Chemistry - STEREO ISOMERISM Question with Solution | TestHub
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) (H₃C)₂HC-CH₃ (2) (H₃C)₂HC-CH₂CH₃
(3) CH₃CH₂-CH₂OH (barrier to rotation = 17 kJ/mol - why?) (4) CH₃CH₂-OH
Options:
Answer:
Solution:
(1) Sight along the C-C bond of (CH₃)₂CH-CH₃; the front carbon carries CH₃, CH₃, H and the back carbon carries H, H, H.
Most stable (staggered) :
Least stable (eclipsed) :
Least stable = 2(CH₃↔H eclipsed) + 1(H↔H eclipsed) = 6 + 6 + 4 = 16 kJ/mol; the staggered form has no eclipsing strain, so the energy difference is 16 kJ/mol.
(2) Sight along the CH-CH₂ bond of (CH₃)₂CH-CH₂CH₃; front carbon : CH₃, CH₃, H; back carbon : CH₃, H, H.
Most stable (CH₃ anti to CH₃) :
Least stable (fully eclipsed) :
Least stable = CH₃↔CH₃ + CH₃↔H + H↔H = 11 + 6 + 4 = 21 kJ/mol; most stable keeps one unavoidable CH₃↔CH₃ gauche interaction (3.8 kJ/mol), so the difference is 21 - 3.8 ≈ 17 kJ/mol.
(3) Sight along the C₁-C₂ bond of CH₃CH₂-CH₂OH; front carbon : CH₃, H, H; back carbon : OH, H, H.
Most stable (CH₃ anti to OH) :
Least stable (eclipsed) :
Least stable = CH₃↔OH eclipsed + 2(H↔H eclipsed) ≈ 9 + 4 + 4 = 17 kJ/mol - the barrier is larger than a simple alkane barrier because eclipsing with the O-H bond and its dipole costs more than a CH₃↔H eclipsing; hence the stated barrier of 17 kJ/mol.
(4) Sight along the C-O bond of CH₃CH₂-OH; the front carbon carries CH₃, H, H and the oxygen carries H and two lone pairs.
Most stable (staggered) :
Least stable (eclipsed) :
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.
