CIE AS Chemistry (9701) 2019-2021

Revision Notes

3.2.1 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-rich regions.

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

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