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

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

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

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

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 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.

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

 

Q3


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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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