Alkane stereochemistry. For the molecules below : (a) provide a three-dimensional structure at the indicated atoms; (b) draw the Newman projection for your structure indicating the direction of sight

Question

Alkane stereochemistry. For the molecules below :

(a) provide a three-dimensional structure at the indicated atoms;

(b) draw the Newman projection for your structure indicating the direction of sight with an arrow;

(c) draw the Newman projection for the most stable conformation;

(d) draw the Newman projection for the least stable conformation;

(e) if possible, calculate the energy difference between the most and the least stable conformations.

(1) $C H_{3} C H_{2} - C H_{2} C H_{3}$ (2) ( $H_{3} C)_{2} HC - CH (C H_{3})_{2}$

(3) $HOC H_{2} - C H_{2} OH$ (the gauche conformer is the most stable - why?) (4) $C H_{3} - OH$

TestHub · Accepted Answer

(1) Sight along the C-C bond of butane; the front carbon carries $C H_{3}$ , H, H and the back carbon carries $C H_{3}$ , H, H.

Most stable (staggered, $C H_{3}$ anti to $C H_{3}$ ) :

structure

Least stable (fully eclipsed) :

structure

Least stable = $C H_{3} \leftrightarrow C H_{3}$ eclipsed + 2(H↔H eclipsed) = 11 + 4 + 4 = 19 kJ/mol; the anti staggered form has no eclipsing or gauche strain, so the energy difference is 19 kJ/mol.

(2) Sight along the $C_{2} - C_{3}$ bond of 2,3-dimethylbutane; front carbon : $C H_{3}$ , $C H_{3}$ , H; back carbon : $C H_{3}$ , $C H_{3}$ , H.

Most stable (one $C H_{3}$ anti to $C H_{3}$ ) :

structure

Least stable (fully eclipsed) :

structure

Least stable = $2 (C H_{3} \leftrightarrow C H_{3}$ eclipsed) + (H↔H eclipsed) = 11 + 11 + 4 = 26 kJ/mol; the most stable staggered form keeps two unavoidable $C H_{3} \leftrightarrow C H_{3}$ gauche interactions (2 × 3.8 = 7.6 kJ/mol), so the difference is 26 - 7.6 ≈ 18 kJ/mol.

(3) Sight along the C-C bond of ethane-1,2-diol; front carbon : $OH$ , H, H; back carbon : $OH$ , H, H.

Most stable (gauche) :

structure

Least stable (fully eclipsed) :

structure

In the gauche conformer the two O-H groups are close enough to form an internal hydrogen bond, which stabilises it by several kJ/mol - enough to outweigh the usual preference for anti; hence the gauche conformer is the most stable (the 'gauche effect').

(4) Sight along the C-O bond of methanol; the front carbon carries H, H, H and the oxygen carries H and two lone pairs.

Most stable (staggered) :

structure

Least stable (eclipsed) :

structure

The barrier here is the smallest of the series : eclipsing involves only the small O-H bond and the diffuse lone pairs on oxygen, which cause little strain compared with eclipsing between full bonds on two carbons.

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