organic chem

Alkanes Structure & Formula

• Alkanes are a group of saturated hydrocarbons

  • The term saturated means that they only have single carbon-carbon bonds, there are no double bonds

• The general formula of the alkanes is C_nH_2n+2

• They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases

• Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller more useful molecules

• Methane is an alkane and is the major component of natural gas

Table of alkanes

Displayed formula	Name	Molecular formula
	methane	CH4
	ethane	C2H6
	propane	C3H8
	butane	C4H10
	pentane	C5H12

Reactions of Alkanes

• Alkanes are hydrocarbons, meaning they contain hydrogen and carbon only

• Hydrocarbons undergo combustion in the presence of air

• Complete combustion occurs to form water and carbon dioxide gas

• For example, the simplest alkane, methane burns as follows:

CH₄ +  2O₂ → CO₂ +  2H₂O

• Gasoline is largely composed of isomers of octane, C₈H₁₈ ,which requires large amounts of oxygen to combust fully

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

• The efficiency of car engines does not usually enable all the gasoline to burn, so car exhaust will contain small amounts of unburnt hydrocarbons as well as other products such as carbon monoxide and soot which lead to environmental problems

• The carbon dioxide produced is a major contributor to global warming and the replacement of combustion engines with electric vehicles is a major on-going challenge for all countries

Alkenes Structure & Formula

• Alkenes are unsaturated hydrocarbons

• All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C

• All alkenes contain a double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot

• The names and structure of the first four alkenes are shown below:

Table of alkenes

Displayed formula	Name	Molecular formula
	ethene	C2H4
	propene	C3H6
	but-1-ene	C4H8
	pent-1-ene	C5H10

The first four members of the alkene homologous series

• Compounds that have a C=C double bond are also called unsaturated compounds

• That means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group

• Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds

• This makes them much more reactive than alkanes

A carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms

Reactions of Alkenes

Combustion of Alkenes

• These compounds undergo complete and incomplete combustion but because of the higher carbon to hydrogen ratio they tend to undergo incomplete combustion, producing a smoky flame in air.

• Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:

C₄H₈           +          6O₂       →      4CO₂       +         4H₂O

butene      +      oxygen    →  carbon dioxide   +  water

• Incomplete combustion occurs when there is insufficient oxygen to burn so a mixture of products can form, e.g:

C₄H₈     +          4O₂          →        4CO           +         4H₂O

butene      +      oxygen    →  carbon monoxide   +  water

• In addition to carbon monoxide, carbon in the form of soot can be produced:

C₄H₈     +          2O₂          →        4C           +         4H₂O

butene      +      oxygen    →  carbon   +     water

• This is more likely to occur in higher alkenes with larger number of carbons

• This is seen as smoky yellow flames when the alkenes burn

Addition Reactions

• The chemistry of the alkenes is determined by the C=C functional group

• Since all members of the alkene homologous series contain the same functional group then they all react similarly

• Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond

• The carbon-carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbon

  

  Diagram showing the general equation for the addition reaction of alkenes

Hydrogenation

• Alkenes undergo addition reactions with hydrogen in which an alkane is formed

• These are hydrogenation reactions and occur at 150ºC using a nickel catalyst

• Hydrogenation reactions are used to manufacture margarine from vegetable oils

  • Vegetable oils are polyunsaturated molecules which are partially hydrogenated to increase the Mr and turn the oils into solid fats

Hydrogen atoms add across the C=C in the hydrogenation of ethene to produce ethane

Halogenation

• The halogens also participate in addition reactions with alkenes

• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond

• The reaction occurs readily at room temperature

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

Bromine Water Test

• Alkanes and alkenes have different molecular structures

• All alkanes are saturated and alkenes are unsaturated

• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot

• This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test

Diagram showing the result of the test using bromine water with alkanes and alkenes

• Bromine water is an orange coloured solution

• When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution

• But when bromine water is added to an alkene, the bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour

Addition Polymers

• Polymers are large molecules of high relative molecular mass and are made by linking together large numbers of smaller molecules called monomers

• Each monomer is a repeat unit and is connected to the adjacent units via covalent bonds

• Polymerisation reactions usually require high pressures and the use of a catalyst

• Many everyday materials such as resins, plastics, polystyrene cups, nylon etc. are polymers

• These are manufactured and are called synthetic polymers

• Nature also produces polymers which are called natural or biological polymers

Diagram showing how lots of monomers bond together to form a polymer

Representing Polymers

• Addition polymers are formed by the joining up of many monomers and only occurs in monomers that contain C=C bonds

• One of the bonds in each C=C bond breaks and forms a bond with the adjacent monomer with the polymer being formed containing single bonds only

