Alkanes can also form cyclic structures
CH2
CH2
CH2
Cyclopropane
CH2
CH2 CH2
CH2
CH2 CH2
CH2
CH2
CH2 CH2
CH2 CH2
CH2
CH2
Cyclobutane
Cyclopentane
CH2
Cyclohexane
General formula for cycloalkanes: CnH2n
Can be conveniently represented using line segment formulae
Note:
H
H C
H
H
C
H
C H
NOT
cyclohexane
H C
H
C
H H
C
H
H
benzene
Cycloalkane nomenclature can be extended to include substitution
CH3
CH3
CH3
Methylcyclohexane
1,3-Dimethylcyclohexane
Only one cycloalkane has a planar structure: cyclopropane
All others have non-planar structure
H
o
109.5
Ideal tetrahedral angle is 109.5o
H
H
C
H
sp3 hybridised carbons with bond angles very different to
109.5o will be less stable (higher in energy)
Cyclopropane
H
H C
H
C H C
o
H
60 H
Bond angle approaching 60o
Cyclopropane is said to suffer
from angle-strain
All C-H bonds in cyclopropane are eclipsed
Cyclopentane has almost zero angle-strain
To relieve torsional strain due to eclipsed C-H bonds,
cyclopentane relaxes into a non-planar structure
H
H
H
H
H
H H
H
H
H
One CH2 group out of the plane of the ring
Cyclohexane
A planar structure would have internal bond
angles of 120o and eclipsed C-H bonds
o
120
Actual structure relaxes into a chair conformation
This reduces the bond angle to 109o
o
~109
Geometry about each Carbon
very close to tetrahedral ideal
•Angle strain ~ zero
HH H
H H
HH
H
H
HH
H
All C-H bonds staggered, i.e. torsional strain ~ zero
H
Newman projection along
any C-C bond
H
H
H
The chair conformation contains two different hydrogen
environments
HH H
H H
HH
H
H
HH
H
6 Axial Hydrogens
HH H
H H
HH
H
H
HH
H
6 Equatorial Hydrogens
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
HH H
H H
HH
H
H
HH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
HH
H
HH
H
H
H
Boat conformation
Exists in trace quantities
Note: hydrogens axial in one chair conformation equatorial in
the other
H
Ball-and-stick model of boat
cyclohexane
What if one of the cyclohexane hydrogens were replaced by
a methyl group?
H
HH
H
H
Cyclohexane
= H
HH
H
H
H
H
CH3
Methylcyclohexane
The two chair conformations are no longer equivalent
One has the methyl group in an axial position; one in an
equatorial position
H
H CH3
H
H
H
HH
H
H
H
H
HH H
H
H
HH
CH3
H
H HH
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
CH3
CH3
Methyl axial
Methyl equatorial
Sources of alkanes
•Lower Mol. Mt. (~ < 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
•< 5 Carbons: gases at room temperature
•5 Carbons < ~18 Carbons: liquids at room temperature
•> 18 Carbons: solids at room temperature
•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
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
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
3. Reaction with halogens
Heat or light
CH4 +
Cl 2
CH3Cl
+
Chloromethane
(Methyl chloride)
With excess Cl2
CH 2Cl2
Dichloromethane
Cl2
CCl4
Tetrachloromethane
(Carbon tetrachloride)
Cl2
CHCl3
Chloroform
(Trichloromethane)
HCl
4. Catalytic cracking
•Fragmentation of alkanes into smaller molecules, e.g:
~ 500oC
CH2 CH2
CH3 CH2 CH2 CH2 CH2 CH3
Catalyst
surface
+
CH3 CH CH2
+ others
+
H2
•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’
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
C C
Carbon-Carbon double bond
Alkynes: contain Carbon-Carbon triple bonds
C C
Carbon-Carbon triple bond
Carbon valency of four maintained in alkenes and alkynes
Alkenes
Older name: Olefins
Characterised by presence of Carbon-Carbon double bonds
General
structural
formula
R
R
C C
R
R
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
o
120
R
R
C C 120o
R
R
R
R
C C
R
R
Three sp2 hybridised orbitals can be arrayed to give trigonal
geometry
120
120o
o
120o
The remaining 2pz orbital is orthogonal to the three sp2 orbitals
2pz orbital
2pz orbital
View along z axis
View along xy plane
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
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
R
R
C
Note:
C
R
R
View along the
Carbon-Carbon bond
R C
R
constitutes one p molecular orbital
i.e. constitutes one p bond when occupied
Carbon-Carbon double bond:
•One s bond; One p bond
•Both occupied by two electrons
p
C C
s
•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 C=C 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
R
R
C C
H
H
or
R
H
C C
H
R
•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
C=C
•The trans isomer is that with like groups on opposite sides
of the C=C
R
R
C C
H
H
R
H
C C
H
R
Cis isomer
Trans isomer
First two members of the alkene series:
H
H
C C
H
H
Ethene
(Ethylene)
Note:
H
CH3
C C
H
H
=
H
CH3
C C
H
H
Propene
(Propylene)
H
H
C C
H
CH3
=
CH3 CH CH2
Nomenclature:
•Prefix indicates number of carbons
•(‘eth…’ = 2C; ‘prop…’ = 3C; etc.)
