Alkenes & Alkynes_Chemistry

General Physics AISM-09/C/ALK

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BRINGiiT on – Study Pack By ASKIITIANS.COM – powered by IITians.

SUBJECT: CHEMISTRY

TOPIC: ALKENES & ALKYNES

COURSE CODE: AISM-09/C/ALK


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Contents- ALKENES AND ALKYNES Alkenes ................................................................................................................................................... 3

General methods of preparation of alkenes ......................................................................................... 6

Physical Properties of Alkenes : ........................................................................................................... 9

Addition reactions of Alkenes: ........................................................................................................... 10

Ozonolysis of alkenes : .......................................................................................................................... 16

Oxidation of Alkenes: ............................................................................................................................ 18

Following are the miscellaneous reactions of alkenes: ....................................................................... 20

Dienes or Diolefins or Alkadienes ....................................................................................................... 23

Physical Properties of Alkenes : ............................................................................................................. 30

Chemical nature of Alkynes: .............................................................................................................. 31

Addition reactions of Alkynes : .......................................................................................................... 32

Oxidation of Alkenes: ............................................................................................................................ 38

Miscellaneous Reactions shown by alkynes: .......................................................................................... 40

Test for unsaturation ............................................................................................................................. 43


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Alkenes (i) General formula CnH2n (ii) Also known as olefins (olefiant = oil forming) because their first member C2H4

forms oily product with chlorine or bromine (iii) Carbon atoms involved in double bond are sp2 hybridized having trigonal

planar structure with an angle of 120oC. CH2==CH2

3 3

The coplanar structure of C=C has been described below, e.g. C2H4 CH3==CH3

I II

I C II C Ground state Ground state

I C II C Excited state Excited state

(sp2)1(sp2)1(sp2)1p1 (sp2)1(sp2)1(sp2)1p1

(iv) Alkenes have the following characteristic bond lengths and bond energy. C=C =C—H

Bond length sp2–sp2( ) sp2–1s ( )


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1.34 A 1.108 A Bond energy 143.1 kcal mol–1 98.69 kcal mol–1

(v) In ethylene the C=C involves one bond formed by head on overlapping of

sp2–sp2 orbitals having bond energy 91.60 kcal as well as one bond formed by

lateral overlapping of p–p orbitals of bond energy 51.5 kcal & thereby producing

total bond energy of 143.1 kcal. That is why C=C (143.1 kcal) is more stronger

than C–C (83 kcal) bond.

(vi) It is important to note that propylene has two types of carbon atoms: CH3—CH=CH2

4 3 3 sp3 sp2 sp2 Thus it has bond lengths C—C = 1.50 A and C=C = 1.34 A sp3–sp2 sp2–sp2 Also it has bond energy C—C 84.48 kcal mol–1 and C=C 143.1 kcal mol–1 sp3–sp2 sp2–sp2 (vii) IUPAC nomenclature :

(a) The IUPAC name is derived from the IUPAC name of alkanes by replacing ending ‘ane’ by ‘ene’ alongwith the position of double bonds. (e.g. alkane – ane + ene = alkene). CH3CH=CH—CH3 is known as but-2-ene or butene–2 or 2–butene.

(b) In case of two double or two triple bonds, the ending ‘ne’ of alkanes is suitably replaced by diene or triene accordingly.

CH3CH=C=CH2 : buta-1, 2-diene; CH2=CH—CH=CH2: buta-1. 3-diene (viii) Alkenyl groups : Residual part left after the removal of one H atom from alkene

is known as alkenyl group. According to IUPAC nomenclature, these groups are named by replacing terminal ‘e’ of alkene by ‘yl’ e.g.

Group Trivial name IUPAC name

2 1 CH2=CH—

vinyl ethenyl

3 2 1 CH2=CH—CH2—

allyl prop-2-enyl

1 2 3 —CH=CH—CH3

— prop-1-enyl

1 2 3 4 —CH2—CH=CH—CH3

crotyl but-2-enyl

The numbering in alkenyl group is started from the carbon with free valencies.


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(ix) Isomerism in alkenes : Alkenes show following isomerisms (i) C3H6 : CH3CH=CH2 & CH2—CH2 Ring chain isomerism CH2 Cyclopropane (ii) C4H8 (a) CH3CH2CH=CH2 but-1-ene (b) CH3CH=CH—CH3 but-2-ene

(c) H3C C=CH2 2-methylpropene H3C

(d) H2C—CH2 H3C—CH2 cyclobutane

(a) and (b) show position isomerism (a) and (c) show chain isomerism (a) and (d) show ring–chain isomerism (b) also shows Geometrical isomerism CH3—C—H CH3—C—H

and CH3—C—H H—C—CH3 The number of isomers increase with increase in carbon atoms more rapidly than corresponding alkanes. (pentane has only 3 isomers) C3H6 C4H8 C6H10

Possible structural isomers 2 4 5 Possible Geometrical isomers – 2 2


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General methods of preparation of alkenes

(i) By cracking :

CH3—CH2—CH3 C2H4 + CH4

(ii) By dehydration of alcohols :

(a) Removal of H2O from a substrate molecule by a suitable dehydrating agent e.g. conc. H2SO4, AI2O3, H3PO4, P2O5. Some other dehydrating agents are KHSO4, BF3, dry HCI, CaCI2 etc.

(b) This involves – elimination.

