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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)
•Reaction mechanism involves two steps
1st Step: alkene p electrons attack Bromine
Bromide ion and a cyclic epibromonium ion results
Br
Br
Br
C C
Br
C C
Cyclic epibromonium ion
•The large size of Bromine w.r.t Carbon (4th row vs. 2nd row)
means that it can span two Carbons
Br
C C
rather than
Br
C C
2nd Step: addition of bromide anion
Anion approaches epibromonium ion from the face opposite
that blocked by bromine
Br
C C
Br
C C
Br
Br
With cyclopentene
Br
H
H
Br
H
H
Br
Br
Epibromonium ion
and bromide
H
Br
Br
H
Trans-1,2-dibromocyclopentane
Chlorine also adds to alkene C=C bonds
Cl2
CH3 CH2 CH CH2
1-Butene
CH3 CH2
Cl Cl
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
•Also, all benzene C-C bond lengths equal: 139 pm
•Comparison: C-C 154 pm; C=C 134 pm
•Planar ring of sp2 hybridised Carbons
•6 pz orbitals overlap to form a continuous cyclic p system
p electron density located
above and below the plane of
the ring
•6 p electrons
•All 6 C-C bonds equivalent
•[Not a representation of benzene
p molecular orbitals]
An orbital representation of the
bonding in benzene.
•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
Some points about this representation
• Neither Kekulé structure alone is an adequate
representation of the p bonding in benzene.
• An adequate representation requires both
structures simultaneously
• The structures are known as resonance forms or
resonance contributors
• Each resonance structure contributes [equally]
to the overall p bonding system
• ‘↔’ is used to show that structures are
resonance forms of each other;
• resonance structures are enclosed in square
brackets
•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
Graphite
Carbon nanotube
Aromatic systems in
pharmaceuticals
HO
O
CO 2H
Me
OH
N
N
OEt HN
N
N
F
Cl
N
O
N
H
N
O
O
S
O
N
N
Cl
Me
Cl
atorvastatin
(Lipitor®)
sildenafil
(Viagra®)
miconazole
Cl
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
1s
2s
2p
2p
2p
Hybridisation
1s
2e-
sp
sp
1e-
1e-
2px
2py
1e-
1e-
•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
•Overlap of sp orbitals on two Carbons results in s bond
formation
s
s
C
C
C C
=
•[s* also formed; not occupied by electrons]
•px orbitals overlap to form a p bond in the xz plane
y
y
y
C C
z
y
C C
p
[p* also formed;
not occupied]
z
•py orbitals overlap to form a p bond in the yz plane
x x
p x x
C C
z
C C
z
[p* also formed;
not occupied]
•C≡C 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
C C
View along the bond axis
p
C
p
A triple bond consists of the end-on overlap of two sp-hybrid orbitals to form
a σ bond and the lateral overlap of the two sets of parallel oriented p orbitals
to form two mutually perpendicular π bonds
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
HC C CH2
1 2 3
CH3
CH CH2 CH2 CH CH CH3
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
H H
Catalyst
R C C R
H H
R C C R
•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
•Possible to modify the catalyst so as to reduce its activity
