6.1 nomenclature, isomerism & alkanes

Question 1

describe the difference between structural formula and displayed formula when representing an organic compound.

Answer 1

structural formula - shows the arrangement of atoms carbon by carbon, with the attached hydrogen atoms, and functional groups.

displayed formula - shows how all of the atoms of the compound are arranged, including the bonds between them.

Question 2

what does ‘nomenclature’ refer to?

Answer 2

nomenclature refers to the process of naming an organic compound.

Question 3

define the term ‘functional group’.

Answer 3

functional group - a group of atoms in a molecule responsible for the characteristic reactions of that compound.

Question 4

define the term ‘homologous series’.

Answer 4

homologous series - a group of organic compounds with the same functional group and general formula.

Question 5

what is a mechanism?

Answer 5

mechanisms are diagrams that break reactions down into individual stages to show how substances react together.

Question 6

give the three main types of mechanism.

Answer 6

• radical substitution.
• electrophilic addition.
• nucleophilic substitution.

Question 7

some mechanisms use curly arrows. explain what a curly arrow shows.

Answer 7

curly arrows show how electron pairs move when bonds are formed or broken.

Question 8

define the term ‘nucleophile’, and give an example of a nucleophile.

Answer 8

• nucleophiles are electron pair donors, which donate a lone pair of electrons to an ‘electron poor’ species.
• nucleophiles are usually negatively charged ions, such as halide ions.

Question 9

define the term ‘electrophile’, and give an example of an electrophile.

Answer 9

• electrophiles are electron pair acceptors, which accept a pair of electrons from an ‘electron rich’ species.
• electrophiles are often positively charged ions, such as H⁺ ions.

Question 10

explain why radicals react with all types of species, regardless of the charge on the species.

Answer 10

• radicals have an unpaired electron, which makes them very reactive.
• as a result, radicals will react with all types of species; positive, negative or neutral.

Question 11

explain why radical substitution reactions are not useful for obtaining a pure product.

Answer 11

• because radicals react with all types of species, radical substitution reactions result in the formation of a mixture of products.
• therefore, radical substitution reactions are not useful for obtaining a pure product.

Question 12

what is an isomer?

Answer 12

isomers refer to structures with the same molecular formula but different atomic arrangements.

Question 13

give the three types of structural isomer.

Answer 13

• chain isomers.
• positional isomers.
• functional group isomers.

Question 14

explain why chain isomers have similar chemical properties but different physical properties, such as melting and boiling points.

Answer 14

• the carbon skeleton of a chain isomer can be arranged in a number of different ways.
• these isomers have similar chemical properties, but their physical properties are different due a change in the shape of the molecule.

Question 15

give the general formula for an alkane.

Answer 15

CₙH₂ₙ₊₂

Question 16

alkanes are saturated hydrocarbons. explain what is meant by the term ‘saturated’.

Answer 16

saturated - refers to a molecule which only contains single carbon-carbon bonds.

Question 17

give the general formula for a cyclic alkane.

Answer 17

CₙH₂ₙ

Question 18

define the term ‘bond fission’.

Answer 18

bond fission - the breaking of a covalent bond.

Question 19

describe the differences between ‘heterolytic fission’ and ‘homolytic fission’.

Answer 19

heterolytic fission

• the covalent bond breaks unevenly, with one of the bonded atoms receiving both electrons from the bonded pair.
• two different substances are formed - a cation and an anion.

homolytic fission

• the covalent bond breaks evenly, and each bonding atom receives one electron from the bonded pair.
• homolytic fission results in the formation of two electrically uncharged radicals.

Question 20

halogens react with alkanes to form halogenoalkanes in which type of reaction?

Answer 20

halogens react with alkanes in photochemical reactions.

Question 21

free radical substitution is a type of photochemical reaction in which a hydrogen atom is substituted by chlorine or bromine. give the reagent required to initiate this reaction.

Answer 21

ultraviolet (UV) light.

Question 22

give the three stages involved in a radical substitution reaction.

Answer 22

• initiation.
• propagation.
• termination.

Question 23

define the term ‘photodissociation’.

