ERT 102 ORGANIC CHEMISTRY SEM 1 2012/2013

CHAPTER 2BIOPROCESS ENGINEERING

Puan Rozaini Abdullah

School of Bioprocess Engineering

Introduction

Alkynes contain a triple bond. General formula is CnH2n-2

Two elements of unsaturation for each triple bond.

Monosubstituted = terminal alkyne Disubstituted = internal alkyne

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Nomenclature

In common nomenclature, alkynes are named as substituted acetylenes:

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• The yne suffix is assigned the lowest number.• Then substituents are assigned the lowest possible set of numbers.

IUPAC nomenclature:

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Naming A Compound That Has More Than One Functional Group

A compound containing two double bonds or two triple bonds:

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Alkene and Alkyne Mixed Functional Groups

Longest chainwith both functional groups

1. Find the longest chain bearing both functional groups.

2. Assign ene and yne suffixes the lowest set of numbers.

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If there is a tie between a double bond and a triple bond, the ene suffix is assigned the lower number:

A chain is numbered to give the lowest possible number to the functional group with the higher priority:

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Complex Mixed Functional Group Examples

Do not forget to designate alkene stereochemistry if necessary.

Longest chainmost functional groups

Vinyl or ethenyl

Ethynyl

Longest chainmost functional groups

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The Structure of Alkynes

A triple bond is composed of a s bond and two p bonds

The Addition Of Hydrogen Halides And The Addition Of Halogens To An Alkyne

Terminal alkyne- the electrophilic addition reaction is regioselective: the H+ adds to sp carbon that is bonded to H.

Markovnikov’s rule is applied for HCl and HBr additions to terminal alkynes

The electrophile adds to the sp carbon that is bonded to the H because the secondary vinylic cation- more stable than the primary vinylic cation.

The addition of a hydrogen halide to an alkyne can be stopped after the addition of 1 equivalent of hydrogen halide because- although alkyne is less reactive than alkene, alkyne is more reactive than the halo-substituted alkene.

Halo-substituted alkene-less reactive: the substituent withdraws electrons inductively (through the σ bond)- decreasing the nucleophilic character of double bond.

Although the addition of hydrogen halide to alkyne can be stopped- a 2nd electrophilic addition reaction will take place if excess hydrogen halides present.

Product of 2nd electrophilic reaction= geminal dihalide (2 halogens on same carbon).

When the excess hydrogen halide adds to double bond, the electrophile (H+) adds to the C with greater H.

The resulted carbocation is more stable- because Br can share the +ve charge with carbon by overlapping 1 of its orbitals that contain a lone pair with the 2p orbital of +vely charged carbon.

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

A secondary vinylic cation-stable as a primary carbocation- generally, primary carbocations are unstable to be formed.

Instead, π-complex is formed as intermediate.

Why π-complex intermediate? From observation that many alkyne addition

reactions- stereoselective.

Addition of a hydrogen halide to an internal alkyne- 2 geminal dihalides- the initial addition of the proton can occur with equal ease to either of the sp carbons.

The halogens Cl2 and Br2 also adds to alkynes

Excess halogen- a 2nd addition reaction occurs.

The solvent is CH2Cl2.

Exercises

Give the major product of each of the following reactions:

- HC CCH3

- CH3C CCH2CH3

HBrexcess

HBrexcess

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Addition of Water to an AlkyneAlkynes undergo acid-catalyzed addition of water- product enol.Enol- compound with carbon-carbon double bond and OH group bonded to sp2 carbons.

• Consider the conversion of a general enol A to the carbonyl compound B. A and B are tautomers: A is the enol form and B is the keto form of the tautomer.

• Equilibrium favors the keto form largely because the C=O is much stronger than a C=C. Tautomerization, the process of converting one tautomer into another, is catalyzed by both acid and base.

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Hg2+ is added to increase the rate of water addition to terminal alkynes:

Mechanism involves a prototropic shift:

Acid Catalyzed Base Catalyzed

Hydroboration—Oxidation

Hydroboration—oxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound.

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

• Hydroboration of a terminal alkyne adds BH2 to the less substituted, terminal carbon. 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

Exercises

For the following alkynes, give products of - Acid catalyzed addition of water - Hydroboration-oxidation

a) 1-butyne

b) 2-butyne There is only one alkyne that forms an

aldehyde when it undergoes the mercuric-ion-catalyzed addition of water. Identify the alkyne.

