1 electrophilic reactions. 2 3 electrophilic reactions addition to unsaturated carbon alkenes +...
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Electrophilic Reactions
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Electrophilic Reactions
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes + Alkynes
Unsaturated hydrocarbonUnsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds
- alkenealkene: contains a carbon-carbon double bond and has the general formula CnH2n
- alkyne- alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2
Ethene(an alkene)
H
C C
H
H H
H-C C-HEthyne
(an alkyne)
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes
IUPAC Nomenclature
-use the infix -enen- to show the presence of a carbon-carbon double bond- number the parent chain to give the 1st carbon of the double bond the lower number- follow IUPAC rules for numbering and naming substituents- for a cycloalkene, the double bond must be numbered 1,2
1-Hexene 4-Methyl-1-hexene2-Ethyl-3-methyl-
1-pentene
1 1
12 2 2
3 3
34 4 4
5 5
56 6
1 2
34
5
3-Methylcyclo-pentene
CH3
1,6-Dimethylcyclo-hexene
CH3
CH31
65
4
3
2
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes
Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names
CH2=CH2 CH3CH=CH2 CH3C=CH2
CH3
IUPAC:IsobutylenePropyleneEthyleneCommon:
2-MethylpropenePropeneEthene
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes
The cis,trans system:The cis,trans system: configuration is determined by the orientation of atoms of the main chain
cis-3,4-Dimethyl-2-pentene
1
2 3
4C
H
C
CH3
CH(CH3)2H3C
trans-3-Hexene
C
H
C
CH2CH3
HCH3 CH2
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes
Configuration: E,Z
Assign a priority to the substituents on each carbon of the double bondif the groups of higher priority are on the same side of the double bond, the configuration is ZZ (German: zusammen, together)
if the groups of higher priority are on opposite sides of the double bond, the configuration is EE (German: entgegen, opposite)
Z (zusammen) E (entgegen)
C
higher
C
higher
lowerlower
C
lower higher
C
lowerhigher
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes – E,Z configuration
Priority rules1. Priority is based on atomic number; the higher the atomic number, the higher the
priority
2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference
3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds
(53)(35)(17)(16)(8)(7)(6)(1)
Increasing priority
-H -CH3 -NH2 -OH -SH -Cl -Br -I
Increasing priority
(8)(7)(6)(1)-CH2-OH-CH2-NH2-CH2-CH3-CH2-H
-CH=CH2
O
-CH
O
H
C
C
O
CC
-CH-CH2is treated as
is treated as
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of hydrogen halides
Alkenes –Addition of water
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of hydrogen halides to double bonds with substituted carbons
Carbocations are planar -> attack from both sides possible!!
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of hydrogen halides to double bonds with substituted carbons-> Addition is regioselective (regioselective reaction:regioselective reaction: a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions)
Markovnikov’s rule:Markovnikov’s rule: in additions of HX, H adds to the carbon with the greater number of hydrogens
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of hydrogen halides to double bonds with substituted carbons
-> Carbocation rearrangements
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens
-> Stereochemical consequences of electrophil addition
-> gives 2,3 dibromobutane as a pair of enantiomers, R,R and S,S
-> a result of anti addition
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens
-> Stereochemical consequences of electrophil addition
-> gives 2,3 dibromobutane as a meso R,S isomer
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens
-> Stereochemical consequences of electrophil addition
-> gives 1,2 dibromocyclohexan as a pair of enantiomers, R,R and S,S
-> a result of anti addition
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens
-> Stereochemical consequences of electrophil addition
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens in water -> Halo alcohol (halohydrin)
Halo alcohols can be used to make -> epoxides
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Electrophilic ReactionsAddition to unsaturated carbon
Alkenes –Addition of halogens in water under basic conditions -> Epoxides
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Electrophilic ReactionsAddition to unsaturated carbon
Alkynes
IUPAC nomenclatureuse the infix -ynyn- to show the presence of a carbon-carbon triple bondnumber the parent chain to give the 1st carbon of the triple bond the lower numberfollow IUPAC rules for numbering and naming substituents
3-Methyl-1-butyne 6,6-Dimethyl-3-heptyne
1 12 2
3
3 44 5
6 7
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Electrophilic ReactionsAddition to unsaturated carbon
Alkynes –Addition of halides
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Electrophilic ReactionsAddition to unsaturated carbon
Alkynes –Addition of halides
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Electrophilic ReactionsAddition to unsaturated carbon
Alkynes – Hydration gives a keton
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Electrophilic ReactionsAddition to conjugated systems
Alkenes – conjugated systems
Dienes, trienes, and polyenesfor an alkene with nn carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n2n cis-trans isomers are possibleconsider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here
C2-C3 C4-C5
Double bond
trans transtrans ciscis transcis cis
trans,trans-2,4-heptadiene
trans,cis-2,4-heptadiene
2 24 4
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Electrophilic ReactionsAddition to conjugated systems
Alkenes – conjugated systems
- vitamin A has five double bonds - vitamin A is the all-trans isomer
Vitamin A aldehyde (retinal)
enzyme-catalyzedoxidation
H
OVitamin A (retinol)
OH
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Electrophilic ReactionsAddition to conjugated systems
Alkenes – addition of halides
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Electrophilic ReactionsAddition to conjugated systems
Alkenes – addition of halides
Conjugate addition
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Electrophilic ReactionsAddition to conjugated systems
Alkenes – addition of halogens
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Electrophilic ReactionsCarbocations as electrophiles
Cationic polymerization
Polystyrene
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Electrophilic ReactionsCarbocations as electrophiles
Cationic polymerization – intramolecular electrophilic addition
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Electrophilic ReactionsCarbocations as electrophiles
Cationic polymerization – intramolecular electrophilic additionTerpenoids and SteroidsTerpenoids and Steroids
Huge group of natural products -> flavouring, aromatherapy oils, Huge group of natural products -> flavouring, aromatherapy oils, vitamins (A, K), steroid hormons (lanosterol, cholesterol)vitamins (A, K), steroid hormons (lanosterol, cholesterol)
Terpene:Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene
2-Methyl-1,3-butadiene (Isoprene)
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34head tail
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Electrophilic ReactionsCarbocations as electrophiles
Cationic polymerization – intramolecular electrophilic additionTerpenoids and SteroidsTerpenoids and Steroids
– myrcene, C10H16, a component of bayberry wax and oils of bay and verbena
– menthol, from peppermint
– Vitamin A
OH
OH
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Electrophilic ReactionsCarbocations as electrophiles
Cationic polymerization – intramolecular electrophilic additionTerpenoids and SteroidsTerpenoids and Steroids
Lemon oil Pine oil Eucalyptus oil
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Catalytic HydrationAddition of hydrogen to alkenes and alkynes
Catalytic hydration -> NOT a radical reaction (Chapter 9: Radical reactions)
Involvement of atomic hydrogen -> mechanism has more in common with radical reaction than with ionic reaction
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Catalytic HydrationAddition of hydrogen to alkenes and alkynes