5. Explain why each of the following is true. You may wish to include drawings a

Question

5. Explain why each of the following is true. You may wish to include drawings and/or examples in your explanations. a. A tertiary (3 degree) carbocation is generally more stable than a secondary (2 degree) carbocation. b. A secondary allylic carbocation is generally more stable than a secondary carbocation. c. A sharp signal corresponding to the carbon-carbon double bond in 1,1-dichloro-2-methyl-1- propene is observed on this compound's IR spectrum, while the carbon-carbon double bond in 2,3- dimethyl-2-butene does not produce a signal on its IR spectrum. d. The lowest-energy chair conformation of cis-1,3-dimethylcyclohexane has both methyl groups in the equatorial position. e. The lowest-energy chair conformation of cis-1,3-cyclohexanediol has both -OH groups in the axial position.

Explanation / Answer

a. In 3' carbocation, the +vely charge is delocalized by the surrounding electron donating alkyl groups, whereas in 2' carbocation, the H atom may leave as H+ ion

b. Because the allylic group is electron withdrawing and therefore prevents the dissociation of H+ ions from the 2' allylic carbocation and hence, it is more stable.

c. Because of the structural symmetry and electron donating CH3 group around the C=C bond in butene, the pi electrons are localised and therefore a peak is missing in IR. However, in propene, the assymmetric structure and electron withdrawing Cl atoms, pulls the pi electrons of C=C double bond towards one side and a strong peak appears.

d. The attaction of methyl group towards the double bond and the bulkiness of the methyl group makes this conformation the lowest energy of the molecule

e. The repulsion of C=C electrons and the electrons on O atoms, the axial position of OH groups becomes the lowest energy level for the molecule to minimize the repulsion.

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