CHAPTER 11 Alkenes and Infrared Spectroscopy

1
CHAPTER 11Alkenes and Infrared Spectroscopy
2
Naming the Alkenes
11-1
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.
3
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.
4
Rule 2 Indicate the location of the double bond
in the 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.
5
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
6
Rule 4 Identify any cis/trans stereoisomers.
These are examples of diastereomers
stereoisomers that are not mirror inages of each
other.
In cycloalkenes, trans isomers are stable only
for the larger ring sizes.
7
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 the same side of the
double bond the molecule is a Z isomer. If they
are on opposite sides of the double bond the
molecule is an E isomer.
8
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.
9
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.
10
Structure and Bonding in Ethene The Pi Bond
11-2
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 ? bonds to two hydrogen atoms
and to the other carbon atom. The remaining
unhybridized p orbital on each carbon overlap to
form a ? bond. The electron density of the ?
bond is equally distributed above and below the
plane of the molecule.
11
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.
12
The relative energies of the bonding and
antibonding ? and ? orbitals can be summarized
13
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 isomeration 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.
14
The measured activation energy for this process
is about 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.
15
Physical Properties of Alkenes
11-3
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.
16
(No Transcript)
17
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.
18
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.
19
Nuclear Magnetic Resonance of Alkenes
11-4
The pi electrons exert a deshielding effect on
alkenyl hydrogens. The proton NMR spectra of
trans-2,2,5,5-tetramethyl-3-hexene shows only two
peaks. The methyl protons and alkenyl protons
are too far from each other to produce detectable
coupling.
The resonanance of the allenyl protons at 5.30
ppm is typical of hydrogens bound to alkenyl
carbons. Terminal alkenyl hydrogens (RRCCH2)
resonate at 4.6-5.0 ppm. Internal alkenyl
hydrogens (RCHCHR) resonate at 5.2-5.7 ppm
20
The deshielding for alkenyl hydrogens has two
causes. Less important is the electron
withdrawing effect of the sp2 hybridized
carbon. More important is the effect of the
external magnetic field on the ? cloud of
electrons. The ? electrons are forced to assume
a circular motion when the magnetic field is
perpendicular to the double bond axis. The
circular motion of the ? electrons induces a
second magnetic field which reinforces the
external field.
21
Cis coupling through the double bond is different
from trans. Unsymmetrically substituted double
bonds lead to non-equivalent alkenyl hydrogens
which leads to spin-spin coupling. Within a set
of cis/trans isomers, the coupling constant, J,
for the trans isomer is always larger than for
the cis isomer.
22
Coupling between hydrogens on adjacent carbons is
called vicinal. Coupling between hydrogens on the
same carbon is called geminal, and is usually
small in alkenes. Coupling to neighboring alkyl
hydrogens (allylic) and 1,4- or long range
coupling is also possible which may produce
complicated spectral patterns.
23
Further coupling leads to more complex spectra.
In 3,3-dimethyl-1-butene Ha resonates at 5.86 ppm
in the form of a doublet with two relatively
large coupling constants (Jab18 Hz, Jac10.5
Hz). Hb and Hc also absorb as doubles due to
their coupling to Ha and their mutual coupling
(Jbc 1.5 Hz)
24
In 1-pentene, there is additional coupling to the
attached alkyl group. In addition, the double
bond causes a slight deshielding of the allylic
CH2 group. The coupling between the allylic
hydrogens and the neighboring alkenyl hydrogen is
about the same as the coupling with the two CH2
hydrogens on the other side. As a result, the
multiplet for the allylic CH2 group appears as a
quartet.
25
Alkenyl carbons are deshielded in 13C
NMR. Relative to alkanes, corresponding alkene
carbons absorb at about 100 ppm lower field.
26
Infrared Spectroscopy
11-5
IR Spectroscopy measures the vibrational
excitation of atoms around the bonds that connect
them. The positions of the absorption lines are
related to the types of functional groups
present. The IR spectrum as a whole is unique for
each individual substance.
27
Absorption of infrared light causes molecular
vibrations. The range of the electromagnetic
spectrum just below visible light is the infrared
region. Absorption of light of this wavelength
causes vibrational excitation of the bonds in a
molecule. Middle infrared light (?2.5-16.7 µm,
or 600-4000 cm-1) has energys from 1 to 10 kcal
mol-1 and is most useful to the chemist.
28
Hookes law relates the parameters affecting the
vibrational frequency of two weights connected by
a spring. The vibrational frequency of two atoms
connected by a bond is also accurately described
by Hookes law
29
The infrared spectrum of a molecule is
significantly more complex than the vibrational
frequencies of all of the bonds present,
however. Various bending motions, and
combinations of stretching and bending are also
excited by IR radiation which leads to
complicated patterns.
30
Fortunatelly,the vibrational bands of many
functional groups appear at characteristic
wavenumbers, and the entire IR spectrum of a
given compound is unique and can be distinguished
from that of any other substance.
31
Functional groups have typical infrared
absorptions.
32
Compare the IR spectra of pentane and hexane
Above 1500 cm-1 the C-H stretching absorptions
typical of alkanes can be seen. Since no function
groups are present, no absorptions are seen in
the region from 2840-3000 cm-1. Below 1500 cm-1,
the fingerprint region, C-C stretching and C-C
and C-H bending motions absorb to give
complicated patterns All saturated hydrocarbons
show peaks at 1460, 1380, and 730 cm-1.
33
Now compare hexane to 1-hexene
An additional peak at 3080 cm-1 can be seen which
is due to the stronger Csp2-H bond. The CC
stretching band should appear between 1620 and
1680 cm-1 and is seen at 1640 cm-1. The two
signals at 915 and 995 cm-1 are characteristic of
a terminal alkene.
34
Several other strong bending modes are
characteristic for the substitution patterns in
alkenes
35
The O-H stretching absorption is the most
characteristic band in the IR spectra of
alcohols. This appears as a broad band over the
range 3200-3650 cm-1. This due to hydrogen
bonding. Dry, dilute alcohols show a sharp narrow
band in the range 3620-3650 cm-1. Haloalkane C-X
stretching frequencies are too low (lt800 cm-1) to
be useful for characterization.
36
Degree of Unsaturation Another Aid to
Identifying Molecular Structure
11-6
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.
37
A fully saturated hydrocarbon will have 2n2
hydrogen atoms for every n carbon atoms. 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.
38
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
39
Catalytic Hydrogenation of Alkenes Relative
Stability of Double Bonds
11-7
Hydrogen gas and an alkene will react when mixed
in the presence of a catalyst such is 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.
40
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 lt cis-2-butene
lt trans-2-butene.
41
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).
42
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 cycloalklene is
trans-cyclooctene which is 9.2 kcal mol-1 less
stable than the cis isomer is highly twisted.
43
Preparation of Alkenes from Haloalkenes and Alkyl
Sulfonates Bimolecular Elimination Revisited
11-8
Two approaches to the synthesis of alkenes are
elimination reactions and the dehydration of
alcohols.
44
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 CC
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.
45
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-2-butene 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.
46
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.
47
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 inteference with
the bulky base. An E2 reaction that generates the
thermodynamically less favored isomer is said to
follow the Hofmann rule.
48
(No Transcript)
49
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 to some extent by the relative
thermodynamic stabilities of the products, the
more stable trans double bond being formed
preferentially. Complete selectivity is rare in
E2 reactions, however.
50
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.
51
Preparation of Alkenes by Dehydration of Alcohols
11-9
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.
52
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.
53
The 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.
54
Important Concepts
11

