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Alkynes.1.1 IntroductionIntroduction
.2.2 Nomenclature of AlkynesNomenclature of Alkynes
.3.3 Physical Properties of AlkynesPhysical Properties of Alkynes
.4.4 Preparation of AlkynesPreparation of Alkynes
.5.5 Reactions of AlkynesReactions of Alkynes
They are unsaturated hydrocarbonsThey are unsaturated hydrocarbons – made up of C – made up of C and H atoms and contain and H atoms and contain one or more Cone or more C ≡≡ CC triple bond triple bond somewhere in their structures.somewhere in their structures.Their general formula is Their general formula is CnHCnH22n-2.n-2.
The alkyne triple bond is composed of The alkyne triple bond is composed of one σ and two one σ and two 2 2 covalent bondscovalent bonds, the triple bond can be , the triple bond can be terminalterminal or or internalinternal. . Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond.Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond.
Structure Of AlkynesStructure Of Alkynes
2
Structure of AlkynesAcetylene (ethyne):
sp Hybrid orbitals
Ground state
2p
2s
1s Promotionof electron
Promotionof electron
Exited state
2p
2s
1s
sp-hybridized state
1s
2p
sp
HybridizationHybridization
Each sp hybrid orbital:
50% s character50% p character
The shape of an sp hybrid obital:
CH CH
Each carbon is sp hybridized with a linear geometry and bond angles of 1800, perpendicular to the two remaining p orbitals.
180°An sp-hybridized carbon atom:
Geometric structure of sp-hybridized C atoms is linear.
In the mole. of acetylene, the formation of C-Cσbond : sp-sp overlap;
The formation C-H σ bonds: sp-1s overlap.
The formation of two C-C π bonds: 2py-2py overlap and 2pz-2pz overlap.
Nomenclature of Alkynes
2. Determine the stem name by selecting the longest possible straight chain containing the C ≡ C triple bond and number the chain to give the triple bond the lower number.
3. Designate the position of the C ≡ C triple bond by using the number of the first atom of the double bond
Nomenclature of AlkenesNomenclature of Alkenes
1. In the IUPAC system, change the –ane ane ending of the parent alkane name to the suffix –yne.yne.
3. Compounds with two triple bonds are named as diynes, those with three are named as triynes and so forth.
CH3 C CHCH3 C C CH2 CH2 Br
Propyne 5-bromo-2-pentyne
CH3 CH
CH3
CH2 C C CH
CH3
CH3
IUPAC :2,6-Dimethyl-3-heptyne
Common :Isobutylisopropylacetylene
7
6-Ethyl-4-nonyne
Pentyne
CCH2CH3HC
1-Butyne
Examples:
Nomenclature of Alkynes
4. Double4. Double and and tripletriple bonds are considered to bonds are considered to have equal priority: have equal priority: thus thus in a molecule with both a double and triple bond, whichever is close in a molecule with both a double and triple bond, whichever is close to the end of the chain determines the direction of numbering.to the end of the chain determines the direction of numbering.
5. In case where double and triple bonds would have the 5. In case where double and triple bonds would have the same same position numberposition number, the , the double bond takes the lower numberdouble bond takes the lower number..
6. Compounds with both a double and triple bond are named as enynes ( ‘’en‘’enee’’ comes before ‘’yne’’’’ comes before ‘’yne’’)).
7. The simplest alkyne, H-CC-H, named in the IUPAC system as ethyne, is more often called acetylene, its common name.
8. The two-carbon alkyl group derived from acetylene is called an ethynyl group.
Nomenclature of Alkynes
CH3CH CH C CH
3-Penten-1-yne
CCH2CHHC CH2
1-penten-4-yne
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
234
Hept-2-en-5-yne
or 2-Hepten-5-yne
Common Nomenclature Of AlkynesCommon Nomenclature Of Alkynes
The simplest alkyne its common name is The simplest alkyne its common name is acetyleneacetylene Therefore the common names of alkynes are derived from acetyleneTherefore the common names of alkynes are derived from acetylene
( e.g. Methyl acetylene( e.g. Methyl acetylene)
Examples:
CH3 C CH CH3 CH
CH3
CH2 C C CH
CH3
CH3
C CH3C
Common : IsobutyisopropylacetyleneCommon : Methyl acetylene
Common: Isopropylmethylacetylene
ExerciseExercise 1. Give the IUPAC and common names of the
following compounds:
11
C CH
C CH2
CH2
C HC C C(CH3)3
a) b)
c) d) -
Physical Properties of Alkynes
12
Alkynes are non polar compounds.Insoluble in water.Soluble in non polar organic solvents.They are less dense than water.Alkynes have low melting points and boiling points.Melting point and boiling point increase as the
number of carbons increases.Terminal alkynesTerminal alkynes, R-C, R-CC-H, are more C-H, are more acidicacidic than than
other hydrocarbons.other hydrocarbons.
Preparation of Alkynes
• Dehydrohalogenation is the elimination of (-2HX) from a dihaloalkane in presence strong base
Elimination ReactionsElimination Reactions
1. Dehydrohalogenation of dihaloalkanes (-2HX)
Br
BrH
H
Br
HNaNH2KOH / alcohol
Heat Heat
/ NH3
2. From reaction of sodium Acetylide with Primary Alkyl Halides
Preparation of Alkynes
C CHR + Na liq NH3 C C
- Na
+R + H21/2
C C- Na
+R + R' X C CR R' + NaX
Sodium acetylide
15
Like alkenes, alkynes undergo addition reactions because they contain relatively weak bonds.
Two sequential reactions can take place: addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a product having four new bonds.
Reactions of Alkenes
Addition ReactionsAddition Reactions
Reactivity: Alkynes < Alkenes
11. Addition of hydrogen ( Hydrogenation. Addition of hydrogen ( Hydrogenation))
16
Alkynes can be partially reduced to Alkynes can be partially reduced to ciscis-alkenes-alkenes with with HH22 in the in the
presence of presence of poisoned catalystspoisoned catalysts..Lindlar catalyst: Pd/ CaCO3, Pb(Ac)2-quinoline
HH
+ H2C C
Pd(BASO4) or (NiB)
Cis- alkene
Reactions of Alkenes
CH3(CH2)3C C(CH2)3CH3H2
Lindlarcatalyst
CH3(CH2)3
C C(CH2)3CH3
H H
17
Alkynes can be reduced toAlkynes can be reduced to transtrans-alkenes -alkenes using using Na or Li in liquid Na or Li in liquid NHNH33 (meaning (meaning Metal-Ammonia ))
H
H
+ H2C C
Na or Li / liq. NH3
Trans -alkene
CH3(CH2)3C C(CH2)3CH3 Li or Na
CH3(CH2)3
C C
(CH2)3CH3H
H
Liq. NH3
78%
2. Addition of halogenAddition of halogen
33. Addition of hydrogen halideAddition of hydrogen halide
Markovnikov’s rule applies to the addition of HX to vinyl halides
18
Reactions of Alkenes
HXR'C CH3
X
X
Excess HXAlkyl dihalideAlkenyl halide
R'C CH HX R'C CH2
X
4. Addition of water: Hydration4. Addition of water: Hydration
Keto-enol tautomerism
C C C
O
H(R)
Transformation of Functional groups:
21
Hydroboration—Oxidation
Hydroboration—oxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound.
22
Hydroboration—oxidation of an internal alkyne forms a ketone.
Hydroboration –oxidation of a terminal alkyne form an aldehyde.
23
Four addition reactions of 1-butyneExamples:
Reactions of Alkenes
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)substitution
4. 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
ExerciseExercise