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Required background:Thermodynamics from general chemistryHybridizationMolecular geometryCurved arrow notationAcidity and basicity Essential for:1. Reactions of elimination2. Selective reactions3. Multistep reaction mechanisms4. Role of petroleum in the economy5. Stereochemistry of reactions6. Role of polymers in the economy7. Chemistry of aromatic compounds
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
C
HH
H
H
-bond
sp3 s
A B
-bond
pp
(sigma)-bonds are symmetrical with respect to rotation around the bond.Rotation of fragments around the bond does not break the bond.
(pi)-bonds are non-symmetrical with respect to rotation around the bond.Rotation of fragments around the bond breaks the bond. The -bonds are normallyweaker, than the -bonds, due to a weaker orbital overlap.
Together,- and - bonds form a double bond.Now we need to choose a hybridization to describe systems, containing a double bond.
The valence shell of the atom of carbon has one s-orbital and three p-orbitals.When the carbon is not bonded by any -bonds, all s- and p-orbitals are involved in the formation of -bonds, through the sp3-hybrid orbitals.
When the carbon is bonded by one-bond, one p-orbital participates in the formation of this - bond, and the remainingone s-orbital and two p-orbitals are involved in the formation of -bonds through the sp2-hybrid orbitals.
The presence of sp2-hybridized carbons (hosts of sp2-orbitals) is characteristic for alkenes.
The presence of a weaker -bond in alkenes accounts for their higher chemical reactivity, comparing with alkanes.
The simplest alkene is ethylene (C2H4). It is the simplest signaling agent in biology, responsible for ripening apples and other fruits.
Propane
C3H8
CnH2n+2
Propene
C3H6
CnH2n
Homologous formula for alkenes
(but not only for them)
CH3CH2
CH3 CH3
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
Due to the planar geometry of the double bond, two substituents can be located at either the same side of the double bond, or at the opposite sides of the double bond.
Note: cis-trans-isomerism is impossible if at least one carbon at the double bond has two identical substituents.
H
R
H
R
R
H
H
R
cis-isomer trans-isomer
R
R
H
R
R
R
H
R
identical
Nomenclature of alkenes
Same rules as for alkanes, except:
1. Replace “-ane” with “-ene”2. The principal carbon chain must contain the double bond3. Numbering of the principal chain:The double bond must have the lowest number4. In the chemical name, indicate position of the double bond
CH2 CH3
CH37
1
2
3 4
5
6
2-propyl-1-heptene
5. If the compound contains more, than one double bond, replace “-ene” with “-diene”, “-triene” etc.6. Indicate stereochemistry (cis- or trans-)
Step 1. For each double bond, assign relative priorities of attached fragments
Step 2. Find highest priority fragments at each carbon. If they are “cis-”, the isomer is “Z”. If they are “trans”, the isomer is “E”.
E-, Z- Nomenclature
CH3
Cl
Br
H
Cl
CH3
Br
H
E-isomer Z-isomer
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
Dipole moments are a little higher, than for alkanes due to polarization of -bonds
Dipole moments for cis-isomers are normally higher, than for trans-isomers
CH2CH3 CH3
CH3
= 0.46D = 0.085D
H
Cl
Cl
H
Cl
H
Cl
H = 0 > 0
More substituted double bonds are more stable, than less substituted double bonds.
Trans-isomers are more stable, than cis-isomers.
To compare relative stability of isomeric alkenes with higher accuracy, heats of hydrogenation can be used instead of heats of combustion.
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
Zaitsev’s Rule: Hydrogen comes off the carbon with the leastnumber of hydrogens attached
Dehydration of alcohols
CH3
CH3
OH
CH2
CH3
+
CH3H
CH3 H
CH3CH3
H H
+ Major products
Minor product
H+
CH3
CH3
O+
H H
CH3
CH+
CH3
H
Dehydrohalogenation
Dehydrogenation
CH2 CH2
Br
H
CH3O -
CH2 CH2 + + CH3OHBr -
CH2 CH2 + H2CH3 CH3
Catalyst, heat
Catalyst = Pt or Ni or other catalysts
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
R1
R2
R3
R4
+ X - Y
R3
R4
Y
X
R2
R1
Addition to double bonds in general
CH2 CH2 + H2 CH3 CH3
Catalyst = Pt or Ni or other catalysts
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
Delocalization of positive charge increases stability of carbocations.So far, we compared stabilities of primary, secondary and tertiaryalkyl carbocations, based on the to p interaction.
