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Alkanes can also form cyclic structures

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Alkanes can also form cyclic structures Cyclopropane Cyclobutane Cyclopentane Cyclohexane General formula for cycloalkanes: CnH2n Can be conveniently represented ... – PowerPoint PPT presentation

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Title: Alkanes can also form cyclic structures


1
Alkanes can also form cyclic structures
Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
General formula for cycloalkanes CnH2n
Can be conveniently represented using line
segment formulae
2
Note
Cycloalkane nomenclature can be extended to
include substitution
Methylcyclohexane
1,3-Dimethylcyclohexane
3
Only one cycloalkane has a planar structure
cyclopropane
All others have non-planar structure
Ideal tetrahedral angle is 109.5o
sp3 hybridised carbons with bond angles very
different to 109.5o will be less stable (higher
in energy)
Bond angle approaching 60o
Cyclopropane
Cyclopropane is said to suffer from angle-strain
All C-H bonds in cyclopropane are eclipsed
4
Cyclopentane has almost zero angle-strain
To relieve torsional strain due to eclipsed C-H
bonds, cyclopentane relaxes into a non-planar
structure
One CH2 group out of the plane of the ring
5
Cyclohexane
A planar structure would have internal bond
angles of 120o and eclipsed C-H bonds
Actual structure relaxes into a chair
conformation This reduces the bond angle to 109o
  • Geometry about each Carbon very close to
    tetrahedral ideal
  • Angle strain zero

6
All C-H bonds staggered, i.e. torsional strain
zero
Newman projection along any C-C bond
The chair conformation contains two different
hydrogen environments
6 Equatorial Hydrogens
6 Axial Hydrogens
7
  • At temperatures below 230 K (-43?C)
  • can observe that two different types of hydrogen
    environment are present on cyclohexane

Above this temperature, observe only one hydrogen
environment
Reason cyclohexane molecules are not static
above 230 K i.e. exist in different
conformations
Undergo ring inversion
Boat conformation Exists in trace quantities
Note hydrogens axial in one chair conformation
equatorial in the other
8
Ball-and-stick model of boat cyclohexane
9
What if one of the cyclohexane hydrogens were
replaced by a methyl group?
Cyclohexane
Methylcyclohexane
The two chair conformations are no longer
equivalent
One has the methyl group in an axial position
one in an equatorial position
10
These interconvert by ring inversion (exist in
equilibrium)
Inversion proceeds through boat conformations
which exist in trace amounts
Can simplify diagram by omitting the C-H bonds
Methyl equatorial
Methyl axial
11
Sources of alkanes
  • Lower Mol. Mt. ( lt 5 Carbons) natural gas
  • Larger Mol. Wt. petroluem of crude oil

Crude oil complex mixture of hydrocarbons
Separated into fractions based on boiling point
ranges
Boiling point related to molecular weight, i.e to
number of carbons
  • lt 5 Carbons gases at room temperature
  • 5 Carbons lt 18 Carbons liquids at room
    temperature
  • gt 18 Carbons solids at room temperature

12
  • Increasing molecular size results in increasing
    tendency to form condensed phases
  • Associated with weak intermolecular interactions
    between alkane molecules
  • London dispersion forces weak electrostatic
    attractions between induced dipoles, i.e. are
  • Van der Waals forces between electrons of one
    molecule and nuclei of another
  • Extent of attraction increases with increasing
    molecular size
  • Weak interactions compared to hydrogen bonding or
    ionic bonding

13
Solubility of alkanes
  • Like dissolves like alkanes soluble in other
    alkanes, e.g petroleum
  • Soluble single liquid phase results upon mixing
  • Alkanes insoluble in water, i.e are hydrophobic
  • Mixtures with water separate into two liquid
    phases aqueous and hydrocarbon

14
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15
Reactions of alkanes
  • Relatively inert contain only stable C-C and C-H
    bonds
  • Some important reactions

1. Combustion, e.g.
2 C4H10 13 O2 ? 8 CO2 10 H2O
DH - 2877 kJ mol-1
i.e. exothermic
2. Steam reforming
CH4 H2O ? 3H2 CO
?
N2
?
NH3
CO2
? Urea