(c) e.g. RCH = CH2

RCH2CH2OH RCH=CH2

RCH = CH2

CH3CH2CH—CH3 CH3CH=CHCH3 +

CH3CH2CH=CH2

Major Minor OH (Cf. Saytzeff rule) (d) The case of dehydration shows the order : tertiary > secondary > primary

alcohols. (iii) By dehydrohalogenation of monohalides:

(a) Removal of HX from a substrate by alcoholic KOH or NaNH2

(b) This too is – elimination

(c) RCH2CH2X RCH=CH2

CH3CH2CHCH3 CH3CH = CHCH3 + CH3CH2CH=CH2

Major Minor X (Cf. Saytzeff rule)

(d) The case of dehydrohalogenation shows the order: For alkyl halides: tertiary > secondary > primary alkyl halides For halogens in halides : iodide > bromide > chloride (iv) By halogenations of dihalides:


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(a) Removal of X2 from a substrate Zn dust/CH3OH or Zn—Cu couple in alcoholic solution.

(b) e.g. CH3CHX2 CH2=CH2 gem dihalide

CH2XCH2X CH2=CH2 vicinal dihalide

Note : Metallic sodium can also be used in place of zinc. (v) By Kolbe’s electrolysis :

(a) Electrolysis of aqueous solutions of sodium or potassium salts of saturated dicarboxylic acids gives alkene.

Anode Cathode (b) e.g. CH2COONa CH2

+ 2CO2 + 2NaOH + H2

CH2COONa CH2 Disodium succinate

(vi) By partial hydrogenation of alkynes:

(a) Hydrogenation of alkynes in presence of Lindlar catalyst gives alkenes.

CH CH CH2=CH2

RC CH RCH=CH2

(b) Lindlar catalyst is Pd on CaCO3 deactivated by lead acetate which

prevents further hydrogenation. (c) Use of Pd or Pd-charcoal poisoned by BaSO4 and quinoline give better

results. (Cram et. Al) (vii) By heating quaternary ammonium compounds :

(C2H5)4 N+ OH– (C2H5)3 N + C2H4 + H2O tetraethyl ammonium hydroxide tertiary ethylamine

(C2H5)4 N+ X– (C2H5)3N + C2H4 + HX


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(CH3CH2CH2CH2)4 N+ X– CH3CH2CH=CH2 + (C4H9)3N + HX

4N+ X– =CH2 +

3N+HX

(viii) By Grignard reagents :

R

Mg + XCH=CH2 RCH=CH2 + MgX2 R (ix) Action of Copper alkyl on Vinyl Chlorides : Vinyl chloride on alkylation with

copper alkyl form higher alkenes.

2H2C = CHCI 2H2C=CHR + CuCI2 alkene

2H2C = CHCI 2H2C=CHR + CuCI2

propene


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Physical Properties of Alkenes :

(a) All are colourless & have no characteristic odour. Ethene has pleasant

smell. (b) Lower members (C2 to C4) are gases, middle one (C3 to C17) are

liquids, higher are solids. (c) The boiling points, melting points, and specific gravities show a

regular increase with increase in molecular weight, however less volatile than corresponding alkanes.

(d) A cis isomer has high boiling and melting point than trans isomer because of more polar nature.

(e) Like alkanes, these too are soluble in non polar solvents. (f) Alkenes are weak polar. The polarity of cis isomer is more than trans

which are either non polar or less polar. (e.g. trans butene-2 is non polar; trans pentene-2 is weak polar).


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Addition reactions of Alkenes:

Following are addition reactions shown by alkenes. (a) Addiion of H2 or hydrogenation :

(i) The mechanism reveals the free radical addition. The process is used to obtained vegetable ghee from hydrogenation of oil.

CH2=CH2 + H2 CH3—CH3

(ii) In presence of Ni as catalyst, reaction occurs at 200—

300oC whereas in presence of Pt or Pd, the hydrogenation of alkenes takes place even at room temperature.

(iii) Hydrogenation of alkene is exothermic in nature. The heat of hydrogenation for alkenes are nearly 30 kcal/mol and their value is most commonly used as a scale for the stability of alkene. The lower the heat of hydrogenation of

an alkene, the more is stability e.g. trans-2-butene ( Hh = 27.6 k cal/mol) is more stable then

cis-2-butene ( Hh = 28.6 k cal/mol) and 1-butene ( Hh = 30.3 kcal/mol)

(b) Addition of halogens :

(i) Addition of CI2 on alkene is free radical addition, whereas addition of Br2 sows electrophilic addition.

CH2=CH2 + CI2 CH2CI—CHCI

CH2=CH2 + Br2 (in solvent) CH2Br—CH2Br The mechanism Br

CH2=CH2 CH2=CH2 CH2+ — CH2

– CH2+ — CH2

CH2 — CH2


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Br Br+ Br+

Br Br

Br Br– Br– (ii) The reactivity order of halogens : CI2 > Br2 > I2

(iii) The addition of Br2 on alkenes provide a useful test for unsaturation in molecule. The brown colour of the bromine being rapidly discharged. Thus decolorization of 5% Br2 in CCI4 by a compound suggests unsaturation in it.

(iv) Additionof bromine on ethylene in aqueous sodium chloride solution gives ethylene dibromide and 1-bromo, 2-chloroethane.

CH2=CH2 + Br2 BrCH2CH2Br + BrCH2CH2CI

Similarly

BrCH2CH2Br + BrCH2CH2I CH2=CH2 + Br2 ——

BrCH2CH2Br + BrCH2CH2ONO2

(c) Addition of halogen acids : (i) Electrophilic addition

(ii) e.g. CH2=CH2 + HX CH3—CH2X (iii) The reactivity order for halogen acids is:

Explained in terms of steric

hinderance of bromine atom

on carbonium ion or in

terms of cyclic bromonium

ion as intermediate i.e.

