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Pre-university Advanced Level Organic Chemistry: Alkenes - reaction with hydrogen - hydrogenation

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Part 2. The chemistry of ALKENES - unsaturated hydrocarbons

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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

doc b oil notes  doc b oil notes H2 doc b oil notes doc b oil notes

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.

alkenes structure and naming (c) doc b  +  2H2  ===> displayed formula butane molecular structure and naming of alkanes nomenclature 

alkenes structure and naming (c) doc b  +  2H2  ===>  structural formula butane skeletal formula alkanes molecular structure naming (c) doc b

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

skeletal formula of saturated fats unsaturated fats structure of fatty acids triglycerides advanced organic chemistry

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.

comparing the skeletal formulae of an unsaturated vegetable oil and saturated animal fat advanced A level organic chemistry doc brown's revision notes

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.

alkene group in cholesterol advanced A level organic chemistry doc brown's revision notesUnsaturated 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 alkenes structure and naming (c) doc b , other isomers include methylcyclopentenes, dimethylcyclobutenes or even trimethylcyclopropenes!

The reaction for cyclohexene to give cyclohexane would be: alkenes structure and naming (c) doc b  +  H2  ===> alkanes structure and naming (c) doc b

 

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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