Participation of higher in energy -orbitals strongly increases stabilityof carbocations at double bonds due to the to p interaction.
Benzylic > allylic > alkyl > vinylic
The first carbocation (triphenylmethyl cation) was synthesizedin 1901 by Noris and Kehrmann.
C+
Existence of carbocations was proved by George Olah (NMR, X-Ray)and brought him the Nobel Prize in 1994.
Carbocations are considered alongside with carboanions, carbenes andradicals among the most reactive intermediates in organic chemistry
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
CH2
R
CH3
R
Cl
CH2 CH2
R
Cl
a
bWhich path is preferred?HCl
Path a
Path b
CH2
R
CH2
R
H+
H+
CH CH3
R
CH3
R
Cl
CH2 CH2
R R
Cl
+
+
Cl-
Cl-
secondary
primary
Markovnikov’s rule (1896)
The halogen of a hydrogen halide attaches to the carbon of the alkene bearing the fewer number of hydrogens and greater number of carbons
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
For the reaction of hydration the Markovnikov’s rule works the best
CH3
CH3
CH2 + H2OH+
CH3
CH3
CH3
OH
+
CH3
CH3
OH
Exclusive product
CH2 CH2 + H2OH3PO4
300 oCCH3 OH
600,000,000 lb of ethanol is produced annually in the US by this reaction.
Hydration is the reaction of dehydration, going backwards.
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
There are several mechanisms of halogenation of alkenes.We will consider electrophilic halogenation.
Consequence of the bromonium cation formation: anti-stereoselectivity of addition
Br+
H
H
Br
Br
+ Br Br- Br -
Step 1
Step 2
Electrophilic attack
Bromonium cation
Nucleophilic addition (anti-)
Br+
H
H
Br -Trans-product
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
This reaction was introduced by Herbert Brown in 1955 andbrought him the Nobel Prize in 1979. Hydroboration, followed by oxidation is used when we need to perform hydration of a double bond against the Markovnikov’s rule.
CH2
R
CH3
R
OH
CH2 CH2
R
OH
a
bWhich path is preferred?
1. BH3
2. H2O2, OH-
Path a
Path b
CH2
R
CH2
R
CH CH2
R
CH3
R
BH2
CH CH2
R R
BH2
+
+
More stabletransition state
BH2 H
BH2H
H2B H
BH2H
CH3
R
OH
R
OH
H2O2, OH-
H2O2, OH-
The intermediate R-BH2 normally reacts further with another molecule of alkene until a trialkylborane R3B is formed. It does not change the reaction product, but enhancesregioselectivity due to the steric hindrance around the fragment R.
The mechanism of hydroboration determines its syn-stereoselectivity:
1. BH3
2. H2O2, OH-CH3
CH3
CH3
CH3
H
OH
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
In alkynes, one- and two - bonds form a triple bond.Now we need to choose a hybridization to describe systems, containing a triple bond.
The valence shell of the atom of carbon has one s-orbital and three p-orbitals.
When the carbon is bonded by two-bonds, each of two p-orbitals participates in the formation of this - bond, and the remainingone s-orbital and one p-orbital are equally involved in the formation of -bonds, giving rise to the sp-hybridization state.
The presence of sp-hybridized carbons is characteristic for alkynes.
Outline1. Bonding in Alkenes2. Nomenclature of Alkenes. Cis-trans-isomerism3. Physical properties of Alkenes4. Preparation of Alkenes5. Catalytic Hydrogenation of Alkenes6. Carbocations7. Hydrohalogenation of Alkenes8. Acid-catalyzed Hydration of Alkenes9. Halogenation of Alkenes10. Hydroboration-Oxidation of Alkenes11. Polymers12. Bonding in Alkynes13. Acidity of Alkynes
The relatively high acidity of alkynes significantly affects their chemical properties.
C C HR1
NaNH2C C NaR1
C CR1 Na+-
+
C C HR1
RMgBrC C MgBrR1 + RH
C C HR1
CuCl, NH3C C CuR1
A colored precipitate,explosive when dry
Same reaction takes place, if Cu is replaced with Ag
An acetylenide (a strong nucleophile)