Alkanes, Haloalkanes, Alcohols and Alkenes Flashcards
define hydrocarbon
compound which contains hydrogen and carbon atoms only
define saturated hydrocarbon
hydrocarbon which contains only single bonds between compounds
what does the bp of hydrocarbons depend on?
the size of the molecule and any side chains as VDW are the only forces that exist between the molecules so their size and how closely together they pack dictates how strong the IM forces are.
what are alkanes soluble in?
they are insoluble in water but soluble in other non polar liquids such as hexane and cyclopentane.
define fractional distillation
the continual evaporation and condensation of a mixture, causing the components to separate due to differences in bp.
explain the process of fractional distillation
- vaporise mixture
- pass into a fractionating column with a temp gradient (much hotter at the bottom than the top)
- the substances with the highest bps will condense first as they rise up the column, eventually separating the mixture into factions.
what are factions?
a group of compounds with similar bp that are therefore removed at a similar level of a fractionating column.
why are long chain hydrocarbons cracked?
shorter chain hydrocarbons are generally more useful, this means that in order to meet the demand for these, the less useful longer chain hydrocarbons are broken up.
What are the conditions for thermal cracking and what are the main types of products?
Very high temp - 1200K
Very high pressure - 70atm
Product is rich in short chain alkenes
Why is catalytic cracking done in short bursts of high temp and pressure?
To prevent the molecule decomposing completely to produce just C and H.
What are the conditions for catalytic cracking and what are the main types of products?
High temp - 1000K
Low pressure - 2atm
Zeolite catalyst in a honeycomb shape for optimum SA:V ratio
Mainly aromatic compounds and motor fuels
How is CO formed during the combustion of alkanes and why is this bad?
Produced when hydrocarbons are burnt is limited oxygen
Reacts with the haemoglobin in our red blood cells preventing them from carrying oxygen. This will kill you.
How is soot formed during the combustion of alkanes and why is this bad?
Soot is pure C which is formed when hydrocarbons are burnt in very limited oxygen.
It does damage to your lungs and causes global dimming
How is NO and NO2 formed during the combustion of alkanes and why is this bad?
The high temp and pressure cause N from the air to react with oxygen.
This causes acid rain and chemical smog.
How is SO2 formed during the combustion of alkanes and why is this bad?
This is formed when fuels with S impurities are burnt.
It causes acid rain
Describe The Greenhouse Effect
Infrared rays from the sun pass through the atmosphere to the earth and warm it. Some of this is then reflected by the earth back into the atmosphere. However, it is prevented from passing out into space by atmospheric gasses such as water vapour and CO2. This further warms the atmosphere and is causing the earth to become hotter. This naturally occurs but is being accelerated by the emission of greenhouse gasses from earth.
How does SO2 react with water vapour to cause acid rain?
SO2 + H2O +1/2O2 -> H2SO4
What is petrochemical smog?
toxic gasses and minature particles suspended in the air close to the surface of earth.
How do catalytic converters work (in words)?
Platinum and rhodium catalysts are spread very thinly over a honeycomb structure. They catalyse the reaction of toxic exhaust gasses together to form less toxic gasses.
What are the catalytic converter equations?
2NO + 2CO -> 2CO2 + N2
C8H18 + 25NO -> 8CO2 + 12.5 N2 + 9H2O
What happens in flue gas desulfurisation (in words)?
The gasses are passed through a wet semisolid slurry containing calcium oxide and calcium carbonate. These are bases and so neutralise the acidic SO2 to form calcium sulphate which is useful in the building industry.
Flue gas desulfurisation equations
CaO + SO2 -> CaSO3
CaCO3 + SO2 -> CaSO3 + CO2
CaSO3 + [O] -> CaSO4
What is the bp of haloalkanes influences by?
- Halogen atom - fluorine is much more electronegative than iodine and so the C-F bond is more polar. This means that any permanent dipole dipole forces are stronger.
- Size - as the length of the carbon chain increases, the VDW forces increase.
What are halogenoalkanes soluble in?
Organic solvents but NOT water.
Describe the process of free rad substitution?
Halogens will react with hydrocarbons in UV light because they have undergone homiletic fission and become two free radical atoms which are extremely reactive. These will react with a hydrogen on the molecule, leaving it as a free radical. It will go on to react with a X2 molecule and form a halogenoalkane. The X free radical is regenerated so further substitutions can occur and a chain reaction begins.
What are the free rad substitution equations?
Initiation:
Cl2 -> 2 Cl~
Propagation:
Cl~ + CH4 -> HCl + ~CH3
~CH3 + Cl2 -> CH3Cl + ~Cl
How will free rad substitution stop?
When two free radicals react together this is a termination step which ends the chain reaction.
What does the ozone do for earth?
It enhances the absorption of harmful UV light by nitrogen and oxygen, prevent it from causing cancer, eye damage and plant tissue damage.
What is happening to the ozone?
Chlorine free radicals are formed when CFCs are released into the atmosphere and are under UV light. These free radicals reacts with the O3 and is gradually destroying the ozone layer.
What are the equations for the depletion of ozone?
Cl~ + O3 -> ClO~ + O2
ClO~ + O3 -> 2O2 + Cl~
Why can halogenoalkanes under go nucleophilic substitution?
The polar C-X bond means the electropositive carbon is susceptible to nucleophilic attack.
What are the conditions for nucleophilic substitution with a hydroxide ion to form an alcohol?
Aqueous NaOH and the haloalkane dissolved in ethanol.
Reflux gently
What are the conditions for nucleophilic substitution with a cyanide ion ion to form a nitrile?
Aqueous KCN and haloalkane dissolved in ethanol.
