Alkene and Alkyne

8/8/2019 Alkene and Alkyne

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ALKENE AND ALKYNE


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Saturated compounds (alkanes): Havethe maximum number of hydrogen atoms

attac

hedto

eac

hc

arbo

nat

om.

Unsaturated compounds: Have fewerhydrogenatomsattachedto the carbonchainthanalkanes.

Containing double bond arealkenes. CnH2n

Containing triple bonds arealkynes. CnH2n-2


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Alkene

Hydrocarbons containing atleast one C=C bond

general formula:CnH2nCnH2n


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Naming alkene

Alkenesarenamed by finding thelongest chain containing thedouble bondand changing thename of the parentalkane from -aneto -ene

rootname +rootname + --enesuffixenesuffix (compare root name + -ane suffix in alkanes)

CH2=CH2 ethene (commonname:ethylene)

location of double bonddouble bondindicated bythelowestlowest--numbered carbonnumbered carbonatom involvedinthe bond.


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Step 1

Findthelongest chain of carbons containing the

double bond. It containssix carbons.


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Step 2-3

Step 2 Namethe parent chain. Its name is hexenehexene..

Step 3 Determinethe position of thedouble bondin

the parent chain. Inthis case,younumberthe parent

chain from rightto left becausethatgivesthe firstgivesthe first

carbon of thedouble bondthelowestnumber.carbon of thedouble bondthelowestnumber.

The parentnameis 2-hexene.


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Becausethedouble bondtakes priority over othersubstituents inanalkene,sometimestheatoms of side chains

and other functional groups mustreceivehighernumbersthanthey otherwisewould.

Forexample,55--methylmethyl--33--heptenehepteneisthe correctname forthefollowing compound,not3-methyl-4-heptene,eventhoughthelatterhasalowernumber forthe methyl group.


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Step 4-5

Step 4 Numberandnamethesubstituents.

The compoundhastwo methyl groups bondedto C4.

Step 5 Puttogetherallthe pieces of thename.

The compound's completenameis4,44,4--dimethyldimethyl--22--

hexene.hexene.


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example

Step 1 Determinethelength of the parent chainand

the positions of thedouble bonds. The parent chainissix carbonslong. Therearetwo double bonds,

andtheir positionsare C1 and C4.


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Step 2 Writethenumbersof thelocations of thedouble bonds followed byahyphen: 1,4.

Step 3 Afterthehyphenwritethename of theparent chainwiththediene ending. The name of

the compound is 1,4-

hexadiene.


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Problem:

Nameeach of the following molecules


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Alkyne

Hydrocarbons containing atleast one CC bond

general formula:CnH2nCnH2n--22


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Structure of Alkyne

Alkynes contain a carboncarbon triple bond.

Terminal alkynes have the triple bond at the end of thecarbon chain so that a hydrogen atom is directly bondedto a carbon atom of the triple bond.

Internal alkynes have a carbon atom bonded to eachcarbon atom of the triple bond.


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Naming Alkyne

Alkynes are named in the same general way that

alkenes are named.

In the IUPAC system, change the ane ending of the

parent alkane name to the suffix yne.

Choose the longest continuous chain that contains both

atoms of the triple bond

Number the chain to give the triple bond the lowernumber.


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Alkenesandalkynesare consideredto haveequal priority Ina moleculewithboth adoubleandatriple bond,

whicheveris closerto theend of the chaindeterminesthe

direction of numbering. Inthe casewhereeachwouldhavethesame position

number,thedouble bondtakesthelowernumber.thedouble bondtakesthelowernumber. Inthename, ene comes before yne because of

alphabetization.


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Example:


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Problem

Writethe IUPAC name foreach of the

following unsaturated compounds:


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Common Names

Despitethe precisionanduniversalacceptance ofIUPAC nomenclature,somealkenes andalkyne(particularlylow-molecular-weightalkenes),are

knownalmostexclusively bytheircommonnames.

CH2 =CH2 CH3 CH=CH2 CH3 C=CH2

PropyleneEthyleneCommon:

IUPAC: 2-MethylpropenePropeneEthene

Isobutylene

CH3


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Cis-Trans Isomers

If alkeneshavetwo differenttwo differentsubstituentssubstituents ateachend of the C=C thenthey canexistasstereoisomers becausethereisrestrictedrotationof thedouble bond.

allterminalalkenes (begin orendwitha C=CH2)do not existascis- andtrans- isomers.

alkeneswiththeR-CH=CH-R unitcan existascis-andtrans- isomers.

Cis-transisomerism isnot possibleif one of thedouble bond carbonsisattachedto two identicalgroups.


