Chapter 12, Alkanes, Organic

12.1,2

What are alkanes (definition, bonds and general formula)?

Alkanes are saturated hydrocarbons that consist of only hydrogen and carbon atoms linked by single/ sigma bonds. The general formula of alkanes are CnH2n+2

Define sigma bonds.

A bond that results from the direct overlap of two different atoms (two orbitals), where two electrons are shared between the bonding atoms, one from each bonding atom.

What is the specific property of a sigma bond and why?

They are free to rotate. Due to a cylindrically symetrical distribution of electron density.

Describe the shape and bond angle of an alkane.

Alkanes have 4 valency electrons, so it can form four bonding areas, making it a tetrahedral shape- 109.5.

Describe how different arrangements of the alkane can affects it boiling point. (2)

1. Chain length- the longer the chain length of an alkane, the higher the boiling point. This is due to a stronger induce dipole dipole (London forces) between the large conact surface area. More energy is needed to overcome the forces.

2. Branching- The more branching areas, the lower the boiling point. This is due to a smaller contact surface area for London forces to act on between molecules. Less energy is needed to overcome these forces.

Explain why alkanes have low reactivity. (3)

1. C-C and C-H bonds have high bond enthalpy ( energy required to break one mole of gaseous bonds) due to strong sigma bonds.

2. C-C bonds are non- polar due to no difference in electronegaivity.

3. C-H bonds are also non- polar due to small or no difference in electronegativity in between.

Explain why alkanes are used as a fuel through complete combustion. (3)

1. Easy to transport

2. Able to produce heat without releasing toxic wastes (only water and carbon dioxide)

3. Really available from natural resources (can be fractional distillated)

Explain how incomplete combustion can be harmful.

Imcomplete combustion of alkanes forms when there is a limited suply of oxygen which produce water and carbon/ carbon monoxide.

Carbon monoxide is odourless and clolurless - bind to haemoglobin- replace oxygen- not enough supply of oxygen in blood.

What happens when alkanes undergoes the free radical substitution reaction with halogens?

Where a hydrogen atom is replaced by a halogen with the presence of UV light.

Explain why free radical substitution only happens when there are UV light.

Because UV light provide high energy that breaks the covalent bond in the halogen molecule to start the reaction.

Describe the mechanism of a free radical substitution reaction. (3)

1. Initialtion- Where UV light break halogen’s covalent bonds. Two halogen radical forms by homolytic fission where electrons are split equally.

2. Propagation- Where radicals react with other species/ alkanes to form alkane radical (missing one hydrogen) and hydrogen halide (HX). The alkane radical then react further with a halogen to form the product needed/ alkane(missing hydrogen) halide and a halogen radical. radical is regenerated.

3. Termination- Final step where the radicals can combine into different possibles and form different types of products (stable molecules).

What are the three main limitations of free radical substitution.

1. Impurities- different products formed in termination step where it is not the desire yield.

2. Further substitution- when the alkane keep having the hydrogen replaced when not neccessary (when the monosunstituted product is already formed) in the propagation step (until all hydrogen are substituted. Which leads to a mixture of products.

3. Substitution on a different carbon atom in the same carbon chain- forms a structural isomer instead of a single, pure product.