Section 2 - Alkenes and Halogenoalkanes

Report
AS Chemistry
Bonding in methane,
ethane and ethene
 and  bonds
Learning Objectives
Candidates should be able to:
•describe covalent bonding in terms of orbital overlap,
giving  and  bonds.
•explain the shape of, and bond angles in, ethane and
ethene molecules in terms of  and  bonds.
Starter activity
Alkenes
pent-2-ene
CH3CH=CHCH2CH3
hex-3-ene
CH3CH2CH=CHCH3
2,3-dimethylpent-2-ene
cyclopenta-1,3-diene
3-ethylhept-1-ene
CH2=CHCH2CH(CH2CH3)CH2CH2CH3
Hybridisation of orbitals
The electronic configuration of a carbon atom is
1s22s22p2
2p
2
2s
1
1s
HYBRIDISATION OF ORBITALS
If you provide a bit of energy you can promote (lift)
one of the s electrons into a p orbital.
The
configuration is now 1s22s12p3
2p
2
2s
1
1s
The extra energy released when the bonds form more than
compensates for the initial input.
Hybridisation of orbitals in alkanes
The four orbitals (an s and three p’s) combine or
HYBRIDISE to give four new orbitals. All four orbitals are
equivalent.
Because one s and three p orbitals are used, it is called sp3
hybridisation.
2s22p2
2s12p3
4 x sp3
Hybridisation of orbitals in alkanes
In ALKANES, the four sp3
orbitals repel each other
into
a
tetrahedral
arrangement.
sp3 orbitals
Bonding in methane
Bonding in ethane
Bonding in ethene
Alternatively, only three orbitals (an s and two p’s)
combine or HYBRIDISE to give three new orbitals. All
three orbitals are equivalent. The remaining 2p orbital is
unchanged.
2s22p2
2s12p3
3 x sp2
2p
What about ethene?
sp2 hybrids
 - bonds
AS Chemistry
Geometric Isomerism
Learning Objectives
Candidates should be able to:
describe cis-trans isomerism in alkenes, and
explain its origin in terms of restricted rotation
due to the presence of π bonds.
deduce the possible isomers for an organic
molecule of known molecular formula.
identify cis-trans isomerism in a molecule of given
structural formula.
Starter activity
What is stereoisomerism?
In stereoisomerism, the atoms making up the isomers
are joined up in the same order, but still manage to have
a different arrangement in space
ISOMERISM
STRUCTURAL ISOMERISM
STEREOISOMERISM
GEOMETRIC ISOMERISM
OPTICAL ISOMERISM
Geometric Isomerism?
GEOMETRIC ISOMERISM
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears
to be a different structure in an alkane is not. Due to
the way structures are written out, they are the
same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’
ROTATION
Animation doesn’t
work in old
versions of
Powerpoint
Geometric Isomerism?
Geometric isomers of but-2-ene
Geometric Isomerism?
X

GEOMETRIC ISOMERISM
How to tell if it exists
Two
different
atoms/groups
attached
Two
different
atoms/groups
attached

