ERT 102 ORGANIC CHEMISTRY

Alkynes

Dr. Syazni Zainul Kamal

School of Bioprocess Engineering

Alkynes

•Alkynes contain a triple bond

•General formula: acyclic alkynes CnH2n-2

cyclic alkynes CnH2n-4

•Systematic name – replace ane ending of alkane with “yne”

•Naming system is the same with other compound

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Nomenclature

1. The longest continuous chain containing C-C triple bond is numbered in the direction that give the functional group suffix/ending the lowest no.

2. triple bond at the end of chain – terminal alkynes3. Triple bond located elsewhere along the chain – internal

alkynes

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1. The longest continuous chain containing C-C triple bond is numbered in the direction that give the functional group suffix the lowest no.2. substituent receive the lowest no. if counting from either direction leads to the same no. for the functional group suffix3. Substituent listed in alphabetical order

IUPAC nomenclature:

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Naming A Compound That Has More Than One Functional Group

1. Compound with 2 triple bond – suffix diyne2. A compound containing two double bonds or two triple

bonds:

Alkene and Alkyne Mixed Functional Groups

1. Find the longest continuous chain contain both functional group

2. Suffix are combined – alkenynes (double&triple bond)3. When the functional groups are double bond & triple

bond, number the chain in the direction that give a name containing lowest no, regardless which functional group get the lowest no.

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3. If there is a tie between a double bond and a triple bond, the double bond get the lower number.

4. Number the chain so that the lowest possible number is given to the functional group with higher priority

Reaction of alkyne

Reactivity of alkyne

•Alkyne is Nucleophile

•Undergo electrophilic addition reaction

•Eg. addition of HCl to alkyne forms an alkene (chlorosubstituted alkene)

alkyne

Chlorosubstituted alkene

•Addition of H+ to alkyne form a vinylic cation

•A vinylic cation has a +ve charge on a vinylic carbon

•Vinylic cation is less stable than similar substituted alkyl cation

•Primary carbocation – too unstable to form

•Same with vinylic cation

•Mechanism for addition of hydrogen halide-intermediate as vinylic cation- not completely correct.

• Instead, π-complex is formed as intermediate.

The Addition Of Hydrogen Halides to an alkyne

•Electrophilic addition reaction between alkyne and HClform alkene

•Second addition can occur if access hydrogen halidepresent

•Second addition : regioselective (the H+ adds to the lesssubstituted sp2 carbon) (the one bonded to H)Markovnikov’s rule

Mechanism for electrophilic addition of a hydrogen halide to an alkyne

1. Alkyne react with electrophile (H+) to form π-complex

2. Chloride ion add to π-complex, forming halo-substituted alkene

3. Secondary addition- addition to halo-substituted alkene (regioselective)

Geminal dihalide

Addition to a terminal alkyne

•Terminal alkyne – the 1st electrophilic addition reaction is also regioselective (H+ add to the less substituted sp carbon)

Addition to an internal alkyne

•Addition of excess hydrogen halide to an unsymmetricalinternal alkyne- 2 geminal dihalides, because the initialaddition of the proton can occur with equal ease to eitherof the sp carbons

• If internal alkyne is symmetrical, only 1 germinal dihalideformed

unsymmetrical

Addition of halogen to an alkyne

•The halogens Cl2 and Br2 also adds to alkynes

•Excess halogen - a 2nd addition reaction occurs.

• Reaction carried out by mixing with inert solvent eg.dichloromethane (not participate in reaction)

2-pentyne

2,3-dichloro-2-pentene 2,2,3,3-tetrachloropentane

•What is the major product of the following reaction:

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Addition of Water to an Alkyne

• Alkynes undergo acid-catalyzed addition of water• H+ add to less substituted sp C• initial product – enol (has C-C double bond with OH at

1 of the sp2 C)• Suffix ‘en’ signify double bond, ol signify OH group• Enol immediately rearranged to a ketone

•Ketone and enol:

- keto-enol tautomers (constitutional isomer that are in rapid equilibrium)

- Differ in position of double bond and hydrogen

•Keto tautomer predominate in solution because it much more stable

•C=O stronger than C=C

Addition to an internal alkyne

•Addition of water to symmetrical internal alkyne form single ketone

•Unsymmetrical internal alkyne, form 2 ketone (initial addition can occur to either sp C)

Addition to a terminal alkyne

•Terminal alkyne less reactive than internal alkyne towards addition of water

•Reaction carried out in the presence of mercuric ion (Hg2+) - catalyst

•What alkyne should be used for the synthesis of the following ketones?

Addition of Borane to alkyne:Hydroboration—Oxidation

- 2-step reaction sequence, converts an alkyne to alkene.

- Boron (BH2) is the electrophile and H- is the nucleophile

1. Addition of borane form organoborane (boron-substituted alkene)2. Oxidation of organoborane with basic hydrogen peroxide form enol3. Enol immediately tautomerize to a ketone

•Hydroboration-oxidation of an internal alkyne forms a ketone.

•Hydroboration-oxidation of a terminal alkyne adds BH2 to the less substituted sp C. After oxidation to the enol, tautomerization yields an aldehyde (a carbonyl compound having a hydrogen atom bonded to the carbonyl carbon)

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Formation of Ketone Vs. Aldehyde

Hydroboration –oxidation of TERMINAL ALKYNE

Mercuric – ion catalyzed addition of water to a terminal alkyne

Carbonyl group is not on the terminal C

Exercises

•For the following alkynes, give products ofHydroboration-oxidation of:

a) 1-butyne

b) 2-butyne

c) 2-pentyne

The Addition Of Hydrogen To An Alkyne

• Initial product- alkene

•Difficult to stop the reaction at this stage because hydrogen’s strong tendency to add to alkenes in the presence of metal catalysts.

•Final product for hydrogenation reaction- alkane.

Forming a cis alkene

•Reaction can be stopped at alkene stage and form cisalkene using partially deactivated metal catalyst- Lindlarcatalyst

• Lindlar catalyst prepared by:

1. precipitating palladium on calcium carbonate

2. treating it with lead (II) acetate and quinoline.

•This treatment modifies the surface of palladium- moreeffective in catalyzing the addition of H to a triple bondthan a double bond.

•The alkyne sits on the surface of metal catalyst and thehydrogens are delivered to the triple bond from thesurface of the catalyst- both H are delivered to the sameside of the double bond.

• syn addition of H (substituent add to same side of bond)

Forming a trans alkene

• Internal alkynes- converted to trans alkenes using sodium (or lithium) in liquid ammonia.

•Reaction stops at alkene stage- Na reacts rapidly with triple bonds than double bonds.

• “dissolving metal reduction”

•Ammonia is gas at RT. Kept as liquid using dry ice/acetone mixture.

•Anti addition – substituent add to opposite side of bond

Mechanism for conversion of an alkyne to a trans alkene

• single e- of Na is transferred to sp carbon of alkyne- radicalanion formed (sp with -ve charge and unpaired electron)• radical anion- strong base that it can remove a proton/Hfrom ammonia. vinylic radical formed- unpaired e- is onvinylic carbon.• another single e- transfer from Na to the vinylic radicalforms a vinylic anion• vinylic anion- strong base – removes proton from anothermolecule of ammonia. Product- trans alkene.

•Describe the alkyne you should start with and the reagent you should use if you want to synthesize

a) Pentane

b) Cis-2-butene

c) Trans-2-pentene