HYDROCARBONS 185

(g) Two p bonds result form overlap of the two remaining (b) Due to stability of vinyl halide by resonance there is partial
unhybridized p orbitals on each carbon atom. These orbitals double bond in which elimination does not take place by
overlap at right angles (90°) to each other, forming one p alc. KOH so stronger base NaNH2 is used.
bond with electron density above and below the C-C sigma
bond, and the other with electron density in front and in (c) Basic strength :  NH 2 is stronger base then RO
back of the sigma bond. This result in a cylindrical p electron
(d) Trans elimination takes place in forming of alkynes.
cloud around s bonded structure.
(e) From Vicinal Dihalides
1.20Å 1.06Å
180° H H H
| | |
alc.KOH
R  C  C  H 
 HX
 R  C  C H
| |
H H H–C–C–H X X
NaNH

 HX
2
 R  C  CH
NOTE
Any type of stereoisomerism does not arise in acetylenic NOTE
bond due to linearity of C  C bond.  Elimination of Vic. dihalides gives also alkadiene (1, 2
4.3 Nomenclature and Isomerism and 1, 3 alkadienes) but the major product is alkyne.
Nomenclature  Non terminal gem dihalide gives 2-Alkyne in presence
of alc. KOH while gives 1-alkyne in presence of NaNH2.
In common system, alkynes are named as derivatives of acetylene
In IUPAC system, they are named as derivatives of the 4.4.2 Dehalogenation of Tetrahaloalkane
corresponding alkanes replacing ‘ane’ by the suffix ‘yne’. By heating 1, 1, 2, 2 - tetra halo alkane with Zn dust.

Structure Common name IUPAC name X X

| |
H  C  CH Acetylene Ethyne 2Zn
R  C  C  H   R  C  CH  2ZnX 2
CH 3CH 2  C  CH Ethylacetylene But  1  yne | |
X X
Isomerism
4.4.3 From Kolbe's Electrolysis
Alkyne shows generally position isomerism
By the electrolysis of aqueous solution of sodium or potassium
CH 3  C  C  CH 3 But  1  yne fumarate or maleate, acetylene is formed at anode.
CH 3 CH 2  C  CH But  2  yne
4.4 General Methods of Preparation + H2

4.4.1 By Dehydrohalogenation
4.4.4 Preparation of Higher Alkynes by Grignard Reagent
(a) From Gem dihalides: Dehydrohalogenating agents are :
By this method lower alkyne is converted in to higher alkyne
NaNH2 (Sodamide) or Alc. KOH or ROH + RONa.
Example

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HYDROCARBONS 186
CaC2 + 2H2O 
 CH CH + Ca(OH)2

Ca+2 +  C C  + 2H+ + 2OH– 

 CH CH + Ca(OH)2
4.4.6 Berthelot's Process
Acetylene is synthesized by striking an electric arc between
carbon electrodes in presence of hydrogen.
1200 C
2C + H2   CH CH
4.4.7 From Haloform [CHI3, CHCl3]
Pure acetylene is obtained when iodoform or chloroform is heated
4.4.5 From Metal Carbide [ Laboratory Method] with Silver powder
Acetylene is prepared in the laboratory by the action of water on
calcium carbide.
Preparation Methods of Alkynes

4.5 Physical Properties doubly bonded carbon atoms and the electrophile is formed.
Since the bond energies of the  bond broken and the new s
 Solubility
bond formed are not much different therefore electrophilic
Alkynes are relatively nonopolar (w.r.t. alkyl halides and substitution reaction are not accompnied by large energy changes.
alcohols) and nearly insoluble in water (but they are more On the other hand in electrophilic addition reactions one weak p-
polar than alkenes and alkanes). They are quite soluble in bond (251 KJ mol –1) is broken and two strong  bonds
most organic solvents, (acetone, ether, emthylene chloride, (2 × 347 = 694 KJ mol–1) are formed. The overall reaction is
chloroform and alcohols). accompnied by a release of about 694-251 = 443 KJ mol–1 of energy.
 Physical state and Boiling point In other words electrophilic addition reactions are energetically
Acetylene, propyne, and butyne are gases at room more favourable than electrophilic substitution reactions Thus
temperature, just like the corresponding alkanes and the typical reactions of alkynes are electrophilic addition reaction
alkenes. In fact, the boiling points of alkynes are nearly the and not the electrophilic substitution reactions.
same as those of alkanes and alkenes with same number of 4.6.1 Catalytic Hydrogenation
carbon atoms. Reduction to Alkenes
4.6 Chemical Reactions of Alkynes (a) By Lindlar's Reagent : Hydrogenation of an alkyne can be
Due to presence of weak p electrons in alkyne, it will go for stopped at the alkene stage by using a “poisioned”
electrophilic reaction. In electrophilic substitution reaction, one (partially deactivated) catalyst made by treating a good
 bond is broken and a new  - bond between one of the

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catalyst with a compound that makes the catalyst less ef- General Reaction:
fective. Lindlar’s catalyst is a poisioned Pd catalyst, com-
posed of powdered barium sulfate coated with Pd, poisoned
with quinoline.
Example

Common Reagents :

