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8/13/2019 36. Hydrocarbon 4
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Alkynes
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Session objectives
1. Preparation of alkynes by
(a) Kolbes method
(b) dehydrohalogenation of vicinal dihalide
(c) dehydrohalogenation of geminal dihalide2. Chemical reactions of alkyne
(a) Addition of halogen, hydrogenhalides.
(b) Ozonolysis
3. Confirmatory test for alkyne
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General characteristics ofalkynes
General formula CnH2n2sp hybridizationCC bond length 1.2 A0
Shows chain and functionalisomerism
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Chain isomerism
CH3
CH2 CH2 C CH CH3 CH C CH
CH3
CH3
C CH CH2 C CH2
Functional group isomerism
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Preparation
Kolbe electrolysis
At anode At cathode
CH
2CO22HNaOH2
CHCOOCH
.
Na2
OH2.
CHCOO.CH.
NaCHCOO CHCOO Na2
NaCOOCH CHCOO C2
CH
Sodium maleate-2e-
e-
Current
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Dehydrohalogenation of vicinaldihalides
CH3
CH CH2
Br Br
CH3
C
2KOH (alc)2H2OCH + 2KBr +
From gem dihalides
CH3
Cl
Cl
KOH.alc
+ 2 KCI + 2 H2O
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Miscellaneous methods
Miscellaneous methodsFrom CaC2
CaC H O HC CH Ca(OH) 2 2 2
Lime
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Chemical properties
Addition Reactions
(i) Addition of hydrogen
CH3C
NiCH
3CH
2CH
3
Propyne
CH + 2H2
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Chemical properties
CH3
H3C
H H
CH3
CH3 C CC C
H2/ Lindlar
Catalyst
H
H
CH3
H3C
CH3CH3C C CC
Na/NH3(liq.)
Trans-2-butene
Lindlar catalyst
(H2/Pd/BaSO4) partially reduces adisubstituted alkyne to a cis-alkene.
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Chemical properties
Addition of halogens
HC CHBr
22
Br Br| |
H C C H
| |
Br Br
Addition of hydrogen halides
HC CH 3 22HBr CH CH Br
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Mechanism
CHCH3 C CH3
CH3 C
+CH
3
Br
CH3
CH2
BrBr
Br
Br
CH3CH3
CH2
Br
CH
HBr
(2) more stable (1) less stable
C CHBr
Major product
(Anti Markownikoff addition)
CH3 CH CHBrCH
3CH +C HBr
Peroxide
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Addition of water
HC CH + H2O
HgSO4
H2SO
4
CH2 CHOH
CH
3CHO
Unstable
CH3
CH2CH3
CH3
CH3
O
C C
H2O/H+
HgSO4
C
CH3C CCH2CH3
O
CH3CCH2CH2CH3CH3CH2CCH2CH3
(Major) (Minor)
O
H2O/H+
HgSO4+
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Hydroboration oxidation
CH3C CHCH
3CH
HC
R2BH
0-10CBR2
H2O2/OH-
CH3
CH
O
CH
H
CH3CH
2CHO
Aldehyde
Tautomerization
: :
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Oxidation of alkynes
HCCOOH
COOH
CH CO2+ H2OStrong heatingalk.
CHCH3C CH3COOH + CO2+ H2O
4KMnO.alk
CH3
C CCH3
CH3COOH + CH
3COOH
alk . KMnO4
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Ozonolysis of alkynes
RC CR + O3
RC C R
O
O O
RCOOH + RCOOH
RC C R + H2O
2
O O
H2O
HC CH + O3 H C C H
O
O O
H C C H + HCOOH
O O
H2O
2
H2
O
MajorMinor
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Formation of alkynides
CH AgNO3
C Ag
+2 RC + 2 NH4OH
(Tollens' reagent)
+2 RC
White ppt
+ 2 NH4CI 2 H2O +
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Formation of alkynides
CH + Cu2Cl
2 CCu2 RC +2 RC 2 NH4Cl + 2H2O
Red ppt
2 NH4OH+ +
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Acidity of Alkynes
H
H
H H
H
H
H H
H
CCC C : :
:
C C- - -
Acetylide anions Vinyl anion Ethyl anion
- - -2 5 2C H > CH = CH > HC C
2 2 3 3HC CH>CH =CH >CH -CH
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Acidity of Alkynes
HC CH Na HC C Na H 21
2sodium. acetylide
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Acidity of Alkynes
The order of boiling point among hydrocarbons is
alkynes > alkenes > alkanes.
