12 - Alkanes Flashcards
What is a sigma bond?
Sigma bonds (σ bonds) are the strongest type of covalent chemical bond. Formed by the overlapping of atomic orbitals. Electron density of a sigma bond is an oval between atoms.
What is the bond angle in an alkane.
Tetrahedron - 109.5 degrees.
How are alkanes separated from crude oil?
Fractional distillation - in a distillation tower, different fractions contains a range of alkanes with similar boiling points. It is cooler higher up the tower so shorter alkanes with lower boiling points condense higher up.
Why does the boiling increase as the chain length of an alkane increases?
As chain length increases, surface area of the molecule increases so the surface contact between each molecule increases so the london forces between molecules will be greater so more energy is required to overcome these molecules.
What effect on boiling point does branching in the alkane have?
More branches on alkanes decreses the boiling point. This is because there are fewer points of surface contact between branched molecules giving less london forces.
Why are alkanes unreactive?
- The sigma bonds between C-H and C-C are strong.
- C-C bonds are non polar.
- The electronegativity between C and H is so similar that the bond between them is considered non polar.
Why are alkanes used as fuels?
They burn in a plentiful supply of oxygen without giving toxic by products, easy to transport and readily available.
Give the general equation for the complete combustion of an alkane.
CxHy + (x+y/4)O2 –> xCO2 + y/2H2O
Describe how an alkane reacts with halogens.
Radical substitution
- Initiation: UV light causes the diatomic molecule to split homolytically to form two halogen radicals.
- Propagation: halogen radical reacts with the CH bond forming a halogen radical and hydrogen halide. Methane radical then reacts with diatomic halide to form a halogen alkane and a halogen radical.
- Termincation, either chain gets doubled or halogen radicals bond again.
