Advanced A Level Organic Chemistry: Reaction mechanisms - hydrogenation of alkenes

Scroll down and take time to study the content and/or follow links or [Use the website search box]

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

Advanced Level Revision Notes PART 10 Summary of organic reaction mechanisms - A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

email doc brown - comments - query?

Index of organic chemistry technical terms and mechanism pages

Index of ALL advanced revision notes on ALKENES

All my advanced A level organic chemistry notes

Index of GCSE level oil and basic organic chemistry notes

Use your mobile phone or ipad etc. in 'landscape' mode

This is a BIG website, you need to take time to explore it [SEARCH BOX]

10.3 Reaction mechanisms of ALKENES

10.3.8 Hydrogenation of alkenes

doc b

  • A HETEROGENEOUS CATALYST IS IN A DIFFERENT PHASE (often solid state') THAN THE REACTANTS (often gaseous or liquid/solution)

  • The catalyst and reactants are in different phases (usually solid catalyst and liquid/gaseous reactant)

  • The reaction occurs on the catalyst surface which may be the transition metal or one of its compounds, examples quoted above. The reactants must be adsorbed onto the catalyst surface at the 'active sites'.

  • This can be physical adsorbed or 'weakly' chemically bonded to the catalyst surface. Either way, it has the effect of concentrating the reactants close to each other and weakening the original intra-molecular bonds within the reactant molecules and so allows a greater chance of 'fruitful' collision.

    • The diagram above illustrates the catalytic conversion of an unsaturated alkene to a saturated alkane

      1. The mixture of hydrogen and alkene gas will both collide with the catalyst surface.

      2. In the manufacture of margarine, the hydrogen gas is bubbled into the vegetable oil mixed with a special nickel based catalyst.

      3. Some of the molecules of both reactants will be physical adsorbed onto the nickel catalyst surface and held there by intermolecular forces.

      4. If the adsorption, via the intermolecular forces, is strong enough, the hydrogen-hydrogen bonds and pi electron bond of the alkene are weakened sufficiently to form weak hydrogen-nickel and alkene-nickel bonds.

      5. If too strongly held, i.e. chemisorption, the reaction will halt as the catalyst surface becomes 'poisoned' with reactant molecules - the active sites on the metal surface are blocked with reactant or product molecules.

      6. The hydrogen molecule can then be split and the resulting hydrogen atoms can then bond with the carbon atoms of the 'opened' double bond of the alkene.

      7. The alkene molecule is temporarily held by the weak intermolecular forces but will then break away from the catalyst surface to be replaced by more reactant molecules.

  • Nickel, Ni(s), catalyses the addition of hydrogen to an alkene double bond, e.g. in the hydrogenation of unsaturated vegetable oils to make more saturated margarine with a slightly higher softening point making it more spreadable.

    • The addition reaction is:

    • R-CH=CH-R + H2 == Ni catalyst ==> R-CH2-CH2-R

    • e.g. ethene is hydrogenated to ethane when it is mixed with hydrogen and passed over a nickel catalyst at 150oC or over a platinum catalyst at room temperature.

    • R is the 'rest' of the molecule in this catalytic hydrogenation reaction in organic chemistry.

    • This is the reaction to 'harden' of polyunsaturated vegetable oils to margarine.

    • The hydrogenated oils become less unsaturated giving them a higher softening point and a more 'spreadable' product for the consumer.

keywords phrases: catalytic hydrogenation reaction nickel catalyst ethene hydrogen conditions formula intermediates organic chemistry reaction mechanisms steps


 COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

Isomerism - chain isomerism, positional isomerism, functional group isomerism & Tautomerism

Stereoisomerism introduction, E/Z isomerism ('ex' Geometric/Geometrical cis/trans Isomerism)

R/S Optical Isomerism and chiral auxiliary synthesis

All Advanced Organic Chemistry Notes

Summary of Organic Functional Groups

Quiz on Organic Structure Recognition

Summary of organic chemistry functional group tests

The shapes and bond angles of simple organic molecules

Aliphatic structure & nomenclature m/c BUMPER QUIZ

Type in name aliphatic structure and nomenclature BUMPER QUIZ


KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry