AlkynesAlkynes Hydrocarbons that have at least one Hydrocarbons that have at least one

triple bond between two adjacent triple bond between two adjacent carbonscarbons

Contain the general formula of CContain the general formula of CnnHH2n-2n-

22

Contains carbon atoms with sp Contains carbon atoms with sp hybrid orbital hybrid orbital (2 p orbitals & 2 sp (2 p orbitals & 2 sp hybrid orbitals)hybrid orbitals)

Undergoes electrophilic additionUndergoes electrophilic additionThe major type reaction are the The major type reaction are the

addition type reactionaddition type reaction

Physical PropertiesPhysical PropertiesThey are insoluble in waterThey are insoluble in waterHas low polarity thus still insoluble in Has low polarity thus still insoluble in

water and quite soluble in solvents of water and quite soluble in solvents of low polarity like ligroin, ether, benzene low polarity like ligroin, ether, benzene and CCland CCl44

They are less dense than waterThey are less dense than waterThere boiling points show the usual There boiling points show the usual

increase with increasing carbon numberincrease with increasing carbon number

IUPAC Name Common Name Formula Melting Pt. °C

Boiling Pt. °C

Density (at 20°)

Acetylene Ethyne C2H2 -82 -75

Propyne Methyl acetylene C3H4 -101.5 -23

1-Butyne Ethyl acetylene C4H6 -122 9

1-Pentyne Propyl acetylene C5H8 -98 40 0.695

1-Hexyne Butyl acetylene C6H10 -124 72 .719

1-Heptyne n-Pentyl acetylene

C7H12 -80 100 .733

1-Nonyne n-Heptyl acetylene

C9H16 -65 151 .763

1-Decyne n-Octyl acetylene C10H18 -36 182 .770

2-Butyne Crotonylene H3C≡CH3 -24 27 .694

2-Pentyne Ethyl methyl acetylene

C5H8 -101 55 .714

3-Methyl-1-butyne Isopropyl acetylene

C5H8 29 .665

2-Hexyne Methyl propyl acetylene

C6H10 -92 84 .730

Chemical PropertiesChemical PropertiesAlkynes are unstable and most Alkynes are unstable and most

reactive compared to alkanes and reactive compared to alkanes and alkenes alkenes because of their shorter because of their shorter bond length & increased electron bond length & increased electron densitydensity..

Electrons in an s orbital benefit from Electrons in an s orbital benefit from closer proximity to the positively closer proximity to the positively charged atom nucleus, and therefore charged atom nucleus, and therefore lower in energylower in energy

The acidity is based to be stabilized The acidity is based to be stabilized as a result of high character of sp as a result of high character of sp orbitalorbital

Terminal & Internal Terminal & Internal alkynesalkynes

Terminal alkynesTerminal alkynes have a hydrogen have a hydrogen atom bonded to at least one of the sp atom bonded to at least one of the sp hybridized carbons (those involve in the hybridized carbons (those involve in the triple bond).triple bond).

ex. Methyl acetyleneex. Methyl acetylene Internal alkynesInternal alkynes have something other have something other

than hydrogen attached to the sp than hydrogen attached to the sp hybridized carbons, usually carbon hybridized carbons, usually carbon atom, but could be a hetero atom.atom, but could be a hetero atom.

ex. 2- pentyneex. 2- pentyne

Preparation of AlkynesPreparation of Alkynes

1. a) From coal and limestone1. a) From coal and limestone

CaCO3heat CaO + CO2

CaO + 3C 2000oC CaC2 + CO

CaC2 + 2H2O CH CH

b) b)

( ( CO and HCO and H2 2 are important side are important side products and needed in the products and needed in the production of alcoholproduction of alcohol))

6CH4 + O2

1500oC

CH CH+ 2CO + 10H22

Alkynes PreparationAlkynes Preparation

2. Dehydrohalogenation of Alkyl Dihalides

( Elimination reaction) This reaction is particularly useful This reaction is particularly useful

since the dihalides are readily since the dihalides are readily obtained from the corresponding obtained from the corresponding alkenes by the addition of halogenalkenes by the addition of halogen

Dehydrohalogenation of Dehydrohalogenation of Alkyl DihalidesAlkyl Dihalides

C

H

CH2

BrBr

CH3KOH or NaNH2 C CHCH3

3. Displacement or 3. Displacement or substitution reactionsubstitution reaction

This reaction involves This reaction involves substitution of acetylide ion for substitution of acetylide ion for halide ionhalide ion

It results from the attack by the It results from the attack by the acetylide ion on carbonacetylide ion on carbon