• Alkenes - Unsaturated molecules.
• IUPAC names derived from longest chain containing
  the double bond as the stem.
• Double bond isomers terminal, internal, cis,
  and trans.
• Tri and tetra substituted alkenes named according
  to the E,Z system.
• Double Bond - Consists of a ? bond and a ? bond.
• ? 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)

55
Important Concepts
11

• Alkene Properties
• Flat, sp2 hybridization.
• Dipoles possible.
• Alkenyl hydrogen is relatively acidic.
• NMR
• Alkenyl hydrogens and carbons appear at low
  field
• 1H (d 4.6 - 5.7 ppm) 13C (d 100 -140 ppm)
• Jtrans gt Jcis jgeminal very small Jallylic
  variable, small.
• 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 (lt1500 cm-1)

56
Important Concepts
11

• Alkane IR
• C-H Stretching 2840 to 3000 cm-1
• CC 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
• Degree of Unsaturation - Number of rings
  number of ? bonds
• Degree of unsaturation (Hsat Hactual)/2
• Hsat 2nC 2 NX NN (disregard oxygen and
  sulfur)

57
Important Concepts
11

• Heats of Hydrogenation Indicate relative
  stability of isomeric alkenes.
• Stability decreases with decreasing substitution
• Trans isomers are more stable than cis.
• Eliminations of Haloalkanes (and other alkyl
  derivatives)
• 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)

58
Important Concepts
11

• Dehydration of Alcohols - Dehydration in the
  presence of strong acid results in a mixture of
  products (major constituent is the most stable
  alkene).