Topic 6: Organic Chemistry I Flashcards
Define displayed formula
- shows how all the atoms are arranged and all the bonds between them
Define skeletal formula
- shows the bonds of the carbon skeleton only, with any functional groups - hydrogen and carbon atoms aren’t shown
Define homologous series
- a group of organic compounds that have the same functional group and general formula
What is a functional group
- a group of atoms in a molecule responsible for the characteristic reactions of that compound
Give the definition of general formula
- an algebraic formula that can describe any member of a family of compounds
Definition of molecular formula
- the actual number of atoms of each element in a molecule
Definition of structural formula
- shows the arrangement of atoms carbon by carbon with the attached hydrogens and functional groups - e.g. CH3CH2CH2CH2OH
What is the definition of empirical formula?
- the simplest whole number ratio of atoms of each element in a compound
What are structural isomers?
- two molecules that have the same molecular formula but different structural formulae
Describe chain isomers
- the carbon skeleton can be arranged differently
- e.g. straight or branched
- these isomers have similar chemical properties but their physical properties will change due to the shape of the molecule
Describe positional isomers
- the skeleton and the functional group could be the same, only with the functional group attached to a different carbon atom
- also have different physical properties and possibly different chemical properties
Describe functional group isomers
- the same atoms can be arranged into different functional groups
- they have very different physical and chemical properties
What is stereoisomerism?
- they have the same structural formulae but have a different spatial arrangement of atoms
- alkenes can exhibit E-Z stereoisomerism
How do E-Z isomers arise?
- there is restricted rotation around the C=C double bond
- there are two different groups/atoms attached both ends of the double bond
What are alkanes
- they are saturated hydrocarbons
- general formula CnH(2n+2)
How does fractional distillation work?
- oil is pre-heated then passed into the column
- fractions condense at different heights
- temperature of column decreases upwards
- separation depends on boiling pint
- boiling point depends on sie f molecules
- the larger the molecule, the larger the London forces
- similar molecules condense together
Order the fractions of alkane molecules in a fractionating column
- fuel gas C1-C4
- petrol
- naphtha C5-C10
- kerosene C10-C16
- diesel oil C15-19
- mineral oil C20-30
- fuel oil
- wax, grease
- bitumen
What is cracking?
- converting large hydrocarbons too smaller molecules by breaking C-C bonds
Why is cracking needed?
- petroleum fractions with shorter C chains are in more demand
- products of cracking are more useful and valuable than starting materials (e.g. ethene is useful)
What are the two types of cracking?
- thermal cracking
- catalytic cracking
Describe thermal cracking
- takes place at high temp of around 1000 ºC
- high pressure (70 atm)
- produces lots of alkenes
- these alkenes make polymers
Describe catalytic cracking
- uses zeolite catalyst (hydrated aluminosilicate)
- at a slight pressure
- high temp 450 degrees
- mostly produces aromatic compounds (contain benzene rings) and motor fuels
- using catalyst cuts cost because it can be done at lower pressure and temp
What is reforming
- converting straight-chain alkanes into branched chain alkanes and cyclic hydrocarbons
- using catalyst (platinum stuck on aluminium oxide)
- at 500 degrees
- H2 gas produced
What is knocking
- where alkanes explode on their own accord
- straight-chain alkanes most likely to cause knocking
- adding branched chains and cyclic hydrocarbons making knocking less likely so combustion more efficient
Are combustion reactions exothermic or endothermic?
- exothermic
What are the products of complete combustion of alkanes?
- CO2 and H2O
What are the products of incomplete combustion of alkanes?
- CO and/or C and H2O
Describe how oxides of sulfur can cause environmental damage
- some molecules in crude oil contain atoms of sulfur
- during combustion of alkanes, sulfur forms sulfur dioxide and then reacts with the atmosphere to form sulfur trioxide
- S + O2 → SO2
- 2SO2 + O2 → 2SO3
- these gases are acidic oxides and dissolve in water in the atmosphere to form sulfurous acid and sulfuric acid:
- SO2 + H2O → H2SO3
- SO3 + H2O → H2SO4
- these contribute to the formation of acid rain, damaging aquatic life, crops, forests, buildings
How can oxides of nitrogen contribute to environmental damage?
- nitrogen oxides form from the reaction between N2 and O2 inside the car engine
- the high temperature and spark provides sufficient energy to break the strong N2 bond
- N2 + O2 → NO
- N2 + 2O2 → NO2
- nitrogen dioxide is acidic and can dissolve in water to form nitrous acid and nitric acid:
- 2NO2 + H2O → HNO2 + HNO3
Describe catalytic converters and how they work
- a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium, giving a large surface area
- removes CO, NOx and unburned hydrocarbons, turning them into CO2, N2, H2O
- e.g. 2NO + 2CO → N2 + 2CO2
What are two examples of biofuels?
