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SolutionsPhysical and Bond Properties

1. A chiral allene

Explain why penta-2,3-diene is chiral whereas propane is not.

Propane, non-chiral, and penta-2,3-diene, chiral
Explanation: propane is a regular tetrahedron with two pairs of identical substituents; in penta-2,3-diene the tetrahedron elongates and a central axis appears, and two different substituents at each end are enough to make object and mirror image non-superimposable

2. Rotation about the central bond

Explain why rotation about the central bond of buta-1,3-diene costs about 3 kcal/mol more than in ethane.

In ethane the 3 kcal/mol barrier comes from the clash of the hydrogens in the eclipsed conformation; in butadiene there is no such clash, but rotation breaks the overlap of the p orbitals of the central bond, which has partial double-bond character, and the barrier rises to 6.7 kcal/mol

A "normal" double bond has a rotation barrier of about 65 kcal/mol: the double-bond character of the central bond of butadiene is small, but noticeable.

3. Bond lengths

Order from shortest to longest: the C2–C3 bond of butadiene, the C=C bond of ethylene and the C–C bond of ethane.

C=C of ethylene (1.34 Å) < C2–C3 of butadiene (1.47 Å) < C–C of ethane (1.54 Å).

The central bond of butadiene is shorter than a normal single bond because it joins two sp2 carbons and has a slight double-bond character.

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