Which Compound Undergoes Unimolecular Substitution Fastest?

[WardenOfTheMint]

Homework Statement

I have these compounds, and I have to predict which one will undergo unimolecular substitution reaction the fastest. And which one will undergo it the slowest.

3 bromo cyclohexene
1 bromo cyclohexene
4 bromo cyclohexene
2 bromo hexane (not cyclohexane/ene)
The solvent is a combination of Ethanol and Silver Nitrate.

So I'm definitely sure this reaction will proceed SN1. I am also unsure of this: what causes the leaving group to leave in a SN1 reaction? . I know a good leaving group is stable because it's large. And in SN2 a leaving group 'leaves' because you have a backside attack by a nucleophile.

Homework Equations

None.



3. The Attempt at a Solution

Fastest: I'm guessing 2 bromo hexane will react the fastest, because when you get it's carbocation, the carbon bearing that cation is 2* substituted.

The rest: I'm not sure about the others. You get a cycloalkene carbocation in each one. But they all have six carbons. They all seem to be 2* substitued.

 

[Kalirren]

Don't forget resonance, which can confer additional stability to carbocations. This effect is often stronger than that of substitution.

 

[Gannon]

It is an Sn1 reaction, and one that is frequently used in labs. Leaving groups are usually substituents on a molecule that benefit less from being a substituent than just taking the electrons from the bond and becoming independent. For example, -OH is a bad leaving group because if it detaches from the molecule, it takes the electrons from the bond to create a hydroxide ion, which is very unstable (quick to react with something), so it would rather stay put. -OH2+, on the other hand, is a good leaving group because it will become H20, which is much more stable.