Two similar
atoms/groups
attached
Two similar
atoms/groups
attached

Two similar
atoms/groups
attached
Two
different
atoms/groups
attached

Two
different
atoms/groups
attached
Two
different
atoms/groups
attached

GEOMETRICAL ISOMERISM
Once you get two similar
atoms/groups
attached
to one end of a C=C,
you
cannot
have
geometrical isomerism
GEOMETRICAL ISOMERISM
GEOMETRIC ISOMERISM
Isomerism in butene
There are 3 structural isomers of C4H8 that are alkenes*. Of these
ONLY ONE exhibits geometrical isomerism.
BUT-1-ENE
cis BUT-2-ENE
trans BUT-2-ENE
2-METHYLPROPENE
* YOU CAN GET ALKANES WITH FORMULA C4H8 IF THE CARBON ATOMS ARE IN A
RING
Summary
To get geometric isomers you must have:
restricted rotation (involving a carbon-carbon double
bond for A-level purposes);
two different groups on the left-hand end of the
bond and two different groups on the right-hand end.
It doesn't matter whether the left-hand groups are
the same as the right-hand ones or not.
The effect of geometric isomerism on
physical properties
isomer
melting
(°C)
point boiling
(°C)
cis
-80
60
trans
-50
48
point
You will notice that:
the trans isomer has the higher melting point;
the cis isomer has the higher boiling point.
Why is the boiling point of the cis
isomers higher?
The difference between the two is that the cis isomer is
a polar molecule whereas the trans isomer is non-polar.
Why is the melting point of the cis
isomers lower?
In order for the intermolecular forces to work well, the
molecules must be able to pack together efficiently in the
solid.
Trans isomers pack better than cis isomers. The "U" shape
of the cis isomer doesn't pack as well as the straighter
shape of the trans isomer.
AS Chemistry
Optical Isomerism
Learning Objectives
Candidates should be able to:
explain what is meant by a chiral centre and that
such a centre gives rise to optical isomerism.
deduce the possible isomers for an organic
molecule of known molecular formula.
identify chiral centres in a molecule of given
structural formula.
Starter activity
Optical isomerism
Chiral centre
Chiral molecule
When four different atoms or groups are attached to a
carbon atom, the molecules can exist in two isomeric forms
known as optical isomers. These are non-superimposable
mirror images.
Optical Isomerism
What is a non-superimposable mirror image?
Animation doesn’t
work in old
versions of
Powerpoint
Optical isomerism
Amino acids (the building blocks of proteins) are optically
active. They affect plane polarised light differently.
Butan-2-ol
Optical Isomerism
The polarimeter
A
B
C
D
E
F
A
B
C
D
E
F
Light source produces light vibrating in all directions
Polarising filter only allows through light vibrating in one direction
Plane polarised light passes through sample
If substance is optically active it rotates the plane polarised light
Analysing filter is turned so that light reaches a maximum
Direction of rotation is measured coming towards the observer
If the light appears to have
turned to the right
DEXTROROTATORY
turned to the left
LAEVOROTATORY
Enantiomers – how do they differ?
Usually have the same chemical and
physical properties – but behave
differently in presence of other
chiral compounds.
Enantiomers – how do they differ?
TYPES OF ISOMERISM
CHAIN ISOMERISM
STRUCTURAL ISOMERISM
Same molecular formula but
different structural
formulae
POSITION ISOMERISM
FUNCTIONAL
GROUP ISOMERISM
GEOMETRICAL ISOMERISM
STEREOISOMERISM
Same molecular
formula but atoms
occupy different
positions in space.
Occurs due to the restricted
rotation of C=C double
bonds... two forms - CIS and
TRANS
OPTICAL ISOMERISM
Occurs when molecules have a
chiral centre. Get two nonsuperimposable mirror images.
AS Chemistry
Electrophilic
Addition to Alkenes
Learning Objectives
Candidates should be able to:
•describe the mechanism of electrophilic addition in
alkenes, using bromine/ethene as an example.
•describe the chemistry of alkenes as exemplified,
where relevant, by the following reactions of ethene:
addition of hydrogen, steam, hydrogen halides and
halogens.
Starter activity
Electrophilic addition
CH2=CH2
+
Br2

