Khadijah Hanim bt Abdul Rahman
School of Bioprocess Engineering, UniMAP
Week 5: 13/10/2011
khadijahhanim@unimap.edu.my
 Structure,
-
nomenclature and naming alkynes
DEFINE and ILLUSTRATE the principle in
naming alkynes on few examples
 Reaction
of alkynes
- EXPLAIN and DISCUSS the reactivity of
alkynes in electrophilic addition reaction.
 Alkyne
is a hydrocarbon
that contains a carboncarbon triple bond.
 Alkyne has 4 fewer
hydrogens than an alkane.
 The general molecular
formula for an acyclic
alkyne is CnH2n-2 and for
cyclic alkyne is CnH2n-4.
 Only
-
a few alkynes are found in nature.
capillin- fungicidal activity
Ichthyothereol- convulsant used by Amazon
people for poisoned arrowheads
Enediynes- antibiotic/anticancer properties
 Few
drugs on the market that contain alkyne
functional groups- not naturally occuring
 Synthesized




Replacing ‘ane’ ending of alkane name with ‘yne’.
The longest continuous chain with functional group is no in
the direction that gives the alkyne functional group suffix
as low no as possible.
Terminal alkynes- triple bond is at the end of the chain
Internal alkynes- triple bonds located elsewhere along the
chain
Common nomenclature-alkynes are named as
substituted acetylenes.
 Common name is obtained by stating the names
of alkyl groups in alphabetical order.
 If counting from either direction leads to the
same no for the functional group suffix, correct
systematic name- the one that contains the
lowest substituent no.
 If substituent- more than 1 substituents- listed in
alphabetical order

 Draw
-
-
the structure for the following:
1-chloro-3-hexyne
Cyclooctyne
Isopropylacetylene
4,4-dimethyl-1-pentyne
 Find
the longest continuous chain containing
both functional groups
 Put both suffixes at the end of the name
 The ‘ene’ ending should be first with
terminal e omitted.
 The no indicating the location of the firststated functional group- placed before the
name of the parent chain.
 No indicating the location of 2nd-stated
functional group is placed immediately
before the suffix for that functional group.
 If
the 2 functional groups are a double bond
and triple bond, chain is numbered to
produce a name containing the lower no.,
regardless of which functional group gets the
lower no.
 If
the same low no is obtained in both
directions, the chain is numbered in the
direction that gives the double bond the
lower no.
 If
the 2nd functional group suffix has a higher
priority than the alkene suffix, the chain is
numbered in the direction that assigns the
lower no to functional group with higherpriority suffix.
 The
highest priority functional group is
assumed to be at the 1-position in cyclic
compounds.
 Give
the systematic name for each of the
following:
 Physical
properties similar to alkanes
 Insoluble in water and soluble in nonpolar
solvents such as benzene or diethyl ether
 Less dense than water
 Boiling points that increase with increasing
molecular weight
 Alkynes are more linear than alkenes- triple
bond is more polarizable than a double bond
 These 2 features- stronger van der waals
interactions
 Alkyne
has higher boiling point than an
alkene with the same no of carbons.
 Internal alkenes have higher boiling points
than terminal alkenes.



Structure of ethyne: each carbon is sp hybridized, each has
2 sp orbitals and 2 p orbitals.
1 sp orbital overlaps the s orbital of a hydrogen, and the
other overlaps an sp orbital of the other carbon.
Because the sp orbitals are oriented as far from each other
as possible to minimize electron repulsion, ethyne is a
linear molecule with bond angle 180o.
 The
triple bond is formed by each of the 2 p
orbitals on 1 sp carbon overlapping the
parallel p orbital on the other sp carbon to
form 2 π bonds.
 The electrostatic potential maps for 2butyne show that it look like a cylinder of
electrons wrapped around the σ bond.
A triple bond is composed of a s bond and two p bonds
 Carbon-carbon
triple bond is shorter and
stronger than carbon-carbon double bond
 A π bond is weaker than σ bond. The
relatively weak π bond allow alkynes to react
easily.
 Alkyl groups stabilize alkynes by
hyperconjugation.
 Therefore, internal alkynes are more stable
than terminal alkynes
 Thus, the alkyl groups stabilize alkenes,
alkynes and carbocation.





Alkyne is an electron-rich molecule- nucleophile
React with electrophile
Mechanism for electrophilic addition reaction:
Z-Stereoisomer
The relatively weak π bond breaks because the π electronattracted to the electrophilic proton
The positively charged carbocation intermediate reacts
rapidly with the negatively charged chloride ion
 Alkynes-
electrophilic addition reactions
 Like alkenes- electrophilic addition to a
terminal alkene is regioselective.
 In addition, the addition reaction of alkynes
have a feature that alkenes do not have:
because the product of the addition of an
electrophilic reagent to an alkyne is alkene,
a 2nd electrophilic addition reaction may
occur if excess hydrogen halide is present.
Alkynes less reactive in electrophilic addition
reactions
 Reactivity depends on ∆G++- which depends on
the stability of reactant and the stability of the
transition state
 For alkyne to be less stable and less reactive
than alkene, 2 conditions must hold:
- The transition state for rate-limiting step of an
electrophilic addition reaction for an alkyne
must be less stable than the transition state for
the first step of electrophilic reaction for an
alkene and
- The difference in the stabilities of the transition
states must be greater than the difference of the
reactants, so that ∆Galkyne >∆Galkene

Why is the transition state for the 1st step of an electrophilic addition
reaction for an alkyne less stable than that for an alkene?
- Hammond predicts- the structure of transition state for the 1st step
reaction resemble the structure of carbocation intermediate-product of
1st step.
-Carbocation formed when a proton adds to an alkyne- vinylic cation.
Whereas, the carbocation formed when proton adds to an alkene- alkyl
cation.
- A vinylic cation has +ve charge on vinylic carbon,which is more
electronegative than the sp2 carbon of alkyl cation- less stable than a
similarly substituted alkyl cation.
A
vinyl cation has +ve charge on an sp
carbon- more electronegative than sp2
carbon of an alkyl cation- less able to bear a
positive charge
 Hyperconjugation-less effective in
stabilizing a charge on a vinylic cation than
on an alkyl cation.