CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.5 Chemical Reactivity of Alkanes

Unreactivity of Alkanes

Strength of C-H bonds

  • Alkanes consist of carbon and hydrogen atoms which are bonded together by single bonds
  • Unless a lot of heat is supplied, it is difficult to break these strong C-C and C-H covalent bonds
  • This decreases the alkanes’ reactivities in chemical reactions

Lack of polarity

  • The electronegativities of the carbon and hydrogen atoms in alkanes are almost the same
  • This means that both atoms share the electrons in the covalent bond almost equally

 

Hydrocarbons Pauling Scale of Elements, downloadable AS & A Level Chemistry revision notes

The Pauling Scale shows that the difference in electronegativity between carbon and hydrogen is only 0.4

 

  • As a result of this, alkanes are nonpolar molecules and have no partial positive or negative charges (δ+ and δrespectively)
  • Alkanes therefore do not react with polar reagents
    • They have no electron-deficient areas to attract nucleophiles
    • And also lack electron-rich areas to attract electrophiles

 

Hydrocarbons Polarity Alkanes, downloadable AS & A Level Chemistry revision notes

Ethane is an example of an alkane that lacks polarity due to almost similar electronegativities of the carbon and hydrogen atoms

 

  • Due to the unreactivity of alkanes, they only react in combustion reactions and undergo substitution by halogens
    • Both processes require the input of a lot of heat (energy) and are therefore endothermic reactions

Exam Tip

Remember: nucleophiles are negatively charged and are attracted to electron-deficient regions.

Electrophiles are positively charged and attracted to electron-rich regions.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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