CH2BrCH2Br
bromine with ethene
CH2=CH2
+ Br2
CH2BrCH2Br
1,2-dibromoethane
hydrogen bromide with ethene
CH2=CH2
+ HBr
CH3CH2Br
bromoethane
Electrophilic addition mechanism
H
H
C
H
C
H
Br
H
+
H
carbocation
C
+
C
H
Br
Br
-
H
Br
Br-
Br 1,2-dibromoethane
H
H
H
C
C
Br
Br
H
Electrophilic addition mechanism
H
H
C
C
H
H
+
H
Br-
H
H
H
carbocation
C
+
C
H
Br-
H
H
H
H
C
C
Br
H
H
bromoethane
Electron flow during electrophilic addition
EQUATION
TEMPERATURE
(OC)
hydrogen
CH2=CH2 + H2 → CH3CH3
~150
steam
CH2=CH2 + H2O→
CH3CH2OH
330
hydrogen
halides
(e.g. HBr)
CH2=CH2 + HBr →
CH3CH2Br
Room
temperature
halogens
CH2=CH2 +Br2→
CH2BrCH2Br
Room
temperature
PRESSURE
CATALYST
Finely divided
nickel
on
support
material
6MPa
Phosphoric (V)
acid
(H3PO4)
adsorbed onto
the surface of
silica.
PHASE
Gas
Gas
Aqueous
solution
Liquid bromine
or
solution
(both aqueous
and non-polar
solvent.
NOTES
Never carried
out industrially.
Analogous
reaction used
to
produce
some
margarines
from oils (see
later).
Major
industrial
process for the
manufacture of
ethanol.
Reactivity
increases from
HF to HI.
Chlorine
and
iodine produce
similar addition
products.
Fluorine is too
powerful
an
oxidizing agent.
Addition reactions of alkenes
Addition to unsymmetrical alkenes
Electrophilic addition to propene
2-bromopropane
1-bromopropane
Addition to unsymmetrical alkenes
In the electrophilic addition to alkenes the major
product is formed via the more stable carbocation
(carbonium ion)
least stable
methyl
<
primary (1°)
most stable
< secondary (2°) < tertiary (3°)
Addition to unsymmetrical alkenes
SECONDARY
CARBOCATION
PATH A
MAJOR PRODUCT
PRIMARY
CARBOCATION
PATH B
MINOR PRODUCT
AS Chemistry
Polymerisation
Learning Objectives
Candidates should be able to:
describe the
polymerisation.
chemistry
of
alkenes
including
describe
the
characteristics
of
addition
polymerisation as exemplified by poly(ethene) and
PVC.
Recognize the difficulty of the disposal of
poly(alkene)s, i.e. non-biodegradability and harmful
combustion products.
Starter activity
Poly(ethene)
Conditions
Temperature:
Pressure:
Initiator:
about 200°C
about 2000 atmospheres
often a small amount of oxygen
as an impurity
Free radical addition
Initiation
Propagation
Propagation
Termination
LDPE or HDPE
LDPE or HDPE
Sandwich bags, cling wrap, car covers, squeeze bottles,
liners for tanks and ponds, moisture barriers in
construction
Freezer bags, water pipes, wire and cable insulation,
extrusion coating
Polymerisation of alkenes
ETHENE
CHLOROETHENE
PROPENE
TETRAFLUOROETHENE
POLY(ETHENE)
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE
PVC
POLY(PROPENE)
POLY(TETRAFLUOROETHENE)
PTFE
“Teflon”
Disposal of polymers
Method
Landfill
Incineration
Recycling
Feedstock
recycling
Comments
Emissions to the atmosphere and water;
vermin; unsightly. Can make use of old
quarries.
Saves on landfill sites and produces
energy. May also release toxic and
greenhouse gases.
high cost of collection and reprocessing.
Use the waste for the production of
useful
organic
compounds.
New
technology can convert waste into
hydrocarbons which can then be turned
back into polymers.
AS Chemistry
Oxidation of
alkenes
Learning Objectives
Candidates should be able to describe the oxidation
of alkenes by:
cold, dilute, acidified manganate(VII) ions to form
the diol, and
hot, concentrated, acidified manganate(VII) ions
leading to the rupture of the carbon-to-carbon
double bond in order to determine the position of
alkene linkages in larger molecules.
Starter activity
Oxidation of alkenes
In the presence of dilute (acidified or alkaline) potassium
manganate (VII).
•Alkenes react readily at room temperature (i.e. in the
cold).
•The purple colour disappears and a diol is formed.
CH2=CH2
+
H 2O
+
[O]