16
3. Reaction with halogens
17
4. Catalytic cracking
  • Fragmentation of alkanes into smaller molecules,
    e.g
  • The products of these reactions are a new type of
    hydrocarbon
  • They are said to be unsaturated compared to
    alkanes
  • i.e., have fewer Hydrogens per Carbon than
    alkanes, which are said to be saturated

18
Unsaturated hydrocarbons contain Carbon-Carbon
multiple bonds
Classes of unsaturated hydrocarbons are defined
by the types of Carbon-Carbon multiple bonds they
contain
Alkenes contain Carbon-Carbon double bonds
Carbon-Carbon double bond
Alkynes contain Carbon-Carbon triple bonds
Carbon-Carbon triple bond
Carbon valency of four maintained in alkenes and
alkynes
19
Alkenes
Older name Olefins
Characterised by presence of Carbon-Carbon double
bonds
General structural formula
Where R Hydrogen or alkyl group
Two Carbons and all four R groups are lying on
the same plane
Bond angles about each Carbon 120o
20
Three sp2 hybridised orbitals can be arrayed to
give trigonal geometry
The remaining 2pz orbital is orthogonal to the
three sp2 orbitals
View along z axis
View along xy plane
21
s bond formation results from overlap of two sp2
hybridised orbitals
s
A s-antibonding orbital is also formed, but this
is not occupied by electrons
Overlap of the pz orbitals results in formation
of a p bond
A p-antibonding orbital is also formed, but this
is not occupied by electrons
p
22
p orbital has a nodal plane on which lies on the
bond axis
p electron density lies above and below the plane
containing the two Carbons and four R groups
View along the Carbon-Carbon bond
Note
constitutes one p molecular orbital
i.e. constitutes one p bond when occupied
p
  • Carbon-Carbon double bond
  • One s bond One p bond

s
  • Both occupied by two electrons

23
  • Rotation about a Carbon-Carbon double bond
    requires opening up of the p bond
  • Requires large input of energy ( 268 kJ mol-1)
  • Hence, rotation about CC bonds does not occur at
    room temperature
  • Consequently, a new form of isomerism becomes
    possible for alkenes
  • Consider an alkene with one Hydrogen and one
    alkyl group R bonded to each Carbon
  • Two structures are possible

or
24
  • This form of isomerism is known as Cis-Trans
    isomerism
  • older term geometrical isomerism
  • The cis isomer is that with like groups on the
    same side of the CC
  • The trans isomer is that with like groups on
    opposite sides of the CC

Cis isomer
Trans isomer
25
First two members of the alkene series
Ethene (Ethylene)
Propene (Propylene)
Note
Nomenclature
  • Prefix indicates number of carbons
  • (eth 2C prop 3C etc.)
  • Suffix ene indicates presence of CC

26
Could have CC between C1 and C2 or between C2
and C3
Butene
1-Butene
2-Butene
Note
1. 1-Butene and 2-butene are structural isomers
2.
3. Number indicates starting point of the CC,
i.e. number through the CC
27
  • 4. Cis-Trans isomerism is possible for 2-butene
  • There are two isomeric 2-butenes

Trans-2-butene b.p. 3.7oC m.p. -139oC
Cis-2-butene b.p. 0.3oC m.p. -106oC
28
Some other alkenes
4-Methyl-2-pentene
2-Methyl-1-butene
1,3-Pentadiene
Cis-3-heptene
Trans-2-decene
29
Can have cycloalkenes
Cyclohexene
Cyclopentene
3-Methylcyclopentene
Note
1,4-Cyclohexadiene
30
Lycopene molecular structure
31
p electrons in alkenes are available to become
involved in bond formation processes
Essential processes in the synthesis of new
molecules formation of new covalent bonds
Covalent bonds pairs of electrons shared between
nuclei (atoms)
In the synthesis of organic molecules, a major
strategy for forming new covalent bonds is
donation of an electron pair by one molecular
species
to form a covalent bond with another, electron
deficient molecular species
Electron pair donating species are known as
nucleophiles
Electron pair accepting species are known as
electrophiles
Reaction of a nucleophile with an electrophile
results in the formation of a new covalent bond
32
Alkene hydrogenation
  • Addition of hydrogen (H2) across a CC

General reaction
  • Alkene p bond is lost, and two new C-H s bonds
    formed
  • Alkene converted to alkane
  • No reaction in absence of catalyst
  • Typical catalysts Palladium (Pd), Platinum (Pt),
    Nickel (Ni), Rhodium (Rh) or other metals
  • Catalysts usually supported on materials such as
    charcoal
  • E.g. Pd/C Palladium on Carbon

