1 1435-1436 2014-2015. learning objectives chapter three discusses the following topics which have...
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Unsaturated HydrocarbonsAlkynes
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1435-14362014-2015
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Learning Objectives
Chapter three discusses the following topics which have to be understood and memorized :
The structure, hybridization and Bonding in alkynes
Common and IUPAC naming of alkynes
Physical properties of alkynes
Preparation of alkynes
Reactions of alkynes: addition reactions and acidity
Alkynes
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Alkynes: Molecular And Structural Formulae The alkynes comprise a series of carbon- and hydrogen- based compounds that
contain at least one triple bond. This group of compounds is a homologous series with the general molecular formula of Cn H2n—2
The alkyne triple bond is composed of one σ and two 2 covalent bonds, the triple bond can be terminal or internal.
The simplest alkyne, ethyne (also known as acetylene), has two carbon atoms and the molecular formula of C2H2. The structural formula for ethyne is:
CH CH3
Alkynes
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sp Hybridization Of Alkynes
This involves the mixing of one s- and one p-orbital forming two sp-hybrid orbitals of equivalent energy. The two sp-hybrid orbitals are oriented in a linear arrangement and bond angle is 180° to minimize the repulsion between them.
The remaining two p orbitals (py and Pz) are unaltered
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Alkynes
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CH CH
Molecular formula of ethyne is C2H2.
In ethyne, each carbon atom is sp-hybridized. In this way, four sp-orbital are generated. One sp- orbital of each carbon atom by overlapping forms a sigma
bond between carbon atoms. Remaining one sp-orbital of each carbon atom overlap with 1s-
orbital of a hydrogen atom to produce two sigma bonds. Py-orbital and Pz-orbitals of each carbon by parallel overlapping
form two pi-bonds between the two carbon atoms. Geometry (shape) of ethyne molecule is linear in which bond
angles are 180o and C=C bond length is 1.20 Å5
Orbital Overlap IN Ethyne
Alkynes
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2s22p2 2s12p3 2 x sp 2p
sp HYBRIDISATION OF ORBITALS in ALKYNES
The electronic configuration of a carbon atom is 1s22s22p2
promotion hybridization
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Alkynes
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Summary
sp hybridization occurs when a C has 2 sigma bonds only sp hybridized orbital has 50% s and 50% p character The 2 sp hybrids point in opposite directions at 180o to each other Each sp hybrid is involved in a(σ)sigma bond The remaining p orbitals form the 2pi bonds The triple bond is one (σ)bond and two pi (∏) bonds.
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Find the longest chain containing both atoms of the triple bond; this gives the root name.
Add the ending –yne to the root name.Number the chain, starting at the end closest to the triple bond. Give branches or other substituents names and numbers to locate their positions.Indicate the number of identical groups by prefixes di, tri, tetra, etc.Place the position numbers and names of the substituent groups in alphabetical
order, before the root name. In alphabetizing ignore prefixes like tert., di, tri, etc. but include iso and cyclo.
Double and triple bonds are considered to have equal priority: thus in a molecule with both a double and triple bond, whichever is close to the end of the chain determines the direction of numbering.
In case where double and triple bonds would have the same position number, the double bond takes the lower number.
In writing the final name ‘’ene’’ comes before ‘’yne’’ regardless which takes the lower number (i.e. alphabetical order).
IUPAC Nomenclature of Alkynes
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ExamplesIUPAC Names Of Alkynes
C CHHC
CH3
CH
F
F
4,4-Difluoro-3-methyl-1-butyne
C CH-CH2-CH3
H
CHC
Hex-3-en-1-yne(triple bond closer to the end of chain)Note: An''e'' is dropped from ''ene''
due to it is followed by a vowel
HC C CH2 CH CH2
Pent-1-en-4-ynedouble and triple bonds have have the same position number
thus ene take lower number
1
2345
6-Ethyl-4-nonyne
C C
Br
H3C
5-Bromo-2-pentyneNot 1-Bromo-3-pentyne
1 2 34
6
1 2
3 4 5
7
8
9
1
5
1 2 3 4 5 6
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Common Nomenclature Of Alkynes The simplest alkyne its common name is acetylene Therefore the common names of alkynes are derived from acetylene
( e.g. Methyl acetylene)
Examples:
C CH3C
CH3 C CH CH3 CH
CH3
CH2 C C CH
CH3
CH3
Common : IsobutyisopropylacetyleneCommon : Methyl acetylene
Common: Isopropylmethylacetylene
HC CH
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Exercise
1) Give the IUPAC and common names of the following compounds
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C CH
C CH2
CH2
C HC C C(CH3)3
a) b)
c) d) -
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Physical Properties
Nonpolar, insoluble in water. Soluble in most organic solvents.The boiling points and melting points of alkynes increase with molecular weight. Branching reduces the boiling point of alkynes Terminal alkynes, R-CC-H, are more acidic than other
hydrocarbons.
