ORGANIC CHEMISTRY I CHEM 2323

CHAPTER 7 - ALKENES I

ALKENE - Unsaturated hydrocarbons (less hydrogen)

STRUCTURE OF ETHYLENE

PROPYLENE

GEOMETRIC ISOMERISM

IUPAC NOMENCLATURE

1. Longest chain containing the double bond

2. Double bond gets lowest number

3. Change the -ane to -ene on the parent name

4. If an -OH is present, the -OH gets priority

COMMON NOMENCLATURE

CIS or TRANS

Cis or trans is used when two groups on each side of C = C are the same

E, Z SYSTEM

Cis and trans work well for disubstituted but when there are four different groups, the E, Z system is used.

PHYSICAL PROPERTIES

The physical properties of alkenes are similar to the physical properties of alkanes.

Less dense than water

Insoluble in water (nonpolar)

BP increases as molecular weight increases (20 - 30^oC/carbon)

INDUSTRIAL SOURCE

SAYTZEFF'S RULE

In a reaction, if there are two possible positions the double bond can form, the preferred position is the alkene which has the greatest number of alkyl groups attached directly to the C = C. If there is no preferred position, then a mixture of two products of about equivalent proportions (depending on reaction conditions) are produced.

PREPARATION

The first form carbon alkenes and found naturally in petroleum. Other alkenes must be prepared by elimination

1. Dehydrohalogenation of alkyl halides

Note: we will discontinue drawing the minor produces from Saytzeff's Rule to save time and space but realize that they are never the less still produced.

2. Dehydration of alcohols

3. Dehalogenation of vicinal dihalides

4. Reduction of alkynes

 

E2 MECHANISM

E2 Mechanism is 2nd order kinetics. Biomolecular elimination

EVIDENCE OF E2

• second order kinetics
• no rearrangement

E1 MECHANISM

E1 Mechanism is a first order kinetics

ELIMINATION VS. SUBSTITUTION