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Chemical Reactivity of Alkanes (CIE AS Chemistry)

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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

The Pauling Scale 

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 δ- respectively)
  • Alkanes therefore do not react with polar reagents
    • They have no electron-deficient areas to attract nucleophiles
    • They also lack electron-rich areas to attract electrophiles

Examining bond polarity in ethane 

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

Examiner Tip

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

Electrophiles are positively charged and attracted to electron-rich regions

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.