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Part 2.
The chemistry of
ALKENES - unsaturated hydrocarbons
Doc Brown's
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK
KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry
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My advanced A level organic
chemistry index of notes on alkenes
Index of GCSE level Oil - Useful Products
Chemistry Revision Notes
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Part 2.5 The
reaction of hydrogen with alkenes - hydrogenation, structure and properties of
oils and
fats and some exemplar calculations
Hydrogenation and the structure and properties of fats
A simple example of this addition reaction is:
propene + hydrogen ==
Ni catalyst ==> propane
CH3–CH=CH2 + H2
===> CH3–CH2–CH3
 H2
In the hydrogenation process, a mixture of the alkene and hydrogen is
passed over a nickel catalyst at ~150oC.
Raney nickel catalyst is a fine powder of a nickel-aluminium alloy.
The mechanism involves addition of hydrogen atoms adding to the C=C
double bond on the surface of the catalyst - which lowers the activation of
he reaction by facilitating the cleavage of strong H-H bonds to provide the
highly reactive hydrogen atoms (illustrated above for ethene).
If there is more than one double bond in the molecule, the number of
moles of hydrogen added increases e.g.
hydrogenation of buta-1,3-diene to butane require 2 moles of hydrogen per
mole of alkene.
+ 2H2 ===>
+ 2H2 ===>
See further down and example of hydrogenating a vegetable oil to make
margarine.
For basic notes see
the structure and
properties of oils, fats and margarine (no need to repeat
here)
See the page with a
detailed description and mechanism
of hydrogenation of alkenes (no need to repeat here)
More
on hydrogenation and the structure of oils and fats - use of skeletal formulae
Triglyceride fats are pretty big
molecules and best represented using skeletal formula
The geometry of the double bond is almost always a Z (cis) configuration in
natural fatty acids.
These molecules
do not compact together very well.
The
intermolecular interactions are slightly more weaker than in saturated molecules.
As
a result, the melting/softening points are lower for unsaturated
fatty acids.
That is why vegetable oils are
partially hydrogenated to raise their softening/melting point.
For more on margarine see
alkenes section 2.9
The Z stereoisomeric geometry and
rigidity of the double bond reduces the ability of the molecules to pack
as close together reducing
the effective intermolecular attractive forces.
Fish oils are highly unsaturated and remain
liquid even in the low temperatures of arctic waters - so the natural oils don't
freeze inside the fish.
 Unsaturated oils are metabolised more
easily than saturated fats and fish oils can help reduce cholesterol levels.
Cholesterol is an essential steroid-sterol to humans but if
too much is produced it can cause heart disease.
The
image on the left gives the skeletal formula structure of cholesterol.
Note the 'alkene' double
bond functional group to the right of the –OH group.
[Cholesterol
image from NIST]
Some calculations based on
hydrogenation
Q1 1.64 g of an
unsaturated hydrocarbon of formula C6H10, combines
with 480 cm3 of hydrogen (H2) at room
temperature and pressure.
(Atomic masses: C = 12, H = 1 and assume
1 mole of gas equals 24.0 dm3)
Calculate (a) how many double bonds the molecule has and
(b) suggest structures for it giving your reasoning.
(a) Formula mass of hydrocarbon = (6 x 12) + 10 = 82
Moles hydrocarbon = 1.64 / 82 = 0.02
Moles hydrogen = 480 / 24 000 = 0.02
The
reactant ratio is 1 : 1, so there must be only one carbon - carbon
double bond to be hydrogenated in the unsaturated hydrocarbon molecule
(b) If it was a non-cyclic alkene with
one double bond, the molecular formula would be based on CnH2n.
This would make it C6H12,
but it is C6H10.
The structure must be therefore a cyclic alkene with
one double bond. Possibilities:
cyclohexene
, other isomers include methylcyclopentenes, dimethylcyclobutenes or
even trimethylcyclopropenes!
The reaction for cyclohexene to give cyclohexane would be:
+ H2 ===>
Q2 0.450 g of a
non-cyclic unsaturated hydrocarbon of formula C5H8,
combines with 317 cm3 of hydrogen (H2) in a
hydrogenation reaction at room temperature and pressure.
(Atomic masses: C = 12, H = 1 and assume
1 mole of gas equals 24.0 dm3)
Calculate (a) how many double bonds the molecule has and
(b) suggest structures for it giving your reasoning.
(a) Formula mass of hydrocarbon = (5 x 12) + 8 = 68
Moles hydrocarbon = 0.45 / 68 =
0.006618 (4 sf) Moles hydrogen
= 317 / 24 000 = 0.01321 (4 sf)
0.01321 / 0.006618 = 1.996 (very close to 2.0)
The reactant ratio is 1 : 2 for C5H8
: H2, so there must be two carbon - carbon double bonds in
the molecule.
(b) If it was a non-cyclic alkene with
one double bond, the molecular formula would be based on CnH2n.
This would make it C5H10,
but it is C5H8 because of the extra double bond
The structure must be therefore a non-cyclic 'diene'
alkene with two double bonds based on C5H8.
There are quite a few isomers of C5H8
e.g.
penta-1,3-diene H3C-CH=CH-CH=CH2,
penta-1,2-diene H2C=C=CH-CH2-CH3,
and other isomers.
On hydrogenation of these two, the saturated alkane
pentane would be formed:
{H3C-CH=CH-CH=CH2
or H2C=C=CH-CH2-CH3} + 2H2
===> H3C-CH2-CH2-CH2-CH3
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