Organic Chemistry

96 CHAPTER 2 An Introduction to Organic Compounds

Table 2.9 Heats of Formation and Total Strain Energies of Cycloalkanes

“Strainless”

Heat of formation heat of formation Total strain energy

(kcal mol) (kJ mol) (kcal mol) (kJ mol) (kcal mol) (kJ mol)

Cylopropane 53.1 27.3 114.2

Cyclobutane 28.5 26.5 110.9

Cyclopentane 6.2 25.9

Cyclohexane 0 0

Cycloheptane 6.2 25.9

Cyclooctane 9.7 40.6

Cyclononane 12.6 52.7

Cyclodecane 12.3 51.5

Cycloundecane -42.9 -179.5 -54.1 -226.4 11.2 46.9

• 36.9 -154.4 -49.2 -205.9


• 31.7 -132.6 -44.3 -185.4


• 29.7 -124.3 -39.4 -164.8


• 28.2 -118.0 -34.4 -143.9


• 29.5 -123.4 -29.5 -123.4


• 18.4 -77.0 -24.6 -102.9

+6.8 -19.7 -82.4

+12.7 -14.6 -61.1

> > > > > >

pull this

carbon down

push this

carbon up

ring flip^2

6

5

4

3

1

6

5

4

2 3

1

Figure 2.8N

The bonds that are axial in one

chair conformer are equatorial in

the other chair conformer. The

bonds that are equatorial in one

chair conformer are axial in the

other chair conformer.

If we assume that cyclohexane is completely free of strain, we can calculate the

total strain energy of the other cyclo-

alkanes. Taking the heat of formationof cyclohexane (Table 2.9) and dividing by 6

for its six groups gives us a value of (or ) for a

“strainless” group (The heat of formationis the heat

given off when a compound is formed from its elements under standard conditions.)

We can now calculate the heat of formation of a “strainless”cycloalkane by multiply-

ing the number of groups in its ring by The total strain in the

compound is the difference between its “strainless”heat of formation and its actual

heat of formation (Table 2.9). For example, cyclopentane has a “strainless”heat of for-

mation of Because its actual heat of formation is

cyclopentane has a total strain energy of 6.2 kcal mol

(Multiplying by 4.184 converts kcal into kJ.)

PROBLEM 31

Calculate the total strain energy of cycloheptane.

Cyclohexane rapidly interconverts between two stable chair conformations be-

cause of the ease of rotation about its carbon–carbon bonds. This interconversion is

known as ring flip(Figure 2.8). When the two chair conformers interconvert, bonds

that are equatorial in one chair conformer become axial in the other chair conformer

and vice versa.

[-18.4- 1 - 24.6 2 =6.2].

• 18.4 kcal>mol, >

1521 - 4.92 2 =-24.6 kcal>mol.

CH 2 - 4.92 kcal>mol.

CH 2 1 - 29.5> 6 =-4.92 2.

CH 2 - 4.92 kcal>mol -20.6 kJ>mol

(angle strain+torsional strain+steric strain)

Bonds that are equatorial in one chair

conformer are axial in the other

chair conformer.

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