Khadijah Hanim bt Abdul Rahman School of Bioprocess Engineering, UniMAP Week 5: 13/10/2011

 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 carbon- carbon triple bond.  Alkyne has 4 fewer hydrogens than an alkane.  The general molecular formula for an acyclic alkyne is CnH 2n-2 and for cyclic alkyne is C n H 2n-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 first- stated functional group- placed before the name of the parent chain.  No indicating the location of 2 nd -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 2 nd 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 higher- priority 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 180 o.

 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 2- butyne show that it look like a cylinder of electrons wrapped around the σ bond. A triple bond is composed of a  bond and two  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:  The relatively weak π bond breaks because the π electron- attracted to the electrophilic proton  The positively charged carbocation intermediate reacts rapidly with the negatively charged chloride ion Z-Stereoisomer

 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 2 nd 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 ∆G alkyne >∆G alkene

Why is the transition state for the 1 st 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 1 st step reaction resemble the structure of carbocation intermediate-product of 1 st 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.