Topic 5E
Reactions of alkenes and alkynes
Reaction Mechanisms 52
In an organic reaction:• we break bonds and form bonds, and• these bonds are covalent; electron pairs are involved• A mechanism describes the sequence in which bond breaking and bond formation occurs as well as how the energy
of the system changes during a process• Use arrow notation to depict movement of electrons and energy profile diagrams to depict energy changes during that
process.
Reaction Energy Profiles 53
Exothermic reaction pathway:
Reaction Coordinate
Energy
Transition state
Reactants
Products
Activation energy barrierA
B
C
Reaction Energy Profiles 53
Endothermic reaction pathway:
Reaction Coordinate
Energy
Transition state
Reactants
Products
Activation energy barrierA
B
C
A two-step reaction 54
• Intermediates lie in shallow energy wells• Rate-determining step— step with highest EA
Reaction Coordinate
Energy
Transition state 1
ReactantsProducts
AB
C
Transition state 2
IntermediateDE
Addition reactions summary 55
RCH CH2
RCH CH2
X H
HX RCH CH2
HO HH+/HOH
RCH CH2
X X
X2
RCH CH2
H H
H2
RCH CH2
HO OH
OsO4 or KMnO4
Addition reactions summary 37
RCH CH2
RCH CH2
X H
HX
Addition of Hydrogen Halides
55
• Hydrogen halides react with alkenes
• Alkyl halides are formed
CH2 CH2 H Br CH3 CH2 BrEthyl bromide
Addition of Hydrogen Halides 56
• Two products are possible• Only 2-chloropropane is formed• H always adds to side of double bond with most hydrogens — the
Markovnikov rule
Cl
CH3CHCH3
CH3–CH=CH2 + HCl
CH3CH2CH2ClNot formed
Mechanism 57
Acids are sources of electrophilic H+ H+ is attacked by the electon pair ofbond leaving a carbocation a positive carbon
(Double headed arrows for electron pairs)
Mechanism animation 57
QuickTime™ and aCinepak Codec by SuperMatch decompressor
are needed to see this picture.
Addition of HCl to 2-methylpropene
Ocol, Bcorgchm CD ROMMcMurry 2.0
Mechanism in detail 57
• Two electrons from bond form bond with H+, an electrophile
• This leaves sp2 carbon with five electrons, a carbocation
CH3 H
HH
CH3 H
H
H
H+
vacant pzorbital
sp3
Mechanism in detail 58
• Finally Cl– , a nucleophile,donates a pair of electrons to carbocation forming a C—Cl bond
• Both carbons are sp3 hybridised.• H—Cl has added across the double bond
CH3
H
H
H
HH
H
HCl
H+
Cl –
sp2
sp3
sp3
CH3
Mechanism in detail 58
CH3CH CH2
+CH3CHCH3 (not CH3CH2CH2
+)
H+
secondarycarbocation
primarycarbocation
C CH2
+H+CH3
CH3
CH3
CH3
C CH3
+CH3
CH3
C CH2
H
not
tertiarycarbocation
primarycarbocation
Why?