• Many polymers can be made by the addition of alkene monomers

• Others are made from alkene monomers with different atoms attached to the monomer such as chlorine or a hydroxyl group

• The name of the polymer is deduced by putting the name of the monomer in brackets and adding poly- as the prefix

• For example if propene is the alkene monomer used, then the name is polypropene

• Polyethene is formed by the addition polymerisation of ethene monomers

Examples of addition polymerisation: polyethene and PVC

Deducing the monomer from the polymer

• Polymer molecules are very large compared with most other molecule

• Repeat units are used when displaying the formula

• To draw a repeat unit, change the double bond in the monomer to a single bond in the repeat unit

• Add a bond to each end of the repeat unit

• The bonds on either side of the polymer must extend outside the brackets (these are called extension or continuation bonds)

• A small subscript n is written on the bottom right hand side to indicate a large number of repeat units

• Add on the rest of the groups in the same order that they surrounded the double bond in the monomer

Diagram showing the concept of drawing a repeat unit of a monomer

Diagram showing the monomer of the repeat unit of polymer

Structure of Condensation Polymers

Higher Tier Only

• Condensation polymers are formed when two different monomers are linked together with the removal of a small molecule, usually water

• This is a key difference between condensation polymers and addition polymers:

  • Addition polymerisation forms the polymer molecule only

  • Condensation polymerisation forms the polymer molecule and one water molecule per linkage

• The monomers have two functional groups present, one on each end, such as diols (an alcohol each end) or dicarboxylic acids (a carboxylic acid each end)

• The functional groups at the ends of one monomer react with the functional group on the end of the other monomer, in so doing creating long chains of alternating monomers, forming an A-B-A-B pattern in the polymer

• It is possible to have monomers with one of each functional group; such as an alcohol one end and a carboxylic acid at the other end (A-A type polymers, such as proteins from amino acids)

• Most synthetic polyesters are formed from two different monomers and produce water

• There are two main types of synthetic condensation polymers; polyesters and polyamides

Forming Polyesters

• Polyesters are made from dicarboxylic acid monomers (a carboxylic with a -COOH group at either end) and diols (an alcohol with an -OH group at either end)

• Each -COOH group reacts with the -OH group on a diol on another monomer

• An ester link, –COO–, is formed with the subsequent loss of one water molecule per link, formed from the combination of a hydrogen ion (proton) (H⁺) and a hydroxide ion (OH^–)

• Terylene is an example of a polyester

The condensation reaction in which the polyester terylene is produced

• The structure of terylene can be represented by drawing out the polymer using boxes to represent the carbon chains

• This can be done for all polyesters

  

Diagram showing a section of a polyester 

Examiner Tips and Tricks

Notice that the sequence of bonding in the polyester is the mirror image at either end of the link, NOT the link repetition due to the monomers containing the same functional group at either end.

Forming Polyamides

• Polyamides are made from dicarboxylic acid monomers (a carboxylic with a -COOH group at either end) and diamines (an amine with an -NH₂ group at either end)

• Each -COOH group reacts with another -NH₂ group on another monomer

• An amide link, –CONH–, is formed with the subsequent loss of one water molecule per link, formed from the combination of a hydrogen ion (proton) (H⁺) and a hydroxide ion (OH^–)

• Nylon is an example of a polyamide

The condensation reaction in which the polyamide, nylon is produced

• The structure of polyamides can be represented by drawing out the polymer using boxes to represent the carbon chains

Diagram showing a section of a polyamide

Formation of Condensation Polymers

Higher Tier Only

Health & Safety Aspects

• The solutions used here are hazardous and contain toxic materials and solvents so care should be taken when handling and disposing of them

• Small quantities are used to minimise the risks associated with these substances

Making Nylon in the Lab

• Two solutions are made, one containing 1,6-diminohexane dissolved in water, and a second solution containing decanedioyl dichloride dissolved in cyclohexane

• 5 cm³ of aqueous 1,6-diaminohexane solution is poured into a 25 cm³ beaker

• 5cm³ of decanedioyl dichloride solution is added carefully on top of the first solution so that mixing is minimised

• The decanedioyl dichloride solution will float on top of the aqueous solution without mixing

• A white coloured film of nylon will form at the interface

• A little of the film can be picked up with a pair of tweezers and wrapped around a glass rod

• As the glass rod is rotated a 'rope' of nylon is extracted from the beaker

The nylon rope trick

• The two monomers contain a reactive group at each end of the molecules

• These groups react together and join up to form long chains, rather like a bead necklace with alternating coloured beads

• The result is nylon, which is actually named after the two cities where it was first discovered- New York and London!