•Suffix ‘…ene’ indicates presence of C=C
Butene
1 2 3 4
C C C C
Could have C=C between C1 and C2
or between C2 and C3
1
2
3
4
CH2 CH CH2 CH3
1-Butene
1
2
3
CH3 CH CH
4
CH3
2-Butene
Note:
1. 1-Butene and 2-butene are structural isomers
2.
CH2 CH CH2 CH3
=
CH3 CH2 CH CH2
= 1-Butene
3. Number indicates starting point of the C=C, i.e. number
through the C=C
4. Cis-Trans isomerism is possible for 2-butene
•There are two isomeric 2-butenes
CH3
H
C
H
CH3
C
C
CH3
Trans-2-butene
b.p. 3.7oC
m.p. -139oC
H
CH3
C
H
Cis-2-butene
b.p. 0.3oC
m.p. -106oC
Some other alkenes
4
2 3
1
CH2 C CH2 CH3
CH3
2-Methyl-1-butene
1
2
3
4
5
CH2 CH CH CH CH3
1,3-Pentadiene
4
2
1
3
5
CH3 CH CH CH CH3
CH3
4-Methyl-2-pentene
2 1
3
H
CH2CH3
C
C
H 4 CH2CH2CH3
5
6 7
Cis-3-heptene
Trans-2-decene
CH3
Can have cycloalkenes
3
4
5
Cyclohexene
Cyclopentene
2
1
3-Methylcyclopentene
6
5
1
Note:
4
2
3
1,4-Cyclohexadiene
=
H H
H C C C H
H
C
C H
C
H H
Lycopene molecular structure
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
Alkene hydrogenation
•Addition of hydrogen (H2) across a C=C
General reaction
R
R
C C
R
R
H2
Catalyst
H
H
R C C R
R
R
•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”
Examples
H2 (g)
CH3 CH2 CH2 CH2 CH2 CH3
CH2 CH CH2 CH2 CH2 CH3
1-Hexene
Pt/C
Hexane
2 H2 (g)
CH3 CH2 CH2 CH2 CH2 CH3
CH2 CH CH CH CH2 CH3
1,3-Hexadiene
CH3
Pt/C
H2 (g)
CH2 C CH2 CH3
2-Methyl-1-butene
Hexane
CH3
CH3 C CH2 CH3
Pt/C
H
2-Methylbutane
•Reaction occurs at the catalyst surface
•H2 molecules adsorbed onto catalyst surface
•Both Hydrogens added to same face of C=C
CH3
H2 (g)
CH3
Pt/C
1,2-Dimethylcyclohexene
H
H
CH3
CH3
Cis-1,2-dimethylcyclohexane
•Both Hydrogens added to the same face of the cyclohexene
C=C
•[Cis/Trans naming system can be extended to cyclic systems]
Addition of HX to alkenes
General reaction
R
R
C C
R
R
HX
X
H
R C C R
R
R
X = Cl, Br, I
•C=C p bond lost; new C-H and C-X s bonds formed
e.g:
HCl
CH2 CH CH3
Propene
CH3
Cl
CH CH3
2-Chloropropane
(only product)
Cl
H2C CH2
CH3
1-Chloropropane
(not formed)
•To explain this, need to consider the reaction mechanism
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
H
C C
•Alkene p electrons
attack proton
•New C-H s bond results
H
C C
•Remaining Carbon short 1
electron
•Carbon positively charged
•Addition of H+ to the alkene p bond forms a new C-H s
bond and a carbocation intermediate
•[or carbonium ion]
2nd Step
X
H
C C
Halide ion attacks
electron deficient
carbon
X
H
C C
New C-X s bond
results
Reaction of HCl with CH3-CH=CH2
1st Step: addition of H+ to form a carbocation intermediate
Two possible modes of addition
H
CH3 CH CH2
CH3 CH CH3
or
H
CH3 CH CH2
CH3 CH2 CH2
I.e. two possible carbocation intermediates
Classification of carbocations
R C H
R C H
R C R
H
R
R
Primary (1o)
Carbocation
Secondary (2o)
Carbocation
Tertiary (3o)
Carbocation
CH3 CH CH3
CH3 CH2 CH2
2o Carbocation
1o Carbocation
The relative order of stability for carbocations is:
Most stable 3o >
2o
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
R> C
R
H
R >C <R
>
H
>
R> C
H
R
Least stabilised
Most stabilised
Addition of HCl to CH3-CH=CH2 proceeds so as to give the
more stable of the two possible carbocation intermediates, i.e:
H
CH3 CH CH2
CH3 CH CH3
CH3 CH2 CH2
Not formed
Addition of chloride then
gives 2-chloropropane
exclusively
Cl-
Cl
CH3 CH CH3
Additions of HX to alkenes which follow this pattern are said to
obey Markovnikov’s rule
“Reaction proceeds via the more stable possible carbocation
intermediate”
Other examples
CH3
CH3
C CH2
CH3
HBr
CH3
CH3 C CH3
not
Br
2-Methylpropene
H
2-Bromo-2-methylpropane
CH3
HCl
1-Methylcyclohexene
CH3 CH CH CH3
2-Butene
(Symmetrical alkene)
CH3
Cl
1-Bromo-2-methylpropane
H
not
1-Chloro-1-methylcyclohexane
