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:

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C CH

C CH2

CH2

C HC C C(CH3)3

a) b)

c) d) -

Physical Properties of Alkynes

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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

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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))

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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

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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

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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:

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Hydroboration—Oxidation

Hydroboration—oxidation is a two step reaction sequence that converts an alkyne to a carbonyl compound.

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Hydroboration—oxidation of an internal alkyne forms a ketone.

Hydroboration –oxidation of a terminal alkyne form an aldehyde.

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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