Classification of carbocations
58
• Three Groups attached to cation centre
CH3
CH3H H
C H CH3 C CH3 CH3 C CH3
+ + +
Primary Secondary Tertiary
• Two Groups attached to cation centre
• One Group attached to cation centre
Stability of carbocations 59
• Alkyl groups push electrons through sigma bonds, they are electron donating• They are positively INDUCTIVE (+I) and charge is stabilised through delocalisation or
dispersion• The more alkyl groups the greater the stability• Inductive effects operate over only one to two bonds
CH3
CH3H H
C H CH3 C CH3 CH3 C CH3
+ + ++ + + + +
+
Primary Secondary Tertiary
RCH2+ << R2CH+ < R3C+
increasing carbocation stability
Mechanism in detail 59
CH3CH CH2
+CH3CHCH3 (not CH3CH2CH2
+)
H+
secondarycarbocation
primarycarbocation
C CH2
+H+CH3
CH3
CH3
CH3
C CH3
+CH3
CH3
C CH2
H
not
tertiarycarbocation
primarycarbocation
Why?RCH2
+ << R2CH+ < R3C+
increasing carbocation stability
Stability of carbocations 59
• More stable carbocation is formed more easily• It is a lower activation energy process
En
ergy
Reaction Coordinate)
CH3CH=CH2 + H +
CH3CH2CH2 not formed
Ea (1°)
CH3CHCH3 observed reactionEa (2°)
Other additions of HX 60
An addition of HBr:
C C
CH3
CH3
CH3
H
CH CH
CH3
CH3
CH3not
+
2carbocation
C CH2CH3
CH3
CH3
3 carbocation
+
H +C
CH2CH3
CH3 Br
CH3Br–
2-bromo-2-methylbutane
Other additions of HX 61
An addition of HI:
C CH2
CH3CH2
CH3CH2
C CH3
CH3CH2
CH3CH2
I
CH3
CH3CH2CCH2CH3I–+H +
3 carbocation (not 1 )
3-iodo-3-methylpentane
An addition of HCl:
CH3CH3
H
CH3
ClCl –+
H+
3 carbocation1-Chloro-1-methylcyclohexane
Summary 61
• Positive part adds to the carbon of the double bond which has the greater number of hydrogens attached to it
• Today this is better stated that addition of an electrophile gives the most stable carbocation
• This is a general rule for addition to alkenes• All alkenes can be expected to react in this manner
Markovnikov's Rule:
Alkene addition reactions 61
RCH CH2
RCH CH2
X H
HX RCH CH2
HO HH+/HOH
Hydration of alkenes 61
CH3CH CH2 CH3CHCH3 H2OH+ OH
2-propanol(an alcohol)
•An example of Markovnikov addition of water
Hydration of alkenes 61
Mechanism:
Alkene addition reactions 63
RCH CH2
RCH CH2
X H
HX RCH CH2
HO HH+/HOH
RCH CH2
X X
X2
Addition of halogen molecules
63
RCH CH2
RCH CH2
X X
X2
CH2 CH2 BrCH2CH2BrBr2
1,2-dibromoethane
CH3CH CH2 + Cl2 CH3CH CH2
Cl Cl
General mechanism 63
CH2
Br Br
H H
H
H
Br
Br
+CH2CH2Br + Br –
H
HH
HBr+
but
then Br –
XCH2
A "bridged bromonium ion" is formed
Addition is trans.
Bromination of bacon fat 63
QuickTime™ and aGraphics decompressor
are needed to see this picture.
Bacon fat contains unsaturated fats which add bromine
Movie from Saunders General Chemistry CD-ROM
Addition of bromine, Br2 63
Addition is trans.
Br
BrH H
trans-1,2-dibromocyclohexane
Br
HBr
HBr2
Diequatorial conformation
Br2 addition to a ring 63
• A "bridged bromonium ion" is formed
• Addition gives the trans product by anti addition
H H
Br
BrH H
trans-1,2-dibromocyclohexane
–Br
Br+
Anti-addition 63
• A "bridged bromonium ion" explains why only trans is formed since only anti addition is possible
• Stepwise addition would give the cis and trans product
Br
H
Br
H
H
Br H
Br
transcis
cis and trans-1,2-dibromo-cyclohexane
BrHH
+
Br–
Br–
H H–Br
Br+
Addition of chlorine 63
• A bridged chloronium ion is formed
• Cl– attacks to give the trans addition product
• The reaction is general for alkenes with halogens
Cl2Cl
Cl–
H
Cl
Cl
H
Addition reactions summary 65
RCH CH2
RCH CH2
X H
HX RCH CH2
HO HH+/HOH
RCH CH2
X X
X2
RCH CH2
HO OH
OsO4 or KMnO4
Cis addition to alkenes 64
• A cyclic osmic ester is first formed• Water converts this to the cis diol
C CR''
R'''
R
R'
O OOs
O O
C C
R''R'''
RR'
O OOs
O O
OsO4
