Reactions of Alkenes with Hydrogen
A concise revision guide to the reaction of alkenes with hydrogen, including the change from alkene to alkane, the use of a nickel catalyst and why this reaction is both an addition reaction and a reduction reaction.
- 4.6-a
- 4.6-d
- 5.4i
What these spec points say
- 4.6-a be able to classify reactions as addition
- 4.6-d be able to classify reactions as reduction
- 5.4i be able to describe the reactions of alkenes, limited to: the addition of hydrogen, using a nickel catalyst, to form an alkane
Hydrogenation on a Catalyst Surface
Choose a step to watch ethene and hydrogen meet on a nickel surface and leave as ethane.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Catalysts and Hardening Oils
Answer three quick GCSE questions before you start.
The Overall Reaction
Alkenes react with hydrogen, H2, in the presence of a nickel catalyst. The reaction converts an alkene into an alkane.
The C=C double bond opens and one hydrogen atom adds to each carbon atom from the original double bond. This removes the alkene functional group and produces a saturated hydrocarbon.
For example, ethene reacts with hydrogen to form ethane.
Key idea: Hydrogen adds across the C=C double bond, so the alkene becomes an alkane.
Ethene reacts with H2; the C=C double bond is converted into a C-C single bond as ethane forms.
Quick Check: How Much Hydrogen?
Apply the conditions to a molecule that has more than one double bond.
Watch: the bond most students forget in hydrogenation
An exam question on hydrogenating ethene: which bonds break? Most students miss one of them.
Why This Is an Addition Reaction
This reaction is an addition reaction because atoms are added across the carbon-carbon double bond.
The alkene starts with a C=C double bond, meaning it is unsaturated. During the reaction, the double bond opens and two new C-H bonds form.
The product is an alkane, which is saturated because it contains only single covalent bonds between carbon atoms.
Hydrogenation: The addition of hydrogen across a C=C double bond, converting an alkene into an alkane.
Remember: In alkene addition reactions, the C=C double bond is the reactive site. The double bond opens so new atoms can bond to the two carbon atoms.
Hydrogen and the alkene are held on the nickel surface, the H–H bond breaks and one hydrogen atom adds to each carbon of the C=C.
Quick Check: Formulae, Names and Amounts
Answer as quickly as you can, using alkenes that are not on this page.
Why This Is Also a Reduction Reaction
The reaction with hydrogen can also be described as a reduction reaction.
In organic chemistry, reduction often means the addition of hydrogen to a molecule. Since the alkene gains hydrogen atoms, the reaction is described as reduction.
This is why the same reaction can be correctly described in two ways: addition, because atoms add across the double bond, and reduction, because hydrogen is added.
Step 1: Hydrogen is the reagent
The alkene reacts with H2 in the presence of a nickel catalyst.
Step 2: The double bond opens
The C=C bond is replaced by a C-C single bond as new C-H bonds form.
Step 3: An alkane forms
The product is saturated because it no longer contains the alkene C=C functional group.
Exam focus: If asked for the reaction type, addition is essential. Reduction is also valid because hydrogen is added.
Adding hydrogen across C=C bonds is both addition and reduction; it turns liquid vegetable oils into solid fats.
Quick Check: Two Names for One Reaction
Drag the words into place to describe the hydrogenation of a ring alkene that is not on this page.
Quick Check: Pick the Accurate Statement
In each round, choose the one statement that is accurate.
Common Exam Mistakes
Students often lose marks by forgetting the nickel catalyst, or by describing the reaction as substitution. This is not substitution because no atom is replaced.
Another common error is to leave the product as an alkene. The product must be an alkane, because the carbon-carbon double bond has been removed.
For a simple reaction question, the safest answer should include the reagent, condition, product type and reaction type.
Exam focus: Link the functional group change to the reaction type: alkene to alkane happens because hydrogen adds across the C=C double bond.
FAQs
These questions secure the catalyst, reaction-type and quantity points for hydrogenation.
Why is a catalyst needed if hydrogen simply adds across the C=C bond?
The reaction has a high activation energy, so it is very slow on its own. Finely divided nickel gives a surface where the alkene and H2 are adsorbed and react, lowering the activation energy. The mixture is usually heated to about 150 °C.
Why is hydrogenation also called reduction?
In organic chemistry, gaining hydrogen counts as reduction. The alkene gains two hydrogen atoms, so the same reaction is both an addition and a reduction.
How much hydrogen reacts with a molecule that has two C=C bonds?
One mole of H2 adds across each C=C bond. Buta-1,3-diene, C4H6, therefore needs 2 mol of H2 to form butane, C4H10.
Where is hydrogenation used in industry?
It is used to harden vegetable oils to make margarine. Hydrogen adds to some of the C=C bonds in the unsaturated fat chains, raising the melting temperature so the oil becomes a soft solid.
Do I need to draw an electrophilic addition mechanism for hydrogenation?
No. H2 is non-polar and the reaction happens on the nickel surface, so no curly-arrow mechanism is expected. State that it is an addition, and a reduction, using a nickel catalyst.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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