(Br2 gets polarized

due to electron

cloud of alkene)


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HI > HBr > HCI > HF (iv) Follow mechanism of reaction & Markownikoff rule in chapter 1. (d) Addition of hypohalous acids : (i) Electrophilic addition (ii) The reactivity order for oxyacids is: HOCI > HOBr > HOI

CH2=CH2 + HOX CH2OH — CH2X (ethylene halo hydrin)

(iii) In place of HOX, (halogen + water) may be used to get above reaction.

(iv) For addition of HOX on unsymmetrical alkene Markownikoff rule is followed.

CH3—CH=CH2 + HO– X+ CH3CHOHCH2X (e) Addition of H2O :

(i) Alkenes show addition of H2SO4 which on hydrolysis yield alcohols.

CH2=CH2 + H2SO4 CH3CH2HSO4 CH3CHOHCH3

(ii) This too is electrophilic & addition on unsymmetrical

alkenes obeys Markownikoff rule. (iii) The reaction is used to separate alkenes from a mixture of

alkane and alkenes.

CH3CH=CH2 CH3CH—CH3 CH3CHOHCH3


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HSO4

Note : (i) Alkene–1 on addition of H2O gives alkanol–2. To obtain alkanol–1, hydroboration of alkene–1 followed with its reaction with H2O2 is made.

6RCH=CH2+B2H6 2(RCH2CH2)3B 6RCH2CH2OH+2H3BO3

(ii) Alkene–1 may also be converted to alkanol–1 as:

RCH=CH2+HBr RCH2–CH2Br RCH2CH2OH

Kharasch effect

(f) Addition of NOCI :

CH2 = CH2 + NO+ – CI– CH2CI—CH2NO ethylene nitrosochloride Note : NO

The addition of NOCI on alkene gives C—C . This product

is stable only when the carbon atom bearing. CI

NO

R NO group does not contain hydrogen e.g. C—C . If H atom

is present on the carbon atom, the product. R CI Undergoes rearrangement to form more stable oxime.

NO


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H

C—C C—C=NOH

CI CI oxime

(g) Addition of O2 : R—CH=CH2 + O2 R—CH—CH2 lower alkenes Ag catalyst O epoxy alkane (or oximes, a class of

compounds)

Alkane epoxides are cyclic ethers which on hydrolysis give diols or glycols.

(h) Addition of HNO3 :

– +

CH2=CH2 + HO — NO2 HOCH2 — CH2NO2 (fuming) 2–nitroethanol (i) Addition of Acetyl Chloride :

– +

CH2=CH2 + CH3COCI CH2CI — CH2COCH3 4–chlorobutan-2-one or methyl-chloroethyl

ketones (j) Addition of isoalkanes or alkylation :

CH3—C=CH2 + CH3—CH—CH3 CH3—C—CH2—CH—CH3

HF


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CH3 CH3 0 to 10^C CH3 CH3 Isooctane

(k) Polymerization :

(i) Alkenes on heating in presence of catalyst (O2, HF or peroxides) at high P undergo addition polymerization i.e. self addition.

(ii) Polymerization is reversible reaction and the strength of polymer depends upon experimental conditions.

nCH2 = CH2 (H2C—CH2)a high P,T polyethene

nCH2 = CH2

polyethene

Note : The derivatives of alkenes of the type CH2=CHX (where X may be halogen, N, OH etc.) also undergo polymerization to

form useful polymers such as poly vinyl cyanide ,

polyvinyl chloride and polyvinyl alcohol

.


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Ozonolysis of alkenes : A test for unsaturation in molecule.

(a) On passing ozone through a solution of alkenes in inert solvent i.e. CHCI3 or CCI4 or ether, addition of ozone takes place round double bond of alkene to form ozonides.

(b) The mono ozonides are highly explosive in nature and are generally decomposed during hydrolysis or by reduction with hydrogen in presence of catalyst to give two molecules of carbonyl compounds.

(c) The complete process of ozonide formation (step a) and then their decomposition to give carbonyl compounds (step b) is known as ozonolysis.

(d) The ozonolysis thus involves the replacement of an olefinic bond C=C by two carbonyl groups C =O. The total number of carbon atoms in two carbonyl compound is equal to total number of carbon atoms in alkene.

(e) The ozonolysis is used to detect the position and nature of unsaturation in a molecule. For this purpose first ozonides are formed. The solution is evaporated to get the ozonides as viscous oil which are then either hydrolysed directly with water using Zn dust as reducing agent or reduced by H2 is presence of Pd or Pt. The Zn dust used during hydrolysis checks the formation of H2O2 which can otherwise oxidize the products (carbonyl compounds) to respective acids. Identification of aldehydes & ketones formed during ozonolysis suggests the nature & position of unsaturation in molecule.

(f) A symmetrical alkene give rise to two molecules of same carbonyl compound.

(g) e.g.CH3CH2CH=CH2 CH3–CH2CH–O–

CH2 CH3CH2CHO + HCHO


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O——–O

CH3CH2CHO + HCHO

Zn dust

CH3CH=CHCH3 CH3—CH—O—CH—CH2 2CH3CHO

above O———O H3C H3C H3C

C=CH2 C—O—CH2 C=O + HCHO

H3C H3C above H3C O———O (h) (i) An alkene of the type RCH=CHR’ gives two

aldehydes RCHO & R’CHO (ii) An alkene of the type R2C = CHR’ gives R2C = O +

R’CHO (iii) An alkene of the type R2C=CR2’ gives ketones only

R2C=O&R2’C=O

(i) Reduction of ozonide can also be made by Zn/Acid, H2-Raney Ni or triphenyl phosphine to carbonyl compounds.

