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ALKYNES

Dr Seemal jelani Chem-114

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• An alkyne is a hydrocarbon with at least one carbon to carbon triple bond.

• Naming an alkyne is similar to the alkenes, except the base name ends in –yne.

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•Alkynes contain a carbon—carbon triple bond.

•Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond.

• Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond.

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• An alkyne has the general molecular formula CnH2n-2, giving it four fewer hydrogens than the maximum possible for the number of carbons present. Thus, the triple bond introduces two degrees of unsaturation.

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•Recall that the triple bond consists of 2 bonds and 1 bond.

•Each carbon is sp hybridized with a linear geometry and bond angles of 1800.

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- and -bonds in alkynes

-bondsp

s

C C HH

p

CH CH

-bond -bond

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•Alkynes are named in the same general way that alkenes are named.

• In the IUPAC system, change the –ane ending of the parent alkane name to the suffix –yne.

•Choose the longest continuous chain that contains both atoms of the triple bond and number the chain to give the triple bond the lower number.

Nomenclature

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•Compounds with two triple bonds are named as diynes, those with three are named as triynes and so forth.

•Compounds with both a double and triple bond are named as enynes. The chain is numbered to give the first site of unsaturation (either C=C or CC) the lower number.

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•The simplest alkyne, H-CC-H, named in the IUPAC system as ethyne, is more often called acetylene, its common name.

•The two-carbon alkyl group derived from acetylene is called an ethynyl group.

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Examples of alkyne nomenclature

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•The physical properties of alkynes resemble those of hydrocarbons of similar shape and molecular weight.

•Alkynes have low melting points and boiling points.

•Melting point and boiling point increase as the number of carbons increases.

•Alkynes are soluble in organic solvents and insoluble in water.

Physical Properties

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• Boiling points of alkynes are close to the boiling points of alkenes and alkanes

• Alkyne have lower densities, than water and they are insoluble in water

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Substitutive nomenclature: Similar to alkenes, but with the following differences:

1. The ending "-ene" is replaced with "-yne"CH3

CH3

C C CH3

4-Methyl-2-pentyne

2. The double bond has a priority over the triple bond when numbered

CCH CH2 CH CH2

1-Pentene-4-yne

12345

3. There are no cis-trans-isomers, because the triple bond is linear

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PREPARATION

• Obsolete commercial process• Pyrolysis of methane ( The modern

method)• Lower haloalkanes

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1. CaC2 + 2H2O + Ca(OH)2CH CH

Calcium carbide

(Obsolete commercial process)

2. 2CH4 + 3H2CH CH Pyrolysis of methane(Modern commercial process)

Hightemperature

3. CH3-CH2-CHBr2 CH3-CH=CHBr C CHCH3

NaNH2

Sodium amide(a very strong base)

NaNH2

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Combustion of acetylene produces very high temperatures, which makes it useful for oxygen-acetylene welding:

2C2H2 + 5O2 = 4CO2 + 2H2O

Alkynes are valuable starting materials for organic synthesis due to their reactivity.

By dehydrohalogenation of vicinal dihalides

H H H | | |— C — C — + KOH — C = C — + KX + H2O | | | X X X

H | — C = C — + NaNH2 — C C — + NaX + NH3 | X

Synthesis of alkynes:

H H | | — C — C — + 2 KOH — C C — + KX + H2O | | heat X X

CH3CH2CHCH2 + KOH; then NaNH2 CH3CH2CCH Br Br

“ + 2 KOH, heat

alkene vicinal dihalide alkyneX2

1. KOH

2. NaNH2

Synthesis of propyne from propane

CH3CH2CH3

Br2, heatCH3CH2CH2-Br + CH3CHCH3

Br

KOH(alc)

CH3CH=CH2Br2

CH3CHCH2Br Br

KOH

CH3CH CHBr

NaNH2CH3C CH

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Reactions• Reduction• Oxidative Cleavage of Alkynes• Addition reactions• Markovnikov’s rule

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• Electrophonic addition to triple bonds proceeds slower, than addition to double bonds and often requires a catalyst.

• The Markovnikov’s rule is as valid as for the addition to double bonds. HBr in the presence of peroxides adds against the rule.

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Alkane formation:

Reduction of an Alkyne to an Alkane

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• Palladium metal is too reactive to allow hydrogenation of an alkyne to stop after one equivalent of H2 adds.

• To stop at a cis alkene, a less active Pd catalyst is used—Pd adsorbed onto CaCO3 with added lead(II) acetate and quinoline. This is called Lindlar’s catalyst.

• Compared to Pd metal, the Lindlar catalyst is deactivated or “poisoned”.

• With the Lindlar catalyst, one equivalent of H2 adds to an alkyne to form the cis product. The cis alkene product is unreactive to further reduction.

Reduction of an Alkyne to a Cis Alkene

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• Reduction of an alkyne to a cis alkene is a stereoselective reaction, because only one stereoisomer is formed.

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• In a dissolving metal reduction (such as Na in NH3), the elements of H2 are added in an anti fashion to form a trans alkene.

Reduction of an Alkyne to a Trans Alkene

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Summary of Alkyne Reductions

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•Alkynes undergo oxidative cleavage of the and both bonds.

• Internal alkynes are oxidized to carboxylic acids (RCOOH).

•Terminal alkynes afford a carboxylic acid and CO2 from the sp hybridized C—H bond.

Oxidative Cleavage of Alkynes

1. Addition of H2

H H | |

— C C — + 2 H2, Ni — C — C — | | H H

alkane

requires catalyst (Ni, Pt or Pd)

HCCH + 2 H2, Pt CH3CH3

HCCH + one mole H2, Pt CH2=CH2 + CH3CH3

H \ /

Na or Li C = C anti- NH3(liq) / \

H— C C —

\ / H2, Pd-C C = C syn-

Lindlar catalyst / \ H H

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Syn addition

• is the addition of two substituents to the same side (or face) of a double bond or triple bond, resulting in a decrease in bond order but an increase in number of substituents

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Anti addition

• Is in direct contrast to syn addition• In anti addition, two substituents are

added to opposite sides (or faces) of a double bond or triple bond, once again resulting in a decrease in bond order but an increase in number of substituents.

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The classical example of this is bromination (any halogenation) of alkenes.

CH3 H \ /

Na or Li C = C anti- NH3(liq) / \

H CH3

trans-2-buteneCH3CCCH3

H H \ /

H2, Pd-C C = C syn- Lindlar catalyst / \ CH3 CH3

cis-2-butene

2. Addition of X2

X X X | | |— C C— + X2 — C = C — + X2 — C — C — | | | X X X

Br Br Br | | |CH3CCH + Br2 CH3C=CH + Br2 CH3-C-C-H | | | Br Br Br