/how does heating help SN1 and SN2 reactions proceed?

Think about what things need to happen when a substitution reaction happens.

Explain out loud what happens step by step as the reaction proceeds.  Do you see anything that might be sped up by adding heat (aka ENERGY *hint hint*) to the situation?

[FutureDoc]

Think about what things need to happen when a substitution reaction happens.

Explain out loud what happens step by step as the reaction proceeds.  Do you see anything that might be sped up by adding heat (aka ENERGY *hint hint*) to the situation?

Bonds must be made and broken, so does increasing the heat allow the C-X (X=halide) bond to be broken more easily?

[macman104]

Exactly!  SN2 and SN1 requires bond breaking, which requires energy.  The reaction is already favored, and will usually happen without heating, however, heating doesn't hurt any.  Good reasoning!

[FutureDoc]

Exactly!  SN2 and SN1 requires bond breaking, which requires energy.  The reaction is already favored, and will usually happen without heating, however, heating doesn't hurt any.  Good reasoning!

Thanks for the *delete me* We haven't yet discussed SN1 and SN2 reactions in great detail, so I was a bit confused about the laboratory experiment we did last week.

[Acid_RefluxDude]

nevermind - delete post