Organic Chemistry Flashcards
How is polymerisation represented?
Double bond brokenSquare brackets aroundNH CH3 | |C = C | |H COOCH3
How are alkenes obtained ?
Cracking and catalytic reforming of crude oil
What is the general formula of an alkene?
CnH2n
What is the single bond represented by?
Sigma o
Do double bonds make a molecule saturated or unsaturated?
Unsaturated
What is the second bond represented by and where is electron density concentrated in this?
PiIn two regions on either side of the axis of the bondDoes not allow rotation around the axis
Why is a double bond less than twice as strong as a single bond?
Electron density further from nuclei
What does the arrangement of electron density explain?
Reactive because of attraction of positively polarised groupsSo geometric isomerism as rotation is not possible around pi bond125
How do alkenes compare equivalent alkanes ?
Physically similar propertiesMelting point little lower
What is a geometric isomer?
When components of the molecule are arranged on different sides of the double bond
What is meant by a cis isomer?
Both methyl groups on one side of bondH H | |C = C | |H3C CH3
What is the shape of a molecule governed by?
Valence shell electron pair repulsion theoryBonding pairs and lone pairs are vitally importantDouble bond counts same as single bond
What is meant by a trans isomer?
Two methyl groups are on opposite side of double bond
What different properties do geometric isomers show?
Melting/boiling pointDensitiesOccasionally reactions
What limitations does the cis-trans system have?
Doesn’t work with all geometric isomersSo E-Z isomerisation
What makes a Z-isomer?
2 higher priority on the same sideZ-but-2-ene
What makes an E-isomer?
Higher priority on opposite sideE-but-2-ene
What are the two main effects of double bonds?
Geometric isomerism as no rotation around C=CAddition reactions where two substances react to form a single product
Why are all keen is more reactive than corresponding alkanes?
Less energy required to break C=C129
Why is C=C attacked when a sigma positive atom approaches?
High electron densitySpecies is electrophilic
What is an electrophile?
An electron deficient species that can form a new covalent bond, using an electron pair provided by the carbon compound (H+)
What is Markovnikov’s rule?
When HX adds across asymmetric double bond, the major product formed is the molecule in which hydrogen adds to the carbon atom in the double bond with the greater number of hydrogen already attached to it
What are reaction mechanisms?
How organic reactions happenPredict outcome of similar reactions
What does a half curly arrow represent?
Movement of 1 electron
What does a full curly arrow represent?
Movement of a pair of electrons
What happens when ?
Pair of elections in pi bond form a new bond with positively polarised attacking group (electrophile)Electrophorus attack on electron rich pi bondCarbocation formed from attackPositive ion attacked by nucleophiles
Where is bromine most likely to go when it reacts with propene?
Middle 2 methyl groups donate electron density to stabilise positive charge
Define polymer
A very large molecule made up of long chains of smaller monomers units joined together
What happens when ethene is polymerised?
Repeating units link together to form long chainDouble bond in ethene breaks to form poly(ethene)
What did Karl Ziegler produce polythene?
Using catalyst60°CAtmospheric pressureProduces HDPE
How was polythene first made?
Polymerisation of ethene with oxygen traces200°C1200 atmProduces LDPE
What is the general formula for alkanes?
CnH2n+2
How are alkanes obtained?
From crude oil and petroleumPrimary distillation changes crude oil into useful chemicalsFractional distillation
What happens during fractional distillation?
Crude oil boiledGases pass up through tower through bubble capsGas condenses at particular temperature on cap
What does cracking produce?
Short chained alkane and alkene
What is used to reduce cost when cracking catalysts? What is this known as?
CatalystsCatalytic/cat cracking carried out in cat cracker
Why are alkanes good for lubricants?
Non corrosive
H
C-C and C-H involve even sharing electrons since electronegativities very closeNot polar so no charge attract polar/ionic
What causes reacts of alkanes?
Formation of free radicals so high activation energies
What is a free radicals?
Unpaired electron
What is bond fission?
Breaking bonds
What is homolytic fission?
Equal sharing of electrons in bondEach receive 1Half curly arrow usedUnpaired electron gained by each atom indicated by Cl•No charge but extremely reactive
What is hetrolytic fission?
Unequal sharing of electrons in covalent bondResults in 2 charged particlesNormally when covalent bond already has degree of polarity
How do alkanes burn?
When in gaseous state so solids and liquids less flammable (vaporised before)
Why are alkanes combusted?
Generate electricity as it produces heat and light energyExothermic process more energy produced by new bonds
Why is propane often used as a fuel?
Liquified under low pressure and low temperatureAllows larger quantities in small spacesSafe because don’t burn in gaseous state
How do you test the oxidation of alkanes?
Add 0.5cm3 of 0.01M potassium maganate and 1M sulphuric acidAdd couple drops of liquid alkane
How should alkanes react when testing for oxidation?
No changeStayed purple
How do you test the action of bromine on alkanes?
Add a few drops of alkane to 0.5cm3 of bromine water
How do you test for the reaction of alkanes with alkalis?
Add 2cm3 of 20% post assign hydroxide dissolved in water to 0.5cm3 or liquid alkaneShake tube
How should alkanes react with bromine when NOT in sunlight?
No change
How should alkanes react when mixed with an alkali?
No reaction
How do you test for the reaction of alkanes with sulphuric acid?
Add 0.5cm3 of alkane to 2cm3 of concentrated sulphuric acid
How should alkanes react when mixed with concentrated sulphuric acid?
No reaction
How do you test for the action of bromine in sunlight?
Add several drops of 2% bromine in an inert solvent to 10cm3 of hexane/cyclohexaneLoosely cork test tube and leave in sunlight
What should be the results of alkanes with bromine in sunlight?
One in dark stayed orangeOne in light turned colourless as Br2 reacts with hexane to produce HBrReacts to form NH4Br
How should you crack an alkane?
Medicinal paraffin soaked in ceramic fibre in test tubeAluminium oxide granulesWater bathHeat aluminium oxide strongly
Why should you discard the first tube of gas collected from cracking alkanes?
Air will expand and be forced out
What should you test cracked alkanes for?
Turned colourless in bromine waterLots of smoke and small flame when lit3 drops KMnO4 2cm3 sulphuric acid, when shaken turns from purple to colourless
What is the procedure for testing combustion of alkenes?
Dip a combustion spoon into alkene and set fire to it
What observations can be made when testing the combustion of alkenes?
Became more orange and smokey than corresponding alkane due to C:H ratioLonger the chain the less volatile therefore smokier and more orange
What is the procedure for testing the action of bromine with alkenes?
Add few drops of alkene to 0.5cm of bromine water
What observations can be made when Alkenes react with bromine?
Rapidly turned from Orange to colourlessProduced 1, 2 -dibropropane
What is the procedure for testing the action of with sulphuric acid alkenes?
Add 0.5cm of alkene to 2cm of concentrated sulphuric acid
What is the procedure for testing the oxidation of alkenes?
0.25 cm potassium maganate 0.01M0.25 cm 1M sulphuric acidFew drops of alkene
What is the procedure for testing the procedure of alkenes?
In fume cupboard
What observations can be made when testing for oxidation with alkenes?
Turns from purple to colourlessNo double bonds
