Chemistry - Topic 6 - Organic Chemistry I

what are structural isomers?

molecules with the same molecular formula but different structural formulae

what are stereoisomers?

molecules with the same molecular formula and structural formula but a different spatial arrangement of atoms

what is chain isomerism?

the carbon chain is different

what is position isomerism?

functional groups can be in different positions on the carbon chain resulting in isomer

what is functional group isomerism?

some compounds with the same molecular formula can contain different functional groups

what are the conditions for E/Z isomerism?

• there must be a double carbon bond

• both Cs from the double carbon bond must have two different groups attached

around which bond is there restricted rotation and why?

• a π bond

• this is because π orbitals have electron density above and below plane but not in the middle (between atoms) so rotation not possible

what are the three steps to the mechanism of free radical substitution?

• initiation

• propagation (has two parts)

• termination

explain the initiation step of the mechanism of free radical substitution

• when exposed to ultraviolet light, a molecule of a halogen breaks apart into two halogen atom free radicals

• the ultraviolet light provides the energy to break the covalent bond between the two halogen atoms

what happens in free radical substitution reactions?

H atoms are replaced by halogen atoms

what is a free radical?

• species with an unpaired electron

• they are very reactive

• represented in mechanisms by a single dot

• is formed by a homolytic fission of a covalent bond and results in the formation of radicals

what is homolytic fission?

homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals

explain the propagation step of the mechanism of free radical substitution

• (molecule + radical → molecule + radical)

• for every H replaced, there is one pair of propagation reactions

• simply: 1 - the radical grabs a H; 2 - the alkyl radical grabs a Br

• step 1 - the alkane/halogenoalkane reacts with the halogen free radical - this removes an H atom from the alkane/halogenoalkane - this produces HF/HCl/etc and a C based free radical

• step 2 - the C based radical from step 1 reacts with the diatomic molecule e.g. F2 to put an atom of e.g. F onto the C based radical - this also produces another halogen free radical to continue the chain reaction in another step 1

explain the termination step of the mechanism of free radical substitution

• ( 2 radicals → molecule)

• if two free radicals collide, they will form a molecule and stop the chain reaction

• any two free radicals formed in the reaction could collide in this way

what is a homologous series?

a family of similar molecules with:

• same chemical properties

• trend in physical properties

• same functional group

• same general formula

what is the functional group of a molecule?

• the atoms and bonds that determine the chemical properties of a compound

• determines which homologous series a molecule belongs to

what are alkanes and cycloalkanes?

saturated hydrocarbons

what are alkenes and cycloalkenes?

unsaturated hydrocarbons

how are alkane fuels obtained?

• fractional distillation

• cracking

• reforming

what is cracking?

• converts heavier fractions, such as diesel oil and fuel oil, into more useful hydrocarbon fuels by breaking up large molecules into smaller ones, and converts straight-chain hydrocarbons to branched and cyclic hydrocarbons, in order to supply more of the smaller molecules

• this is because shorter chained alkanes are in more demand than longer chained alkanes

• also produces alkenes for plastic/polymer productions

• cracking is the thermal decomposition of alkanes

• converts carbon chains with 12 or more carbon atoms into a mixture of branched alkanes, cycloalkanes, alkenes and branched alkenes

what is fractional distillation?

• first stage in refining crude oil

• separates crude oil into different fractions

• fuel fractions must be refined to remove sulphur compounds, which would cause air pollution when they burn

• for smooth running, petrol must burn smoothly in the engines of vehicles and not in fits and starts. to ensure smooth combustion, companies produce fuel with a high octane number by increasing the proportions of branched alkanes and arenes, or blending-in oxygen compounds

the three main methods to increase the octane number of fuels are?

• cracking

• reforming

• adding ethanol and ethers

how to produce a higher proportion of alkenes through cracking?

conduct cracking at higher temperatures in the presence of steam

how to produce a higher yields of branched and cyclic alkanes? state the conditions and explain why such a temperature is needed.

• conducting cracking in the presence of a catalyst (catalytic cracking)

• either a zeolite catalyst or silica + alumina (SiO₂ + Al₂O₃)

• 650°C - high temperatures required to break the strong C-C covalent bonds

what is reforming?

• increases the proportion of branched, cyclic and aromatic (benzene) hydrocarbons (from straight chain alkanes and cyclic alkanes)

• branched and cyclichydrocarbons are required to increase the octane number of fuel to allow smoother burning/more efficient combustion

• hydrogen is a valuable by-product of the process - it can be used elsewhere at the refinery

what are the conditions for reforming?