(j) Reduction of ozonide by LiAIH4 or sodium borohydride gives corresponding alcohols.

R’CH—O—CHR” R’CH2OH + R”CH2OH

O———O


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Oxidation of Alkenes:

(a) Combustion : The combustion of alkenes is also exothermic with high calorific values and thus used for welding purposes in oxy-ethylene welding.

CH2=CH2 + 3O2 2CO2 + 2H2O; H = –ve

CnH2n + (3n/2)O2 nCO2 + nH2O; H = –ve (b) Oxidation by Baeyers’ reagent or hydroxylation : A test for

unsaturation Alkenes on passing through dilute alkaline, 1% cold KMnO4

(i.e. Baeyers reagent) decolorize the pink colour of KMnO4 and forms dihydroxy compounds (e.g. glycols)

CH2 CH2OH

+ H2O + [O] CH2 reagent CH2OH ethylene glycol

CH3—CH=CH2 + H2O + [O] CH3—CH—CH2

OH OH propylene glycol (c) Oxidation by alkaline KMnO4 : Oxidation of alkenes by hot

alkaline KMnO4 gives two acid salts showing fission of C=C bond

RCH=CHR’ RCOOK + R’COOK alk. KMnO

4


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(d) Oxidation by acidic KMnO4 or K2Cr2O7 :

(i) Oxidation of alkenes by acidic KMnO4 or K2Cr2O7 gives carboxylic acids. If HCOOH is acid, it is further oxidized to CO2 & H2O.

CH2=CH2 HCCOH + HCOOH H2O + CO2 (for HCOOH only

acidic KMnO4

CH3—CH = CH2 CH3COOH + HCOOH

RCH = CHR’ RCOOH + R’COOH R R R

C=C CO R R R (ii) Same products are obtained if oxidation is made by per

iodic acid or lead tetra acetate. (iii) The nature of acid formed decides the position of

unsaturation in molecule.

Note : Alkenes on oxidation by osmium tetraxide gives an intermediate product which on refluxing with NaHSO3 (alc.) gives glycols.


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Following are the miscellaneous reactions of alkenes:

1.Isomerization

(i) Alkenes on heating to 500 to 700oC or on heating in presence of catalyst {AICI3 or AI2(SO4)3} undergo isomerization.

(ii) The isomerisation involves migration of olefinic bond or alkyl group.

CH3—CH2—CH = CH2 CH3—CH=CH—CH3 or (CH3)2C=CH2

butene-1 butene-2 isobutene 2. Allylic substitution

(i) The alkyl group of the alkene (except C2H4) undergoes substitution at high temperature in presence of CI2 or Br2.

(ii) It is free radical substitution

(iii) The substitution occurs at –carbon to the double bond.

CH3—CH=CH2 + CI2 CH3CICH=CH2 + HCI

CH3CH2CH=CH2 + CI2 CH3CHCICH = CH2 + HCI

Note : At normal temperature halogens show addition reactions

with alkenes. 3. Wohl-Ziegler bromination. (Allylic bromination with NBS) O O

CH2—C CH2—C

H—CH2—CH=CH2 + N—Br Br—CH2—CH=CH2 + NH


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CH2—C ally bromide CH2—C

O O N-bromisuccinimide 4. Oxy-mercuration-demercuration

This method involves synthesis of alcohols from alkenes. Here addition occurs according to Markownikoff’s rule

H H

R—C==C—H R—CH—CH3

(ii) NaBH4, OH– OH alkene alcohol CH3 CH3 CH3

CH3—CH2—C=CH2 CH3CH2—C—CH2 CH3—CH2—C—CH3 + Hg

THF-H2O OH–

2-methylbutane-1 MeCOO HgOOCMe OH 2-methylbutan-2-ol Uses : (i) In plastic formation i.e., polyethene, polypropene etc. and

synthetic rubber formation. (ii) In oxy-ethylene welding. (iii) As food preservatives (C2H4) and ripening of fruits. (iv) As general anaesthetic (C2H4 with 10% O2)


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(v) In preparation of mustard gas – An oily liquid having high vaporizing tendency. Its vapours have high penetrating power and penetrate even thick boots and causes painful blisters on skin as well as inside the body, causing death ultimately. It was used I world war.

CH2 CH2 CH2CI CH2CI

+ S2CI2 + + S

CH2 CH2 CH2—S—CH2 sulphur mustard gas or

monochloride


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Dienes or Diolefins or Alkadienes

Hydrocarbons having two double bonds ae known as alkadienes. Alkadienes are classified into three categories on the basis of location of two double bonds.

(i) Cumulative dienes: Two double bonds are on adjacent carbon atoms e.g.

CH2 = C = CH2 allene or 1, 2-propadiene

(ii) Conjugated dienes: Molecules having alternate single & double bonds e.g.

CH2 = CH—CH = CH2 buta-1, 3-diene CH2 = C—CH = CH2 2-methylbuta-1, 3-diene or isoprene

CH3

(iii) Isolated diene : Molecules having two double bonds separated by more than one single bond.

CH2=CH—CH2—CH=CH2 1, 4-pentadiene CH=CH—CH2—CH2—CH=CH2 1, 5-hexadiene

Among the three types of dienes, conjugated alkadienes have some characteristic nature and undergo addition reactions in a peculiar manner. The simplest conjugated alkadiene is buta-1, 3-diene. It has following note worthy features.