• This particular type of nylon is called nylon 6,10, which refers to the length of the two monomer pieces containing links that are 6 carbons long (from the diamine) and 10 carbons long (from the diacyl chloride), respectively

DNA

• Deoxyribonucleic acid (DNA) is a large molecule which is essential to all life

• It contains genetic information which it encodes as instructions which organisms need to develop and function correctly

• DNA consists of four different monomers called nucleotides which contain small molecules called bases and which are abbreviated to A, T, C, and G which are bound together by polymerisation

• The nucleotides form two strands that intertwine, giving the famous double helix shape of DNA

• The bases on either polymer chain pair up in specific sequences forming cross links that hold the strands together, giving rise to the double helix shape

• It is a complex molecule that contains genetic information which is stored in the order in which the bases organise themselves, which is a code for the organisms gene

• Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula C_x(H₂O)_y

• There are simple carbohydrates and complex carbohydrates

• Simple carbohydrates are called monosaccharides and are sugars such as fructose and glucose

• Complex carbohydrates are called polysaccharides such as starch and cellulose

• The monomers from which starch and cellulose are made are both sugars

• Starch is used to store energy and cellulose is a stiff polymer used in plant cell walls to provide support

• Complex carbohydrates are condensation polymers formed from simple sugar monomers and, unlike proteins, are usually made up of the same monomers

• An H₂O molecule is eliminated when simple sugars polymerise

  • The linkage formed is an -O- linkage and is called a glycosidic linkage

Diagram of the starch amylose showing glycosidic linkages (-O-) which bind the monomers together, Amylose makes up approximately 20-30% of starch

Natural Polymers with Biological Functions

• Proteins are also important natural polymers with specific biological functions

• Some examples of proteins and their functions include:

  • Haemoglobin which transports oxygen in the blood

  • Antibodies in the immune system help protect the body from viruses and bacteria

  • Enzymes which are biological catalysts

Amino Acids 

• Amino acids are small molecules containing the amino, NH₂, and carboxylic acid, COOH, functional groups

• The NH₂ group is basic and behaves in a similar way to ammonia

• The COOH group is acidic and is called a carboxyl group

• There are twenty naturally occurring amino acids and they all have the same general structure

• Different combinations of amino acids (monomers) join together to form very long protein molecules 

The structure of naturally occurring amino acids have an amino group on the second carbon along from the carboxyl group. The R represents a varying side group.

Alcohols Structure & Formula

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions

• Alcohols are a homologous series of compounds that have the general formula C_nH_2n+1OH

• They differ by one -CH₂ in the molecular formulae from one member to the next

Diagram of the side chain and -OH group in ethanol which characterizes its chemistry

The First Four Alcohols

• The names and structures of the first four alcohols are shown below

• In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’

Table showing the Formulae and Structures of the First Four Alcohols

Reactions of Alcohols

• Alcohols are colourless liquids that dissolve in water to form neutral solutions

• The first four alcohols are commonly used as fuels

• School laboratories use ethanol in spirit burners as it burns cleanly and without strong odours

• Methanol and ethanol are also used extensively as solvents

• This is because they can dissolve many substances that water cannot such as fats and oils, but can also dissolve most of the substances that water can

• Alcohols undergo combustion to form carbon dioxide and water

• The complete combustion of ethanol is as follows:

CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O

• Alcohols undergo oxidation to produce carboxylic acids, an organic acid

• This is achieved by heating the alcohols with acidified potassium manganate(VII)

  CH₃CH₂OH + 2[O]     →       CH₃COOH  + H₂O

ethanol                         ethanoic acid

• The [O]  is a symbolic representation of an oxidising agent which helps to simplify the equation

Carboxylic Acids Structure & Formula

• Carboxylic acids are a homologous series of compounds that have the general formula of

C_nH_2n+1 COOH 

• They differ by one -CH₂ in the molecular formulae from one member to the next

• They show a gradation in their physical properties:

  • Boiling points increase with increased carbon chain length

  • Viscosity increases with increased carbon chain length

• They have similar chemical properties

Diagram of the general structure of a carboxylic acid. The structures and formulae of the first four carboxylic acids

Crude Oil

• Crude oil is a finite resource which we find in the Earth's crust

• It is also called petroleum and is a complex mixture of hydrocarbons which also contains natural gas

  • Hydrocarbons are compounds that are made of carbon and hydrogen atoms only

• The hydrocarbon molecules in crude oil consist of a carbon backbone which can be in a ring or chain, with hydrogen atoms attached to the carbon atoms

• The mixture contains molecules with many different ring sizes and chain lengths

• It is a thick, sticky, black liquid that is found in porous rock (under the ground and under the sea)

• Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of plants and animals

• It is being used up much faster than it is being formed, which is why we say crude oil is a finite resource

Crude oil found under the sea

Examiner Tips and Tricks

Crude oil is also sometimes referred to as petroleum. Some fractions may have different names in the UK and the USA, e.g. gasoline is the name used in the USA for petrol. You may be asked to give a definition of the term hydrocarbon - be careful! You must say a compound which contains carbon and hydrogen atoms only. If you do not say only, then you might not get the mark.