HCl
CH3 C CH2 Br
Cl
H
CH3
Cl
1-Chloro-2-methylcyclohexane
CH3 CH2 CH CH3
2-Chlorobutane
Cl
CH3 CH CH2 CH3
Same structure
Addition of water to alkenes
•Follows same pattern as addition of HX
•Acid catalysis required
H
CH3 CH CH2
+ H2O
Propene
catalyst
OH
CH3 CH CH3
2-Hydroxypropane
(2-Propanol)
Mechanism:
1. Protonation of C=C so as to give the more stable
carbocation intermediate
H
CH3 CH CH2
CH3 CH CH3
2. Attack on the carbocation by water acting as a nucleophile
H
O
H
H
CH3 CH CH3
O
H
CH3 CH CH3
3. Loss of proton to give the product and regenerate the catalyst
H
O
H
CH3 CH CH3
OH
+ H
CH3 CH CH3
•Acid catalysed addition of water often difficult to control
•A Mercury (II) mediated version often used - oxymercuration
i) (CH3CO2)2Hg, H2O
OH
CH3
CH3
1-Methylcyclopentene
ii) NaBH4
(Sodium borohydride)
•Gives exclusively Markovnikov addition
1-Hydroxy-1-methylcyclopentane
Hydroboration
i) "BH3" (Borane)
H OH
CH3
1-Methylcyclopentene
CH3
H
ii) H2O2, NaOH
1-Hydroxy-2-methylcyclopentane
•Gives exclusively anti-Markovnikov addition
Mechanisms of these reactions beyond the scope of this module
Alkene hydroxylation
R
R
C C
R
R
KMnO 4
or OsO4
OH
HO
R C C R
R
R
•Alkene p bond lost; two new C-OH s bonds formed
Alkene epoxidation
R
R
C C
R
R
RCO3H
(Peroxy acids)
R O R
C C
R
R
Epoxides
•Alkene p bond lost; two new C-O s bonds are formed to
the same Oxygen
Examples
OsO4
OH OH
CH3 CH CH2
CH3 CH CH2
Propene
Propane-1,2-diol
CH3CO3H (Peroxyacetic acid)
CH3
O
CH CH2
1,2-Epoxypropane
H
O
CH3CO3H
H
1,2-Epoxycyclopentane
H
OsO 4
H
OH
H
Cyclopentene
OH
H
Cis-1,2-cyclopentanediol
Ozonolysis of alkenes
•Ozone (O3): strong oxidising agent
•Adds to C=C with loss of both the p and s bonds
•Products formed are known as ozonides
R
R
C C
R
R
O3
O O
R
R
C
C
R
R
O
Ozonide
•Ozonides usually not isolated, but further reacted with
reducing agents
O O
R
C
C
R
R
O
R
Zn
R
C O
R
+
R
O C
R
•Formation of two molecules each containing C=O
(Carbonyl) groups
Overall process:
R
R
C C
R
R
i) O3
R
C O
R
ii) Zn
+
R
O C
R
Examples
i) O3
CH3 CH2 CH CH2
ii) Zn
1-Butene
CH3
CH3
C C
CH3
+
CH3 CH2 CH O
CH3
2,3-Dimethyl-2-butene
Aldehydes
i) O3
CH3
2
ii) Zn
C
O
CH3
Ketone
O CH2
Addition of bromine (Br2) to alkenes
General reaction
Br2
R
R
C C
R
R
Br
Br
R C C R
R
R
•Alkene p bond lost; two new C-Br s bonds formed
•Stereospecific reaction observed with cycloalkenes
H
H
Cyclopentene
Br2
H
Br
Br
H
Trans-1,2-dibromocyclopentane
(no cis-isomer)
Chlorine also adds to alkene C=C bonds
Cl2
CH3 CH2 CH CH2
1-Butene
Cl
Cl
CH3 CH2 CH CH2
1,2-Dichlorobutane
Benzene
•Molecular formula C6H6
•All Carbons and Hydrogens equivalent
H
Kekulé structure (1865)
H
C
C
H
C
C
H
C
C
H
=
H
•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
Br2
FeBr3
(Catalyst)
Br
H Br
not
H
Br
H
H
C C
H
Styrene
Br
Br2
Br
H C C H
H
•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
•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
Alkynes
Older name: Acetylenes
•Characterised by the presence of Carbon-Carbon triple bonds
C C
•General structure of alkynes
R C C R
•Groups R, C, C and R are co-linear
•Neither sp3 nor sp2 hybridised Carbon consistent with this
geometry
•Two sp hybridised orbitals can be arrayed to give linear geometry
o
180
o
180
•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]
y
x
z
consists of one s bond and two p bonds
•The s bond lies along the C-C bond axis
•C≡C
•The bond axis lies along the intersection of orthogonal planes
•One p bond lies in each plane, with a node along the bond axis
C C
View along the bond axis
p
C
p
First two members of the series of alkynes
H C C CH3
H C C H
Ethyne
(Acetylene)
Propyne
Nomenclature
•Prefix indicates number of carbons (‘eth…’, ‘prop…’, etc.)
