Transcript Double Bond

CHAPTER 11
Alkenes; Infrared Spectroscopy and
Mass Spectroscopy
11-1 Naming the Alkenes
Alkenes are characterized by the presence of a double bond.
The general formula of an alkene is CnH2n, the same as for a
cycloalkane.
Common nomenclature for alkenes replaces the corresponding
alkane suffix –ane with -ylene.
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IUPAC nomenclature replaces the alkane suffix –ane with –ene
(ethene, propene, etc.).
Rules for naming alkenes:
Rule 1: Find the longest chain that includes both carbons
of the double bond.
Rule 2: Indicate the location of the double bond in main chain by
number starting at the end of the chain closest to the double bond.
The two double bond carbons in cycloalkenes are numbered 1
and 2.
Alkenes with the same formula but differing in the location of the
double bond are called double-bond isomers.
A 1-alkene is referred to as a terminal alkene; the others are called
internal.
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Rule 3: Add substituents and their positions as prefixes to the
alkene stem.
If the stem is symmetric, begin from the end giving the
first substituent the lowest possible number.
Rule 4: Identify any cis/trans stereoisomers. These are
examples of diastereomers, or stereoisomers that are not
mirror images of each other.
In cycloalkenes, trans isomers are stable only for the larger ring
sizes.
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Rule 5: Use the IUPAC E,Z system when cis/trans labels are
not applicable (3 or 4 different substituents attached to the
double-bond carbons).
Apply the sequence rules devised for R,S substituent
priorities to the two groups on each double-bond carbon.
If the two groups of highest priority are on opposite sides
of the double bond, the molecule is an E isomer. If they
are on the same side of the double bond, the molecule is a
Z isomer.
Rule 6: Give the hydroxy functional group precedence over
the double bond in numbering a chain.
Alcohols containing double bonds are named alkenols.
The stem incorporating both functions is numbered to give
the OH carbon the lowest possible assignment.
The last “e” and “alkene” is dropped in naming alkenols.
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Rule 7: Substituents containing a double bond are named
alkenyl.
The numbering of a substituent chain containing a double bond
begins at the point of attachment to the basic stem.
11-2 Structure and Bonding in Ethene: The Pi Bond
The double bond consists of sigma and pi components.
Ethene is planar. It contains two trigonal carbon atoms having
bond angles close to 120o.
The hybridization of the carbon atoms is best described as sp2.
The three sp2 orbital on each carbon form three  bonds to two
hydrogen atoms and to the other carbon atom.
The remaining unhybridized p orbital on each carbon overlap to
form one  bond. The electron density of the  bond is equally
distributed above and below the plane of the molecule.
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The pi bond in ethene is relatively weak.
The overlap of the two sp2 orbitals to form the  bond connecting
the two carbon atoms is much greater than the overlap of the two
p orbitals to form the  bond.
As a consequence, the  bond contributes more to the double
bond strength than does the  bond.
The relative energies of the bonding and antibonding  and 
orbitals can be summarized:
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Thermal isomerization allows us to measure the
strength of the pi bond.
Thermal isomerization involves the interconversion of the cis form
and the trans form of a double bond at high temperature.
During the isomerization process, the  bond between the two
carbon atoms is broken and the p orbitals on the two carbon
atoms become perpendicular to each other (transition state).
The activation energy for this process is roughly the same as the
 contribution to the double-bond energy.
Measured activation energy for this process is ~ 65 kcal mol-1.
The total energy of the ethene double bond is 173 kcal mol-1,
which means the  bond energy must be about 108 kcal mol-1.
The alkenyl hydrogens are more tightly held in alkenes than the
C-H bonds in the corresponding alkanes. As a result, addition to
the weaker  bond characterizes the reactivity of alkenes in
radical reactions, rather than hydrogen abstraction.
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11-3 Physical Properties of Alkenes
The boiling points of alkenes are very
similar to the corresponding alkanes.
The melting points of alkenes are lower
than those of the corresponding alkanes.
The presence of a trans double bond
lowers the melting point slightly, while
the presence of a cis double bond
lowers the melting point significantly
more.
