ALKYNES. 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. Alkynes contain a carbon—carbon triple bond.
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ALKYNES Dr Seemal jelani Chem-114
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. Dr Seemal Jelani Chem-114
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. Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
s- and p-bonds in alkynes Dr Seemal jelani Chem-114
Nomenclature • Alkynes are named in the same general way that alkenes are named. • In the IUPAC system, change the –aneending 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. Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
Examples of alkyne nomenclature Dr Seemal jelani Chem-114
Physical Properties • 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. Dr Seemal jelani Chem-114
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 Dr Seemal jelani Chem-114
Substitutive nomenclature: Similar to alkenes, but with the following differences: Dr Seemal jelani Chem-114
PREPARATION • Obsolete commercial process • Pyrolysis of methane ( The modern method) • Lower haloalkanes Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
Synthesis of alkynes: • By dehydrohalogenation of vicinal dihalides • H HH • | | | • — C — C — + KOH — C = C — + KX + H2O • | | | • X XX • H • | • — C = C — + NaNH2 — C C — + NaX + NH3 • | • X
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 1. KOH alkyne 2. NaNH2 X2
Reactions • Reduction • Oxidative Cleavage of Alkynes • Addition reactions • Markovnikov’s rule Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
Reduction of an Alkyne to an Alkane Alkane formation:
Reduction of an Alkyne to a Cis Alkene • 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 is a stereoselective reaction, because only one stereoisomer is formed.
Reduction of an Alkyne to a Trans Alkene • 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.
Oxidative Cleavage of Alkynes • 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.
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
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 Dr Seemal jelani Chem-114
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. Dr Seemal jelani Chem-114
The classical example of this is bromination (any halogenation) of alkenes. Dr Seemal jelani Chem-114
CH3H \ / Na or Li C = C anti- NH3(liq) / \ H CH3 trans-2-butene CH3CCCH3 H H \ / H2, Pd-C C = C syn- Lindlar catalyst / \ CH3CH3 cis-2-butene
Addition of X2 • X XX • | | | • — C C— + X2 — C = C — + X2 — C — C — • | | | • X XX • Br BrBr • | | | • CH3CCH + Br2 CH3C=CH + Br2 CH3-C-C-H • | | | • Br BrBr