• palladium catalyst

• 500°C

• high pressure

• platinum or rhodium catalyst supported on an inert material such as aluminium oxide

engines that burn petrol or diesel fuels can pollute the air for three main reasons:

• incomplete combustion

• fuel contains impurities

• they run at such a high temperature that nitrogen and oxgen in the air can react

which pollutants are formed during the combustion of alkane fuels?

• carbon monoxide (toxic)

• oxides of nitrogen and sulphur (acid rain)

• carbon particulates

• unburned hydrocarbons (through incomplete combustion or evaporation of fuel - includes benzene which is carcinogenic)

suggest a reason why oil companies reform alkanes such as heptane?

has more efficient combustion/allows smoother burning/increases octane number/reduces knocking/is a more efficient fuel

what is an electrophile?

an electron deficient species that can accept a lone pair of electrons

how do catalytic converters solve some problems caused by pollutants?

they remove CO, NO_x and unburned hydrocarbons from the exhaust gases, turning them into CO₂, N₂ and H₂O

what property allows crude oil to be separated by fractional distillation?

crude oil is a mixture with boiling ranges

why should we use fuels such as biodiesel as opposed to petrol and diesel?

• save fossil fuels

• more sustainable

• uses renewable resources

• plants more carbon neutral

• biodiesel has a smaller carbon footprint

what induces a dipole in Br₂?

the pi bond in the double carbon bond

why do alkenes undergo addition reactions?

they contain a double carbon bond with a pi bond that is relatively weak and can break and bond to an electrophile

what is the positive inductive effect?

alkyl groups can stabilise a carbocation by pushing electrons towards the positive charge

what is polymerisation?

a reaction where many monomers join together

what is a monomer

a small molecule that can join together to form a polymer

what is a polymer?

a giant molecule formed from many monomers

what is the name given to the class of compounds used as radical initiators in addition polymerisation?

• peroxides

• (composed of O-O single bond) → likely to break with homolytic fission

waste polymers can be separated into which 3 specific types of polymers?

• sorting (using IR camera) and recycling

• incineration to release energy

• use as a feedstock for cracking

advantages and disadvantages of processing method 1 of polymers?

• sorting (using infrared camera) and recycling

• reduces landfill waste, reduces consumption of raw/precious material

• household plastics contain a mixture of polymers - hard and expensive to sort, also involves the use of energy (to make the new product)

advantages and disadvantages of processing method 2 of polymers?

• incineration to release energy

• reduces landfill waste + releases energy for generating electricity (generates energy for other processes)

• releases greenhouse gases, releases toxic gases such as SO₂, NO_x, HCl that need to be removed

advantages and disadvantages of processing method 3 of polymers?

• feedstock for cracking

• reduces landfill waste, converts waste products into useful chemical feedstock

• high energy input required (from crude oil), some polymers, e.g. PVC, release toxic gases like HCl when heated strongly

how can chemists limit the problems caused by polymer disposal?

• develop biodegradable polymers (including photodegradable)

• advantages - break down or decompose naturally in a quicker time (within 9 months), will not require landfill as it can degrade in sunlight

• disadvantages - cannot be reused, cannot be used in repeated outdoor applications e.g. window frames

• removing toxic waste gases caused by incineration of plastics - calcium oxide or calcium hydroxide react with acidic gases

oxidation of alkene, observations, catalysts, conditions and type of product

• KMnO₄ and H₂SO₄ - acidified potassium manganate (VII)

• room temperature

• purple to colourless

• produces a diol

electrophilic addition reaction (goes via a carbocation) - addition of steam to an alkene with acid catalyst, observations, catalysts and type of product

• phosphoric acid catalyst (H₃PO₄)

• reacts with H₂O gas

• produces alcohol

• known as acidic elimination

catalytic hydrogenation - observations, conditions, catalysts and type of product + special reaction

• nickel catalyst at 150°C or platinum catalyst at room temperature

• reacts with hydrogen

• alkane product

• also a reduction reaction as adding hydrogen

• this reaction is used to manufacture margarine from unsaturated vegetable oils

electrophilic addition reaction (goes via a carbocation) - alkene with bromine liquid, observations, catalysts and type of product

• the pi bond induces a dipole in the Br₂

• forms a dibromoalkane

• orange to colourless

• can form a dihalogenoalkane using another halogen or a mixed interhalogen compound e.g. ICl

electrophilic addition reaction (goes via a carbocation) - alkene with bromine water, Br₂(aq), observations, catalysts and type of product

• makes some type of bromoalcohol + HBr

• orange to colourless

• UV light required

what is heterolytic bond fission?