The effect of a double bond on melting
point is due to the disruption of
packing of molecules in the crystal
lattice compared to the packing of
saturated molecules.
cis double bonds often exhibit weak dipolar character.
The degree of s orbital character in a sp2 carbon is larger than in
an sp3 carbon (alkane) which makes the sp2 carbon a weak
electron withdrawing group.
trans double bonds, on the other hand, generally have little
dipolar nature since the dipoles involved oppose each other.
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The electron-attracting character of the sp2 carbon also accounts
for the increased acidity of the alkenyl hydrogen, compared to its
saturated counterpart.
Ethene is still a very poor source of protons compared to
alcohols or carboxylic acids.
11-6 Degree of Unsaturation
Knowledge of the degree of unsaturation, defined as the
numbers of rings and  bonds present in a molecule, is useful
information when determining the structure of a compound.
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A fully saturated hydrocarbon will have 2n+2 hydrogen atoms for
every n carbon atom.
Consider the compounds in the class C5H8. This compound is 4
hydrogens short of being saturated, so its degree of
unsaturation is 4/2 = 2.
All molecules having this formula must have a combination of
rings and  bonds adding up to 2.
The presence of heteroatoms may affect the calculation.
The presence of a halogen atom decreases the number of
hydrogens by one.
The presence of a nitrogen atom increases the number of
hydrogens by one.
The presence of oxygen or sulfur does not affect the number
of hydrogens.
To determine the degree of unsaturation:
Step 1: Hsat = 2nC + 2 – nX + nN
Step 2: Degree of unsaturation = (Hsat – Hactual)/2
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11-7 Catalytic Hydrogenation of Alkenes: Relative
Stability of Double Bonds
Hydrogen gas and an alkene will react when mixed in the
presence of a catalyst such as platinum or palladium.
Two hydrogen atoms are added to the alkene in a reaction called
hydrogenation, which is very exothermic.
The heat released is called the “heat of hydrogenation” and has a
typical value of about -30 kcal mol-1 per double bond.
The heat of hydrogenation is a measure of stability.
The relative stabilities of related alkenes can be determined by
measuring their heats of combustion.
The thermodynamic stability of the butenes increases in the
order: 1-butene < cis-2-butene < trans-2-butene.
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Highly substituted alkenes are most stable; trans
isomers are more stable than cis.
The relative stability of the
alkenes increases with
increasing substitution
(hyperconjugation), and
trans isomers are usually
more stable than cis isomers
(crowding).
An exception to this stability rule is in medium-ring and smaller
cycloalkenes. The trans isomers of cycloalkenes are much more
strained than are the corresponding cis isomers.
The smallest isolated simple trans cycloalkene is transcyclooctene which is 9.2 kcal mol-1 less stable than the cis isomer
is very twisted.
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11-8 Preparation of Alkenes from Haloalkenes and Alkyl
Sulfonates: Bimolecular Elimination Revisited
Two approaches to the synthesis of alkenes are elimination
reactions and the dehydration of alcohols.
Regioselectivity in E2 reactions depends on the base.
Haloalkanes (or alkyl sulfonates) in the presence of strong base
can undergo elimination of HX with the simultaneous formation of
a C=C double bond.
In the cases where the hydrogen atom can be removed from
more than one carbon atom in the structure, the regioselectivity
of the reaction can be controlled to a limited extent.
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Consider the dehydrobromination of 2-bromo-2-methylbutane.
Elimination of HBr proceeds through attack by
the base on one of the neighboring hydrogens
situated anti to the leaving group.
The transition state leading to 2-methyl-2butene is slightly more stabilized than the one
leading to 2-methyl-1-butene.
The more stable product is formed faster
because the structure of the transition state
resembles that of the products.
Elimination reactions that lead to the more highly substituted
alkene are said to follow the Saytzev rule.
The double bond preferentially forms between the carbon that
contained the leaving group and the most highly substituted
adjacent carbon that bears a hydrogen.
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When a more hindered base is used, more of the
thermodynamically less favored terminal alkene is generated.
Removal of a secondary hydrogen (C3 in the starting bromide) is
sterically more difficult than abstracting a more exposed methyl
hydrogen when a hindered base is used.
The transition state leading to the more stable product is
increased in energy by steric interference with the bulky base.