HOCH2CH2OH
ethane – 1,2-diol
Oxidation of alkenes
In the presence of a hot, concentrated solution of
acidified potassium manganate (VII), any diol formed is
split into two fragments which are oxidized further to
carbon dioxide, a ketone or a carboxylic acid.
Fragment
=CH2
Product
CO2
R-CH=
Aldehyde
R2C=
Ketone
→
carboxylic acid
Oxidation of alkenes
1. CH2=CH2
2. CH3CH=CH2
3. (CH3)2C=CH2
2 products – both
contain ketone
1 product only
2 products – one contains
2 ketone groups and one
contains 2 acid groups.
AS Chemistry
Halogenoalkanes
Learning Objectives
Candidates should be able to recall the chemistry of
halogenoalkanes as exemplified by the following
nucleophilic substitution reactions of bromoethane:
hydrolysis;
formation of nitriles;
formation of primary amines by reaction with
ammonia.
Starter activity
Naming Halogenoalkanes
a. CHCl3
b. CH3CHClCH3
c. CF3CCl3
trichloromethane
2-chloropropane
1,1,1-trichloro-2,2,2-trifluoroethane
F
Cl
F
Cl
F
Cl
Physical Properties
a. 1-chloropropane is polar and has permanent dipoledipole intermolecular forces that are stronger than
the temporary dipole-induced dipole forces in nonpolar butane.
b. 1-chloropropane is polar and has permanent dipoledipole intermolecular forces that are stronger than
the temporary dipole-induced dipole forces in nonpolar butane.
Nucleophilic substitution
negotiate clever
electronegative
alp or
polar
cadet tart
attracted
eat given
negative
enticed if
deficient
chenille soup
nucleophiles
had lie
halide
stubs tuition
substitution
Nucleophilic substitution
This is known as an SN2 reaction.
S stands for substitution,
N for nucleophilic, and
 2 because the initial stage of
the reaction involves two species.
Nucleophilic substitution - mechanism
Attack by nucleophile is to the back of the molecule –
away from the negatively charged halogen atom.
ANIMATION SHOWING THE SN2 MECHANISM
Rate of reaction
Halogen
Electronegativity
F
4.0
Cl
3.0
Br
2.8
I
2.5
Bond strength (C-X)
kJ mol-1
484
338
276
238
You may expect the fluoroalkane to react more quickly as the C-F bond is the
most polar and therefore more susceptible to attack by nucleophiles. However,
the C-F bond is the strongest. A nucleophile may be more attracted more
strongly to the carbon atom but, unless it forms a stronger bond to carbon, it
will not displace the halogen.
Actually the reaction with the iodoalkane is the most rapid. This suggests that
the strength of the C-X bond is more important than its polarity. Note that
the C-I bond is not polar. However, it is easily polarisable.
Measuring the rate of reaction
Experiment
Water is a poor nucleophile but it can slowly displace halide
ions
C2H5Br(l)
+
H2O(l)

