Explain the low reactivity of alkanes
Relate alkane unreactivity to strong, nearly non-polar C–H bonds and weak attraction to polar reagents.
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
Which description best fits a typical alkane C–H bond?
What you will learn
- Relate C–H bond strength to reaction energy
- Describe the limited polarity of C–H bonds
- Explain why many polar reagents do not attack alkanes readily
Strong bonds are difficult to break
Alkanes contain strong C–C and C–H sigma bonds. A reaction that breaks these bonds needs an input of energy; the C–H bonds are not easily broken under mild conditions.
A strong bond does not mean a reaction is impossible. Suitable conditions, such as ultraviolet light in halogen substitution, can initiate a pathway that forms reactive radicals.
The bonds have little polarity
Carbon and hydrogen have similar electronegativities, so a C–H bond has only a small polarity. Alkanes have no strongly polar functional group for an ionic or polar reagent to attack.
The syllabus explanation combines bond strength with relative lack of polarity. Avoid saying every alkane bond is perfectly non-polar or that alkanes never react.
Pause and explain
Why is a polar reagent often unreactive toward an alkane under mild conditions?
Worked example
Explain why an alkane usually does not react readily with an aqueous ionic reagent under mild conditions.
Show the worked solution
- Alkanes have strong C–C and C–H bonds.
- Their C–H bonds are only weakly polar and they have no strongly polar functional group.
- There is little attraction for the ionic reagent and bond breaking has an energy cost.
Answer Alkanes are relatively unreactive because their strong C–C/C–H bonds are difficult to break and their nearly non-polar molecules interact weakly with polar reagents.
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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.
Give two reasons an alkane is relatively unreactive toward polar reagents.
Give me a hint
Include a bond-energy reason and a polarity reason.
Compare my reasoning
- C–C and C–H bonds are strong.
- C–H bonds have little polarity.
- The molecule therefore offers little electrostatic attraction and bonds are not readily broken under mild conditions.
Strong C–C/C–H bonds are difficult to break, and the nearly non-polar alkane has little attraction for polar reagents.
Look for these in your work
- Bond strength included
- Polarity included
- Both linked to reactivity
A learner claims alkanes never react. Use halogen substitution to refine the statement.
Give me a hint
Distinguish relative reactivity from an absolute claim.
Compare my reasoning
- Alkanes are relatively unreactive under mild conditions.
- UV light initiates a radical substitution with chlorine or bromine.
- Therefore they can react under suitable conditions.
Alkanes are relatively rather than completely unreactive; ultraviolet light can initiate halogen substitution.
Look for these in your work
- Relative term used
- Suitable condition named
- Counterexample applied
Explain why an alkene reacts with bromine more readily than an alkane does under the standard test conditions.
Give me a hint
Compare a reactive π bond with the alkane bond framework.
Compare my reasoning
- An alkene has an electron-rich C=C π bond.
- The π electrons can attract an electrophile and participate in addition.
- An alkane lacks this π bond and has strong, nearly non-polar sigma bonds.
The alkene π bond is available for electrophilic addition; the alkane has only strong, nearly non-polar C–C/C–H sigma bonds and is relatively unreactive under those conditions.
Look for these in your work
- π bond identified
- Addition linked
- Alkane comparison accurate
Common mistakes
- Saying alkanes are inert in all conditions
- Claiming C–H bonds are ionic
- Using polarity alone and ignoring bond strength
What can you explain now?
Which description best fits a typical alkane C–H bond?
Compare with the explanation
Strong and only slightly polar
Both bond strength and limited polarity help explain alkane unreactivity.
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Make it stick
Which two molecular features combine to explain why alkanes are relatively unreactive?
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.