- biodiesel: produced by reacting vegetable oils with a mixture of alkali and methanol
- bioalcohols: produced from the fermentation of sugars from plants
What are the advantages and disadvantages of biofuel?
Advantages:
- renewable, so reduction of use of finite fossil fuels
- more carbon-neutral
- allows fossil fuels to be used as feedstock for organic compounds
- no risk of large scale pollution from exploitation of fossil fuels
Disadvantages:
- less food crops can be grown
- rainforests have to be cut down for land
- shortage of fertile soils
What do alkanes react with halogens to form?
- halogenoalkanes
What are radicals?
- a reactive species with an unpaired electron
Describe free radical substitution reactions of alkanes
Initiation:
- UV light supplies the energy to break the Cl-Cl bond by homolytic fission
- this forms two chlorine free radicals
- Cl2 → 2Cl•
Propagation:
- the chlorine free radical removes an H from alkane forming an alkyl free radical
- the alkyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical
- CH4 + Cl• → HCl + •CH3
- •CH3 + Cl2 → CH3Cl + Cl•
Termination:
- collision of two free radicals does not generate further free radicals
What is the problem with free radical substitution and how to solve?
- it’s hard to get a particular product as you end up with a mixture of products
- you may end up with dichloromethane, trichloromethane, tetrachloromethane, chloromethane
- solve by having an excess of methane
- another problem is that radical substitution can occur at any point along the carbon chain so a mixture of structural isomers can be formed
How do you write propagation steps for substituting a halogen on a ‘middle’ carbon?
- make sure to put the free radical dot on the correct carbon
Describe the bonding in alkenes
- C=C double covalent bond consists of one sigma and one pi bond
- pi bonds are exposed and have high electron density (they are electrophiles)
What is the strongest type of covalent bond?-
- sigma bonds
- high electron density between the nuclei means there is a strong electrostatic attraction between the nuclei and shared pair of electrons
- sigma bonds have a high bond enthalpy
What type of bond are single covalent bonds in organic molecules?
- sigma bonds
- when two orbitals overlap
- gives highest possible electron density
Which bond has restricted rotation?
- pi bonds
What is an addition reaction and what molecules tend to undergo these?
- a reaction where two molecules react together to produce one
- alkenes tend to undergo addition reaction because the pi-bond electrons are used to form new bonds with an attacking molecule
- the product contains sigma bonds, not pi bonds, so the bonds in the product are stronger and more stable
Describe reaction of alkenes → alcohols
- alkenes are hydrated by steam
- at 300 degrees
- 60-70 atm
- solid phosphoric (V) acid catalyst
- done industrially, high atom economy
Describe the reaction of alkenes with hydrogen
- alkenes → alkanes
- reagent: hydrogen
- conditions: nickel catalyst
- addition/reduction reaction
What are electrophiles?
- electron pair acceptors
- often positively charged ions or partially positive areas
- electron poor, so attracted to electron rich areas
- react with negative ions, atoms with lone pairs and electron rich area around C=C double bond
What is electrophilic addition?
- happens to alkenes
- the alkene double bond opens up and atom are added to the carbon atoms
- happens because double bond has got many electrons and is attacked by electrophiles
Describe the reaction of alkenes with bromine/chlorine
- alkene → dihalogenoalkane
- reagent: bromine/chlorine
- conditions: room temperature
- mechanism: electrophilic addition
Briefly describe the addition of hydrogen halides to alkenes
- alkene → halogenoalkane
- reagent: HCl or HBr
- conditions: room temp
- electrophilic addition
How do you oxidise an alkene?
- alkene → diol
- reagent: acidified KMnO4
- conditions: room temp
- observation: purple colour of MnO4- ion will decolourise to colourless
- a test for alkene functional group
What does bromine water test for?
- presence of C=C
- colour change from orange to colourless
Describe the products of unsymmetrical alkenes addition reactions
- there is a major and minor product
- a primary and a secondary (or tertiary) carbocation is formed
- carbocation stability is 3>2>1
- because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion, stabilising it
What is addition polymerisation?
- double bonds in alkenes open and join together to as make long chain called polymers
What are polyethene and polypropene used for?
- polyethene: flexible, easily moulded, waterproof, low density
- plastic bags
- buckets
- bottles
- polypropene: stiffer
- utensils
- ropes
- carpets
Describe and explain methods of disposal of waste polymers
Incineration:
- rubbish is burnt and energy produced used in generating electricity
- toxins are released on incineration
- greenhouse gases emitted
- volume of rubbish greatly reduced
Recycling:
- saves raw materials
- polymers need collecting/sorting, expensive
- careful sorting needed
Feedstock for cracking:
- Polymers can be cracked into small molecules which can be used to make other chemicals and new polymers
- saves raw materials
What are nucleophiles
- they are electron pair donors - often negatively charged ions or a species that contain a lone pair of electrons - electron rich so attracted to electron poor areas - therefore, likely to reaction with positive ions and partially positive areas in molecules with polar bonds
Give examples of nucleophiles that react readily with halogenalkanes
- OH-
- NH3
- CN-
- H2O
What makes halogenoalkanes reactive?
