Reactions of Alkenes: Addition Reactions
Feb 23, 2016
Reactions of Alkenes:Addition Reactions
Addition is the opposite of elimination.
Here are some of the many addition reactions that will be considered.
Addition Reactions
Addition of Halogensto Alkenes
+ X2 X X
electrophilic addition to double bondforms a vicinal dihalide
C CC C
General features
CH3CHCHCH(CH3)2
(100%)
CHCl3
0°C
CHCH(CH3)2CH3CHBr2
Example
Br Br
F2 addition proceeds with explosive violence
I2 addition is endothermic: vicinal diiodides
dissociate to an alkene and I2
limited to Cl2 and Br2
Scope
Halogenation
anti addition
Stereochemistry of Halogen Addition
Br2
trans-1,2-Dibromocyclopentane80% yield; only product
Example
H
H
Br
Br
H
H
Cl2
trans-1,2-Dichlorocyclooctane73% yield; only product
Example
H
H
H
H
Cl
Cl
anti addition
Mechanism of Halogen Addition to Alkenes: Halonium Ions
Br2 is not polar, but it is polarizable
two steps(1) formation of bromonium ion(2) nucleophilic attack on
bromonium ion by bromide
Mechanism is electrophilic addition
Addition of Halogens to Alkenes
ethylene H2C=CH2 1
propene CH3CH=CH2 61
2-methylpropene (CH3)2C=CH2 5400
2,3-dimethyl-2-butene (CH3)2C=C(CH3)2
920,000
More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore “electron rich.”
Relative Rates of Bromination
Question
Arrange the alkenes in order of decreasing rate of reaction toward bromine addition:2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-buteneA) 2-methyl-1-butene > 3-methyl-1-butene >
2-methyl-2-buteneB) 3-methyl-1-butene > 2-methyl-1-butene >
2-methyl-2-buteneC) 2-methyl-2-butene > 2-methyl-1-butene >
3-methyl-1-buteneD) 2-methyl-2-butene > 3-methyl-1-butene >
2-methyl-1-butene
Br2
trans-1,2-dibromocyclopentane80% yield; 2 asymmetric carbon atoms,
Chiral product,Optically inactive,Racemic mixture, only products formed
Example
H
H
Br
Br
H
H
Diastereomers
In general, diastereomers fall into two categories:geometric isomers
cis-trans
stereoisomers containing two or more asymmetric atoms; (that are not enantiomers)
diastereomers
(S)
(R)
(R)
(R)
Diastereomers
Diastereomers have different physical properties:BP, MP, density, refractive index, solubilityCan be separated through conventional
means (distillation, recrystallization, chromatography)
MP = 158oC MP = 256oC
Diastereomers
A compound with “n” asymmetric carbon atoms can have a maximum of 2n stereoisomers.
2,3-dibromobutane
Enantiomers Same compound (meso)diastereomers
s
(S)
(S) (R)
(R)
(R)
(S)
(S)
(R)
Only 3 stereoisomers for 2,3-dibromobutane.
Three stereoisomers of 2,3-dibromobutane
2R,3R 2S,3S 2R,3S
chiral chiral achiral
CH3
CH3
BrH
HBrH Br
CH3
CH3
HBr H
CH3
CH3
Br
BrH
The addition of bromine to cis-2-butene produces:
A) A single enantiomerB) A product with one asymmetric carbon atomC) An optically inactive meso productD) A racemic mixtureE) 4 different stereoisomers each with 2 chiral
carbon atoms
Question
The addition of bromine to trans-2-butene produces:
A) A single enantiomerB) A product with one asymmetric carbon atomC) An optically inactive meso productD) A racemic mixtureE) 4 different stereoisomers each with 2 chiral
carbon atoms
Question
Diastereomers
When naming compounds containing multiple chiral atoms, you must give the configuration around each chiral atom:position number and configuration of each
chiral atom, separated by commas, all in ( ) at the start of the compound name
(S)
(S)(2S, 3S)-2-bromo-3-chlorobutane
Diastereomers
Which ones are chiral? Name each one. Give the stereochemical relationship between them.
All of them are chiral.
Diastereomers
(2S, 3R)-2,3-dichloropentane (2R, 3S)-2,3-dichloropentane
(2S, 3S)-2,3-dichloropentane (2R, 3R)-2,3-dichloropentane
Diastereomers
A B C DA and B: enantiomersC and D: enantiomers
A and C: diastereomersA and D: diastereomersB and C: diastereomersB and D: diastereomers
Hydrogenation of AlkenesHeterogeneous Catalysis
The characteristic reaction of alkenes is addition to the double bond.
