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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
Part 2.5 The
reaction of hydrogen with alkenes - hydrogenation
The structure and properties of
oils and
fats
INDEX of ALKENE revision notes
All Advanced A Level Organic
Chemistry Notes
GCSE/IGCSE Oil - useful products, organic chemistry Notes
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Hydrogenation and the structure and properties of fats
A simple example is:
propene + hydrogen ==
Ni catalyst ==> propane
CH3–CH=CH2 + H2
===> CH3–CH2–CH3
 H2
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)
Some extra notes for advanced organic chemistry students
More
on hydrogenation and the structure of oils and fats
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 much weaker than 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.
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
Q3
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What
next?
Index of all the ALKENE revision notes
All Advanced Level Organic
Chemistry Notes
Index of GCSE/IGCSE Oil - Useful Products
Chemistry Revision Notes
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