33
Examples
1-Hexene
Hexane
Hexane
1,3-Hexadiene
2-Methyl-1-butene
2-Methylbutane
34
  • Reaction occurs at the catalyst surface
  • H2 molecules adsorbed onto catalyst surface
  • Both Hydrogens added to same face of CC

1,2-Dimethylcyclohexene
Cis-1,2-dimethylcyclohexane
  • Both Hydrogens added to the same face of the
    cyclohexene CC
  • Cis/Trans naming system can be extended to
    cyclic systems

35
Addition of HX to alkenes
General reaction
X Cl, Br, I
  • CC p bond lost new C-H and C-X s bonds formed

e.g
2-Chloropropane (only product)
1-Chloropropane (not formed)
Propene
  • To explain this, need to consider the reaction
    mechanism

36
  • Reaction mechanism
  • detailed sequence of bond breaking and bond
    formation in going from reactants to products
  • Addition of HX to alkenes reaction involves two
    steps

1st Step Addition of proton (H)
2nd Step Addition of halide (X-)
1st Step
  • Alkene p electrons attack proton
  • New C-H s bond results
  • Remaining Carbon short 1 electron
  • Carbon positively charged

37
  • Addition of H to the alkene p bond forms a new
    C-H s bond and a carbocation intermediate
  • or carbonium ion

2nd Step
New C-X s bond results
Halide ion attacks electron deficient carbon
38
Reaction of HCl with CH3-CHCH2
1st Step addition of H to form a carbocation
intermediate
Two possible modes of addition
or
I.e. two possible carbocation intermediates
39
Classification of carbocations
Primary (1o) Carbocation
Secondary (2o) Carbocation
Tertiary (3o) Carbocation
2o Carbocation
1o Carbocation
40
The relative order of stability for carbocations
is
Most stable 3o gt 2o gt 1o Least
stable
  • This is because carbocations can draw electron
    density along s bonds known as an inductive
    effect
  • This effect is significant for alkyl
    substituents, but weak for Hydrogens

Least stabilised
Most stabilised
41
Addition of HCl to CH3-CHCH2 proceeds so as to
give the more stable of the two possible
carbocation intermediates, i.e
Not formed
Addition of chloride then gives 2-chloropropane
exclusively
Cl-
Additions of HX to alkenes which follow this
pattern are said to obey Markovnikovs rule
Reaction proceeds via the more stable possible
carbocation intermediate
42
Other examples
not
2-Methylpropene
2-Bromo-2-methyl- propane
1-Bromo-2-methyl- propane
not
1-Methylcyclohexene
1-Chloro-1-methyl- cyclohexane
1-Chloro-2-methyl- cyclohexane
2-Butene
2-Chlorobutane
(Symmetrical alkene)
Same structure
43
Addition of water to alkenes
  • Follows same pattern as addition of HX
  • Acid catalysis required

Propene
2-Hydroxypropane (2-Propanol)
Mechanism
1. Protonation of CC so as to give the more
stable carbocation intermediate
44
2. Attack on the carbocation by water acting as a
nucleophile
3. Loss of proton to give the product and
regenerate the catalyst
45
  • Acid catalysed addition of water often difficult
    to control
  • A Mercury (II) mediated version often used -
    oxymercuration

1-Methylcyclopentene
1-Hydroxy-1-methyl- cyclopentane
  • Gives exclusively Markovnikov addition

Hydroboration
1-Methylcyclopentene
1-Hydroxy-2-methyl- cyclopentane
  • Gives exclusively anti-Markovnikov addition

Mechanisms of these reactions beyond the scope of
this module
46
Alkene hydroxylation
  • Alkene p bond lost two new C-OH s bonds formed

Alkene epoxidation
Epoxides
  • Alkene p bond lost two new C-O s bonds are
    formed to the same Oxygen

47
Examples
Propene
Propane-1,2-diol
1,2-Epoxypropane
Cyclopentene
1,2-Epoxycyclopentane
Cis-1,2-cyclopentanediol
48
Ozonolysis of alkenes
  • Ozone (O3) strong oxidising agent
  • Adds to CC with loss of both the p and s bonds
  • Products formed are known as ozonides