+ A—BC C A C C B
Reactions of Alkenes
+ H—Hs
ss
exergonic H° = –136 kJ/mol
catalyzed by finely divided (heterogeneous) metals
(Insoluble) Pt, Pd, Rh, Ni
C C H C C
H H
H H
H
H
H
H
H
Hydrogenation of Ethylene
H2, Pt
(73%)
Example
CH2H3C
H3C
CH3
HH3C
H3C
What three alkenes yield 2-methylbutane on
catalytic hydrogenation?
H2, Pt
Problem
Question
Which one of the following terms best applies to the hydrogenation of an alkene in thepresence of finely divided platinum? (in ethanol as solvent)A) anti additionB) concerted reactionC) heterogeneous catalysisD) endothermic reaction
If catalysis takes place on the surface of a solid surrounded by solution, the catalyst is HETEROGENEOUS.
HOMOGENEOUS catalysts also exist.
What advantage might a homogeneous catalyst have?
Catalytic Hydrogenation
Hydrogenation of AlkenesHomogeneous Catalysis
In 1968, Knowles modified Wilkinson’s catalyst by using a chiral phosphine ligand.
A chiral catalyst can produce one desired enantiomer over another.
Asymmetric Hydrogenation
A chiral catalyst allows one enantiomer to be formed.
Some chiral catalysts give better enantioselectivity than others.
Asymmetric Hydrogenation
BINAP is a chiral ligand that gives very pronounced enantioselectivity.
For any reaction, stereoselectivity can only be occur if at least one reagent (reactant or catalyst) is chiral.
Asymmetric Hydrogenation
Question
Which of the alkenes below will produce 2-methylbutane on catalytic hydrogenation?
A) 1 and 3B) 1, 2 and 3C) 2 and 4D) 2, 3 and 4
can be used to measure relative stability of isomeric alkenescorrelation with structure is same as when heats of combustion are measured
Heats of Hydrogenation
CH3CH2CH2CH3
126
119115
Heats of Hydrogenation of Isomers
Ethylene 136
Monosubstituted 125-126
cis-Disubstituted 117-119
trans-Disubstituted 114-115
Terminally disubstituted 116-117
Trisubstituted 112
Tetrasubstituted 110
Heats of Hydrogenation (kJ/mol)
Question
Rank the following alkenes in order of decreasing heat of hydrogenation.
A) 1 > 3 > 2B) 3 > 2 > 1C) 2 > 3 > 1D) 1 > 2 > 3
Match each alkene with its correctheat of hydrogenation.
126 kJ/mol
118 kJ/mol
112 kJ/mol
highest heat ofhydrogenation;least stable isomer
lowest heat ofhydrogenation;most stable isomer
Problem
Question
Which alkene has the lowest heat of hydrogenation?
A) B)
C) D)
Stereochemistry of Alkene Hydrogenation
Catalytic Hydrogenation of an Alkene
Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation
(1) syn addition of both H atoms to double bond (adds from the same side)(2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond
Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation
(1) syn addition of both H atoms to double bond
syn addition anti addition
syn Addition versus anti Addition
CO2CH3
CO2CH3
(100%)
H2, Pt
Example of syn-Addition
CO2CH3
CO2CH3
H
H
CO2CH3
CO2CH3
H2, Pt
Question
CO2CH3
CO2CH3
H
H
Select one: True (A) or False (B) for the following statement.
“The hydrogenation product above is chiral.”
Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation
(1) syn addition of both H atoms to double bond(2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond
Two Spatial (stereochemical) Aspects ofAlkene Hydrogenation
(1) syn addition of both H atoms to double bond(2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond
A reaction in which a single starting materialcan give two or more stereoisomeric productsbut yields one of them in greater amounts thanthe other (or even to the exclusion of the other)is said to be stereoselective.
H3C CH3
H3C
H
H2, cat
Both productscorrespond tosyn additionof H2.
Example of Stereoselective
Reaction
CH3H3C
H3CH
H
HCH3
H3C
H3C
HH
H
H2, cat
But only thisone is formed.
Example of Stereoselective
Reaction
H3C CH3
H3C
H
CH3H3C
H3CH
H
H
CH3H3C
H3CH
H
H
H3C CH3
H3C
H
H2, catTop face of doublebond blocked bythis methyl group
Example of Stereoselective
Reaction
H2, cat
H2 adds to bottom face of double bond.
Example of Stereoselective
Reaction
CH3H3C
H3CH
H
H
H3C CH3
H3C
H
Electrophilic Addition of Hydrogen Halides to
Alkenes
Electrophilic Additions: Alkenes
+ H—X –
C C H XC C
When the Electrophile is a Hydrogen Halide
CH3CH2 CH2CH3
H H
CH3CH2CH2CHCH2CH3
Br
(76%)
CHCl3, -30°CC C
HBr
Example
Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate.