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If two groups occur onthesameside of thedouble

bond the compoundisciscis

If theyare onoppositesidesthe compoundistranstrans


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Example

Namethese following compounds:

C C

CH3

H

CH3

CH2

CH3

C C

Cl

H

CH2

Cl

CH3


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Rule 2. If the firstatom dont giveyouananswerthen movedownthe chainuntilyou findadifference.


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Rule3. Multiple bondsatomsareequivalentto

thesamenumber of single bonds.


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Br

H

Cl

F

(Z)-2-Bromo-1-chloro-1-floroethene

Higher priorityHigher priority Br

H

F

Cl

(E)-2-Bromo-1-chloro-1-floroethene

Higher priority


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Problem

Nameeachalkene andspecifyitsconfiguration bytheE,Z system.

(a) (b)

(d)(c)

Cl

Cl

Br

Cl


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Physical Properties

.

Alkenes and alkynes have boiling points and melting points similar to

alkanes ofcomparable size. The unsaturated hydrocarbons are only

very slightly soluble in water and have densities less than water

0.766174-36

0.746125-79

0.71671-132

0.69040-90

0.69127-32

(a gas)8-126

(a gas)-23-102

(a gas)-84-81

Density

at 20C(g/mL)

oiling

Point(C)

Melting

Point(C)FormulaName

1-Decyne

1-Octyne

1-Hexyne

1-Pentyne

2- utyne

1- utyne

Propyne

Ethyne HC CH

CH3 C CH

CH3 C CCH3CH 3 ( CH2 ) 2 C CH

CH 3 CH2 C CH

CH 3 ( CH2 ) 3 C CH

CH 3 ( CH2 ) 5 C CH

CH3

( CH2

)7

C CH


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Reactions: Addition Reactions of

Alkenesand Alkynes

Addition (combination)reactionshavethe formA + B p AB

Foralkenesthe generalreactionhasthe form

R2C=CR2 + A-Bp

R2AC-CBR2(where R = anyalkyl group or H)

Additionreactionsarethe most commontypes ofreactions foralkenesandalkynes

The bondsareeasily broken,andthat pair of

electrons can form anewW bond Thereactionsare favorable becausethe products

(allW bonds)are morestablethanthereactants


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Hydrogenation

Addsahydrogenatom to each carbonatom of a multiple

bond

H2 can beaddedto alkenes oralkynesto form alkanes

Usuallya metal catalyst (Pt, Pd or Ni)isusedto speedupthereaction (thereaction generallydoesntworkwithoutacatalyst)

H2C C H2 + H2

Examples:

H C C H + 2H2

H

H

H

H

H

H

H

H

H

H

H

H

H


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Example

Whatisthe product of adding H2 (Ni catalyst)to 1-

butene?

CH3CH

2CH

2CH

3


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Hydrohalogenation of Alkenes

Hydrogenhalides (HCl, HBr or HI) canaddto alkenestoform haloalkanes

Whenahydrogenhalideaddsto asubstitutedalkene,thehalide goesto the moresubstituted C

((MarkovnikovsMarkovnikovs rule)rule)Examples:

C C

H

H H

H

+ HBr C C

H

H

H

B r

H

H

C C

H

H H

C H3

+ HCl C C

H

H

H

C l

C H3

H

C H3CH3


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Markovnikovs rule

Whenadding HX to adouble bondthe

hydrogen of HX goesto the carbon

whichalreadyhasthe mosthydrogens

..... conversely,theanion X addsto the most

highlysubstituted carbon ( the carbonwith

mostalkyl groupsattached).


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Hydration: Addition of Waterto

Alkenes

Inthe presence of astrong acid catalyst (HCl, H2SO4etc.)alkenesreactwith H2Oto form alcohols

Hydrationreactions follow Markovnikovs rule

Exampl :

Acidat.

C C

H

H

H

O H

H

H+ H 2O

C C

H

H C H3

H

AcidCat.

C C

H

H

H

O H

H

C H3+ H 2O

C H3 Acid

Cat.+ H 2O

C H3

O H


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Halogenation of Alkenesand Alkynes

Halogens (Cl2 orBr2) canaddto alkenes oralkynesto form haloalkanes

Alkenes form dihaloalkanes;alkynes form

tetrahaloalkanesExa ples:

B r

B r

+ Br2

+ Br2

B r

B r

B r B r

Exa ples:

B r

B r

+ Br2

+ Br2

B r

B r

3 + 2Br2

B r

B r

B r

B r

3


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Take Home Message

Oxidation of Alkeneandalkyne

Synthesis of Alkeneand Alkyne


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Alkene and Alkyne