Cycloheptane

Template:Chembox Cycloheptane, also known as Suberane, is an organic compound, which belongs to the group of cycloalkanes. The compound can occur in different conformers.

Production

Cycloheptane occurs naturally in petroleum and can be extracted from it. It is synthesized by a Clemmensen reduction from cycloheptanone.

Properties

Cycloheptane is a colorless liquid with a mild, aromatic odor. The boiling point at normal pressure is 119°C. The molar enthalpy of vaporization is 38.5 kJ mol⁻¹. According to the Antoine equation, the vapor pressure function is given by log₁₀(P) = A−(B/(T+C)) (P in bar, T in kelvins) with A = 3.97710, B = 1330.402 and C = −56.946 in the temperature range from 341.3 K to 432.2 K.

In the solid phase, cycloheptane occurs in four polymorphic forms. The transformation temperatures for the conversion from form IV to form III are −138°C, from form III to form II −75°C and from form II to form I −61°C. Form I melts at −8°C.

Summary of the most important thermodynamic properties

Property	Type	Value [Unit]	Note
Standard enthalpy of formation	ΔfH0liquid	−156.4 kJ·mol−1
Standard entropy	S0liquid	242.55 J·mol−1·K−1	as a liquid
Heat of combustion	ΔcH0liquid	−4598.9 kJ·mol−1
Heat capacity	cp	180.614 J·mol−1·K−1 (25 °C)	as a liquid
		132.0 J·mol−1·K−1 (25 °C)	as a gas
Triple point	Ttriple	265.12 K
Critical temperature	Tc	604.2 K
Critical pressure	pc	38.2 bar
Critical volume	Vc	0.53 l·mol−1
Critical density	ρc	2.83 mol·l−1

Chemical properties

Cycloheptane can be thermally rearranged to methylcyclohexane in the presence of aluminum trichloride.

File:Cycloheptane rearrangement to methylcyclohexane.svg

Functionalization can be achieved by chlorination with N-chlorosuccinimide.

File:Cycloheptane NCS chlorination.svg

The compound is flammable and forms flammable vapour-air mixtures with air. The flash point is 6 °C, the lower explosion limit is 1.1 vol.%.

Conformation

Cycloheptane is not a flat molecule, because that would give C-C-C bond angles much greater than the tetrahedral angle of around 109.5°. Instead it is puckered and three-dimensional. One can ask the question of what conformations would have the same angle everywhere (near 109.5°) and all bond lengths equal. If we think of an open chain of seven bonds, there are five dihedral angles that can be chosen, for the sequences (1,2,3,4), (2,3,4,5), and so on. The last bond though should end where the first began, and should form the correct angle with the first bond. This imposes four constraints, but we have five dihedral angles to play with, so there is one degree of freedom. It turns out that there are two continua of solutions. One is a circular series of fourteen "boat" conformations interspersed with "twist-boat" conformations, and the other is a circular series of fourteen "chair" conformations interspersed with "twist-chair" conformations. The boat and chair conformations have mirror symmetry, while the twist-boat and twist-chair have two-fold rotational symmetry. Conformations between boat and twist-boat or between chair and twist-chair have no symmetry. The passage along the continuum boat→twist-boat→boat→twist-boat→boat constitutes a pseudorotation, as does chair→twist-chair→chair→twist-chair→chair.

Uses

Cycloheptane can be used as a non-polar solvent. In organic synthesis, the cycloheptyl functional groups can be introduced into organic molecules, e.g. pharmaceutical active ingredients, after functionalization.

Hazards

An irritating effect on the eyes and respiratory tract is mentioned in the literature. The toxic effect is more comparable to that of methylcyclohexane, which only slightly irritates the mucous membranes. Animal experiments showed only a slight irritating effect on the skin. Systemically, cycloheptane has a depressant effect on the central nervous system.