Question

this procedure is a microscale hydration of norbornene.

1. What was the stereochemical outcome of the reaction? what evidence permitted a definitive assignment of stereochemistry?

2. Assemble a molecular model of norbornene and use it to explain why the obsvered product was obtained rather than the isomer.

3. The norbornyl carbocation does not rearrange to a tertiary carbocation even though there is a tertiary carbon adjacent to the secondary carbon bearing the positive charge. Suggest a possible explantion.

4.Norbornene is a strained alkene. It is very reactive toward hydration in aqueous acid. Explain how strain contributes to the high reactivity.

Answer 1

1. and 2.

a) Hydration of norbornene is the acid catalysed reaction. It follows electrophilic addition reaction mechanism in which proton first adds to C=C and forms the carbocation- a planar structure then the H2O water molecule attacks the carbocation. The approach of H2O is from the side of bridged group as the rear side attack is hindere because of flag pole hydrogens. This gives rise to the endo product formation as major or almost exclusively.

b) The physical constan values like M.P and IR spectral data will give definite assignment of the product formed.

For endo isomer : M.P. is ~150 ^oC and IR O-H strech is 1030 cm^-1 .

For exo isomer : M.P. is ~125 ^oC and IR O-H stretch at 1000 cm^-1.

M.P is more useful tool than IR data.

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3. Rearrangement of the secondary carbocation to the most stable tertiary carbocation by hydride transfer do not occur because,

i. The carbocation has neccessarily a planar structure and the attainment of planarity at the bridghead position is not possible and this makes the would be forming tertiary carbocation less stable or we can say highly unstable.

ii. This is also known as Bredt's rule states that the no carbocation formation possible at bridghead position.

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4.

In the nrbornene structure we can see that there is C=C in six membere ring with the bridging of CH2 group in C3-C6 carbon. C1 C2 are sp2 hybrid atoms. This makes the ring highly strained and difficult to attain more stable chair form- which is most stable conformation for six membered C atom ring i.e cyclohexane ring.

The hydration causes the conversion of C=C to C-C bonding through addition and hence the strain is considerably removed.

(Still chair form can't be attained by the structure.)

Hence the contributuion of strain in the norbornene in making it more reactive towards hydration.