(i) All carbon atoms in CH2=CH—CH=CH2 (buta-1, 3-diene) are sp2 hybridized

(ii) The delocalization of electrons results in resonance in molecule to show extra stable nature than corresponding non conjugated alkadienes (Resonance energy of buta-1, 3-diene is 3 kcal mol–1)

CH2=CH—CH=CH2 CH2+ —CH=CH—CH2 CH2—CH=CH—CH2

(iii) The addition of H2, Br2 or HBr …….. etc on conjugated alkadienes takes

place in two ways either 1, 2 or 1, 4-addition. 1, 2-addition 1, 4-addition


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CH2=CH—CH=CH2 CH3CH2CH=CH2 + CH3–CH=CH–CH3

CH2=CH—CH=CH2 + Br2 CH2CHBrCH=CH2 + CH2–CH=CH–CH2

Br Br Br

CH2=CH—CH=CH2 + HBr CH3CHBrCH=CH2 + CH3–CH=CHCH2Br

(iv) Non ionizing solvent favours 1, 2-addition whereas ionizing solvent

favours 1, 4-addition. However in each case mixture of both type of addition products are formed, the one predominating on the other as the case may be.

(v) It undergoes polymerization in presence of peroxides to give polybutadiene (Buna Rubber).

nCH2=CH—CH=CH2 (CH2—CH=CH—CH2)n

Buna rubber


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Alkenes

(i) General formula CnH2n (ii) Also known as olefins (olefiant = oil forming) because their first member C2H4

forms oily product with chlorine or bromine (iii) Carbon atoms involved in double bond are sp2 hybridized having trigonal

planar structure with an angle of 120oC. CH2==CH2

3 3

The coplanar structure of C=C has been described below, e.g. C2H4 CH3==CH3

I II

I C II C Ground state Ground state

I C II C Excited state Excited state

(sp2)1(sp2)1(sp2)1p1 (sp2)1(sp2)1(sp2)1p1

(iv) Alkenes have the following characteristic bond lengths and bond energy. C=C =C—H

Bond length sp2–sp2( ) sp2–1s ( ) 1.34 A 1.108 A Bond energy 143.1 kcal mol–1 98.69 kcal mol–1


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(v) In ethylene the C=C involves one bond formed by head on overlapping of

sp2–sp2 orbitals having bond energy 91.60 kcal as well as one bond formed by

lateral overlapping of p–p orbitals of bond energy 51.5 kcal & thereby producing

total bond energy of 143.1 kcal. That is why C=C (143.1 kcal) is more stronger

than C–C (83 kcal) bond.

(vi) It is important to note that propylene has two types of carbon atoms: CH3—CH=CH2

4 3 3 sp3 sp2 sp2 Thus it has bond lengths C—C = 1.50 A and C=C = 1.34 A sp3–sp2 sp2–sp2 Also it has bond energy C—C 84.48 kcal mol–1 and C=C 143.1 kcal mol–1 sp3–sp2 sp2–sp2 (vii) IUPAC nomenclature :

(a) The IUPAC name is derived from the IUPAC name of alkanes by replacing ending ‘ane’ by ‘ene’ alongwith the position of double bonds.

(e.g. alkane – ane + ene = alkene). CH3CH=CH—CH3 is known as but-2-ene or butene–2 or 2–butene.

(b) In case of two double or two triple bonds, the ending ‘ne’ of alkanes is suitably replaced by diene or triene accordingly.

CH3CH=C=CH2 : buta-1, 2-diene; CH2=CH—CH=CH2: buta-1. 3-diene (viii) Alkenyl groups : Residual part left after the removal of one H atom from alkene

is known as alkenyl group. According to IUPAC nomenclature, these groups are named by replacing terminal ‘e’ of alkene by ‘yl’ e.g.

Group Trivial name IUPAC name

2 1 CH2=CH—

vinyl ethenyl

3 2 1 CH2=CH—CH2—

allyl prop-2-enyl

1 2 3 —CH=CH—CH3

— prop-1-enyl

1 2 3 4 —CH2—CH=CH—CH3

crotyl but-2-enyl

The numbering in alkenyl group is started from the carbon with free valencies. (ix) Isomerism in alkenes : Alkenes show following isomerisms


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(i) C3H6 : CH3CH=CH2 & CH2—CH2 Ring chain isomerism CH2 Cyclopropane (ii) C4H8 (a) CH3CH2CH=CH2 but-1-ene (b) CH3CH=CH—CH3 but-2-ene

(c) H3C

C=CH2 2-methylpropene

H3C

(d) H2C—CH2

H3C—CH2 cyclobutane

(a) and (b) show position isomerism (a) and (c) show chain isomerism

(a) and (d) show ring–chain isomerism

(b) also shows Geometrical isomerism

CH3—C—H CH3—C—H

and

CH3—C—H H—C—CH3

The number of isomers increase with increase in carbon atoms more rapidly than

corresponding alkanes. (pentane has only 3 isomers)

C3H6 C4H8 C6H10


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Possible structural isomers 2 4 5

Possible Geometrical isomers – 2 2

Preparation of Alkynes

(i) By dehydrohalogenation of dihalides :

CH3 CH2X CHX CH

or CHX2 CH2X –HX CH2 –HX CH vinyl halide acetylene Vinyl halide being less reactive & thus to get better yield a stronger base sodalime NaNH2 is used in II step. (ii) By dehalogenation of tetrahalides :

CHX2

CH CH CHX2

or strong electro

positive metals (iii) By haloform : only for acetylene

CHX3 + 6Ag + X3HC CH CH + 6AgX Powder

(iv) By Kolbe’s electrolytic method :

CHCOOK Anode Cathode

CH CH + 2CO2 + 2KOH + H2 CHX(aq)

pot fumarate or

maleiate (v) Laboratory method : Acetylene is prepared in laboratory by the action of water

on calcium carbide.

