4.2 ALKANES

Question 1

Alkane basics.

Answer 1

Alkane basics:
-general formula CnH2n+2
-saturated hydrocarbons
-each carbon atom is joined to four other atoms by single covalent bonds
-covalent bonds are all sigma bonds which can freely rotate
-used as fuels to release heat energy during combustion with oxygen

Question 2

What are sigma bonds?

Answer 2

Sigma bonds are a bond formed by the overlap of one orbital from each bonding atom, consisting of two electrons and with the electron density centred around a line directly between the nuclei of the two atoms

Question 3

How does chain length effect boiling point of isomers?

Answer 3

As chain length of isomers increase, surface area and strength of London forces also increase. This means there is more contact possible between molecules and more energy is required to overcome intermolecular forces
no branches = highest boiling point

Question 4

What is fractional distillation?

Answer 4

Fractional distillation:
-crude oil is made up of many long chained hydrocarbons
-fractional distillation is used to separate hydrocarbons with similar boiling points into fractions in a column
-hydrocarbons with smaller chains are collected at the cooler end of the column as they have lower boiling points; due to smaller surface area and chain length, London forces are weaker and the intermolecular forces are broken easier
-longer chained hydrocarbons are collected at the hotter end of the column as they have higher boiling points; due to larger surface area and larger chain length, London forces are greater, intermolecular forces are harder to overcome

Question 5

Reactivity of alkanes.

Answer 5

Reactivity of alkanes:
-combust readily and are therefore commonly used as fuels- butane in lighters, octane in motor vehicles
-will react with halogens under suitable conditions

Question 6

Issues with products of combustion.

Answer 6

Issues with products of combustion:
-Co2 = greenhouse gas, contributes to global warming
-CO = colourless, odourless, toxic gas which combines with haemoglobin in red blood cells, preventing oxygen transportation around the body (carbon monoxide)
-carbon particulates = make surfaces dirty and contribute to global dimming

Question 7

Reactions with halogens.

Answer 7

Reactions with halogens:
-FREE RADICAL SUBSTITUTION REACTIONS
-UV radiation (present in sunlight) provides initial energy required for a reaction between a halogen and an alkane to take place = photochemical reaction
-also called substitution reaction as hydrogen atoms of the alkane are replaced by halogen atoms

Question 8

Reaction mechanism- chlorination of alkanes.

Answer 8

Reaction mechanism- chlorination of alkanes:
STEP 1- initiation = covalent bond in chlorine molecule broken down by homolytic fission (energy provided by UV radiation), each Cl atom takes one electron from shared pair, forms two highly reactive chlorine radicals
STEP 2- propagation = Cl radical reacts with C-H bond in methane; forming methyl radical and molecule of hydrogen chloride
= each methyl radical reacts with another Cl molecules forming chloromethane and a new Cl radical
STEP 3- termination = propagation is terminated when two radicals collide (can be after millions of propagation cycles) forming a molecule with all electrons paired

Question 9

Limitations in organic synthesis.

Answer 9

Limitations in organic synthesis:
-monosubstituted products (H replaced with Cl)
-further substitution can occur until all H atoms have been replaced with Cl. results in a mixture of organic products which is not desirable