Cyclopentane

Cyclohexane

The common cycloalkanes are cyclopentane and cyclohexane.

Using the Born-Haber cycle we can establish the amount of
energy needed to produce a mole of alkane, C_nH_(2n+2);

	n C(s)	→	n C(g)	n x 716.7 kJ
	(2n+2)/2 H2(g)	→	(2n+2) H(g)	(2n+2) x 218.0 kJ
	n C(g) + (2n+2) H(g)	→	CnH(2n+2)(g)	Total bond formation energy
SUM	n C(s) + (2n+2)/2 H2(g)	→	CnH(2n+2)(g)	Enthalpy of formation

Note: Bond formation energy = − bond energy, since bond energy is the energy needed to break the bond.

So enthalpy of formation = (n x 716.7 kJ + (2n+2) x 218.0 kJ) + total bond formation energy.

Tutorial 2.6

ALKANES ΔcHΘ / kJ mol‾�

Methane − 802.3

Ethane − 1427.2

Propane − 2043.9

Butane − 2656.9

Pentane(g) − 3271.9

Hexane(g) − 3886.9

Δ_fH^Θ(CO₂) = 393.5 kJ mol‾� and Δ_fH^Θ(H₂0_(g)) = 241.8 kJ mol‾�.

With the data given above, compute the standard enthalpy of formation of the alkanes. Hint    Answer

Assuming the bond formation energy for the methyl radical CH₃−   be 1579 kJ mol‾� that for a CC σ−bond be 334 kJ mol‾� and that for a C−H be 420 kJ mol‾�; the enthalpy of formation for propane computed would be  −103.9 kJ mol‾�. This is in fairly good agreement with that measured.

For cycloalkane the enthalpy of formation can be represented by;

Enthalpy of formation = (n x 716.7 + 2n x 218.0) kJ + total bond formation energy

The enthalpy of formation for cyclopropane would be  −63.9 kJ mol‾�. This means that  −63.9 kJ of energy should be released to form a mole of the compound. The actual value measured was 53 kJ mol‾�. So a total of 117 kJ (64 kJ + 53 kJ) must be supplied to form a mole of cyclopropane. This is extremely high. Very often it is referred to as a molecule under stress.

Similarly it is with cyclobutane. The enthalpies of formation of some corresponding alkanes and cycloalkanes are tabulated below. (Try computing the values. If you have any problem contact me.)

Difference: The energy needed to form the molecule on top of the energy needed to form the bonds.

The problem with cycloalkanes is the bond angle. Sp� orbitals require the C−H bonds angle to be ca 109�. This is difficult with cyclopropane and cyclobutane. Only cyclopentane and cyclohexane meet this requirement. Although this is not such a serious problem with larger cycloalkanes you will find that it is still a problem if you try constructing the molecule with a model.

So cycloalkanes are easy to prepare. The only cycloalkanes most students would come across would be cyclopentane and cyclohexane and their chemical properties are no different from that of the straight chain alkanes.