Alkanes

Question 1

What is a hydrocarbon

Answer 1

Compounds that contain only carbon and hydrogen atoms

Question 2

Different types formulae (Empricical, molecular, general, structural, displayed and skeletal)

Answer 2

Empirical Formula
the simplest whole number ratio of atoms of each element in a compound

Molecular Formula
The true number of atoms of each element in a compound

General Formula
All members of a homologous organic series follow the general formula. For example, alkanes have the general formula C_nH_2n+2

Structural Formula
Shows the structural arrangement of atoms within a molecule e.g. CH₃CH₃ (formula for ethane)

Displayed Formula
Shows every atom and every bond in an organic compound

Skeletal Formula
Shows only the bonds in a compound and any non-carbon atoms. The verticies are carbon atoms and hydrogen is assumed to be bonded to them unless stated otherwise

Question 3

What is a ‘homologous series’?

Answer 3

A series of organic compounds that follow a general formula and react in a very similar way.

Each consecutive member differs by CH₂, and boiling point increases with chain length

Question 4

What is a ‘functional group’?

Answer 4

Each homologous series has a functional group that allows the molecule to be recognised to be apart of that group because it makes it react in a certain way

Question 5

How to name compounds (IUPAC)

Answer 5

1. Stem
Find the longest unbroken carbon chain in the compound (e.g. methane, hexane, etc.)

2. Functional groups
The ending of the compound name tells you the functional group present.
(e.g. alkane = -ane, alcohol = -ol)

If there is more than one present, it is added onto the end (e.g. eth-an-ol, meth-an-oic acid)

If a halogen is present it is added as a prefix (e.g. Bromo-methane, Chloro-ethene)

3. Side chains
* Functional groups and side chains are given with the name of the carbon they are attached to
* Numbers are seperated by commas
* Numbers and words are seperates by hyphens
* If more than one of that group/side chain is present then prefixes are added: di- (2) tri- (3) tetra- (4) etc
* The carbon chain is numbered in ascending order from the end of the chain nearest a functional group
* If multiple prefixes are present, they are put in alphabetical order

Question 6

Prefixes for compounds up to C₁₀

Answer 6

C₁ = meth-
C₂ = eth-
C₃ = prop-
C₄ = but-
C₅ = pent-
C₆ = hex-
C₇ = hept-
C₈ = oct-
C₉ = non-
C₁₀ = dec-

Question 7

Reaction mechanisms
(addition, substitution, oxidation, reduction, polymerisation)

Answer 7

Types of reactions
Addition:
The reactants combine to form a single product
Substitution:
One functional group is replaced by a different functional group
Oxidation:
A species loses at least one electron, and is oxidised
Reduction:
A species gains at least one electron, and is reduced
Polymerisation:
Many small molecules (monomers) join together to form a long, repeating molecule called a polymer

Question 8

What is isomerism? (structural, position, functional group)

Answer 8

Isomerism: molecules with the same molecular formula but a different arrangement of atoms within the molecule

Structural isomerism:
same molecuclar formula but a different structural arrangement of atoms. They can be straight chains or branched chains.

position isomers:
The same functional group is in a different position on the carbon chain

functional group isomerism:
same molecular formula but molecules have a different functional group

Question 9

What is stereoisomerism?

Answer 9

Molecules with the same molecular formula but different spatial arrangement
OO
OO

OO
OO

Question 10

The general formula for alkanes

Alkanes

Answer 10

C_nH_2n+2

Question 11

Why are alkanes and cycloalkanes saturated hydrocarbons?

Alkanes

Answer 11

they contain only single bonds between carbon atoms

Question 12

Fractional distillation of crude oil

Alkanes

Answer 12

Crude oil is a mixture of different hydrocarbons. Fractional distillation seperates it into seperate molecules, as the different chain lengths of molecules result in them having different boiling points

Steps:
* Mixture is vapourised and fed into the fractionating column
* Vapours rise, cool and condense
* Products are siphoned off for different uses

Products with short carbon chains have lower boiling points, meaning they rise higher up the fractionating column before reaching their boiling point and condensing, and are collected at the top

Compounds collected from the fractionating column can then be broken down further via cracking

Question 13

Cracking of crude oil

Alkanes

Answer 13

Longer chains are not very useful, so they are broken down to form smaller, more useful molecules. The carbon-carbon bonds are broken, requiring harsh reaction conditions.