Ozonide
  • Ozonides usually not isolated, but further
    reacted with reducing agents
  • Formation of two molecules each containing CO
    (Carbonyl) groups

49
Overall process
Examples
1-Butene
Aldehydes
Ketone
2,3-Dimethyl-2-butene
50
Addition of bromine (Br2) to alkenes
General reaction
  • Alkene p bond lost two new C-Br s bonds formed
  • Stereospecific reaction observed with cycloalkenes

Cyclopentene
Trans-1,2-dibromo- cyclopentane (no cis-isomer)
51
Chlorine also adds to alkene CC bonds
1,2-Dichlorobutane
1-Butene
52
  • Molecular formula C6H6

Benzene
  • All Carbons and Hydrogens equivalent

Kekulé structure (1865)
  • However, does not behave like a typical alkene
  • Less reactive than typical alkenes
  • Only reacts with bromine in presence of a catalyst
  • A substitution rather than an addition reaction
    occurs

not
53
Styrene
54
  • Arrangement of 6 p electrons in a closed cyclic p
    systems is especially stable
  • Said to possess aromaticity
  • Aromatic systems very common (e.g. benzene and
    its derivatives)

Representing the p system in benzene
  • Represents p system well
  • Of limited use in describing reactivity
  • Better to use a combination of Kekulé structures

55
  • These are NOT independent species existing in
    equilibrium
  • The p electrons in benzene are said to be
    resonance delocalised over the entire ring system
  • Resonance delocalisation is generally
    energetically favourable
  • Resonance delocalisation of 6 p electrons in a
    closed ring system is especially favourable
    aromaticity

56
Alkynes
Older name Acetylenes
  • Characterised by the presence of Carbon-Carbon
    triple bonds
  • General structure of alkynes
  • Groups R, C, C and R are co-linear
  • Neither sp3 nor sp2 hybridised Carbon consistent
    with this geometry

57
  • Two sp hybridised orbitals can be arrayed to give
    linear geometry
  • Two remaining 2p orbitals are mutually orthogonal
    and orthogonal to the two sp hybridised orbitals
  • If the two sp orbitals lies along the z axis,
    2px lies along the x axis and 2py along the y
    axis

58
  • CC consists of one s bond and two p bonds
  • The s bond lies along the C-C bond axis
  • The bond axis lies along the intersection of
    orthogonal planes
  • One p bond lies in each plane, with a node along
    the bond axis

View along the bond axis
59
First two members of the series of alkynes
Ethyne (Acetylene)
Propyne
Nomenclature
  • Prefix indicates number of carbons (eth,
    prop, etc.)
  • Suffix yne indicates presence of CC

Butyne
Can have CC between C1 and C2 or between C2 and
C3
1-Butyne
2-Butyne
  • These are structural isomers

60
6-Methyl-3-octyne
1-Heptene-6-yne
4-Methyl-7-nonen-1-yne
61
Linear geometry of alkynes difficult to
accommodate in a cyclic structure
Hence relatively few cycloalkynes
Smallest stable cycloalkyne is cyclononyne
Cyclononyne
62
Hydrogenation of alkynes
  • Standard hydrogenation conditions completely
    remove the p bonds
  • Both p bonds lost four new C-H s bonds formed

Heptane
3-Heptyne
  • Conversion of alkyne to alkane

63
Alkyne
Cis-alkene
3-Heptyne
Cis-3-heptene
64
  • Alkynes can also be converted into alkenes by
    reaction with sodium or lithium metal in liquid
    ammonia
  • Na, liq. NH3 or Li, liq. NH3
  • This gives specifically Trans-alkenes

3-Heptyne
Trans-3-heptene
65
Cis-2-hexene
Trans-2-hexene
66
Addition of bromine (Br2) to alkynes
  • Can have addition to one or both alkyne p bonds

Alkyne
Trans-1,2-dibromo- alkene
1,1,2,2-tetra- bromoalkane
1,1,2,2-Tetrabromoethane
Ethyne (Acetylene)
Trans-1,2-dibromo- 1-butene
1-Butyne
67
Hydration of 1-alkynes
  • Addition of water
  • Requires catalysis by mercury (II) salts

1-Alkyne
Ketones
4-Methyl-1-hexyne
Ketone
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