An E2 reaction that generates the thermodynamically less favored
isomer is said to follow the Hofmann rule.
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E2 reactions often favor trans over cis.
The E2 reaction can lead to cis/trans alkene mixtures, in some
cases with selectivity.
This and related reactions appear to be controlled by the relative
thermodynamic stabilities of the products. The more stable
trans double bond is formed preferentially.
Complete selectivity is rare in E2 reactions, however.
Some E2 processes are stereospecific.
The preferred transition state
of elimination places the
proton to be removed and the
leaving group anti with
respect to each other.
When Z or E isomers are
possible, stereospecific
reactions may occur.
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11-9 Preparation of Alkenes by Dehydration of Alcohols
When alcohols are treated
with mineral acid at
elevated temperatures,
dehydration (E1 or E2)
occurs, resulting in alkene
formation.
As the hydroxy
bearing carbon
becomes more
substituted, the
ease of
elimination of
water increases.
Secondary and tertiary alcohols dehydrate by an E1 mechanism.
The protonated hydroxy forms an alkyloxonium ion providing
a good leaving group: water.
Loss of water forms a secondary or tertiary carbocation.
Deprotonation forms the alkene.
Carbocation side reactions (hydrogen shifts, alkyl shifts, etc.)
are possible.
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Thermodynamically most stable alkene or alkene mixture usually
results from unimolecular dehydration in the presence of acid.
Whenever possible, the most highly substituted system is
generated.
Trans-substituted alkenes predominate if there is a choice.
Treatment of primary alcohols with mineral acids at high
temperatures also leads to alkenes.
The reaction of propanol yields propene.
The reaction proceeds by protonation of the alcohol, followed by
attack by HSO4- or another alcohol molecule (E2 reaction) to
remove a proton from one carbon atom and water from the other.
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Important Concepts
1. Alkenes – Unsaturated molecules
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IUPAC names are derived from the longest chain
containing the double bond as the stem.
Double bond isomers: terminal, internal, cis, and
trans
Tri- and tetra-substituted alkenes are named
according to the E,Z system.
2. Double Bond – Consists of a  bond and a  bond
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 bond: overlap of two sp2 hybrid lobes on carbon
 bond: overlap of two remaining p orbitals
 bond E (~65 kcal/mol);  bond E (~108 kcal/mol)
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Important Concepts
3. Alkene Properties –
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Flat, sp2 hybridization
Dipoles possible
Alkenyl hydrogen is relatively acidic.
4. NMR –
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Alkenyl hydrogens and carbons appear at low field:
1H (δ = 4.6 - 5.7 ppm); 13C (δ = 100 -140 ppm)
Jtrans > Jcis; jgeminal very small; Jallylic variable, small.
5. IR – Measures vibration excitation
• 1-10 kcal/mol (2.5-16.7 μm; 600-4000 cm-1)
• Characteristic peaks for stretching, bending and
other vibrational modes
• Fingerprint region (<1500 cm-1)
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Important Concepts
6. Alkane IR –
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C-H Stretching: 2840 to 3000 cm-1
C=C Stretching: 1620 to 1680 cm-1
Alkenyl C-H Stretching: ~3100 cm-1
Bending Modes: below 1500 cm-1
Alcohols: Broad O-H stretch: between 3200 and
3650 cm-1
7. Degree of Unsaturation – Number of rings +
number of  bonds:
• Degree of unsaturation = (Hsat – Hactual)/2
• Hsat = 2nC + 2 – NX + NN (disregard oxygen and
sulfur)
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Important Concepts
8. Heats of Hydrogenation – indicate relative
stability of isomeric alkenes.
• Stability decreases with decreasing substitution.
• Trans isomers are more stable than cis.
9. Eliminations of Haloalkanes (and other
alkyl derivatives) –
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Follow the Sayzex rule (non-bulky base, internal
alkene formation) or the Hofmann rule (bulky base,
terminal alkene formation).
Trans alkene products predominate over cis.
Elimination is stereospecific (dictated by the anti
transition state).
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Important Concepts
10. Dehydration of Alcohols – Dehydration in the
presence of strong acid results in a mixture of products
(major constituent is the most stable alkene).
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