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Chapter 10Alkenes
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Alkenes
Introduction—Structure and Bonding
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Alkenes
Strength of the p Bonding
Restricted rotation:
Stereoisomerism:
Stability:
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Alkenes
• Cycloalkenes having fewer than eight carbon atoms have a cis geometry.
• trans-Cyclooctene is the smallest isolable trans cycloalkene
less stable than cis-cyclooctene, making it one of the few alkenes having a higher energy trans isomer.
Introduction—Structure and Bonding
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Alkenes
• general structural formula CnH2n : acyclic alkenes, Cycloalkanes• Each bond or ring removes two hydrogen atoms from a molecule, and this
introduces one degree of unsaturation.• The number of degrees of unsaturation for a given molecular formula can be
calculated by comparing the actual number of H atoms in a compound to the maximum number of H atoms possible for the number of carbons present if the molecule were a straight chain alkane.
• This procedure gives the total number of rings and/or bonds in a molecule.
Calculating Degrees of Unsaturation : n-m for CnH2(n-m)
Quick assessment of molecular structure from molecular formula
n-m = # of rings + # of p bonds
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Alkenes
Calculating Degrees of Unsaturation w/ hetero atoms
Halogens (F, Cl, Br, I) - Add the number of halogens to the
number of hydrogens in the formula.Oxygen – Ignore the number of oxygens in the fomula.Nitrogen – Subtract the number of nitrogens from the number of hydrogens in the formula.
Examples. Deduce the number of degrees of unsaturation (d.u.) in the following molecular formulas and suggest one possible structure for each:
(a) C6H11Cl; (b) C5H8O; (c) C8H9N.
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AlkenesNomenclature of Alkenes: alkenes are identified by the suffix –ene.
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AlkenesNomenclature of Alkenes• Compounds with two double bonds : the suffix “–adiene”.• three double bonds trienes, and so forth.• Always choose the longest chain that contains both atoms of the
double bond.• In cycloalkenes, the double bond is located between C1 and C2, and
the “1” is usually omitted in the name. • The ring is numbered clockwise or counterclockwise to give the first
substituent the lower number.• Compounds that contain both a double bond and a hydroxy group
are named as alkenols and the chain (or ring) is numbered to give the OH group the lower number.
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Nomenclature of Alkenes : stereoisomers
Alkenes
Zusammen (together)Entgegen (opposite)
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Alkenes
• Some alkene or alkenyl substituents have common names.• The simplest alkene, CH2=CH2, named in the IUPAC system as
ethene, is often called ethylene.
Nomenclature of Alkenes
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Alkenes
• physical properties are similar to alkanes of comparable molecular weight.
• Alkenes have low melting points and boiling points.• Melting and boiling points increase as the number of carbons
increases because of increased surface area.• Alkenes are soluble in organic solvents and insoluble in water.• The C—C single bond between an alkyl group and one of the
double bond carbons of an alkene is slightly polar because the sp3 hybridized alkyl carbon donates electron density to the sp2 hybridized alkenyl carbon.
Physical Properties
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Alkenes
• A consequence of this dipole is that cis and trans isomeric alkenes often have somewhat different physical properties.
• cis-2-Butene has a higher boiling point (4°C) than trans-2-butene (1°C).
• In the cis isomer, the two Csp3—Csp
2 bond dipoles reinforce each other, yielding a small net molecular dipole. In the trans isomer, the two bond dipoles cancel.
Physical Properties
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Alkenes
Interesting Alkenes
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Alkenes
• Triacyl glycerols are hydrolyzed to glycerol and three fatty acids of general structure RCOOH.
Lipids
• As the number of double bonds in the fatty acid increases, the melting point decreases.
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Alkenes
Lipids
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Alkenes
• Fats and oils are triglycerols with different physical properties.
• Fats have higher melting points—they are solids at room temperature. Usually from animal sources
• Oils have lower melting points—they are liquids at room temperature. Usually from vegitable sources
• The identity of the three fatty acids in the triacylglycerol determines whether it is a fat or an oil.
• An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains.
Lipids
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Alkenes
• Increasing the number of double bonds in the fatty acid side chains decreases the melting point of the triacylglycerol.
• Fats are derived from fatty acids having few or no double bonds.
• Oils are derived from fatty acids having a larger number of double bonds.
• An exception to this generalization is coconut oil, which is largely composed of saturated alkyl side chains.
Lipids
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Alkenes
alkenes can be prepared from alkyl halides and alcohols via elimination reactions.
Preparation of Alkenes
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Alkenes
• elimination reactions are stereoselective and regioselective.