The reaction is limited to used of The reaction is limited to used of primary halides because of the primary halides because of the general tendency for secondary general tendency for secondary and tertiary halides to undergo a and tertiary halides to undergo a side reaction, eliminationside reaction, elimination

a.)Reaction of sodium acetylides with primary alkyl halides

Reaction of sodium Reaction of sodium acetylides with primary acetylides with primary

alkyl halidesalkyl halides

Example:Example:HC C:HC C:--NaNa++ + CH + CH33CHCH22CHCH22CHCH22Br HC C(CHBr HC C(CH22))33CHCH33

b.) using HMPT( hexamethylphosphoric b.) using HMPT( hexamethylphosphoric triamide)triamide)

CH2CH2CH2CH2CH3CH nC4H9LiHMPT

C4H10 + C(CH2)3CH3CLi

C(CH2)3CH3CLi CH3(CH2)4Cl C(CH2)4CH3H3C(H2C)3C

4. Dehalogenation of 4. Dehalogenation of tetrahalidestetrahalides

This reaction is to eliminate the This reaction is to eliminate the halogens to form a tetrahalideshalogens to form a tetrahalides

The groups are eliminated and The groups are eliminated and the reagent used are the reagent used are essentially the same as the essentially the same as the preparations of alkenespreparations of alkenes

Dehalogenation of Dehalogenation of tetrahalidestetrahalides

Example:Example:

Br BrBr Br

CHCH33 C CH C CH ZnZn CH CH33 C CH C CH

Br BrBr Br

Reaction of AlkynesReaction of Alkynes

1. Addition of Hydrogen1. Addition of Hydrogen The addition of hydrogen atoms The addition of hydrogen atoms

react to the triple bond carbon that react to the triple bond carbon that can easily form a C-H bond can easily form a C-H bond formation generating the alkenes formation generating the alkenes

ex. ex.

Alkynes convert to alkenes by the addition Alkynes convert to alkenes by the addition of Hydrogen of Hydrogen

H+ H -

CH CH

Na, Li in NH3

H2, Pd/NiBr

trans

cis

CCH3H3CC 2H2, Ni CH3CH2CH2CH3

CCH2CH3H3CH2CC

Na, Li in NH3

H2

Pd/NiBr

C

CH2CH3

H

C

H

H3CH2C

C

H

CH2CH3

C

H

H3CH2C

2. Addition 2. Addition Halogens( halogenation)Halogens( halogenation)

ex.ex.

Acetylene was attacked by Acetylene was attacked by halogens resulting to form 1-2, halogens resulting to form 1-2, dibromoethenedibromoethene

Br+ Br -

Br+

Br -

3. Addition of Hydrogen Halides3. Addition of Hydrogen Halides ex.ex.

The hydrogen halides reacts to the The hydrogen halides reacts to the alkynes to form bromopropene alkynes to form bromopropene

The reaction of an excess hydrogen The reaction of an excess hydrogen halides, a second addition will occur to halides, a second addition will occur to the product of alkene giving a giminal the product of alkene giving a giminal dihalide. dihalide.

CHH3CC + HCl CH2C

Cl

CH3HI CH3C

Cl

CH3

I

4. Addition of water. Hydration4. Addition of water. Hydration The addition of water to acetylene to The addition of water to acetylene to

form acetyldehyde, which can be form acetyldehyde, which can be oxidized to acetic acid , is an oxidized to acetic acid , is an extremely important industrial extremely important industrial processprocess

ex.ex.

By the addition of water the alkyne By the addition of water the alkyne was reacted to form a acetyldehyde was reacted to form a acetyldehyde

Formation of Heavy metal acetylidesFormation of Heavy metal acetylides The acidic acetylene will react with The acidic acetylene will react with

certain heavy metals such as the silver certain heavy metals such as the silver (Ag(Ag++) to form insoluble acetylides) to form insoluble acetylides

ex. ex. H H H H AgAg

C C  + C C  + 2Ag2Ag alcohol C C  + 2H alcohol C C  + 2H++

AgAg

Acetylene will react to a heavy metal to form silver Acetylene will react to a heavy metal to form silver acetylidesacetylides

Ag+ Ag -

5. Formation of Heavy metal acetylides

6. Formation of alkali metal 6. Formation of alkali metal acetylidesacetylides

The acetylene will react to a alkali The acetylene will react to a alkali metal which is the sodium enable metal which is the sodium enable liberate the hydrogen gas to form liberate the hydrogen gas to form sodium acetylidessodium acetylides

ex. ex. HC C- H + Na HC C: HC C- H + Na HC C: --NaNa++ + ½H + ½H22

Or metal acetylide can react with ketone Or metal acetylide can react with ketone and further with an acid to form alcoholand further with an acid to form alcoholCLiRC + C

CH3

OCH3CC

CH3

CH3

O-

CHH

+

CC

CH3

CH3

OHCH

DienesDienes

Dienes are hydrocarbons Dienes are hydrocarbons which contain two double which contain two double bondsbonds

Intermediate between Intermediate between alkanes and polyenesalkanes and polyenes

