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

2. 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

3. 9/16/2018
Dr Seemal Jelani

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

5. Linear arrangement in Carbon-Carbon triple bond Methane 109.50
Ethene
Ethyne
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

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

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

8. Rules for naming are same except the ending “yne”
Acetylene with Oxygen gives oxyacetylene which has temperature 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

9. 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

10. H H
| |
— C — C — KOH  — C  C — KX + H2O
| | heat
X X
CH3CH2CHCH KOH; then NaNH2  CH3CH2CCH
Br Br
“ KOH, heat

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

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

14. HCCH H2, Pt  CH3CH3
[ HCCH + one mole H2, Pt  CH3CH 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

15. 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

16. 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=CH HCl  CH3CCH3
Cl Cl

17. 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

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