here you will learn more about alkynes and their hydrogenation

1. ALKYNES AND
HYDROGENATION OF
ALKYNES

2. What are Alkynes?
• Alkynes are a category of hydrocarbons containing a Carbon-
Carbon triple bond.
• Due to the triple bond , an Alkyne has 4 fewer hydrogen
atoms than it corresponding alkane
• Alkynes form both cyclic and non-cyclic molecular structures.

3. GENERAL MOLECULAR FORMULA OF ALKYNES
• Acyclic alkynes have the general molecular formula
of CnH2n-2
• Alkynes that have triple bonds at the end of the
Carbon chain are called monosubstituted alkynes
or terminal alkynes which is termed as the most
acidic hydro carbon group.
• Disubstituted alkynes have internal triple bonds

4. ALKYNES PREPARATION
The carbon-carbon triple bond of the alkynes is formed the same way as the double
bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons.
The groups that are eliminated and the reagents used are essentially the same as in
the preparation of alkenes.

6. CHEMICAL PROPRTIES OF ALKYNES
• Molecular structure:- i)The Carbon-Carbon triple bond
has the linear geometry.
ii) The triply bonded carbon atoms are sp hybridized
• Reactivity of alkynes:- Alkynes show many reactions with
chemical reagents most of them are addition reactions.
However, terminal alkynes undergo substitution
reactions too.

7. PHYSICAL PROPERTIES OF ALKYNES
Alkynes are compounds which have low polarity and have physical properties
that are similar to that of alkanes and alkenes.
• They are insoluble in water.
• They are quite soluble in the usual organic solvents of low polarity (eg.
Ether, carbon tetrachloride, etc.).
• They are less dense than water.
• Their boiling points show an increase with increase in the number of
carbon.
• They generally have a slightly higher boiling points that the corresponding
alkanes and alkenes .

8. NOMENCLATURE: IUPAC
• Find the longest chain containing the triple bond.
• Change –ane to –yne.
• Number the chain, starting at the end closest to the
triple bond.
• Give branches or other substituents a number to
locate their position.

9. EXAMPLES OF NOMENCLATURE
• All other functional groups, except ethers and halides have a higher
priority than alkynes.

10. BOND LENGTHS
• Triple bonds are shorter than double or single bonds
because of the two overlapping pi orbitals

11. CATALYTIC HYDROGENATION OF
ALKYNES
• Two molecules of
hydrogen can add across
the triple bond to form the
corresponding alkane.
• A catalyst such as Pd, Pt or
Ni needs to be used for the
reaction to occur.
• Under these conditions the
alkyne will be completely
reduced, the alkene
intermediate cannot be
isolated.

12. HYDROGENATION WITH
LINDLAR’S CATALYST
• The catalyst used for the
hydrogenation reaction is
partially deactivated
(poisoned), the reaction can
be stopped after the addition
of only one mole of hydrogen.
• The catalyst used is commonly
known as Lindlar’s catalyst and
it is composed of powdered
barium sulfate, coated with
palladium poisoned with
quinoline
• The reaction produces alkenes
with cis stereochemistry

13. MECHANISM OF
LINDLAR’S CATALYST
• Both substrates, the
hydrogen and alkyne , have
to be absorbed on the
catalyst for the reaction to
occur
• Once absorbed the
hydrogens add to the same
side of the double bond
giving the product a cis
stereochemistry.