Alkanes

• What are alkanes?

  The main components of natural gas and crude oil. Amongst the most stable organic compounds, and their lack of reactivity has allowed crude oil deposits to remain in the earth for many millions of years. Mainly used as fuels - exploiting their reaction with oxygen to generate heat
• What is the bonding in alkanes?

  carbon and hydrogen atoms joined together by single covalent bonds. each carbon atom in an alkane is joined to 4 other atoms by a type of single covalent bond known as a sigma bond
• What is a sigma bond?

  A covalent bond is defined as a shared pair of electrons. A sigma bond is the result of the overlap of 2 orbitals, one from each bonding atom. Each overlapping orbital contains one electron, so the sigma bond has 2 electrons that are shared between the bonding atoms. A sigma bond is positioned on a line directly between bonding atoms
• How many sigma bonds does each carbon atom in an alkane have?

  4
• Shapes of alkanes?

  each carbon atom is surrounded by 4 electron pairs in 2 sigma bonds. repulsion between these bonds results in a 3D tetrahedral arrangement around each carbon atom - 109.5 degrees.
• What do sigma bonds act as and what does this result in?

  axes around which the atoms can rotate freely - these shapes are not rigid
• Boiling points increase as chain length increases
• Why does boiling point increase?

  Due to weak intermolecular forces - London forces. These forces hold molecules together in solids and liquids, but once broken, the molecules move apart from each other and the alkane become a gas. The greater the intermolecular forces, the higher the boiling point.
• What is the effect of chain length on boiling point?

  London forces act between molecules that are in close surface contact. As chain length increases, the molecules have a large surface area, so more surface contact is possible between molecules. The London forces between molecules will be greater so more energy is required to over come the forces.
• What is the effect of branching on boiling point?

  Isomers of alkanes have the same molecular mass, if you compare boiling points of branched isomers with straight-chain isomers, you find that the branched isomers have lower boiling points. This is again because of London forces - there are fewer surface points of contact between molecules of the branched alkanes, giving fewer London forces. Another factor is the shape of the molecules. The branches get in the way and prevent branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further