CIE A Level Chemistry复习笔记3.2.5 Chemical Reactivity 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 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
  • And also lack electron-rich areas to attract electrophiles

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

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