• when a covalent bond breaks, leaving one atom with both electrons from the covalent bond pair and hence the other atom does not retain any electrons from the bond pair

• this leads to the formation of ions

what is a nucleophile?

• a lone pair donor

• tends to be delta negative or negative, must have a lone pair

what is a hydrolysis reaction?

water or hydroxide ions break a chemical compound into two compounds

which is a poor nucleophile, water or OH^- ions

water

what does the reactivity of halogenoalkanes depend on?

• the strength of the C-X bond

• the halogenoalkane with the weakest C-X bond will have the fastest rate of reaction

why is it that the more branched a halogenoalkane gets, the faster the substitution reaction occurs?

• in the tertiary halogenoalkane, the delta positive carbon is bonded to three electron-releasing alkyl groups. this reduces the magnitude of the positive charge on the carbon atom

• this gives the intermediate carbocation greater stability

• and allows easier escape of the leaving group (negative halide ion)

• hence the C-X bond is weaker in branched halogenoalkanes and the substitution reaction faster

name the type of reaction, mechanism and product formed when a halogenoalkane reacts with dilute aqueous potassium hydroxide and is heated under reflux.

• hydrolysis reaction

• nucleophilic substitution reaction

• alcohol

order the halogenoalkanes from most reactive to least reactive and explain why

• iodoalkane

• bromoalkane

• chloroalkane

• the reactivity of the halogenoalkane depends on the stength of the C-X bond

• the halogenoalkane with the weakest C-X bond will have the fastest rate of reaction

explain the trend in boiling points as a molecule becomes increasingly branched

• the boiling points decrease with increased branching

• more branching gives less surface area of contact and therefore weaker London dispersion forces

• less energy required to overcome London dispersion forces/intermolecular forces

explain why MTBE is added to petrol

to increase the octane number/to ensure smooth burning/to reduce knocking

explain why compound G has stereoisomers

• there is restricted rotation around the C=C bond

• and two different groups attached to each carbon atom of the C=C bond

state why the empirical formula of a poly(alkene) is the same as that of the monomer from which it is produced.

no atoms lost or gained

suggest why a small amount of UV light can result in the formation of a large amount of product.

because a chlorine radical is regenerated

suggest why calcium oxide reacts with sulphur dioxide

calcium oxide is basic/sulphur dioxide is an acidic gas

suggest why the calcium oxide is coated on a mesh

bigger surface area to react

why can the C=C double bond not be rotated?

the C=C double bond cannot be rotated due to the side-on overlap of p orbitals - this occurs if both the carbons have different groups attached to them.

suggest the name of a different haloalkane that could be used to improve the atom economy. explain your decision.

1-chloropropane or 1-fluoropropane as the total Mr of the products will be less (so the atom economy will be greater).

why is it that as the length of the carbon chain increases, the solubility of alcohols in water decreases?

• as the alkyl group increases in size, there are stronger london dispersion forces between the alcohol molecules

• these forces are weaker than the intermolecular forces between water and alcohol molecules therefore lower solubility

• although the OH group can still form hydrogen bonds with water, this is outweighed by the long, non-polar alkyl chain

explain two points of procedure that would maximise the yield of the aldehyde (2)

• heat and distil (to remove the aldehyde from the mixture before it reacts further)

• use a stoichiometric amount of oxidising agent (rather than excess)

explain why alkanes are unreactive. (2)

• C-H bond enthalpy is large or C-H bonds are non-polar

• so alkanes are not attacked

give a test to identify HCl gas

rod dipped in NH₃ → should see white clouds (solid - NH₄Cl)

why should anti-bumping granules be added to the round-bottomed flask?

• prevents superheating/localised heating

• prevents violent boiling

• promotes smooth and uniform boiling

explain fully why ethanal, rather than ethanoic acid, is produced and collected when the apparatus for distillation is used. (2)

• ethanal has a low boiling point, lower than ethanoic acid, due to not having an O-H bond therefore cannot form hydrogen bonding

• ethanal is taken away from the mixture so cannot be oxidised further.

explain why the production of carbon monoxide and soot is hazardous

• CO is toxic because it binds to haemoglobin and limits the ability of blood to transport oxygen around the body

• soot particles are toxic because particulates enter the body through airways and damage the lungs

explain why nitrogen monoxide is formed in car engines but is not formed at room temperature and pressure

in car engines there is a high temperature therefore a greater fraction of nitrogen and oxygen molecules collide with energy greater than the high activation energy leading to a higher frequency of successful collisions and a faster rate of reaction

nitrogen monoxide is also formed in aircraft engines which is released into the atmosphere. explain how this leads to the formation of acid rain using chemical equations to support your answer.