Mechanism
Electrons flow fromthe system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.
Mechanism
XH
C C
..
..:
Mechanism
HC C....X:
..
..:X: –
HC C+
Regioselectivity of Hydrogen Halide Addition(Markovnikov's Rule)
When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.
Markovnikov's Rule
acetic acidBr
CH3CH2CHCH3
Example 1
CH2CH3CH2CHHBr
(80%)
Markovnikov's Rule
CH3
CH3
CH3 C
Br(90%)
C C
Example 2
acetic acid
HBrCH3
CH3
H
H
Markovnikov's Rule
(100%)
Example 3
HCl
Markovnikov's Rule
CH3
CH3
Cl0°C
Question
What is the product of the reaction of 1-methylcyclohexene with HCl?
A) B)
C) D)
Protonation of double bond occurs in direction that gives more stable of two
possible carbocations.
Mechanistic Basis for Markovnikov's Rule
In many addition reactions, chirality centers are formed.
There are two possible Markovnikov products:
Stereochemical Aspects
The geometry of the carbocation:
CONCEPTUAL CHECKPOINT 9.6.
Stereochemical Aspects
Br
CH3CH2CHCH3CH2CH3CH2CHacetic acid
HBr
Mechanistic Basis for Markovnikov's Rule:Example 1
Br
CH3CH2CHCH3CH2CH3CH2CH
HBr
CH3CH2CH—CH3 + Br – +
CH3CH2CH2—CH2
+
primary carbocation is less stable: not formed
Mechanistic Basis for Markovnikov's Rule:Example 1
Question
Which carbocation forms when 3-methyl-2-pentene is protonated?
A) B)
Mechanistic Basis for Markovnikov's Rule:Example 3
H
CH3
CH3
Cl
HCl
0°C
Question
The reaction of 3-methyl-1-butene with HBr produces 2-bromo-3-methylbutane and which other alkyl halide?
A) B)
C) D)
HCl
H
CH3
CH3
Cl
H H
CH3+
secondary carbocation is less stable: not formed
CH3
H
+
H
Cl–
Question
Which alkene reacts with HCl (electrophilic addition) at the fastest rate?
A) B)
C) D)
Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes
HCl, 0°C
CH3CHCH(CH3)2 +
H2C CHCH(CH3)2
+CH3CHC(CH3)2
H
CH3CHCH(CH3)2
Cl (40%)
CH3CH2C(CH3)2
Cl(60%)
Rearrangements Can Occur
Carbocation Rearrangements
1,2-hydride shift a more stable carbocation
a more stable carbocation1,2-methyl shift
Carbocation Rearrangements
Carbocation Rearrangement Mechanism
a more stablecarbocation
Ring Expansion
Carbocation Rearrangements
NOTE: Carbocation does not always rearrange …
The "peroxide effect"
Free-radical Addition of HBr to Alkenes
acetic acidBr
CH3CH2CHCH3CH2CH3CH2CHHBr
(80%)
Markovnikov's Rule
CH3CH2CH2CH2Br
Carbocation product :only one in absence of peroxides
Free Radical product only when peroxides added to reaction mixture
Br
CH3CH2CHCH3
CH2CH3CH2CH
HBr
Addition of HBr to 1-Butene
CH3CH2CH2CH2Br
only product when peroxides added to reaction mixture
CH2CH3CH2CH
HBr
Addition opposite to Markovnikov's rule
Addition of HBr to 1-Butene
Addition of HBr with a regiochemistry oppositeto Markovnikov's rule also occur wheninitiated with light with or without added peroxides.
+ HBrh
(60%)
CH2
CH2Br
H
Photochemical Addition of HBr
Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism.
Initiation steps:
Mechanism
..
..O RR O....
O .R ....
..
..O R.+
+ BrO .R ....
H ....
: +OR ....
H . Br....
:
Propagation steps: Mechanism
+CH3CH2CH CH2. Br..
..:
.CH3CH2CH CH2 Br:
....
..
+.CH3CH2CH CH2 Br:
....
H Br....
:
CH3CH2CH2CH2 Br:..
. Br....
:
(More stable free radical)
Examples of Termination Steps(Initiation Propagation Termination)
Question
What is the correct IUPAC name of the compound isolated from the reaction of2-methyl-2-pentene with HBr in the presence of peroxides?A) 3-bromo-4-methylpentaneB) 3-bromo-2-methylpentaneC) 2-bromo-2-methylpentaneD) 2,3-dibromo-2-methylpentane