alkane facts

• What are alkanes?

  saturated (all the carbon-carbon bonds are single bonds) hydrocarbons (contain only carbon and hydrogen atoms)

  has the general formula CnH2n+2

  every carbon atom in an alkane has four single bonds with other atoms
• Alkanes:

  • alkane molecules are tetrahedral around each carbon
  • each carbon atom has 4 pairs of bonding electrons around it
  • they all repel each other equally
  • each bond angle is 109.5 degrees
• Alkanes have covalent bonds inside the molecules

  Between the molecules, there are induced dipole-dipole interactions (LDF) which hold them all together

  The longer the carbon chain, the stronger the induced dipole-dipole interactions as there is more surface contact and electrons to interact

  As the molecules get longer, it takes more energy to overcome the induced dipole-dipole interactions, hence the boiling point rises
• Why does a branched chain alkane have a lower boiling point than its straight chain isomer?

  As branched chain alkanes cannot pack closely together so they have less surface contact between molecules and they have smaller molecular surface areas, so the induced dipole-dipole interactions are reduced, thus require less energy to break the IMF and change state.
• What is the general formula of cycloalkanes?

  CnH2n
• Why do boiling points increase of cycloalkanes (cyclopropane -> cyclopentane -> cyclohexane)?

  More carbons in the ring and more surface contact, so stronger London dispersion forces, thus more energy is required to break the intermolecular forces.

  (Increasing molecular size and surface area.)
• What is an alkane?

  a saturated hydrocarbon containing C-H bonds only
• What is the general formula of an alkane?

  CnH2n+2
• Are alkane bonds polar? Why/Why not?

  No. Alkane bonds are nonpolar because carbon and hydrogen have similar electronegativities.

  (the electronegativity difference between carbon and hydrogen is very small)
• What is the shape and angle of an alkane?

  tetrahedral

  109.5 degrees

  (4 bonded electron pairs, electron pairs repel to give the largest bond angle possible)
• What is the sigma bond in alkanes?

  the sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms
• What type of intermolecular forces do alkanes have, why?

  London dispersion forces (induced dipole-dipole interaction), as the bonds are non-polar.
• What happens to the boiling point as alkane chain length increases, why?

  The bp increases as there is more surface area so more induced dipole-dipole interaction

  thus, more energy is required to overcome the attraction
• Are alkanes soluble in water, why?

  alkanes are insoluble in water as hydrogen bonds in water are stronger than alkane's London dispersion forces of attraction
• What reactions will alkanes undergo, even though they are very unreactive?

  combustion and reactions with halogens
• What are alkanes used as & why?

  fuels

  as they readily burn in the presence of oxygen
• what is the general formula of cycloalkanes?

  CnH2n