(i) + X2 (ii) + HOX

(iii) + HX (iv) + H2O/H+

(b) By Birch Reduction
(a) Hydration of Alkynes
General Reaction
(i) Mercuric ion Catalyzed Hydration :
Alkynes undergo acid–catalyzed addition of water across
the triple bond in the presence of mercuric ion as a catalyst.
A mixture of mercuric sulfate in aqueous sulfuric acid is
Example commonly used as the reagent.
General Reaction

(c) By Hydroboration Reduction Example

Example

(ii) Hydroboration Oxidation of Alkynes :

4.6.2 Electrophilic Addition to Alkynes Alkynes react with BH3 or B2H6 + THF to give trivinyl bo-
rane which upon subsequent treatment with alkaline H2O2,
Many of the reactions of alkynes are similar to the corresponding
gives alcohols corresponding to anti-markovnikov's addi-
reactions of alkenes. Like the pi bond of an alkene, the pi bonds
tion of H2O to alkynes, which on tautomerisation give cor-
of an alkyne are electron-rich, and they readily undergo addition
responding aldehydes or ketone. Terminal alkynes give al-
reaction. The bond energy of the alkyne triple bond is about 226
dehyde whereas internal alkynes give ketone.
kJ (54 kcal) more than the bond energy of an alkene double bond.
Since sigma bonds are generally stronger than pi bonds, the (b) Addition of Hydrogen Halides (+HX)
reaction is usually exothermic. Alkynes have two pi bonds, so Markownikoff’s Rule:
upto two molecules can add across the triple bond. We must When reagent (asymmetrical HX, H2O) adds to asymmetrical
consider the possibility of a double addition whenever a reagent alkene eg, propene isobutene etc. the addition occurs such
adds across the triple bond of an alkyne. Some conditions may that the nucleophile attaches itself to the carbon atom of
allow the reaction to stop after a single addition, while other
conditions give double addition.

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HYDROCARBONS 188
the alkene bearing the least number of hydrogen, or (c) Addition of Halogen (Halogenation)
electrophile adds to the sp2 carbon that is bonded with the
Alkyne + 2X2  Tetrahalide
greater number of hydrogen.
General Reaction
Alkyne + 2HX  Geminal Dihalides
Hydrogen halides add across the triple bond of an alkyne
in much the same way they add across the alkene double
bond. The initial product is a vinyl halide. When a hydrogen
halide adds to a terminal alkyne, the product has the
orientation predicted by Markownikoff’s rule. A second
molecule of HX can add, usually with the same orientation
as the first.
Example
Nature of Addition: Anti in both step

NOTE
(d) Addition of HOX
 Markownikoff’s Addition in both steps.
 If two moles of HX are added the final product is Alkynes + HOX  ' - dihaloketone + -
Gemdihalide. haloketone
Example
 Electrophilic addition to terminal alkyne is regioselective.
Example

HCl
C H 3 C H 2  C  C  H  

1  B utyne

Cl
|
CH 3CH 2  C  CH 3
HCl |
 
Cl
2, 2  Dichlorobutane

Example

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NOTE
 Two molecules of HOX can be added, the end product is
Dihaloketone.
 The intermediate product is an enol which gives a minor
product haloketone.
4.6.3 Polymerisation
(a) Linear Polymerisation :
Dimerisation : When two molecules of acetylene passed
through a solution of Cu2Cl2 and NH4Cl a vinyl
acetylene is obtained.
Cu Cl
2HC C—H 
NH 4 Cl  CH2
2 2
CH—C C—H
mono vinyl acetylene
4.6.4 Isomerisation
When vinyl acetylene react with HCl then chloroprene is
obtained. (a) When 1-alkyne is treated with alcoholic KOH 2-alkyne is
formed.
CH 2  CH  C  C  H  
HCl
 CH 2  CH  C  CH 2
| R  CH 2  C  CH 
Alco.KOH

 R  C  C  CH3
Cl 1 alkyne 2  alkyne

2- chloro-1,3-butadiene [chloroprene]
(b) When 2-alkyne is treated with sodamide then it is converted
Polymerisation
 Neoprene (Synthetic rubber) into 1-alkyne.
Trimerisation: 3 molecules of acetylene.
CH 3  C  C  CH 3 
NaNH 2
 NH 3
 CH 3  CH 2  C  CH
Cu2 Cl2
3CH CH 
NH 4 Cl  CH2 CH—C C—CH CH2 
H2 O
 NaOH
CH 3  CH 2  C  CH

Divinyl acetylene 4.6.5 Laboratory test of Terminal Alkynes (Acidic nature)

(b) Cyclic Polymerisation : When alkyne is passed through When triple bond comes at the end of a carbon chain.
red hot metallic tube, cyclic polymerisation takes place with The alkyne is called a terminal alkyne.
the formation of aromatic compound
acetylenic hydrogen

H - C C - CH2CH3
1-Butyne, a terminal alkyne

(i) Decolourization of Br2 in CCl4 solution.

(ii) Decolourisation of 1% alkaline KMnO4 solution.
(iii) 1- alkynes give white ppt. with ammonical AgNO3 and red
ppt with ammonical cuprous chloride solution.
NOTE
(i) and (ii) tests are used for determination of unsaturation
(i.e, presence of double or triple bond in any compound)
(iii) Test is used for distinguish between alkenes and
1-alkynes or 1-alkyne and 2-alkyne.

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4.6.6 Laboratory Test for Alkynes

Chemical Properties of Alkynes

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