(poisoning)
Lindlar’s catalyst
Pd/PbO/CaCO3
•Pd: catalytic metal
•PbO: poison
•CaCO3: supporting material
•Hydrogenation of alkynes using Lindlar’s catalyst removes
only one p bond
•[Only two Hydrogens added to C≡C; products are alkenes]
•Reaction occurs on catalyst surface; both Hydrogens added
to same face of alkyne
•Specifically Cis-alkenes produced
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
CH3
4-Methyl-1-hexyne
O
H2O, H 2SO4
HgSO4
Ketones
O
CH3 CH2 CH CH2 C CH3
CH3
Ketone
Review: quantifying acid strength: pKa
HA
H
Acid
Proton
A
+
Conjugate
base
•Extent of dissociation is medium dependent; hence medium
should be defined
•If not otherwise stated, assume medium is water
HA
Acid
+
H2O
H3O
Base
Conjugate
acid
+
A
Conjugate
base
Can define an equilibrium constant Ka’
H3O
Ka'
A
=
HA
H2O
•Assume concentration of water stays constant; remove
[H2O] term to give the dissociation constant Ka
H3O
Ka
A
=
HA
•The stronger the acid HA, the greater the dissociation
•The stronger the acid, the greater the value of Ka
•Range of Ka values is vast; inconvenient numbers
•For convenience, take logs; define:
pKa
=
- log10Ka
•Stronger acid; greater Ka; smaller pKa
•Weaker acid; smaller Ka; greater pKa
•‘Strong acid’: HCl pKa = -7.0
•‘Weak acid’: CH3CO2H pKa = 4.76
CH3 CH3
CH2 CH2
HC CH
ethane
ethene
ethyne
50.0
44.0
25.0
pKa
Conjugate
bases
CH3 CH2
CH2 CH
HC C
•Ethane and ethene are effectively devoid of acidity
•Ethyne dissociates to a miniscule extent
•Reflects the relative stability of the conjugate bases
CH3 CH2
Least stable
<
CH2 CH
<<
HC C
Most stable
•Order of stability is related to the hybridisation of the
Carbons bearing the negative charge
Least stable
CH3 CH2
sp3
Most stable
<
CH2 CH
<<
2
HC C
sp
sp
•Increasing s character assists in stabilising negative charge
on Carbon
•s orbitals locate the excess electron density closer to the
positively charged nucleus
•By comparison, p orbitals have nodal points at the nucleus
s
p
HC≡CH pKa 25
•Extent of dissociation almost negligible
•However, dissociation can be driven to completion by
reaction with very strong base
Na
NH2
Sodium amide
(Sodamide)
HC CH
+
Na
NH2
HC C
Na
Sodium
acetylide
•This reaction goes entirely to completion
+
NH3
The process is general for 1-alkynes
NH2
R C CH + Na
R C C
+
Na
NH3
Sodium acetylides
•Reaction of 1-alkynes with sodium amide gives complete
conversion into sodium acetylides
NaNH2
CH3CH2CH2 C C
CH3CH2CH2 C CH
Na
+
1-Pentyne
CH3
HC C CH
CH3
NaNH2
3-Methyl-1-butyne
CH3
HC C C
CH3
Na
Acetylide anions
+
NH3
NH3
•Acetylide anions are strong Carbon nucleophiles
•React with Carbon electrophiles to form new CarbonCarbon bonds
Acetylide anion attacks
methyl Carbon
R C C
CH3 Cl
Chloride anion
displaced
R C C CH3
Chloromethane
New C-C bond
formed
+ Cl
CH3 Cl
CH3 CH2 C C
Na
CH3 CH2 C C CH3
+ NaCl
2-Pentyne
CH3
HC C C
CH3
CH3 Cl
CH3
HC C C CH3
CH3
Na
+ NaCl
2-Methyl-3-pentyne
CH3 Cl
NaNH2
CH3 C CH
Propyne
CH3 C C
+
NH3
Na
CH3 C C CH3
+ NaCl
2-Butyne
Recall:
Heat or light
CH4 +
Cl 2
CH3Cl
+
HCl
Chloromethane
(Methyl choride)
With excess Cl2
CH 2Cl2
Dichloromethane
Etc.
