Newman projections: butane template

Butane viewed down C2–C3: anti (180°), gauche (60°), eclipsed (120°) and syn (0°) Newman projections and the energy against dihedral angle (anti 0, gauche 3.8, eclipsed 16, syn 19 kJ mol⁻¹).

What the template shows

• Rotation about C2–C3: anti, gauche, eclipsed and syn, and the torsional energy curve
• Conformations of butane (C2–C3)
• butane: view down the C2–C3 bond
• CH₃–C2–C3–CH₃ dihedral angle
• Energy / kJ mol⁻¹
• Anti (CH₃ groups 180° apart) is the most stable conformation; the two gauche forms (60°) are about 3.8 kJ mol⁻¹ higher (steric strain between the CH₃ groups).
• Eclipsed forms are maxima: CH₃/H eclipsed about 16 kJ mol⁻¹, syn (CH₃/CH₃ eclipsed, 0°) about 19 kJ mol⁻¹.
• At 25 °C butane is roughly 70 % anti and 30 % gauche; the two gauche forms are mirror images.
• The same gauche interaction is the 1,3-diaxial strain in cyclohexane chairs.
• Wedge = toward you, hash = away from you · every part of this figure is editable

Drawn with 4 atoms, 565 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 16 Textbook
• Eliel & Wilen, Stereochemistry of Organic Compounds, Wiley 1994, ch. 10 Textbook