C2H5OH(l)
+
H+(aq) + Br¯(aq)
If aqueous silver nitrate is shaken with a halogenoalkane
(they are immiscible) the displaced halide combines with a
silver ion to form a precipitate of a silver halide. The
weaker the C-X bond the quicker the precipitate appears.
hydroxide ion with bromoethane
CH3CH2Br +
OH- (aqueous)
warm
CH3CH2OH + Brethanol
Water with bromoethane
CH3CH2Br + H2O
(aqueous)
warm
CH3CH2OH + HBr
ethanol
This is a slower reaction – water is not such a good nucleophile.
Nucleophilic substitution mechanism
H
+
CH3
C
H
OH-
Br
H
CH3
C
H
ethanol
OH
-
Br
Nucleophilic substitution mechanism
H
+
CH3 C
Br
H
CH3
H
C
+
OH
H
H
-
Br
H2O
H
CH3
ethanol
C
H
OH
HBr
Nucleophilic substitution
cyanide ion with bromoethane
CH3CH2Br
+
CN-(ethanol)
reflux
CH3CH2CN + Brpropanenitrile
ammonia with bromoethane
CH3CH2Br + NH3(ethanol)
CH3CH2Br + 2 NH3(ethanol)
Heat /
pressure
Heat /
pressure
CH3CH2NH2 + HBr
aminoethane
CH3CH2NH2 + NH4+Br-
Nucleophilic substitution mechanism
H
+
CH3
C
H
CN-
Br
H
CH3
C
CN
H
propanenitrile
-
Br
Nucleophilic substitution mechanism
H
+
CH3 C
Br
H
CH3
H
C
+
NH2
H
H
-
Br
NH3
NH3
H
CH3
aminoethane
C
H
NH2
H
NH3+Br -
Past paper question
Cl2
U.V. /sunlight
Ethanolic KCN
reflux
Br2
U.V. /sunlight
AS Chemistry
Substitution vs.
Elimination
Learning Objectives
Candidates should be able to:
recall the chemistry of halogenoalkanes as
exemplified by the elimination of hydrogen bromide
from 2-bromopropane.
describe the mechanism of nucleophilic substitution
(by
both
SN1
and
SN2
mechanisms)
in
halogenoalkanes.
Starter activity
Type of
halogenoalkane
Position of
halogeno- group
Example
primary
at end of chain:
bromoethane
secondary
in middle of chain:
2-bromopropane
tertiary
attached to a carbon atom which 2-bromo-2-methylpropane
carries no H atoms:
SN1 – tertiary halogenoalkanes
Nucleophilic attack at the back of the molecule is
hindered by bulky CH3 groups. Tertiary carbocation is
stabilised by electron donating effect of CH3 groups.
SN1 or SN2 ?
Halogenoalkane
Primary
Secondary
Tertiary
Mechanism
SN2
SN1 and SN2
SN1
Elimination
You need to be aware that the hydroxide ion can act as a
strong base as well as a nucleophile.
An alternative reaction can take place in which HBr is
removed and an alkene is formed. This is known as
elimination.
CH3CH2Br + NaOH  CH2=CH2 + NaBr
+ H2O
Elimination of HBr from 2-bromopropane
CH3CHBrCH3 + OH(in ethanol)
CH3
H
H
C
C
Br
H
CH3CH=CH2 + H2O + Br-
H
H
H
CH3
OH acting as a base
propene
C
C
H
Br
-
H OH
nucleophilic substitution
+ OH- (aqueous)
alcohol
RCH3CH2OH + Br-
hydroxide acts as a nucleophile
RCH2CH2X
+ OH- (ethanol)
elimination
hydroxide acts as a base
RCH=CH2 + H2O + Xalkene
92
AS Chemistry
Pros and Cons
Learning Objectives
Candidates should be able to:
interpret
the
different
reactivities
of
halogenoalkanes e.g. CFCs; anaesthetics; flame
retardants; plastics with particular reference to
hydrolysis and to the relative strengths of the C-Hal
bonds;
explain
the
uses
hydrofluorooalkanes in
chemical inertness;
of
fluoroalkanes
and
terms of their relative
recognise the concern about the
chlorofluoroalkanes on the ozone layer.
effect
of
Starter activity
Properties:
.

Non-flammable

Low toxicity

Unreactive

Liquefy easily when compressed
Refrigerants
Propellants for aerosols

Solvents (including dry-cleaning)
Degreasers

Natural ozone layer
Replacements
•Hydrochlorofluorocarbons, HCFCs: shorter life in the
atmosphere.
•Hydrofluorocarbons, HFCs: don’t contain chlorine so
zero affect on ozone layer.
•Hydrocarbons: zero effect on ozone layer but
flammable and lead to photochemical smog.
C. Why is BCF good at extinguishing fires?
The presence of a bromine confers flame – retarding qualities on the
product.
The high temperature in fires break this compound down, producing free
radicals such as Br∙. These react with other free radicals produced during
combustion, quenching the flames.

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