Alkynes and dienes compounds

1. ALKYNES AND DIENES

2. Higher alkynes. Nomenclature
Like the alkanes and alkenes, the alkynes form a homologous series, the increment
again being CH2 . The alkynes are named according to two systems. In one, they are
considered o be derived from acetylene by replacement of one or both hydrogen
atoms by alkyl groups.
Alkynes
Industrial source of acetylene
Preparation of acetylene
The simplest member of the alkyne family is acetylene, C2H2 . The general formula
for this family is C n H 2n -2

3. For more complicated alkynes the IUPAC names are used. The rules are exactly the same as for the naming
of alkenes, except that the ending -yne replaces-ene. The parent structure is the longest continuous chain that
contains the triple bond, and the positions both of substituents and of the triple bond are indicated by
numbers.
The triple bond is given the number of Ihe first triply-bonded carbon encountered, starting from the
end of the chain nearest the triple bond.
Physical properties of alkynes

4. Preparation of alkynes
A carbon-carbon triple bond is formed in the same way as a double bond: elimination
of atoms or groups from two adjacent carbons. The groups eliminated and the
reagents used are essentially the same as in the preparations of alkenes.
1. Dehydrohalogenation of alkyl dihalides:
2. Reaction of sodium acetylides with primary alkyl halides:

5. 3. Dehalogenation of tetrahalides:
REACTIONS OF ALKYNES
1. Addition of hydrogen
Addition Reactions

6. 2. Addition of halogens
3. Addition of hydrogen halides

7. 4. Addition of water Hydration
5. Formation of heavy metal acetylides
6. Formation of alkali metal acetylides

8. DIENES
Structure and nomenclature of dienes
Dienes are simply alkenes that contain two carbon-carbon double bonds. They
therefore have essentially the same properties as the alkenes we have already
studied. For certain of the dienes, these alkene properties are modified in important
ways; we shall focus our attention on these modifications. Although we shall
consider chiefly dienes in this section, what we shall say applies equally well to
compounds with more than two double bonds. Dienes are named by the IUPAC
system in the same way as alkenes, except that the ending -diene is used, with two
numbers to indicate the positions of the two double bonds. This system is easily
extended to compounds containing any number of double bonds.
Dienes are divided into two important classes according to the arrangement of the
double bonds, Double bonds that alternate with single bonds are said to be
conjugated; double bonds that are separated by more than one single bond are said
to be isolated.

9. A third class of dienes, of increasing interest to organic chemists, contain cumulated
double bonds; these compounds are known as allenes:
Preparation and properties of dienes
Dienes are usually prepared by adaptations of the methods used to make simple
alkenes. For example, the most important diene, 1,3-butadiene (used to make
synthetic rubber), has been made in this country by a cracking process, and in
Germany by dehydration of an alcohol containing two OH groups:

10. Resonance in conjugated dienes
Let us focus our attention on the four key carbon atoms of any conjugated diene
system. We ordinarily write the C1—C2 and C3—C4 bonds as double, and the
C2—C3 bond as single:
Using the language of conventional valence-bond structures, we say that a
conjugated diene is a resonance hybrid of I and II. The dotted line in II represents
Electrophilic addition to conjugated dienes. 1,4-Addition
When 1,4-pentadiene is treated with bromine under conditions (what are they?) that
favor formation of the dihalide, there is obtained the expected product, 4,5-
dibromo-1-pentene. Addition of more bromine yields the 1,2,4,5-tetrabromo-
pentane.

11. This is typical of the behavior of dienes containing isolated double bonds: the double
bonds react independently, as though they were in different molecules. ‘ When 1,3-
butadiene is treated with bromine under similar conditions, there is obtained not only
the expected 3,4-dibromo-l-butene, but also l,4-dibromo-2- butene.
Treatment with HC1 yields not only 3-chloro-l-butene, but also l-chloro-2-butene.
Hydrogenation yields not only 1-butene but also 2-butene.
Study of many conjugated dienes and many reagents shows that such behavior is
typical: in additions to conjugated dienes, a reagent may attach itself not only to a
pair of adjacent carbons (1,2-addition), but also to the carbons at the two ends of the
conjugated system (1,4-addition). Very often the 1,4-addition product is the major
one.