alkene and alkyne
TRANSCRIPT
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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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