Organic Chemistry

3.1 Molecular Formula and the Degree

of Unsaturation

In Chapter 2, you learned that the general molecular formula for a noncyclic alkane

is You also learned that the general molecular formula for a cyclic alkane is

because the cyclic structure reduces the number of hydrogens by two. Noncyclic

compounds are also called acycliccompounds (“a”is Greek for “non”or “not”).

The general molecular formula for an acyclic alkeneis also because, as a re-

sult of the carbon–carbon double bond, an alkene has two fewer hydrogens than an

alkane with the same number of carbon atoms. Thus, the general molecular formula

for a cyclic alkenemust be We can, therefore, make the following state-

ment:The general molecular formula for a hydrocarbon is minus two hy-

drogens for every bond and/or ring in the molecule.

Therefore, if we know the molecular formula of a hydrocarbon, we can deter-

mine how many rings and/or bonds it has because, for every twohydrogens that

are missing from the general molecular formula a hydrocarbon has either

a bond or a ring. For example, a compound with a molecular formula of

needs four more hydrogens to become Consequently, the com-

pound has either (1) two double bonds, (2) a ring and a double bond, (3) two rings,

or (4) a triple bond. Remember that a triple bond consists of two bonds and a

bond (Section 1.9).

Because alkanes contain the maximum number of carbon–hydrogen bonds possi-

ble—that is, they are saturated with hydrogen—they are called saturated hydrocar-

bons. In contrast, alkenes are called unsaturated hydrocarbons, because they have

fewer than the maximum number of hydrogens. The total number of bonds and rings

in an alkene is called its degree of unsaturation.

PROBLEM 1 SOLVED

Determine the molecular formula for each of the following:

a. a 5-carbon hydrocarbon with one bond and one ring

b. a 4-carbon hydrocarbon with two bonds and no rings

c. a 10-carbon hydrocarbon with one bond and two rings

d. an 8-carbon hydrocarbon with three bonds and one ring

SOLUTION TO 1a For a 5-carbon hydrocarbon with no bonds and no rings,

A 5-carbon hydrocarbon with a degree of unsaturation of 2 has four

fewer hydrogens, because two hydrogens are subtracted for every bond or ring present in

the molecule. Its molecular formula, therefore, is C 5 H 8.

p

CnH 2 n+ 2 =C 5 H 12.

p

p

p

p

p

CH 3 CH 2 CH 2 CH 3 CH 3 CH CHCH 3

a saturated hydrocarbon an unsaturated hydrocarbon

p

CH 3 CH CH(CH 2 ) 3 CH CH 2 CH 3 (CH 2 ) 5 C CH CH^2 CH^3

Several compounds with molecular formula C 8 H 14

p s

C 8 H 18 (C 8 H 2 * 8 + 2 ).

p C 8 H 14

CnH 2 n+ 2 ,

p

CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH CH 2

an alkane

C 5 H 12

CnH 2 n+ 2

an alkene

C 5 H 10

CnH 2 n

a cyclic alkane

C 5 H 10

CnH 2 n

a cyclic alkene

C 5 H 8

CnH 2 n− 2

p

CnH 2 n+ 2 ,

CnH 2 n- 2.

CnH 2 n

CnH 2 n

CnH 2 n+ 2.

112 CHAPTER 3 Alkenes • Thermodynamics and Kinetics

The general molecular formula for

a hydrocarbon is minus two

hydrogens for every bond or

ring present in the molecule.

P

CnH 2 n+ 2 ,

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