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Alkynes
C CH
C CH2
CH2
C HC C C(CH3)3
a) b)
c) d) -
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Preparation of alkynes
1. By dehydrohalogenation of geminal or vicinal dihaloalkanes
(-2HX):
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Alkynes
C C
XX
Strong base
- 2 HXC C
Br
Br
1,2-dihaloalkane
Excess NaNH2
- 2HBr
Br
Br
Br
KOH/ alcohol
heat
NaNH2
heat
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2. From reaction of sodium Acetylide with Primary Alkyl Halides (Alkylationof acetylide with primary alkyl halid)
Alkynes
C CHR + Na liq NH3 C C
- Na
+R + H21/2
C C- Na
+R + R' X C CR R' + NaX
Sodium acetylide
For Terminal CC (use only Acetylene H-CC-H)
For Non Terminal CC (use Monosub. R-CC-H)
Na
NaNH2/ NH3 CH3Br
- NaBr
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Reactions of alkynes
1. Addition of hydrogen ( Hydrogenation)
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Alkynes can be reduced to trans-alkenes using Na or Li in liquid NH3
Alkynes can be partially reduced to cis-alkenes with H2 in the presence of poisoned catalysts.
HH
+ H2C C
Pd(BASO4) or (NiB)
Cis- alkene
H
H
+ H2C C
Na or Li / liq. NH3
Trans -alkene
Electrophilic Addition Reaction
Alkynes
H2/ Pd
or (Z)
or (E)
Pd(BaSO4): Lindlar’s Catalyst
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2. Addition of halogen (Halogenation)
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HCCH + X2 HXC=CXH + X2 H XX C–CXXH
If we used one mole ……(alkene) two mole …….alkane
Alkynes
C C C C
X
X
+ X2 + X2 C C
X X
XX
Tetrahalide(X= Br or Cl)
Trans
+ Br2
Br
Br
+ Br2
Br
Br
Br
Br
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3 .Addition of hydrogen halide
HC CR + HX HHC=CX R + HX HHHC – CXXR
+ HBr
Br
+ HBr
Br
Br
Alkynes
C C C C
H X
+ HX + C C
H X
XH
A gem dihalide(X= Br , Cl, I)
HX
A haloalkene
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4 .Addition of water: Hydration
Alkynes
HC
Propyne (an unstable enol)
C CH3
/ H2SO4, HgSO4 H-OH
Acetone(a stable carbonyl compound)
H
C
H
C
CH3
O HCH3C
O
CH3
H C
Acetylene
Specific example:
C H/ H2SO4, HgSO4 H-OH H
H
Acetaldehyde(a stable carbonyl compound)
HH
C
H
Vinyl alcohol(an unstable enol)
C
H
O HC
O
CH
HO-HH2SO4 / HgSO4C C +
O
H
an enol unstable
O
H
carbonyl more stable
H
Hketo-enol tautomerism
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1. An alkyne’s name ends with(a) –ane (b) -ene(c) –yne (d) diene2. An alkyne function has …….. pi bond(s).(a) one (b) two(c) three (d) four3. Alkynes react with HCl by a mechanism called(a) elimination (b) Markovnikov addition(c) (d) substitution4. Alkynes react with water in the presence of a catalyst to give(a) a dialcohol (diol) (b) an alkane(c) an enol (d) a dibromide5. The conversion of alkynes to alkanes is an example of(a) oxidation (b) reduction(c) chlorination (d) dehydration
Alkynes
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Thank You for your kind attention !
Questions?
Comments
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Alkynes