AQA A level - Organic Chemistry

Organic compounds

Compounds that contain a chain of carbon atoms

Number of electrons in carbons outer shell

4 so can form 4 covalent bonds

Carbon-Carbon bonds are...

non-polar and relatively strong

Empirical formula

the simplest whole number ratio of atoms of each element in a compound

molecular formula

a formula that tells us the actual number of atoms of each different element in a compound

structural formula

a way of writing the formulas of an organic compound in which bonds aren't shown but each carbon is written seperately.

skeletal formula

a way of showing the structure of a complex molecule. only labels the carbon and side groups.

nomenculture

the universl rules about naming compounds

why use nomenculture?

so all scientists use the same names and others can understand what compounds they have used.

IUPAC ruling system

the way that the IUPAC has developed a naming convention that chemists all follow.

side chains are shown by...

a prefix of the root of the longest carbon chain in the side chain.

Alkanes general formula

CnH2n+2

Alkene FG

C=C double bond anywhere

alcohol FG

-OH at end or on a branch

Aldehydes FG

- CHO at the end of the chain

ketones FG

- CO - carbonyl group in the middle of the chain

Carboxylic acids FG

- COOH terminates the chain

Esthers FG

- COO - in the middle

Nitriles FG

C triple bonded to N terminates the chain

Alkynes FG

C triple bonded to C

Homologous series

a family of organic compounds with the same functional group, but different carbon chain length.

Similar rules of homologous series?

have the same general formula and their formula will differ from the next member of the series.

Isomers

molecules with the same molecular formula but different arrangements of their atoms.

Structural Isomers

have the same molecular but different structural formulae.

positional isomers

have the same functional group but they are attached to the hydrocarbon chain in different places.

functional group isomers

have the same molecular formula but arranged as different functional groups

chain isomers

have the same molecular formula but the chain is arranged differently.

Alkane uses

fuels, lubricants and important in industry

Crude Oil

a mixture of lots of different alkanes and so they must be separated before they can be used.

alkanes are..

unreactive but release a lot of heat energy when burned.

Alkanes

saturated hydrocarbons and un-polar. they are held tigether by weak van der waals forces.

hydrocarbon

a compound made up of hydrogen and carbon atoms only.

suffix for alkanes

-ane

Methane

CH4

Ethane

C2H6

propane

C3H8

Butane

C4H10

Pentane

C5H12

to name a branched alkane

1. find the longest carbon chain

2. the branches are added as prefixes with number to show what carbon they are attached to.

Polarity of alkanes

non-polar as the electro-negativity of hydrogen and carbon are so similar.

Melting and boiling point of alkanes

as the chain length increases, the size of the melting and boiling point increases due to the increasing strength of van der waals forces in the larger molecules.

Solubility of Alkanes

not soluble in water due to the hydrogen bonds that hold water together are stronger than van der waals forces.

Reactivity of alkanes

low reactivity due to their strong carbon-carbon and carbon-hydrogen bonds. they will burn and react with halogens.

Fractional Distillation

Method to separate a liquid mixture into its individual components using the boiling and condensing point of the liquids

crude oil is non-renewable because

the process takes a very long time.

the shorter the hydrocarbon chain

the lower its boiling point

different sized hydrocarbons condense at different levels

so each fraction is a mixture of hydrocarbons of similar sizes.

Cracking

the process of splitting longer chain hydrocarbons into shorter more useful hydrocarbons

Economic reasons for cracking

more demand for shorter hydrocarbons, more valuable.

2 types of cracking

Thermal and Catalytic

Combustion of alkanes

burn with a less smoky flame and combust completely when enough oxygen

fuel

a substance that releases heat energy when it is burnt. this energy can be used to power machines.

why are alkanes good fuels?

they release a large amount of energy when they burn. negative enthalpies of combustion - the more carbon in the alkane the greater the amount of energy released.

Halogenoalkanes

reaction between alkanes and halogens.

free-radical substitution reaction

when a halogenoalkane is formed

Free radical

atom with an unpaired electron and are very reactive

Initiation stage

homolytic breaking of covalent bonds and electrons in the bonds between halogen atoms

Propagation stage

the halogen free radical takes a hydrogen ion from the alkane and forms a stable H-halogen and a free radical. this reacts with another halogen free radical to form a stable halogenoalkane molecule.

termination step

the step where the free radicals are removed.

halogenoalkanes

organic compounds that consist of a hydrocarbon chain with one or more halpgen atoms attached.

Halogenalkanes aren't natural

but they are the basis of many synthetic compounds.

general formula for halogenoalkanes

CnH2n+1X (X being the halogen)

naming halogenoalkanes

named with the prefix that identifies the halogen

halogenoalkane reactivity

the C-X bond is polar therefore they are more reactive than alkanes.

Halogenoalkanes polarity

polar as the halogen is more electronegative than the carbon - the carbon is slightly positive which attracts nucleophiles.

nucleophiles

electron-rich molecules

Bond enthalpy of halogenalkanes

the enthalpy decreases down group 7. this is because the higher up the group, the more polar so less energy needed to break the bonds.

reactivity of the bonds increases down the halogen group showing

the bond enthalpy is more important than the bond polarity

alkenes

unsaturated hydrocarbons. they are not saturated with hydrogen and have at least one C=C bond.

gneral formula for alkenes

CnH2n

position isomers of alkenes

isomers have different positions of the double bond in the hydrocarbon chain. the longer the chain, the more isomers there are.

Geometrical Isomers

the same structural formula but different arrangement of the bonds around teh C=C bond.

Reactivity of alkenes

very reactive because of the double bonds. double bonds have a higher bond enthalpy, but they are nucleophiles so can be easily attacked by positive electrophiles.

Polymers

long molecules formed of lots of the same molecules, called a monomer.

Addition polymerisation

the joining of monomers

3 steps of fractional distillation

evaporation, condensation and collection

Fractional Distillation

the process of separating crude oil into groups of hydrocarbons with similar number of carbon atoms

long chain hydrocarbons

higher boiling point so evaporate and condense earlier on in the column

short chain hydrocarbons

lower boiling point so evaporate and condense further up the column

Temperature is highest

at the bottom of the column

why cracking?

shorter chain alkanes are more valuable and useful

thermal cracking

uses harsh conditions like high temperature and high pressure.

Catalytic Cracking

uses lower temperatures and pressure to reduce costs and make the reaction faster

Conditions of Catalytic Cracking

450 degrees C and 1-2atm

Catalyst used in catalytic cracking

zeolite

zeolite

An iconic compound containing aluminium, silicon and oxygen

Combustion

the burning of materials

complete combustion results

blue flame and produces CO2 and H2O

Incomplete combustion results

orange smoky flame and produces H2O CO2 and Carbon

Why orange flame?

due to small particles of pure carbon glowing red hot

Combustion in enines products

nitrous oxides, unburned hydrocarbons, carbon monoxide

Flue gases

gases that come out of chimneys. like Sulfur Dioxide.

Removing sulfur dioxide

by reacting it with calcium oxide or calcium carbonate

CaO + SO2 =

CaSO3

Catlytic conversters can

remove unburned hydrocarbons and nitrous oxides from fuel

Chlorination

the first reaction mechanism involving free radiacls

steps of substitution reactions

initiation, propogation and termination

Halogenalkanes

a halogenated alkane - one or more hydrogen atoms have been replaced by a halogen atom

halogenalkanes polar?

halogens are more electronegative than carbon, so they carry a partial negative charge and the carbon is partially positive.

Nucleophiles

a molecule that is attracted to the nuclues