ClassesClasses Cumulated dienesCumulated dienes dienesdienes have the have the

neighboring double bondsneighboring double bonds

ex. 1,2 Butadieneex. 1,2 Butadiene Conjugated dienesConjugated dienes have conjugated have conjugated

double bonds separated by one single double bonds separated by one single bond.bond.

ex. 2- methyl-1,3- butadieneex. 2- methyl-1,3- butadieneIsolated dienes Isolated dienes havehave double bonds double bonds

that are separated by more than one that are separated by more than one single bondsingle bond

ex. 1,3-Butadiene ex. 1,3-Butadiene

Properties of DienesProperties of Dienes The chemical properties of diene The chemical properties of diene

depend upon the arrangement of its depend upon the arrangement of its bonds bonds

Isolated diene are identical with the Isolated diene are identical with the simple alkenessimple alkenes

In conjugated dienes they differ In conjugated dienes they differ from simple alkenes in three ways: from simple alkenes in three ways: (a) they are more stable, (b) they (a) they are more stable, (b) they favor 1,4-addtion than 1,2-addition, favor 1,4-addtion than 1,2-addition, (c) toward free radical addition, they (c) toward free radical addition, they are more reactiveare more reactive

STRUCTURE COMMON NAME

IUPAC NAME

CH2=C=CH2 Allene Propadiene

CH2=CH-CH=CH2

Divinyl 1,3-Butadiene

CH2=CH-CH2-CH2CH=CH2

Diallyl 1,5-Hexadiene

CH3-CH=C=CH2

Methylallee 1,2-Butadiene

Preparation of DienesPreparation of Dienes Dienes are usually prepared by Dienes are usually prepared by

adaptations of the methods used to adaptations of the methods used to make simple alkenesmake simple alkenes

1. By catalytic cracking 1. By catalytic cracking ( dehydrogenation)( dehydrogenation)

CHCH33CHCH22CHCH22CHCH33 heat, catalyst heat, catalyst

CHCH33CH=CH=CHCH=CH=CH22

2. Dehydration2. Dehydration loss of water by chemical loss of water by chemical compound: the process by which a compound: the process by which a chemical compound loses water chemical compound loses water moleculesmolecules

ex. CHex. CH22CHCH22CHCH22CHCH22 HEAT /ACIDHEAT /ACID CH CH22=CH-CH=CH=CH-CH=CH22 OH OH

1, 3-Butadiene

By dehydration process, By dehydration process, simultaneously happened. OH simultaneously happened. OH will dissociate, become OHwill dissociate, become OH--, , forming Hforming H22O. Since C1 and C4 is O. Since C1 and C4 is electron deficient, the two electron deficient, the two carbon will be pulled an electron. carbon will be pulled an electron. Therefore the C1-C2 and C3-C4 Therefore the C1-C2 and C3-C4 forming double bonds. That have forming double bonds. That have a product of 1,3-Butadienea product of 1,3-Butadiene

ELECTROPHILIC ADDITION TO ELECTROPHILIC ADDITION TO CONJUGATED DIENES-1,4-CONJUGATED DIENES-1,4-

ADDITIONADDITION

In addition to conjugated dienes; a In addition to conjugated dienes; a rearrangement may attach itself rearrangement may attach itself not only to a pair of adjacent not only to a pair of adjacent carbons (1,2-addition), but also to carbons (1,2-addition), but also to the carbons at the two ends of the the carbons at the two ends of the conjugated system (1,4-conjugated system (1,4-addition).Electrophilic addition to addition).Electrophilic addition to conjugated dienes yields a mixture conjugated dienes yields a mixture of 1,2- and 1,4- addition products of 1,2- and 1,4- addition products

MechanismMechanism

Stability of Stability of ConjugatedConjugated DienesDienes

More stable than non conjugated dienesMore stable than non conjugated dienes

1. Stability observed as less heat release 1. Stability observed as less heat release by hydrogenation than non conjugated by hydrogenation than non conjugated dienes dienes

Stability observed in more substituted Stability observed in more substituted compounds either conjugated or notcompounds either conjugated or not

2. Bond length also indicates stability due 2. Bond length also indicates stability due to overlap of hybrid orbitalto overlap of hybrid orbital

CH2=CH-CH2

-CH=CH2 + H2 CH3CH2CH2CH2CH3

CH2=CH-CH=

-CH

-CH3 + H2 CH3CH2CH2CH2CH3

∆H= 60.8 Kcal/mol

∆H = 54.1 kcal/mol

Stability of Conjugated Stability of Conjugated DienesDienes

3. Structurally3. Structurally

a. Delocalizationa. Delocalization

b. Hyperconjugationb. Hyperconjugation

Reactions of DienesReactions of Dienes

1.1. HydrogenationHydrogenation

2.2. OzonolysisOzonolysis

3.3. Glycol formationGlycol formation

4.4. HalogenationHalogenation