Question

How to make these molecules from cyclopentene?

Starting from cyclopentene how would you prepare these molecules:

a) chlorocyclopentane
b) 3-bromocyclopentane
c) cyclopentanol
d) cyclopentylcyclopentan

Answer 1

`a)`

Chlorocyclopentane will have no specific designation as to which carbon is carbon-1, so it does not matter which atom in the double bond is chlorinated. Aqueous `"HCl"` would do the trick.

`b)`

3-bromocyclopentane is the same as bromocyclopentane. I think you may mean 3-bromocyclopentene, where the 3 has a meaning.

To do this, using `N`-bromosuccinimide with a peroxide and light or heat puts a bromine atom on the allylic carbon (one atom away from an alkene carbon).

Since cyclopentene has a reflection plane bisecting its double bond, the regular product and the resonance product (where the double bond conjugates down to be across carbon-3 and carbon-2) are identical.

If you mean the alkane, just finish by subsequently adding `H_2` catalyzed by `Pd` over `C`.

`c)`

Similar to `(a)`, it does not matter which alkene carbon gets the `"OH"`, so simple acid-catalyzed hydration---which is Marknovnikov---works just fine (rather than, say, hydroboration-oxidation, which is anti-Markovnikov), i.e. `"H"_2"O"` and `"H"_2"SO"_4`.

It would work better if the `"H"_2"SO"_4` were dilute.

`d)`

This one's kinda tricky, and requires you to look for a specific reaction to do in one step.

You can do this with a Gilman reagent, which is most useful for coupling two alkyl groups starting from an alkyl halide and an alkyllithium reagent.

If you prepare your Gilman reagent:

`2"cyclopentylLi" + "CuI" stackrel("THF"" ")(->) ("cyclopentyl")_2"CuLi" + "LiI"`

then you can use it with the newly-formed bromocyclopentane dissolved in `"THF"` to dimerize and form your cyclopentylcyclopentane.