ORGANIC CHEMISTRY I CHEM 2323

ALKYNES

ALKYNE - Unsaturated hydrocarbons (less hydrogen)

STRUCTURE OF ACETYLENE

IUPAC NOMENCLATURE

1. Longest chain containing the triple bond

2. Triple bond gets lowest number

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

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

COMMON NOMENCLATURE

The common names of alkynes are derived from the acetylene group. List the one or two alkyl groups on each side of the two carbons with the triple bond between them.

PHYSICAL PROPERTIES

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

Less dense than water

Insoluble in water (nonpolar)

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

INDUSTRIAL SOURCE

PREPARATION

1. Dehydrohalogenation of alkyl dihalides

2. Reaction of metal acetylides with primary alkyl halides

ADDITION REACTIONS

A reaction in which two molecules combine to yield a single molecule of product is called an addition reaction.

The triple bond (pi bonds) serves as a source of electrons (base). The compounds which it reacts with are electron deficient (acid). These acidic reagents that are seeking a pair of electrons are called electrophilic reagents.

The typical reaction of an alkyne is electrophilic addition.

1. Additions of Hydrogen (hydrogenation)

2. Addition of Halogens

3. Addition of hydrogen halides

Markovnikov's Rule - in the addition of an acid (HX) to the carbon-carbon triple or double bond , the hydrogen (H) of the acid attaches itself to the carbon that already holds the greater number of hydrogens.

If both carbons in the carbon-carbon bond holds the same number of hydrogens, neither product will predominate and roughly equal qualities of the two isomers are actually obtained.

If peroxides are present in the system, anti-Markovnikov addition occurs. (The acid hydrogen goes to the carbon with the least number of hydrogens)

4. Addition of Water: hydration

5. Formation of metal acetylides