cis-1,2-diol
C C
R''R'''
RR'
HO OH
H2O
OsO2
Cis diol (glycol) formation with osmium tetroxide:
Diol (glycol) formation 65
• A cyclic manganese ester is formed• Water converts this to the cis diol• Brown MnO2 is generated (purple colour of KMnO4 lost)
C CR''
R'''
R
R'
O OMn
O O
C C
R''R'''
RR'
O OMn–
O O
MnO4–
cis-1,2-diol
C C
R''R'''
RR'
HO OH
H2O
MnO2Permanganate reacts similarly:
Diol formation — cyclic alkenes
65
• Syn-addition to cyclic alkenes affords hydroxyl groups on the same face
• Cyclic alkenes afford the cis-1,2-diol
H
OH
H
OH
H
O
H
OMnO2
–
cis-1,2-cyclohexanediol
Addition reactions summary 65
RCH CH2
RCH CH2
X H
HX RCH CH2
HO HH+/HOH
RCH CH2
X X
X2
RCH CH2
H H
H2
RCH CH2
HO OH
OsO4 or KMnO4
Addition of hydrogen, H2 65
• Addition of hydrogen is exothermic by 120kJmol–1
• Addition has a very high activation energy though• With catalysts, addition occurs via a low energy path• Pt, Pd, Ni, Rh and Ru
C CR''
R'''
R
R'
C C
R''
R'''R
R'
H H
+ H2
High EA
Catalytic action 66
• Catalytic pathways may be multistep• Overall activation energy is lower
En
ergy
Progress of reaction
uncatalysed high EA
Catalysed (multistep, low EA each step)
Schematic of catalytic hydrogenation
66
• Hydrogen is absorbed onto the surface
• H—H bond weakened and hydrogens becomeatom-like
H HH H
metalsurface
metalsurface
Schematic of catalytic hydrogenation
66
• Hydrogens react with -bond (stepwise)
• Hydrogens attach on same face (syn addition) to give cis product.
metalsurface
metalsurface
H H
R R
R R R RR R
H H cis-addition
H
R R
R R R RR R
H H
Cis addition to cyclic alkenes
67
• Hydrogens add to one face of the alkene• The result is cis addition
CH3
CH3
CH3
CH3
H
HH2/Pt
cis-1,2-dimethylcyclohexane
Reaction of alkynes
• Very similar to alkenes:– They add halogens twice, X2
– They add hydrogen halides twice, HX– They add hydrogen twice, H2
– They add water with acid (hydration)– Markovnikov's rule applies
• They are slightly acidic and react with strong bases
Addition of bromine
• Two molecules of bromine add successively
H C C HH
C CBr
Br
H+ Br 2
C C
Br
H
H
Br
Br2CH CHBr2
1,1,2,2-tetrabromoethane
+ Br 2
Addition of HBr
• Two molecules of HBr add successively• Markovnikov addition in both steps
CH3C CH CH3C CH2
Br
CH3CCH3
Br
Br
HBr HBr
2,2-dibromopropane
Addition of H2
• Normal catalysis leads to double addition
• Less active catalysts allow syn addition of one molecule
R C C R + H2 C C
R
H
R
H
R CH2 CH2 R'
special catalyst
normal catalyst
Reactions as an acid
• Very strong base required
R C C H R C C +–
NH3–
NH2
Strong base Alkynide ion
Liq. NH3
Alkynide formation
• Anions (electron pairs) in sp hybrid orbitals arecloser to the carbon nucleus
• More stable than anions in sp2 or sp3 orbitals
% s character:
Electrons further from positive nucleus
s sp sp2 sp3
100 253350
Index of hydrogen deficiency
(Double bond equivalents)67
• Hexane C6H14
• Hydrogen deficiency is TWO• 2H is equivalent to either a double bond or a ring
cyclohexane1-pentene
C6H12
Index of hydrogen deficiency
68
• TWO double bond or • ONE ring and ONE double bond or• TWO rings
cyclopentene1-pentyne
C5H12 – C5H8 = 2 x H2
Equivalent to either
H2
C C
CH2
H2
C C
H
H
Combustion analysis 68
• Microanalysis gives the percentages of carbon and hydrogen• These, when divided by the atomic numbers, give the relative
proportion of each element
C: 88.16%
H: 11.84%
Then C = 88.1612.01 = 7.341
H = 11.841.008 = 11.75
Combustion analysis 69
• This the empirical formula• Compare to the weight of the compound• If the same, this is the molecular formula• If not the molecular formula will be a factor of the empirical formula
Divide by lowest number:
CH1.6 x 2 = C2H3.2
x 3 C3H4.8
x 4 C4H6.4
x 5 C5H8 molar mass 68