- the C-X bond is polar due to differences in electronegativity
- carbon atom is slightly positive, so they attract nucleophiles
What halogenoalkane are subsituted fastest and why?
- iodoalkanes
- the weaker the bonod, the easier it is to break and the faster the reaction
What affects how quickly different halogenoalkanes hydrolyse?
- the carbon-halogen bond enthalpy
- weaker carbon-halogen bonds breaks more easily, so react faster
- bond enthalpy depends on the size of the halogen
- the larger the halogen, the longer the C-X bond, the lower the bond enthalpy
- the size of the halogen increases down group 7, so iodoalkanes have the weakest bonds, so hydrolysed fastest
Describe the reaction of halogenoalkanes with aqueous hydroxide ions
- halogenoalkane → alcohol
- reagent: potassium hydroxide
- heat under reflux
- nucleophilic substitution
Which halogenoalkane undergoes SN2 mechanism and draw?
- primary halogenoalkanes
Which halogenoalkane undergoes SN1 mechanism and draw?
- tertiary halogenoalkanes
Describe the reaction of halogenoalkanes with ammonia
- halogenoalkane → amine
- reagent: NH3 dissolved in ethanol
- heat under pressure
- further substitution reactions can account between halogenoalkanes and amines
Describe halogenalkanes reaction with alcoholic hydroxide ions
- halogenoalkane → alkene
- reagents: potassium hydroxide
- conditions: in ethanol, heat
- mechanism: elimination
How do cyanide ions react with halogenoalkanes to form nitriles?
- reflux a halogenoablkane with potassium cyanide in ethanol to form a nitrile by nucleophilic substitution
What are the uses of halogenoalkanes?
- refrigerants, fire retardants, pesticides, aerosol propellants
- chloroalkanes are used as solvents
- they are toxic and destroy the ozone layer
Describe the combustion of alcohols
- combust with a clean flame
- CH3CH2OH + 3O2 → 2CO2 + 3H2O
How do alcohols form halogenoalkanes?
- Various halogenating compounds are used to substitute OH group for a halogen
-
How do you form a bromoalkane from an alcohol?
- substitution reaction with HBr
- reaction requires an acid catalyst e.g. 50% concentrated H2SO4 and KBr
How do you make iodoalkanes from alcohol?
- reacting with phosphorus triiodide (PI3)
- 3ROH + PI3 → 3RI + H3PO3
What happens when you react alcohols with PCl5
- produces chloroalkanes
- ROH + PCl5 → RCl + HCl + POCl3
- misty fumes produced (HCl)
What happens when you react alcohols with HCl?
- produces chloroalkanes
- ROH + HCl → RCl + H2O
- fastest rate with tertiary alcohols
- slowest rate with primary alcohol
Describe the dehydration of alcohols
- alcohol → alkene
- reagents: concentrated phosphoric acid
- conditions: warm under reflux
- elimination reaction
- secondary and tertiary alcohols can give more than one product
Why are there two possible alkene products when water from alcohols are eliminated?
- depends on which side of the hydroxyl group the hydrogen is eliminated from
What are carbonyl compounds?
- have the functional group C=O
Describe the oxidation of primary alcohols (partial then full)
- reaction: primary alcohol → aldehyde → carboxylic acid
- reagent: potassium dichromate(VI) solution and dilute sulfuric acid
- conditions: warm gently and distil/heat under reflux
- orange dichromate ion reduces to green Cr3+ ion
Describe the oxidation of secondary alcohols
- Reaction: secondary alcohol → ketone
- reagent: potassium dichromate(VI) and dilute sulfuric acid
- heat under reflux
- orange to green
How can you control how far the alcohol is oxidised?
- gently heating ethanol with potassium dichromate(VI) solution and sulphuric acid in a test tube should produce apple smelling ethanal , but you could end up with ethanoic acid
– to get just an aldehyde, you need to get it out of the oxidising solution as soon as it is formed, you can do this by heating excess alcohol with a controlled amount of oxidising agent in distillation apparatus, so aldehyde can be distilled immediately
– to produce the carboxylic acid, the alcohol has to be vigorously oxidised, so heated with excess oxidising agent and under reflux
What do tertiary alcohol oxidise to?
- they don’t oxidise
- you must burn them
How can you test whether a compound is an aldehyde or a ketone?
- using benedict’s solution
- which is a blue solution of copper(II) ions dissolved in sodium carbonate
- if it’s heated with an aldehyde, the blue copper(II) ions are reduced to a brick-red precipitate of copper (I) oxide
- if it’s heated with a ketone, nothing happens as ketones cannot be easily oxidised