CaC2 + 2H2O Ca(OH)2 + C2H2 (Wohler reaction)


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The air of the flask in which acetylene is to be prepared is displaced with oil gas as acetylene forms an explosive mixture with air. Acetylene evolved is collected over water. The acetylene so prepared is contaminated with small amounts of impurities such as PH3, H2S, AsH3 and NH3 which are removed by passing the mixture through CuSO4 solution before its collection over water.

(vi) Manufacture :

old method : 2C + H2 C2H2 (Bertholots reaction) Modern method : 6CH4 + O2 2CH CH + 2CO + 10H2

or 2CH4 CH CH + 3H2 (vii) Preparation of higher alkynes : Higher alkynes may be obtained b acetylene.

(a) CH CH + Na CH CNa + H2

CH CNa + RX CH CR + NaX

(b) CH CH + RMgX XMg—C CH R—C CH + MgX2


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Physical Properties of Alkenes : (a) All are colourless & have no characteristic odour. Ethene has pleasant

smell. (b) Lower members (C2 to C4) are gases, middle one (C3 to C17) are

liquids, higher are solids. (c) The boiling points, melting points, and specific gravities show a

regular increase with increase in molecular weight, however less volatile than corresponding alkanes.

(d) A cis isomer has high boiling and melting point than trans isomer because of more polar nature.

(e) Like alkanes, these too are soluble in non polar solvents. (f) Alkenes are weak polar. The polarity of cis isomer is more than trans

which are either non polar or less polar. (e.g. trans butene-2 is non polar; trans pentene-2 is weak polar).


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Chemical nature of Alkynes:

An alkyne molecule (except ethyne) has three parts

R —C C H Alkyl part Alkyne Acidic H Part (i) Alkyl part : The alkyl part being inert in alkyne & thus does not show

substitution reaction.

(ii) Alkyne part : It consists of one & 2 bonds. Due to more strain than alkene,

bonds are highly reactive and less stable. The polarization of bonds in alkynes leads to addition reaction. Unlike alkene, alkyne reacts with two molecules of additive to form saturated molecule.

(iii) Acidic hydrogen part : H atom attached to sp hybridized carbon or triply bonded carbon atom is acidic in nature. The acidic character is due to the fact than an increase in s character of carbon atom give rise to higher electonegativity to it and thus H atom attached on sp hybridized carbon acquires polarity to show acidic nature.

Type of bond C—H =C—H —C—H

s character 50% or 33.3% or 25% or

Thus R—C C—H+ + Base R—C C– + H–Base The hybridized carbon atom being more electronegative, is best able to accommodate the electron pair in the anion left after the removal of proton.

Relative acidic nature : HOH HOR > HC CR > HNH2 > H2C=CH2 > CH3—CH3

Relative basic nature : OH– OR– < C CR < NH2 < CH=CH2 < CH2—CH3


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Addition reactions of Alkynes :

(a) Addition of H2 : Addition of two molecules of H2 takes place on alkyne.

CH CH + H2 CH2=CH2 CH3—CH3

However the addition of 2nd H2 molecule can be checked if Lindlar catalyst is used.

Dialkyl acetylenes may be catalytically reduced to a mixture of cis and trans alkenes, the former is formed predominantly if Lindlar catalyst is used.

R R H R H C C C

C C C R R H R H (cis) (trans)

However reduction with sodium in liq. NH3 or by LiAIH4 produces trans alkene.

(b) Electrophilic addition : Acetylenic bond in alkyne is a combination of one

sigma bond and two bonds. Like alkenes, alkynes also show characteristic electrophilic addition reactions which take place in two stages involving the formation of olefinic intermediate. Thus alkynes shows addition of two molecules of addendum.

—C C— + E—Nu —C C— —C—C—

E Nu E Nu olefinic intermediate


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However the rate of electrophilic addition in acetylene is rather

show than that of ethene inspite of the fact that alkynes has excess of electron. This fact is also supported than in many of electrophilic addition reaction, presence of catalyst such as Hg2+ ions is needed. The low reactivity of acetylene is not yet clear.

(ii) Addition of Halogens :

CH CH + CI2(g) CICH=CHCI CHCI2CHCI2 or Acetylene dichloride Acetylene

tetrachloride Br2 (or western) lime or BaCI2

CHCI CCI2 Westrosol

(a) Western and westrosol are good industrial solvents for rubber, fats and varnishes. Western also have some insecticidal action.

(b) The rate of reaction increases in presence of light. (c) The reactivity order for halogens is : CI2 > Br2 > I2

CH CH + Br2 CHBr = CHBr (only) water

CH CH + Br2 CHBr2—CHBr2 in CCI4

CH CH + I2 CHI=CHI in alcohol (d) Direct combination of acetylene with chlorine may be accompanied with

explosions, but it is prevented by the presence of metallic chloride as catalyst.

(e) The predominant product during addition of one molecule of halogen on alkyne ins trans isomer.

(iii) Addition of halogen acids :


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CH CH + HX CH2=CHX CH3—CHX2 vinyl halide ethylidene dihalide The reactivity order is : HI > HBr > HCI

Acetylene reacts with dil HCI in presence of Hg2+ at 65oC to give vinyl chloride, used in preparation of poly vinyl chloride, a synthetic polymer.