•Reaction mechanisms so far have involved nucleophiles
reacting with electrophiles…
•…and ionic intermediates
•Covalent bond formation the occurs as a result of
movement of pairs of electrons
•Such mechanisms are known as polar mechanisms
•New covalent bonds can also be formed by processes in
which…
•…each molecular species involved donates one electron
•Chlorination of alkanes proceeds by such mechanisms

2 Cl atoms
Cl Cl
or light

Cl
Cl
Cl
Homolytic
cleavage
+
or light
•[Heterolytic cleavage: cleavage into ions]
Cl
H
H C
H
H
H
H C
H
Cl
+ HCl
Methane (CH4)
Methyl radical
•Methyl radical is a neutral species bearing an unpaired electron
•Is said to be a ‘free radical’
•Methyl radical can react with further chlorine molecules
H
H C
H
H
Cl Cl
H C Cl
+
Cl
H
•This step generates product and further chlorine atom
•Overall process is a chain reaction
CH4
HCl
Propagation
Initiation
Cl Cl
Cl
CH3
Propagation
CH3Cl
Cl2
Chlorination of alkanes other than methane
e.g. 2-Methylbutane
CH3
CH3
C CH2 CH3
H
•Substrate contains primary (1o), secondary (2o) and tertiary
(3o) Hydrogens
H
R C H
H
R
R C H
H
R
R C R
H
1o
2o C-H
3o C-H
C-H
CH3
CH3
C CH2 CH3
H
3o
CH3
CH3 H
C C
H H
2o
CH3
CH3
C CH2 CH3
H
1o
CH3
Monochlorination of 2-methylbutane: four products obtained
CH3
CH3
C CH2 CH3
H
CH3
CH3 C CH2 CH2 Cl
H
1-Chloro-3-methylbutane
[2]
Cl2
300oC
CH3
CH3
Cl CH2 C CH2 CH3
H
1-Chloro-2-methylbutane
CH3
C CH CH3
H Cl
2-Chloro-3-methylbutane
[3]
CH3
[1]
CH3
C CH2 CH3
Cl
2-Chloro-2-methylbutane
[4]
•Four products obtained in unequal amounts
•If all Hydrogens on the substrate were equally reactive
towards chlorine atom, would expect:
Based on
[1]
[2]
[3]
[4]
50%
25%
17%
8%
[1]
CH3
CH3
C
[2]
CH2
CH3
H
[3]
[4]
Expected ratio [1]:[2]:[3]:[4] = 6:3:2:1
Observed ratio of products
[1]
[2]
[3]
34% 16% 28%
[4]
22%
•Less of products [1] and [2] than expected
•More of product [3] than expected
•Substantially more of product [4] than expected
Conclusion: Hydrogens not all equally reactive towards chlorine
Relative reactivity
1.0
CH3
CH3
C
1.0
CH2
CH3
H
3.3
4.4
most reactive 3o > 2o > 1o
least reactive
This trend reflects the
relative stabilities of the
intermediate free radicals
R
R C
o
3 Radical
R
more stable than
R
R C
o
2 Radical
H
More stable than
H
R C
H
o
1 Radical
•Primary, secondary, tertiary system used to distinguish
between substitutents of the same number of Carbons
R C3H7
Propyl group
Two possibilities
R CH2 CH2 CH3
CH3
R CH
CH3
1-Propyl (‘Propyl’)
2-Propyl or Isopropyl
R C4H9
Butyl group
Four possibilities
R CH2 CH2 CH2 CH3
1-Butyl (‘Butyl’)
CH3
R C CH3
CH3
tert-Butyl
(“tertiary- Butyl”)
[or 2-Methyl-2-propyl]
CH3
R CH
CH2 CH3
2-Butyl
or sec-Butyl
(“secondary-Butyl”)
R CH2
CH3
CH
CH3
Isobutyl
Free-Radical Polymerization (of Alkenes)
n
R
R
n
R
radical
initiator
=
R
Examples
Monomer
n
Polymer
R
R
R
R
Monomer
Polymer
CH2 CH Cl
CH2 CH
n
Cl
polyvinyl chloride
vinyl chloride
CH2 CH2
ethylene
R
CH2 CH2
n
polyethylene
n
CH2 CH CH3
propylene
CH2 CH
n
CH3
polypropylene
styrene
polystyrene
Free radical polymerization mechanism
Require a free radical initiator (In•)
In
In
In
R
R
R
R
Termination
R
R
R
In
R
R
R
R
R
R
R
R
R
R
R
or
R
H
R
+
R
R
R
R
R
R
SC slides now available on ChemWeb
etc.