Add hydrogen halides to alkenes
Describe room-temperature electrophilic addition of gaseous HX to an alkene.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the product class when HX adds to an alkene?
What you will learn
- Recognise hydrogen-halide addition
- Write a balanced addition product
- Apply the major-product orientation to propene
HX adds across the alkene bond
A hydrogen halide, HX(g), adds to C=C at room temperature. H bonds to one alkene carbon and X bonds to the other; the carbon skeleton remains intact.
Ethene gives bromoethane with HBr: CH₂=CH₂ + HBr → CH₃CH₂Br. For an unsymmetrical alkene, the major product can be predicted from carbocation stability and Markovnikov addition, developed in a later lesson.
Predict products without changing the carbon count
For propene, addition of HBr gives 2-bromopropane as the major product in the standard orientation.
The equation records the major product; avoid claiming a universal absence of minor products unless the question explicitly asks for the main product.
Pause and explain
What major product forms from propene and HBr?
Worked example
Predict the major product when propene reacts with HBr(g) at room temperature.
Show the worked solution
- The alkene has a terminal and an internal carbon in C=C.
- The major orientation places Br on carbon 2.
- Name the three-carbon halogenoalkane.
Answer CH₃CH=CH₂ + HBr → CH₃CHBrCH₃; 2-bromopropane is the major product.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the equation for HCl adding to ethene.
Give me a hint
Add H and Cl to the two alkene carbons.
Compare my reasoning
- Retain two carbon atoms.
- Change the C=C to C–C.
- Place H and Cl on the product chain.
CH₂=CH₂ + HCl → CH₃CH₂Cl.
Look for these in your work
- Two-carbon skeleton retained
- H and Cl added
- Equation balanced
Predict the major product of HBr addition to but-1-ene.
Give me a hint
Apply the usual orientation to the unsymmetrical alkene.
Compare my reasoning
- H adds to the terminal alkene carbon.
- The secondary carbocation orientation is favored at carbon 2.
- Br attaches to carbon 2.
2-bromobutane is the major product.
Look for these in your work
- Correct carbon skeleton
- Major orientation reasoned
- Product named
A student says hydrogen bromide turns propene into propane. Explain the error.
Give me a hint
Check what atom group is supplied by HBr.
Compare my reasoning
- HBr supplies both H and Br.
- The carbon skeleton receives a bromine substituent.
- Propane would require H₂ addition, not HBr.
HBr forms a bromopropane, not propane; hydrogenation with H₂ forms propane.
Look for these in your work
- Reagent composition used
- Halogenoalkane identified
- Hydrogenation distinguished
Common mistakes
- Breaking the carbon skeleton
- Placing Br on the terminal carbon as the major product without reasoning
- Using UV light as a condition for alkene HX addition
What can you explain now?
What is the product class when HX adds to an alkene?
Compare with the explanation
A halogenoalkane
The hydrogen halide adds H and X across the double bond.
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Make it stick
How does HX addition differ from hydrogenation in the reagent and product?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.