• 2NO + O₂ → 2NO₂

• 4NO₂ + O₂ + 2H₂O → 4HNO₃

why is cracking important in industry?

• it produces more petrol

• short chain alkanes are more useful products

• demand is greater for shorter chain alkanes

• it produces ethane/short chain alkenes for making poly(ethene)

• smaller alkanes give less pollution/burn more efficiently

• recycles waste products

• as a source of hydrogen

the alkene funntional group consists of a C=C bond. this bond contains two different types of covalent bond. draw an annotated diagram to show how there two different types of covalent bond are formed in this functional group.

• sigma bond - head-on overlap of s orbitals with electron density between two nuclei

• pi bond - sideways overlap of p orbitals with electron density above and below plane of nuclei

what does a positive Fehling’s solution test signify?

• blue solution to brick-red ppt (Cu₂O)

• aldehyde - CHO present

what does steamy fumes and vigorous effervescence with PCl₅ imply?

OH present (alcohol or carboxylic acid)

what does no reaction with hot acidified potassium dichromate (VI) solution signify?

tertiary alcohol or carboxylic acid

give a possible reason why HCl gas forms steamy fumes when it mixes with moist air

droplets of HCl form

explain why poly(but-1-ene) is considered to be unsustainable. describe ways that addition polymers such as poly(but-1-ene) can be disposed

• but-1-ene is derived from crude oil, which is a finite, non-renewable resource

  methods of disposal include:

• incineration to provide energy

• cracking polymers to produce useful chemical feedstock

• sorting (with an IR camera) and recycling

draw and annotate the apparatus used for reflux

• continual boiling and condensing of reaction mixture to ensure the reaction takes place without the contents of the flask boiling dry

• for prolonged, intense heating

• cold water in the condenser

draw and annotate the apparatus used for distillation

• contents of flask are heated and react then boil off and condense into a separate vessel

• distillation is for shorter periods of gentler heating

• cold water in the condenser

core practical 4: devise an experiment to compare the rates of the hydrolysis of 2-chlorobutane, 2-bromobutane and 2-iodobutane.

state the trend in the rates of reaction. justify your answer. (6)

suggest two ways you could improve the accuracy and one way to improve the reliability of your results

water bath at 50°C

improve accuracy by:

• using colder water in water bath so it takes a longer time for the precipitate to form, so there is a smaller percentage uncertainty in the measurement of time

• use a more accurate timer (more highly callibrated) as this will reduce percentage uncertainty in time measurement

improve reliability by…

• repeating the measurement of time three times for each halogenoalkane and take a mean of the time values for each halogenoalkane

when is steam distillation used?

for more temperature sensitive molecules

state the purpose of adding ethanol to each of the test tubes when hydrolysis halogenoalkanes. (1)

ethanol is added to dissolve both the halogenoalkane and water

identify further reagents that can be added, including relevant observations, to confirm the identity of the halogen atom present in each halogenoalkane. (2)

core practical 5: the oxidation of ethanol

why are the concentrated sulphuric acid and sodium dichromate (VI) added a drop at a time in the laboratory process? (1)

to prevent the mixture heating too rapidly/to prevent ethanol evaporating

they have a (very) high boiling temperature/are ionic compounds

(oxidation of ethanol using concentrated sulphuric acid and sodium dichromate (VI)

• cool the mixture because the reaction with concentrated sulphuric acid is very exothermic/releases a lot of heat

• anti-bumping granules are added to prevent violent boiling OR promote smooth boiling OR to provide nucleation centres for bubble formation OR to prevent superheating

• heating under reflux is used to prevent the loss of any volatile substances

• brown vapour is Br₂

• bromide ions are oxidised by concentrated sulphuric acid

bromoethane was prepared from the reaction of ethanol with sulphuric acid and potassium bromide.

suggest why the flask containing the ethanol was frequently placed in a stream of cold running water as the concentrated sulphuric acid was gradually added. (1)

ethanol would evaporate/is volatile/has a low boiling temperature

bromoethane was prepared from the reaction of ethanol with sulphuric acid and potassium bromide.

the potassium bromide was initially lumpy and not a fine powder. state the apparatus that would be suitable for breaking up the lumps of potassium bromide into a powder. (1)

a pestle and mortar

bromoethane was prepared from the reaction of ethanol with sulphuric acid and potassium bromide.

the round-bottomed flask contains ethanol, concentrated sulphuric acid and potassium bromide.

explain why an orange colour was seen in the round-bottomed flask when it was first gently heated. (2)

• orange colour is Br₂

• that is formed by the oxidation of bromide ions by sulphuric acid