CH CH + dil. HCI CH2=CHCI

Note : (i) Peroxides have the same effect on the addition of HBr to unsymmetrical alkynes as they have on alkenes.

(ii) Because of –I effect of the bromine atom, the availability of the electrons during the second molecule addition becomes much slower than ethylene.

(iv) Addition of hypohalous acids :

CH CH + HOCI CHOH=CHCI CH(OH)2CHCI2 CHOCHCI2 unstable dichloro acetaldehyde OH

CH3—C CH+HOCI CH3—C=CHCI CH3—C—CHCI2 CH3COCHCI2

dichloro acetaldehyde OH OH Unstable

Note : (i) Presence of two or more OH gp on one carbon atom makes it unstable and the molecules loses H2O molecule.

(ii) However two exceptions to this rule; one is chloral hydrate

CCI3CH(OH)2 and the other is carbonic acid HO—C—OH. Chloral

O

hydrate is extra stable inspite of two OH gp on one carbon atom due to H-bonding.

(v) Addition of H2SO4 :


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CH CH + H2SO4 CH2=CHHSO4 CH3CH(HSO4)2 (cold & conc.) vinyl hydrogen Ethylidine sulphate dihydrogen sulphate

CH3CH(HSO4)2 CH3CH(OH)2 CH3CHO –H2SO4 The above reaction can also be made as: H

CH CH CH2=C—C H—C—C—H (or CH3CHO)

60% OH H O

Vinl alcohol CH2=CHOH, which is rapidly converted into an equilibrium mixture that is almost CH3CHO is an example of keto-enol tautomerism.

CH3C CH CH3COCH3 60%

Note : Only C2H2 on addition of H2O gives aldehyde and rest all alkynes give ketone.

(vi) Addition of AsCI3 :

CH CH + AsCI3 CHCI=CHAsCI2 chlorovinyl dichloroarsenic (Lewsite),

A poisonous gas, more poisonous than mustard gas

(vii) Addition of HCN :

CH CH + HCN CH2=CHCN vinyl cyanide or acrylonitrile used for preparation of (i) Buna-N rubber

a

copolymer of vinyl cyanide & butadiene and (ii) synthetic fibre orlon.

(viii) Addition of acetic acid :


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CH CH+CH3COOH CH2=CH(OOCCH3) CH3CH(OOCCH3)2 (CH3CO)2O+CH3CHO vinyl acetate ethylidene diacetate

used in preparation of

polymer polyvinyl acetate

(ix) Addition of CO and H2O :

CH CH + CO + H2O CH2=CHCOOH acrylic acid

(x) Polymerisation or self addition : Alkynes undergo polymerization yielding different types of polymeric compounds under different conditions.

(a) Cyclic polymerization :

(i) 3CH CH(g) metal tube

benzene CH—CH CH CH

(ii) 4CH CH Tetra hydro furan CH CH (solvent for C2H2)

High P CH—CH Cyclooctatetraene

(iii) CH3—C CH

mesitylene (b) Linear Polymerisation :

CH CH+CH CH CH2=CH–C CH CH2=CH–C C–CH=CH2

monovinyl acetylene divinyl acetylene

Vinyl acetylene on reaction with HCI forms 2-chloro, 1, 3-butadiene (or chloroprene) which on exposure to air polymerizes to give synthetic rubber neoprene


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CH2=CH–C CH+HCI CH2=CH–

C=CH2

CI chlorprene neoprene (rubber)

Note : Acetylene on heating with spongy copper or its oxide gives a cork like substance, used in manufacture of linoleum.

Ozonolysis : O

CH CH+O3 CH3—C—CH CH3CO CHO+H2O2 CH3COOH+HCOOH

O—-O

Acetylene monoozonide O

CH3–C CH+O3 CH3—C—

CH CH3COCHO+H2O2 CH3COOH+HCOOH

O—-O O

R—C C—R’ R—C—CR’ T—C—CR’ + H2O2 RCOOH + R’COOH O—-O O O

(i) Addition of O3 on alkynes gives their monoozonides which on hydrolysis forms dicarbonyl compounds which are further oxidized to carboxylic acids.

(ii) In alkenes two molecules of carbonyl compounds are formed during ozonolysis and in alkyne one molecule of dicarbonyl compound is formed which is further oxidized to two acids.


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Oxidation of Alkenes:

(a) Combustion : The combustion of alkenes is also exothermic with high calorific values and thus used for welding purposes in oxy-ethylene welding.

CH2=CH2 + 3O2 2CO2 + 2H2O; H = –ve

CnH2n + (3n/2)O2 nCO2 + nH2O; H = –ve (b) Oxidation by Baeyers’ reagent or hydroxylation : A test for

unsaturation Alkenes on passing through dilute alkaline, 1% cold KMnO4

(i.e. Baeyers reagent) decolorize the pink colour of KMnO4 and forms dihydroxy compounds (e.g. glycols)

CH2 CH2OH

+ H2O + [O] CH2 reagent CH2OH ethylene glycol

CH3—CH=CH2 + H2O + [O] CH3—CH—CH2

OH OH propylene glycol (c) Oxidation by alkaline KMnO4 : Oxidation of alkenes by hot

alkaline KMnO4 gives two acid salts showing fission of C=C bond

RCH=CHR’ RCOOK + R’COOK


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alk. KMnO4

(d) Oxidation by acidic KMnO4 or K2Cr2O7 :

(i) Oxidation of alkenes by acidic KMnO4 or K2Cr2O7 gives carboxylic acids. If HCOOH is acid, it is further oxidized to CO2 & H2O.