Preparation of Alkenes
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Reactions of Alkenes
• The characteristic reaction of alkenes is addition—the bond is broken and two new bonds are formed.
Introduction to Addition Reactions
• Alkenes are electron rich. • Because alkenes are electron rich, simple alkenes do not react
with nucleophiles or bases, reagents that are themselves electron rich. Alkenes react with electrophiles.
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• Stereochemical outcome of addition to alkenes
Introduction to Addition Reactions
When the addition is selective, only one set of enantiomers forms.
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Addition Reactions to Alkenes
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Alkenes
Hydrohalogenation—Electrophilic Addition of HX
Addition reactions are exothermic
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Alkenes
Hydrohalogenation—Electrophilic Addition of HX
What about the stereochemistry & Regiochemistry of the reaction ?
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Alkenes
• The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions.
Mechanism of Hydrohalogenation
C C
CH3 CH3
H H
H Br
slow
C
CH3
H
C CH3
H
H
++ Br
_
Carbocation
1st new bond
Step 1 Electrophilic attack of H+ on the bond
Step 2 Nucleophilic attack of Br- on carbocation
+
_Br
C CH3
H
H
C
Br
H
CH3
C CH3
H
HH
CH3
C
fast
2nd new bond
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AlkenesHydrohalogenation—Electrophilic Addition of HX
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Alkenes
Markovnikov’s Rule : Regioselectivity of addition
Markovnikov’s rule (1869) in the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms to begin with.
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Alkenes
• The basis of Markovnikov’s rule : the formation of a carbocation in the rate-determining step of the mechanism.
Markovnikov’s Rule
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Alkenes
According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process.
Hydrohalogenation—Markovnikov’s Rule
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• The Hammond postulate relates reaction rate to stability. • It provides a quantitative estimate of the energy of a transition
state.• The Hammond postulate : the transition state of a reaction
resembles the structure of the species (reactant or product) to which it is closer in energy.
in an endothermic step, TS resembles the products,in an exothermic step, TS resembles the reactants.
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Alkenes
According to the Hammond postulate, Path [2] is faster because formation of the carbocation is an endothermic process.
Hydrohalogenation—Markovnikov’s Rule
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NOT 1-Chloropropane2-Chloropropane ONLY
CCH3
H
C
H
H
H Cl
C C
CH3
H
H
H
HNOT
Br
CH3
1-bromo-1-methylcyclohexane ONLY
CH3
Br
CCH3
H
Cl
C
H
H
H
1-methylcyclohexene
Propene
diethylether
HBr
diethylether
HCl
CH3
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Alkenes
• Recall that trigonal planar atoms react with reagents from two directions with equal probability.
• Achiral starting materials yield achiral products or racemic mixture.
Hydrohalogenation—Reaction Stereochemistry
A racemic mixture
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Hydrohalogenation—Reaction Stereochemistry
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Alkenes
Hydrohalogenation—Summary
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Alkenes
• Hydration forms an alcohol.
Hydration—Electrophilic Addition of Water
- Reaction mechanism is very siminar to hydrohalogenation -
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Alkenes
• Alcohols add to alkenes, forming ethers by the same mechanism in presence of acid.
Hydration—Electrophilic Addition of Alcohols
• Note that there are three consequences to the formation of carbocation intermediates:1. Markovnikov’s rule holds.2. Addition of H and OH occurs in both syn and anti
fashion.3. Carbocation rearrangements can occur.
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Alkenes
• Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal dihalide.
Halogenation—Addition of Halogen
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Alkenes
• Halogens add to bonds because halogens are polarizable.
• Two facts demonstrate that halogenation follows a different mechanism from that of hydrohalogenation or hydration.
No rearrangements occurOnly anti addition of X2 is observed
These facts suggest that carbocations are not intermediates.
Halogenation—Addition of Halogen
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C CH H
CH3 CH3
C CH CH3
CH3 H
Br2
Br2
C
Br
C
HCH3
CH3
Br
C
Br
C
H
CH3
CH3
H
Br
CC
H
H
CH3
CH3
Br
Br
C
Br
C
H
CH3
CH3
H
Br
+
+
Racemic mixture of enantiomers
Achiral meso compound
cis-2-Butene
trans-2-Butene
Addition reaction of Br2 to alkenes : the fact!
How can we explain this outcome ?
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AlkenesMechanism of Halogenation—Addition of Halogen
Carbocations are unstable because they have only six electrons around carbon.
Halonium ions are unstable because of ring strain.
Step 2
Step 1
C C
X
X
Fast
:X:..
