4.3.5

Structure of Alkanes

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The Structure of Alkanes

Alkanes are hydrocarbons, so contain only hydrogen and carbon atoms. Alkanes are fully saturated molecules, meaning that they only contain C-H and C-C bonds.

Sigma (σ) bonds

Sigma (σ) bonds

  • The C-H and C-C bonds found in alkanes are known as σ bonds.
  • In σ bonds, the electron orbitals overlap directly between the two atomic nuclei.
  • σ bonds also allow free rotation of the atoms at either end.
General formula

General formula

  • For every n carbon atoms, there will be 2n+2 hydrogen atoms in an alkane.
  • CnH2n+2 is the general formula for the entire homologous series of alkanes, whether straight-chain or branched.
Carbon atom bond angles

Carbon atom bond angles

  • Every carbon atom in an alkane is bonded to four other atoms (either H or C) via single covalent bonds.
  • Electron pair repulsion theory states that regions of negative charge about an atom will repel to be as far apart from each other as is possible in a sphere.
  • Four regions of negative charge around each carbon atom in an alkane repel to form a tetrahedral arrangement of bonds, with the angle between the bonds at 109.5o.
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Principles of Science I

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