Thermal cracking:
-produces a high proportion of alkanes and alkenes
-needs high temperatures around 1200k and pressures around 7000kPa used to crack the carbon chains
-always produces an alkane, and the remaining atoms form at least one alkene

Catalytic cracking:
-produces aromatic compounds with carbon rings
-lower temperatures around 720K are needed along with normal pressure
-requires a zeolite catalyst to compensate for these less harsh conditions

Question 14

Reforming of crude oil

Alkanes

Answer 14

Reforming is the conversion of straight chained hydrocarbons into branched and cyclic hydrocarbons by heating with a platinum catalyst

Question 15

Combustion of alkanes

Answer 15

Alkanes make good fuels because they release a lot of energy when burned.

With sufficient oxygen present, they undergo complete combustion, producing carbon dioxide and water:
CH₄ + 2O₂ —> CO₂ + 2H₂O

In insufficient oxygen, incomplete combustion takes place, where carbon monoxide or carbon particulates are formed alongside water:
2CH₄ + 3O₂ —> 2CO + 4H₂O

Question 16

Which pollutants can be formed during the combustion of alkane fuels?

Alkanes

Answer 16

• Carbon monoxide (CO):
  A toxic, odourless, colourless, gaseous product
• Nitrogen dioxide
• Sulphur dioxide

Question 17

The problems arising from pollutants from the combustion of fuels

Alkanes

Answer 17

Oxides of nitrogen and sulphur are also produced as a byproduct of alkane combustion along with carbon particulates from unburnt fuel. In clouds, these oxides can react with water and form dilute acids, which result in acid rain.

Question 18

How does a catalytic converter solve some problems caused by pollutants

Alkanes

Answer 18

Gaseous pollutants produced from the combustion of fuels can be removed from systems using a catalytic converter.

They use rhodium catalysts to convert harmful products into more stable products such as CO₂ or H₂O

Question 19

The use of alternative fuels (comparing biofuel to non-renewable fossil fuels)

Alkanes

Answer 19

Biofuels release fewer, less harmful products when burned

Carbon dioxide is released during fuel combustion. It is a greenhouse gas so it causes global warming and contributes to climate change

Ethanol is a common biofuel. It is ‘carbon neutral’ as the carbon given out when burned is equal to the carbon taken in by the crop during the growing process.

Ethanol is produced by fermentation, where enzymes break down starch from crops into sugars which can then be fermented to form ethanol.

Biofuels are also sustainable. This means their supply can be maintained at the rate they are being used, so they will not run out - unlike fossil fuels

Question 20

Free radical substitution mechanism steps

Alkanes

Answer 20

Initiation:
1. A halide molecule’s bond is broken by U.V. light, producing two halide free radicals

Propagration:
2. A halide free radical reacts with an alkane, producing an alkane free radical and a halide-hydrogen (as the hydrogen from the alkane is bonded to the halide instead)
3. The alkane free radical reacts with another halide molecule, producing a halide free radical and an alkane-halide. (the free radicals from these steps of propagation form a chain reaction)

Termination:
4. Halide free radical + halide free radical —> halide molecule
5. Halide free radical + alkane free radical —> Alkane-halide molecule
6. Alkane free radical + alkane free radical —> different alkane (free radical formula x2)

Question 21

What is homolytic fission?

Answer 21

Describes the breaking of a covalent bond so that the bonding electrons are shared equally between the atoms involved

Question 22

What condition is required in the first step of the free radical substitution mechanism?

Answer 22

U.v light to break the halogen molecule bond

Question 23

The reactions of alkanes with:
* Oxygen in the air (combustion)
* Halogens (in terms of radical substitution)

Answer 23

Alkanes with oxygen:
* In good oxygen supply, the combustion of alkanes produces carbon dioxide and water
* In insufficient oxygen supply, carbon monoxide or carbon particulates can be produced alongside water

Alkanes with halogens:
produces halognenoalkanes via free radical substitution
ex. C₂H₆ + Br₂ –> CH₅Br + BrH

Question 24

The limitations of the use of free radical substitution reactions in the synthesis of organic molecules

Answer 24

1. It forms a mixture of products
2. can take place at any point along the carbon chain
3. low atom economy
4. difficult to control

Question 25

What the different catalysts are used for

Answer 25

Rhodium:
Catalytic converters
Zeolite:
Catalytic cracking
Platinum:
Reforming of crude oil