Fractional Distillation

• Crude oil as a mixture is not a very useful substance but the different hydrocarbons that make up the mixture, called fractions, are enormously valuable, with each fraction having many different applications

• Each fraction consists of groups of hydrocarbons of similar chain lengths

• The fractions in petroleum are separated from each other in a process called fractional distillation

• The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain

• The size and length of each hydrocarbon molecule determines in which fraction it will be separated into

• The size of each molecule is directly related to how many carbon and hydrogen atoms the molecule contains

• Most fractions contain mainly alkanes, which are compounds of carbon and hydrogen with only single bonds between them

Diagram showing the process of fractional distillation to separate crude oil in a fractionating column

• Fractional distillation is carried out in a fractionating column which is very hot at the bottom and cool at the top

• Crude oil enters the fractionating column and is heated so vapours rise

• Vapours of hydrocarbons with very high boiling points will immediately condense into liquid at the higher temperatures lower down and are tapped off at the bottom of the column

• Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off

• The different fractions condense at different heights according to their boiling points and are tapped off as liquids

• The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases

• The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column

• As the size of the hydrocarbon increases, the boiling point increases because the intermolecular forces get stronger and require more energy to break

Cracking

• Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms

• Alkanes are saturated compounds and alkenes are unsaturated compounds

• Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules

• A process called cracking is used to convert them into short chain molecules which are more useful

• Small alkenes and hydrogen are produced using this process

• Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen

Decane is cracked to produce octane for petrol and ethene for ethanol synthesis

• Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them

• The vapours then pass over a hot powdered catalyst of alumina or silica

• This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions

• The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes

• Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure

Writing Equations for Cracking

• We can use the general formulae for alkanes and alkenes to check that we have correctly balanced equations for cracking

• Hexane for example, can be cracked to form butane and ethene, both of which are very useful molecules

• Ethene as the starting material for the production of alcohol and butane is used as a fuel

• The equation for this cracking reaction is:

C₆H₁₄ ⟶ C₄H₁₀ + C₂H₄

• Note that the starting compound for this reaction is an alkane and thus the general formula C_nH_2n+2 applies

• Butane is also an alkane and so the same rule applies

• Ethene is an alkene and so its formula will follow the C_nH_2n rule

Examiner Tips and Tricks

Always check that sum of the carbons and hydrogens adds up on each side of the equation AND that you have made alkanes or alkenes.

Chemical Cells

• A simple cell is a source of electrical energy

• The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit

• A common example is zinc and copper

• Zinc is the more reactive metal and forms ions more easily, readily releasing electrons

• The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes

• The electrons then flow around the circuit to the copper electrode which is now the more positive electrode

• The difference in the ability of the electrodes to release electrons causes a voltage to be produced

• The greater the difference in the metals reactivity then the greater the voltage produced

• The electrolyte used also affects the voltage as different ions react with the electrodes in different ways

• Cells produce a voltage until one of the reactants is used up

Simple cell made with Cu and Mg. These metals are further apart on the reactivity series than Cu and Zn and produce a greater voltage

Hydrogen Fuel Cells

• A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

• These cells are becoming more common in the automotive industry to replace petrol or diesel engines

• As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell

• Different electrolytes and fuels can be used to set up different types of fuel cells

• An important cell is the hydrogen-oxygen fuel cell which combines both elements to release energy and water

Diagram showing the movement of hydrogen, oxygen and electrons in a hydrogen-oxygen fuel cell

• At the anode, hydrogen molecules lose electrons and become hydrogen ions:

2H₂ (g) → 4H⁺ (aq) + 4e^-

• The electrons flow through the external circuit to the cathode

• Hydrogen ions migrate through a special membrane separating the anode and cathode

• At the cathode hydrogen ions gain electrons and react with oxygen to form water:

4H⁺ (aq) + O₂ (g) + 4e^-→  H₂O (g)

• The overall reaction is:

2H₂ (g) + O₂ (g) → 2H₂O (l)

Pros & Cons of Hydrogen Fuel Cells

Pros

• They do not produce any pollution

• They produce more energy per kilogram than either petrol or diesel

• No power is lost in transmission as there are no moving parts, unlike an internal combustion engine

• No batteries to dispose of which is better for the environment

• Continuous process and will keep producing energy as long as fuel is supplied

Cons

• Materials used in producing fuel cells are expensive

• High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle

• Fuel cells are affected by low temperatures, becoming less efficient

• Hydrogen is expensive to produce and store