Organic Chemistry

Summary 137

Summary

Alkenesare hydrocarbons that contain a double bond. The

double bond is the functional groupor center of reactivity

of the alkene. The functional group suffixof an alkene is

“ene.”The general molecular formula for a hydrocarbon is

minus two hydrogens for every bond or ring in

the molecule. The number of bonds and rings is called

the degree of unsaturation. Because alkenes contain fewer

than the maximum number of hydrogens, they are called

unsaturated hydrocarbons.

Because of restricted rotation about the double bond, an

alkene can exist as cis–trans isomers. The cis isomerhas

its hydrogens on the same side of the double bond; the

trans isomerhas its hydrogens on opposite sides of the

double bond. The Zisomerhas the high-priority groups on

the same side of the double bond; the Eisomerhas the

high-priority groups on opposite sides of the double bond.

p

CnH 2 n+ 2 , p

The relative priorities depend on the atomic numbers of the

atoms bonded directly to the carbons.

All compounds with a particular functional groupreact

similarly. Due to the cloud of electrons above and below its

bond, an alkene is an electron-rich molecule, or

nucleophile. Nucleophiles are attracted to electron-

deficient atoms or molecules, called electrophiles. Alkenes

undergo electrophilic addition reactions. The description

of the step-by-step process by which reactants are changed

into products is called the mechanism of the reaction.

Curved arrowsshow which bonds are formed and which

are broken and the direction of the electron flow that ac-

company these changes.

Thermodynamicsdescribes a reaction at equilibrium;

kineticsdescribes how fast the reaction occurs. A reaction

coordinate diagramshows the energy changes that take

p

sp^2

PROBLEM 25

a. Which step in the following reaction has the greatest free energy of activation?

b. Is the first-formed intermediate more apt to revert to reactants or go on to form products?

c. Which step in the reaction sequence is rate determining?

PROBLEM 26

Draw a reaction coordinate diagram for the following reaction in which C is the most sta-

ble and B the least stable of the three species and the transition state going from A to B is

more stable than the transition state going from B to C:

a. How many intermediates are there?

b. How many transition states are there?

c. Which step has the greater rate constant in the forward direction?

d. Which step has the greater rate constant in the reverse direction?

e. Of the four steps, which is the fastest?

f. Which is the rate-determining step in the forward direction?

g. Which is the rate-determining step in the reverse direction?

A

k 1

GHk

• 1

B

k 2

GHk

• 2

C

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