.. _
_..
..:X:+
X
CC
+....
Bridged Halonium ionStrained ring
Slow
....
+X
CC
..
......: :X X
C C
: :
..
..
: :
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CCH3
CH3
F
C
HCH3
Br:
SbF5
SbF5
SO2(l)
Br
CH3
CH3 CH3
H
+SbF6
-
Non-classical carbocation By George Olah
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::..
:
C C
X X:: ....
..
..
X
CC
+....
Slow
....
+X
CC
+ :X:..
.. _
..
Fast
Step 1
Step 2
H2O:
H2O
X
CC+
(Step 3)protontransfer X
CC
HO
Halohydrin
Even though X¯ is formed in step [1] of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X¯ is the nucleophile.
Halohydrin Formation : addition of X-OH
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Alkenes
Halohydrin Formation• Bromohydrins are also formed with
N-bromosuccinimide (NBS) in aqueous DMSO [(CH3)2S=O].
• In H2O, NBS decomposes to form Br2, which then goes on to form a bromohydrin by the same reaction mechanism.
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AlkenesHalohydrin Formation : Selectivity
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Halohydrin Formation
application
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Alkenes
Halohydrin Formation
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Alkenes
Hydroboration—Oxidation
Hydroboration—oxidation is a two-step reaction sequence that converts an alkene into an alcohol.
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Alkenes
Hydroboration—Oxidation
Hydroboration—oxidation results in the addition of H2O to an alkene.
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Alkenes
Hydroboration—OxidationBH3 is a reactive gas that exists mostly as a dimer, diborane (B2H6). Borane is a strong Lewis acid that reacts readily with Lewis bases.
The first step in hydroboration
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Alkenes
Hydroboration—Oxidation• The proposed mechanism involves concerted addition of
H and BH2 from the same side of the planar double bond:
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Alkenes
Hydroboration—Oxidation
BH3 can react with three equivalents of alkene to form a trialkylborane.
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Alkenes
Hydroboration—Oxidation
9-borabicyclo[3.3.1]nonane (9-BBN)
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Hydroboration—Selectivity
With unsymmetrical alkenes, the boron atom bonds to the less substituted carbon atom.
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Alkenes
Hydroboration—Oxidation
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Alkenes
Hydroboration—Oxidation
• Oxidation replaces the C—B bond with a C—O bond, forming a new OH group with retention of configuration.
• The overall result of this two-step sequence is syn addition of the elements of H and OH to a double bond in an “anti-Markovnikov” fashion.
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Alkenes
Hydroboration—Oxidation
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Summary of Addition Reactions of Alkenes
Reagent Product Mechanism Regioselectivity Stereochemistry
HX Alkyl halide
2 step via rate-determining formation of R+.Rearrangements possible
Markovnikov Syn and anti addition
H2O Alcohol As above As above As above
X2 Vicinal (1,2) dihalide
2 step via rate-determining formation of bridged halonium ionNo rearrangements
- Anti additionStereospecific
X2/H2O Halohydrin (2-haloalcohol)
3 steps, but similar to halogenation
Markovnikov: X+ bonds to less substituted C.
Anti addition
BH3 (or equivalent), then H2O2/OH-
Alcohol 2 steps: one-step hydroboration, then oxidation.No rearrangements
Anti-Markovnikov
Syn addition in hydroboration step; retention of configuration in oxidation step
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keeping track of all the reactions?
Alkyl halides – Substitution and elimination (they have good leaving groups).Alcohols – As alkyl halides, but only if OH group has been converted to a good leaving group.Alkenes – Addition ( bond is easily broken)
Firstly, check the basic reaction types for a functional group. This provides an overall organization of reactions.
Then, learn the specific reagents for each reaction. This helps to classify the reagent according to its major properties.
Is it basic or acidic? Is it electrophilic or nucleophilic? Is it an oxidizing agent or a reducing agent?
Think mechanism ! – reasonable ones….
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AlkenesAlkenes in Organic Synthesis: combination of reactionsSuppose we wish to synthesize 1,2-dibromocyclohexane from cyclohexanol.
To solve this problem we must:
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AlkenesAlkenes in Organic SynthesisWorking backwards from the product to determine the starting material from which it is made is called retrosynthetic analysis.
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10.40, 10.46, 10.51, 10.52, 10.53, 10.57, 10.61, 10.63,
10.64
Homework
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Preview of Chapter 11
Alkynes
Extension of chapter 10
Preparation of alkynes
dehydrohalogenation
Reactions of alkynes
Addition reaction
Reaction of acetylide anions
