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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
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

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 C
2
H
2
. The structural formula for ethyne is:
H C CH
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 This involves the mixing of one s- and one p-orbital forming two sphybrid 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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Orbital Overlap IN Ethyne H C CH


Molecular formula of ethyne is C
2
H
2
.
In ethyne, each carbon atom is sp-hybridized.
 In this way, four sp-orbitals 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
180 o .
 The length of CΞC is 1.20 A°
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s p HYBRIDISATION OF ORBITALS in ALKYNES
The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2
2s 1 2p 3
2s 2
2p 2 promotion hybridization
2 x sp 2p
Carbon in ground state
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 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 180 o 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).
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F
3
F
4
2
5-Bromo-2-pentyne
1
1
4,4-Difluoro-3-methyl-1-butyne
3 4
5
6
7 8
9
2
6-Ethyl-non-4-yne
1
2 3
5
Br
4
5-Bromo-2-pentyne
Not 1-Bromo-3-pentyne
2
1
5 4
3
Pent-1-en-4-yne double and triple bonds have have the same position number thus ene take lower number
HC C
6
H
C CH-CH
2
-CH
3
1
4
2
5 3
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
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 The simplest alkyne its common name is acetylene
 Therefore the common names of alkynes are derived from acetylene
( e.g. Methyl acetylene)
CH
3
C CH
Common : Methyl acetylene
CH
3
CH
3
CH CH
2
C
CH
C CH
3
CH
3
Common : Isobutyisopropylacetylene
CH
3
H
3
C C C
CH
3
Common: Isopropylmethylacetylene
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1) Give the IUPAC and common names of the following compounds: a) b) c) d) HC C C(CH
3
)
3
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 Nonpolar, insoluble in water.
 Soluble in most organic solvents.
 Branching reduces the boiling point of alkynes
 Terminal alkynes , R-C

C-H, are more acidic than other hydrocarbons.
 For example; compounds b & d are more acidic than a & c.
b) C CH a) c) d) HC C C(CH
3
)
3
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1. By dehydrohalogenation of geminal or vicinal dihaloalkanes
(-2HX):
Br
Br
Geminal dihaloalkane
KOH/ alcohol heat
Br
NaNH
2 heat
Strong base
C C
- 2 HX
X X
Vicinal 1,2-dihaloalkane
Br
Excess NaNH
2
- 2HBr
Br
C C
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2. From reaction of sodium Acetylide with Primary Alkyl
Halides
R C CH
+
Na liq NH
3 R C C
-
Na
+
+
1/2 H
2
Sodium acetylide
R C C
-
Na
+ +
R' X R C C R'
+ NaX
R= H For , for prep. of Terminal alkynes ( use Acetylene HC

C -H as satrting material)
R= alkyl or aryl for prep of Non Terminal alkynes (use Monosubstituted alkynes RC

C-H)
NaNH
2
/ NH
3
Na
CH
3
Br
- NaBr
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Electrophilic addition reactions:
1
. Addition of hydrogen ( Hydrogenation
)
 Alkynes can be partially reduced to cis-alkenes with H
2 poisoned catalysts. in the presence of
R C C R
+ H
2
R, R'= H or R or Ar
Pd(BaSO
4
) or (NiB)
Lindar's catalyst
R R'
Cis alkene or Z-alkene
 Alkynes can be reduced to trans-alkenes using Na or Li in liquid NH
3
H
Na or Li / liq. NH
3
C C + H
2
H
Trans -alkene
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2. Addition of halogen If we used one mole of reagent an alkene is produced two moles an alkane is produced
HC

CH + X
2
C C
+
X
2
(X= Br or Cl)
+ Br
2
H X C=C X H + X
2
C
X
X
C
+ X
2
Br
Br
+ Br
2
H X X C–C X X H
X
C
X
X
C
X
Tetrahalide
Br
Br
Br
Br
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3 . Addition of hydrogen halide
HC

CR + H X
+ HBr
H HC=C X R + H X
Br
+ HBr
H H HC – C X X R
Br
Br
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4. Addition of water: Hydration
H
H
2
SO
4
/ HgSO
4
C C + HO-H keto-enol tautomerism
O an enol unstable
H
Specific example:
H C C H
H-OH / H
2
SO
4
, HgSO
4
Acetylene
HC C CH
3
H-OH / H
2
SO
4
, HgSO
4
Propyne
H
H
O carbonyl more stable
H
H H
H
H
C C
O H
H C
H
C
O
Vinyl alcohol
(an unstable enol)
H
CH
3
C C
H
O H
(an unstable enol)
Acetaldehyde
(a stable carbonyl compound)
O
H
3
C C
Acetone
CH
3
(a stable carbonyl compound)
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Practice problems:
1. An alkyne’s name ends with
(a) –ane
(c) –yne
(b) -ene
(d) diene
2. An alkyne function has …….. pi bond(s).
(a) one (b) two
(c) three (d) four
3. Alkynes react with HCl by a mechanism called
(a) elimination (b) Markovnikov addition
(c) AntiMarkovnikov addition (d) substitution
4. Alkynes react with water in the presence of a catalyst to give
(a) a dialcohol (diol)
(c) an enol
(b) an alkane
(d) a dibromide
5. The conversion of alkynes to alkanes is an example of
(a) oxidation (b) reduction
(c) chlorination (d) dehydration
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