These hydrocarbons also possess low dipole moment.
= 0.80 D
CH CH C CH CH CH CH CH CH C C CH 3 2 3 2 2 3 3
= 0.30 D = 0.0 D
Acetylene having two hydrogen atoms is more acidic
than a mono substituted acetylene.
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Class exercise
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Class exercise 1
Hence, the answer is (c)
Solution:
How will you effect the followingconversions?
CH4 CH2 CH2(a)
CH3C CH3COCH3CH(b)
(a) (i) Cl2+ hn (ii) Wurtz reaction
(iii) Cl2+ hn (iv) alc. KOH, heat
(b) (i) HgSO4, H2SO4(20%)
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Class exercise 2
Write the structural formula of themain organic product.
Hence, the answer is (b)
Solution:
C CH
CH3
(i)
H2O/H+
HgSO4
CH3C CH(ii)2 HOCl
CH3C CCH
2CH
2CH
3(iii)
H2/Lindlar's
Catalyst
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Solution
O
(i)
CH3CCHCl
2
O
(ii)
H H
(iii)
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Class exercise 3
Give a chemical test to distinguishbetween 1-butyne and 2-butyne.
Solution:
1 butyne will give white ppt. with ammonical AgNO3.
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Class exercise 4
A cylinder contains one of thefollowing gases propyne,
propene, propane. What chemicaltest would you apply to identifythe gases?
Solution:
AgNO3(ammonical) is used for alkyne, alk. KMnO4,and heat is applied for alkene. No reaction takes placewith propane with the above mentioned reagents.
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Class exercise 5
Write the product of thefollowing reaction.
Solution:
CH3CH
2CHCl
2
Alkali
Boil
CH3CH2CHO
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Class exercise 6
Solution:
What reaction of an appropriatealkyne would lead to the following
product?
H3C
Br
Br
CH2CH3
C C
CH3
C CCH2CH
3
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Class exercise 7
An organic compound (A) havingmolecular formula when treated
with NaNH2/ NH3followed by reactionwith n-propyl bromide yielded (B) .Compound (A) gives a ketone (C)when treated with acidified HgSO4
.(B) on oxidation with hot alk. KMnO
4
gives two isomeric acids (D) and (E).Deduce structures from (A) to (E).
Solution:
A: B:O
CCOOH
D
COOHE:
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Class exercise 8
Write the correct product of thefollowing reaction.
Solution:
HC CH+2HOCl ? Product
Cl2CHCHO
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Class exercise 9
Write the products of the followingreaction.
Solution:
H C C HNa C2H5Br
KMnO4, Hot
O3, distillation
(A) (B)
(C) + (D) + (E)
(F) + (G)
HC C NaA is +
H C C C2H5B is
C is CO2
D is C2H5CO2H
E is H2O
F is HCOOH
G is C2H5COOH
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Class exercise 10
Write the intermediate steps of thefollowing reaction
Solution:
C6H
5CHC CH C6H5CH CHCHO
OH
H3O+
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Solution
C6H
5CH C CH
OH
C6H
5CH C CH
OH2
C6H
5CH C CHC6H5CH
C
CH
H2O , H
C6H
5CH C CH OH
C6H
5CH CH CH O
+
H2O
+
+
+
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Thank you