Reflux gently
What are the conditions for nucleophilic substitution with ammonia to form an amine?
Conc ammonia and haloalkane dissolved in ethanol.
What are the conditions for elimination reactions of haloalkanes?
The hydroxide ions are dissolved in ethanol (ethanolic) and heated
How can haloalkanes react with hydroxide ions in addition and elimination?
Then dissolved in water, -OH will act as a nucleophile. When dissolved in ethanol, the hydroxide ion will act as a base and accept a H atom of the alkane rather than attacking the electropositive C.
What is an unsaturated hydrocarbon?
A hydrocarbon with at least one C=C.
Why are alkenes more reactive than alkanes?
The C=C bond becomes an area of high electron density.
Define stereoisomers
Molecules that ha e the same molecular formula but a different 3d arrangement of atoms in space.
What are E Z isomers?
Isomers of the same alkene that have a different arrangement of groups around the C=C and so a different arrangement in space as the C=C is non rotational.
How are E and Z isomers different?
Z - highest priority groups on same side of C=C
E - highest priority groups on opposite side of C=C
What happens in addition polymerisation?
The double bond in the monomers break allowing them to form a long chain.
What effects the bp of polymers?
- Size - longer chain means stronger VDW
- Shape - lots of side chains means polymers cannot pack together as well and so the VDW forces aren’t as strong.
What are the different types of poly(ethene) used for?
HDPE -> kitchenware
LDPE -> plastic bags and bottles
What is poly(chloroethene) or PVC used as?
Un-plasticised - drainpipes, window frames
Plasticised - Wellys, raincoats
Why can alkenes undergo electrophilic addition?
In polar molecules, the electropositive side is attracted towarsds the area of electron density around the double bond and are electrophiles so can accept the pair of electrons.
What is the breaking of the bond in the polar substance that acts as the electrophile called?
Heterolytic fission as the covalent bond has broken unevenly, the electronegative atom has gotten both of the electrons.
What are the conditions required for electrophilic addition?
It will occur even below room temp as long as there is a polar electrophile.
How is electrophilic addition used to make alcohols from alkenes? What are the conditions required for this?
Alkenes will undergo electrophilic addition with concentrated H3PO4 and the product of this can be reacted with water to form an alcohol.
Conc acid catalyst, 60atm and 600K
How can electrophilic additions occur with non polar electrophiles?
The area of high electron density repels electrons in the reactant and allows it to form a dipole.
What is the chemical test for alkenes?
Add to bromine water and shake, the Br water should decolourise.
What dictates the products of electrophile addition with asymmetrical alkenes?
The structure of the carbocation intermediary.
A tertiary carbocation intermediate is more stable than a secondary which is more stable than a primary. The more stable intermediaries are more likely to form and so form the major product.
Why are tertiary carbocation intermediaries most stable?
Alkyl groups have an electron donating effect meaning they push electrons towards the positive charge. So, the positive charge becomes less and the carbocation is more stable.
Why do more stable carbocations form major products?
They remain in the reaction mixture for a longer time and are therefore more likely to react with the anion to form the product.
Why do alcohols have higher bps than a molecule of their size should?
Hydrogen bonding
Why are short chain alcohols soluble in water?
Hydrogen bonding
What do primary alcohols oxidise into and how
React with acidified potassium dichromate.
First, aldehydes (immediately distill).
Then, carboxylic acids (reflux).
What do secondary alcohols oxidise into?
Keytones (reflux with acidified potassium dichromate).
What visible change occurs when alcohols are oxidised?
The orange Cr2O7 2- ion is reduced to the green Cr 3+ ion.
What are the two tests for aldehydes and ketones?
Tollens’ reagent will form a silver mirror on the walls of the test tube if aldehydes are present.
Fehling’s solution will go from blue to having a brick red ppt.
Why does tollens reagent go silver?
Ag+ ions are reduced to Ag (silver)
Why does fehlings solution form a red ppt?
Cu 2+ ions are reduced to Cu+ ions.
How are alcohols dehydrated?
Elimination reaction occurs when alcohols are heated with a conc sulphuric acid catalyst. An alkene is formed. The alcohol vapour can also be passed over an Al2O3 catalyst at 600 degrees for same results.
What may cause EZ isomers to be formed?
Elimination of non symmetrical isomers.
Under what conditions is ethanol produced via fermentation?
Yeast is present.
Anaerobic conditions.
35 degrees
neutral pH
supply of glucose
What is the equation for the fermentation of glucose?
C6H12O6 -> 2C2H5OH + 2CO2
What are the economic cons of fermentation for production of ethanol?
Batch process so inefficient
Very slow
Produces impure ethanol which needs to be further processed.
What is a batch process?
Everything must be put into a container and left until the reaction is complete. The container must then be cleared out before a new reaction mixture can be added.
What are the environmental pros of fermentation for the production of alcohols?
Gentle temps and atmospheric pressure used
Renewable resources.
What are the economic pros of hydration of ethene for ethanol production?
Continuous flow process - more efficient
Very fast
Much purer ethanol produced.
What are the environmental cons of the hydration of ethene for the production of ethanol?
Uses high temp and pressure which require high energy input
Uses finite resource - crude oil.
What is biofuel?
Fuel made from living organisms or their waste.
Define carbon neutral.
A process or activity in which no net emissions of CO2 into the atmosphere.
Why does biofuel promise to be carbon neutral and why is this false?
The mass of CO2 released = the mass of CO2 taken in growing the crops to make biofuel. However, energy is used producing fertilisers, transporting and extracting the fuel. This energy likely comes from the burning of fossil fuels.