CH2=CH2 HCCOH + HCOOH H2O + CO2 (for HCOOH only

acidic KMnO4

CH3—CH = CH2 CH3COOH + HCOOH

RCH = CHR’ RCOOH + R’COOH R R R

C=C CO R R R (ii) Same products are obtained if oxidation is made by per

iodic acid or lead tetra acetate. (iii) The nature of acid formed decides the position of

unsaturation in molecule.

Note : Alkenes on oxidation by osmium tetraxide gives an intermediate product which on refluxing with NaHSO3 (alc.) gives glycols.


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Miscellaneous Reactions shown by alkynes: 1. Isomerization :

CH3CH2C CH CH3—C C—CH3 but-1-yne Triple bond shifts from

corner to centre

NaNH2,

Triple bond shifts from centre to corner

2. Action of N2 :

CH CH + N2 2HCN 3. Formation of heterolytic compounds :

CH CH CH——CH

+ NH3 +

CH CH CH CH N H Pyrrole CH CH CH——CH

+ S + CH CH Fe Pyrite CH CH S Thiophene 4. Nucleophilic addition : Acetylene undergoes nucleophilic addition with CH3OH

in presence of CH3ONa.

CH CH + CH3OH CH2=CHOCH3 vinyl mehyl ether

CH CH CH+ = CH– CH==CH– CH=CH2


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H+ H+ H+

OCH3 OCH3

OCH3– OCH3

– OCH3–

5. Substitution reaction : Acetylene on passing through sodium hypochlorite

solution at 0oC in absence of light shows substitution of H by chlorine atom. (a) Formation of sodium acetylide or alkynides :

CH CH CH CNa + H2

liq. NH3 mono sod. acetylide

CH CH + Na CH CNa NaC CNa

di. sod. acetylide (b) Formation of Acetylenic Grignard reagent :

CH CH + R MgX CH CMgX

(c) Formation of copper alkynides : on passing alkynes through ammoniacal cuprous chlorides solution, a red precipitate of cuprous alkynide is obtained.

CH CH + Cu2CI2 + 2NH4OH CuC CCu + 2NH4CI + 2H2O cuprous acetylide red ppt.

2RC CH + Cu2CI2 + 2NH4OH 2RC CCu + 2NH4CI 2H2O red ppt. (d) Formation of silver alkynides : On passing alkynes through ammoniacal

silver nitrate solution (Tollens reagent) a white precipitate of silver alkynides is obtained

CH CH + 2AgNO3 + 2NH4OH AgC CAg + 2NH4NO3 + 2H2O silver acetylide (while ppt.)


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RC CH + AgNO3 + NH3OH RC CAg + NH4NO3 + H2O Note : (i) These alkynides are ionic in nature (ii) Alkynides are generally explosive and unstable when dry.

(iii) Copper and silver alkynides are very sensitive to shock when dry & may explode

(iv) These alkynides are easily converted to original alkynes when

treated with dilute acids.

NaC CNa + 2HNO3 HC CH + 2NaNO3

(v) Acidic nature of alkyne can be utilized to separate, purify and

identify alkyne-1 from other hydrocarbons.

Uses : Among alkynes, acetylene has got wide applications in industries.

(i) As oxy-acetylene flame for welding.

(ii) As illuminating agent in hawker’s lamps and light houses.

(iii) In artificial ripening of fruits. (iv) In preparation of monomeric unit (vinyl chloride, vinyl cyanide, vinyl

cyanide, vinyl acetate, vinyl acetylene, etc.) to get polymers (PVC, PVA, chloroprene, Buna-S etc.) widely used in textile, plastic, shoe and rubber industries.

(v) In preparation of poisonous gas, Lewiste. (vi) In preparation of solvents such as westron, westrosol and other useful

chemicals e.g., C6H6, acetaldehyde, acetone etc. (vii) It is used as general anaesthetic under the name Naracylene


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Test for unsaturation (i) By Br2 in CCI4 : The decolorization of 5% Br2 in CCI4 by a compound confirms

the presence of unsaturation in molecule. (ii) By Baeyers reagent : The decolorization of pink colour of 1% cold alkaline

KMnO4 by a compound confirms the presence of unsaturation in molecule. Note : (i) Alkenes without any hydrogen atom on olefinic bond do not show these

tests e.g. (ii) Aldehydes, primary and secondary alcohols which are readily oxidized by

alk. KMnO4 and thus decolorize alk. KMnO4

To locate the position of unsaturation : By ozonolysis To distinguish

(i) Alkane and alkene : By Br2 in CCI4 or By Baeyers reagent (ii) Alkane and alkyne : As above (iii) Alkene-1 and alkene-2 : By ozonolysis (iv) Alkyne-1 and alkyne-2 : By Amm. AgNO4 or Amm. Cu2CI2

To separation a mixture of alkane, alkene and alkyne-1

The mixture is passed through amm. Cu2CI2 or amm. AgNO3 where alkyne-1 are retained in it and alkane, alkene mixture comes out. The mixture is then passed through conc. H2SO4 which absorbs alkene and alkane comes out.

R—C CH + AgNO3 + NH4OH R—C CAg + NH4NO3 + H2O white ppt

RC CH by dil acid Alkyne-1

RCH=CH2 + H2SO4 RCH2—CH2 RCH=CH2

on heating to 170^oC Alkene-1 HSO4

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Alkenes & Alkynes_Chemistry