Answer 23

the breaking of bonds, which lead to the breakdown of a molecule, by photons.

Question 24

radical substitution reactions result in the formation of a number of different by-products alongside the desired product. give one way in which the formation of by-products in radical substitution reactions could be reduced.

Answer 24

• one way in which the formation of by-products could be reduced is by having an excess of methane present in the reaction.
• an excess of methane means that there is a greater chance of a radical colliding with only a methane molecule and not a chloromethane / bromomethane molecule.

Question 25

explain why free radical substitution can result in the formation of a number of structural isomers.

Answer 25

• free radical substitution can take place at any point along the carbon chain.
• because of this, free radical substitution can result in the formation of a number of structural isomers.

Question 26

give the process used to separate crude oil.

Answer 26

fractional distillation.

Question 27

what is cracking?

Answer 27

the process of breaking long-chain alkanes into smaller hydrocarbons, by breaking the C-C bonds.

Question 28

give the two types of cracking used to break long-chain alkanes into short-chain alkanes.

Answer 28

• thermal cracking.
• catalytic cracking.

Question 29

give the conditions required for thermal cracking to take place.

Answer 29

• a temperature of 1000°C.
• a pressure of 70 atm.

Question 30

give the conditions required for catalytic cracking to take place.

Answer 30

• a zeolite catalyst.
• a temperature of 450°C.
• a pressure of 1 atm.

Question 31

give one product of thermal cracking.

Answer 31

alkenes.

Question 32

give one product of catalytic cracking.

Answer 32

aromatic hydrocarbons.

Question 33

give one advantage of using a catalyst to ‘crack’ long-chain hydrocarbons.

Answer 33

• using a catalyst reduces costs by speeding up the reaction.
• performing the reaction under a low pressure and lower temperature conserves both energy and money.

Question 34

what is ‘catalytic reforming’?

Answer 34

the process of converting straight-chain alkanes into branched-chain alkanes and cyclic hydrocarbons, using a catalyst.

Question 35

give an example of a catalyst which could be used in this reaction.

Answer 35

platinum.

Question 36

give the products of complete combustion.

Answer 36

CO₂ (g) and H₂O (l)

Question 37

give the products of incomplete combustion.

Answer 37

C (s), CO (g), and H₂O (l)

Question 38

give an example of a pollutant produced from burning fossil fuels.

Answer 38

carbon monoxide (CO)

Question 39

explain how high concentrations of carbon monoxide in the bloodstream can lead to carbon monoxide poisoning.

Answer 39

• oxygen in the bloodstream is carried by the protein complex haemoglobin.
• carbon monoxide binds more readily to haemoglobin than oxygen does, so will bind to the haemoglobin at a much faster rate.
• as a result, less oxygen will be able to bind to the haemoglobin, leading to oxygen deprivation and carbon monoxide poisoning.

Question 40

explain how acid rain can be caused by burning fossil fuels that contain sulphur, and give one consequence of acid rain.

Answer 40

• acid rain can be caused by burning fossil fuels that contain sulphur.
• the sulphur burns to produce sulphur dioxide gas, which then enters the atmosphere and dissolves in the moist air, where it is converted into sulphuric acid.
• this falls as acid rain, which destroys trees and vegetation.

Question 41

give the name of the substance that removes pollutants from car emissions, by converting them into less harmful gases.

Answer 41

a catalytic converter.

Question 42

give an example of a catalytic converter.

Answer 42

a platinum catalyst.

Question 43

state a renewable source of energy which can be used as an alternative to fossil fuels.

Answer 43

biofuels.

Question 44

give an example of a biofuel, and explain how it is produced.

Answer 44

bioethanol is an alcohol-based biofuel made by the fermentation of sugar from crops, such as maize.

Question 45

give one advantage and one disadvantage of switching to biofuels over fossil fuels.

Answer 45

advantage

• biofuels are renewable and can be produced over short periods of time, unlike fossil fuels which take several millennia to form.

disadvantage

• switching to biofuels would require the modification of petrol car engines to enable them to use fuels with high concentrations of ethanol - this would be very expensive.