The Addition Of Hydrogen To An Alkyne

Alkynes undergo catalytic hydrogenation- initial product-alkene.

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

Final product for hydrogenation reaction- alkane.

Reaction can be stopped at alkene stage using partially deactivated metal catalyst- Lindlar catalyst

Lindlar catalyst- prepared by precipitating palladium on calcium carbonate and treating it with lead (II) acetate and quinoline.

This treatment modifies the surface of palladium- more effective in catalyzing the addition of H to a triple bond than a double bond.

Lindlar Catalyst

The components of Lindlar catalyst:

Palladium MetalCalcium CarbonateLead Oxide

Origin of the cis stereochemistry:

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The poison

The substrate

The catalyst

Delivery of hydrogen atoms to one side by the solid-phase catalyst

The alkyne sits on the surface of metal catalyst and the hydrogens are delivered to the triple bond from the surface of the catalyst- both H are delivered to the same side of the double bond.

Only syn addition of H. Syn addition of H to an internal alkyne forms a cis

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.

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

Mechanism for conversion of an alkyne to a trans alkene

• single electron from s orbital of Na is transferred to sp carbon of alkyne- radical anion- negative charge and unpaired electron.• radical anion- strong base that it can remove a proton from ammonia. Results in the formation of a vinylic radical- radical’s unpaired electron is on vinylic carbon.• another single-electron transfer from Na to the vinylic radical forms a vinylic anion• vinylic anion- strong base – removes proton from another molecule of ammonia. Product- trans alkene.

A hydrogen bonded to an sp carbon is ‘acidic’

Electronegativity of an atom depends on its hybridization.

sp carbon is more electronegative than sp2 carbon, which is more electronegative than sp3.

Acidic compound- compound with most hydrogen attached to the most electronegative atom.

Ethyne is stronger acid then ethene and ethene is a stronger acid than ethane.

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Acidity of a Hydrogen Bonded to an sp Hybridized Carbon

High s character, sp orbital penetrates

nucleus.

Lower s character, sp3 orbital

penetrates nucleus to a lesser degree

In order to remove a proton from acid, the base that removes the proton must be stronger than base that is generated.

Start with a stronger base than the base that will be formed. NH3 is a weaker acid than terminal alkyne, the conjugate base

of NH3 (NH2-) is a stronger base than the carbocation- called

acetylide ion- formed when a hydrogen is removed from sp carbon of terminal alkyne.

Therefore, an amide ion (-NH2) can be used to remove a proton from a terminal alkyne to prepare an acetylide ion.

The stronger the acid, the weaker its conjugate base:

In contrast, if hydroxide ion were used as base, the reaction would strongly favor the reactants because- OH ion is weaker base than acetylide ion that would be formed.

Amide ion cannot remove a hydrogen bonded to an sp2 or an sp3 carbon.

Only a hydrogen bonded to an sp carbon is sufficiently acidic to be removed by an amide ion.

Hydrogen bonded to an sp carbon referred as ‘acidic’ hydrogen.

‘acidic’ property that differs the reactivity of terminal alkynes from alkenes.

It is more acidic than most other hydrogens, but it is much less acidic than hydrogen of water molecule- water is weakly acidic compound.

CH3CH2- H2C CH- H2N

- HC C- HO- F-> > > > >

relative base strength

weakestbase

strongestbase

Synthesis Using Acetylide Ions

Reactions that form C-C bonds are important- to synthesis a molecule with larger carbon skeletons.

Reaction that forms a carbon-carbon bond is the reaction of an acetylide ions with alkyl halide.

Only primary alkyl halides or methyl halides should be used in this reaction.

We can convert terminal alkynes to longer internal alkynes by addition of alkyl halides

The mechanism: Br is more electronegative than carbon and as a result, the electrons in the C-Br bond are not shared equally by 2 atoms.

There is a partial +ve charge on carbon and a partial –ve charge on bromine.

The –vely charged acetylide ion (nucleophile) attracted to the partially +vely charged carbon (electrophile) of alkyl halide.

As electron of acetylide ion approach the C to form new C-C bond, they push out the Br and its bonding electrons- carbon can only bond to 4 atoms.

Known as an SN2 reaction

The reaction- alkylation reaction- attaches an alkyl group to a species.

Terminal alkynes can be converted into internal alkynes of any desired length by choosing an alkyl halide of the appropriate structure.

Count the no of carbons in terminal alkyne and the no of carbons in the product to see how many carbons are needed in alkyl halide.