•Suffix ‘…yne’ indicates presence of C≡C
Butyne 1
2 3 4
C C C C
Can have C≡C between C1 and C2
or between C2 and C3
1 2 3
4
HC C CH2 CH3
1
2 3 4
CH3 C C CH3
1-Butyne
2-Butyne
•These are structural isomers
5
4 3 2
6
1
8
7
CH3 CH2 CH CH2 C C CH2 CH3
CH3
6-Methyl-3-octyne
2
1
7 6 5
3
4
HC C CH2 CH2 CH2 CH CH2
1-Heptene-6-yne
CH3
HC C CH2 CH CH2 CH2 CH CH CH3
1 2 3
5
4
6
7
8
9
4-Methyl-7-nonen-1-yne
Linear geometry of alkynes difficult to accommodate in a
cyclic structure
Hence relatively few cycloalkynes
Smallest stable cycloalkyne is cyclononyne
CH2
CH2
CH2
CH2
CH2
CH2
C C
CH2
Cyclononyne
Hydrogenation of alkynes
•Standard hydrogenation conditions completely remove the p
bonds
xs. H 2
R C C
R
H H
R C C
Catalyst
R
H H
•Both p bonds lost; four new C-H s bonds formed
xs. H 2
CH3 CH2 CH2 C C CH2 CH3
3-Heptyne
CH3 CH2 CH2 CH2 CH2 CH2 CH3
Pd/C
•[Conversion of alkyne to alkane]
Heptane
H2
R C C
Alkyne
R
Pd/PbO/CaCO3
(Lindlar's catalyst)
CH3 CH2 CH2 C C CH2 CH3
H2
H
H
C C
R
R
Cis-alkene
3-Heptyne
Pd/PbO/CaCO3
(Lindlar's catalyst)
H
H
C C
CH3 CH2 CH2 CH2 CH3
Cis-3-heptene
•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
CH3 CH2 CH2 C C CH2 CH3
Li
3-Heptyne
liq. NH3
H
CH2 CH3
C C
CH3 CH2 CH2 H
Trans-3-heptene
H
H
C C
CH2 CH2 CH3
CH3
H2
Cis-2-hexene
Pd/PbO/CaCO3
(Lindlar's catalyst)
xs. H 2
CH3 C C CH2 CH2 CH3
2-Hexyne
Li
CH3 CH2 CH2 CH2 CH2 CH3
Pd/C
liq. NH3
H
CH2 CH2 CH3
C C
Trans-2-hexene
H
CH3
Hexane
Addition of bromine (Br2) to alkynes
•Can have addition to one or both alkyne p bonds
Br2
R C C
R
C C
R
Br
Br
R
Alkyne
Br2
Trans-1,2-dibromoalkene
2 Br2
Br
Br
HC CH
Br
Br
HC CH
Ethyne
(Acetylene)
Br2
CH3 CH2 C
1-Butyne
CH
Br Br
R C C
R
Br Br
1,1,2,2-tetrabromoalkane
1,1,2,2-Tetrabromoethane
H
C C
CH3CH2
Br
Br
Trans-1,2-dibromo1-butene
Hydration of 1-alkynes
•[Addition of water]
•Requires catalysis by mercury (II) salts
H2O, H 2SO4
R C
CH
1-Alkyne
R C CH3
Hg (II) salt
CH3 CH2 CH CH2 C CH
H2O, H 2SO4
CH3
4-Methyl-1-hexyne
O
HgSO4
Ketones
O
CH3 CH2 CH CH2 C CH3
CH3
Ketone