Alkynes Dr Seemal Jelani Chem-160 9/16/2018

General formula Cn H2n-2 or C2H2 , C3 H4 An acyclic unsaturated hydrocarbon in which one or more Carbon-Carbon triple bonds are present General formula Cn H2n-2 or C2H2 , C3 H4 9/16/2018 Dr Seemal Jelani

9/16/2018 Dr Seemal Jelani

“Terminal” alkynes have the triple bond at the end of the chain: CH3CH2CCH HCCCHCH2CH3 1-butyne 3-methyl-1-pentyne Ethylacetylene sec-Butylacetylene

Linear arrangement in Carbon-Carbon triple bond Methane 109.50 Ethene 1200 Ethyne 1800 Cis and trans isomerism is not possible in alkynes The simplest alkyne is called Acetylene. It’s a colourless gas, with BP -840 is prepared 9/16/2018 Dr Seemal Jelani

Physical properties: weakly or non-polar, no H-bonding relatively low mp/bp water insoluble

Simplest alkyne Acetylene Colorless, BP -840, is prepared by the reaction of calcium carbide and water 9/16/2018 Dr Seemal Jelani

Rules for naming are same except the ending “yne” Acetylene with Oxygen gives oxyacetylene which has temperature 10000 C and used for welding purposes Rules for naming are same except the ending “yne” Physical properties are similar to alkenes and alkanes Insoluble in water but soluble in organic solvents Low density BP increases with molecular mass Low molecular mass alkynes are gases at room temperature 9/16/2018 Dr Seemal Jelani

Synthesis of alkynes 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

H H | | — C — C — + 2 KOH  — C  C — + KX + H2O | | heat X X CH3CH2CHCH2 + KOH; then NaNH2  CH3CH2CCH Br Br “ + 2 KOH, heat

Reactions, alkynes: addition of H2 (reduction) addition of X2 addition of HX addition of H2O, H+

Addition of H2 H H | | — C  C — + 2 H2, Ni  — C — C — alkane requires catalyst (Ni, Pt or Pd)

HCCH + 2 H2, Pt  CH3CH3 [ HCCH + one mole H2, Pt  CH3CH3 + CH2=CH2 + HCCH ] H \ / Na or Li C = C anti- NH3(liq) / \ — C  C — \ / H2, Pd-C C = C syn- Lindlar catalyst / \ H H

CH3 H \ / Na or Li C = C anti- NH3(liq) / \ H CH3 trans-2-butene CH3CCCH3 H H H2, Pd-C C = C syn- Lindlar catalyst / \ CH3 CH3 cis-2-butene

Addition of hydrogen halides: H H X | | | — C C— + HX  — C = C — + HX  — C — C — | | | X H X HX = HI, HBr, HCl Markovnikov orientation Cl CH3CCH + HCl  CH3C=CH2 + HCl  CH3CCH3 Cl Cl

Addition of X2 X X X | | | — C C— + X2  — C = C — + X2  — C — C — | | | X X X Br Br Br CH3CCH + Br2  CH3C=CH + Br2  CH3-C-CH Br Br Br

Chemical reactivity is similar to alkenes 9/16/2018 Dr Seemal Jelani