Alkanes

alkanes are saturated hydrocarbons

only contain carbon and hydrogen atoms and each carbon atom forms 4 single bonds

crude oil can be separated via fractional distillation

into smaller more useful hydrocarbons

fractional distillation of crude oil:

• crude oil is vapourised at 350℃

• vapourised crude oil goes into the bottom of the fractioning column and rises up through the trays

• the largest hydrocarbon chains are vapourised at all because their boiling point is too high

• as crude oil vapour travels up the fractioning column it gets cooler creating a temperature gradient

• because boiling points of alkanes increase as the molecule gets longer, each fraction condenses at different temperatures

• hydrocarbons with the lowest boiling points dont condense, they’re drawy off as gases at the top of the column

long chain hydrocarbons are broken down into smaller pieces in a process called cracking

to meet the high demand of short chain hydrocarbons

there are two types of cracking

• thermal

• catalytic

thermal cracking

• occurs at temperatures up to 1000℃

• high pressure up to 70 atm

• produces lots of alkenes (useful for polymers)

catalytic cracking

• zeolite catalyst

• slight pressure

• high temperature 500℃

• produces lots of aromatic hydrocarbons and alkanes for motor fuels

using a catalyst cuts costs

because the reaction can be done at a lower pressure/temperature and saves time+money

complete combustion of alkanes

if you burn/ oxidise alkanes with plenty of oxygen, you get carbon dioxide and water

incomplete combustion of alkanes

when there’s not enough oxygen present when you burn hydrocarbons you produce particulate carbon )soot) and carbon monoxide

carbon monoxide is poisonous

• carbon monoxide binds to the same sites on haemoglobin in RBCs as oxygen

• this prevents oxygen from being carried around the body

carbon monoxide can be removed from exhause gases

by catalytic converters on cars

nitrogen odixes are a series of toxic and poisinous molecules with the general formula NO_x

Nitroge monoxide is produced when at high temp/pressure in a car engine cause hitrogen and oxygen atoms in the air to react

unburnt hydrocarbons can react with nitrogen oxides in the presence of sunlight to form ground level ozone O₃

ground level ozone irritates people’s eyes, aggrivates respritory problems and causes lung damage

3 main pollutants from vehicle exchaust

• nitrogen oxides

• unhurnt hydrocarbons

• carbon monoxide

catalytic converters can remove vehicle pollutants from the exhaust

sulfur from fossil fuels reacts to form sulfur dioxide

sulfure dioxide dissolves the moisture in the atmosphere and is converted into sulfuric acid AKA acid rain

when nitrogen dioxide escapes into the atmosphere

nitric acid is created

acid rain

destroys trees, vegitation and corrodes buildings and statures and kills fish in lakes

sulfur dioxide can be removed before it gets into the atmosphere

using powdered CALCIUM CARBONATE/CALCIUM OXIDE MIXED W WATER

flue gases + alkaline slurry= harmless salt

burning fossil fuels produces carbon dioxide

carbon dioxide is a green house gas

green house gases cause the green house gass efect

absorb infrared energy and emit some of the energy they absorb back towards the earth keeping it warm

halogens react with alkanes in photochemical reactions

forming halogenoalkanes

photochemical reactions

reactions started by ultraviolet light

a hydrogen atom is replayed by chlorine/ bromine

free radical substitution

free radicals

particles with an unpairewd electron

free radicals form when

a covalent bond splits equally giving 1 electron to each species

the unpaired electrons make them very reactive

methene and chlorine exposed to UV light forms chloromethane

free radical substitution is split into 3 phases

• INITIATION

• PROPOGATION

• TERMINATION

Initiation

produces the free radical via photodissocation

propogation

the free radical are used up in a chain reaction, first it will attack the normal molecule creating a new free radical which attack another molecule to for the origional free radical

(substitution)

if there is excess reactants you get all sorts of reaction

termination

all free radicals are mopped up to create stable molecules

chlorofluorocarbons

halogenoalhakes when all the hydrogen atoms have been replaced by chlorine and fluorine

ozone forms naturally when a oxygen molecule is roken down into 2 free radicaks by ultraviolete radiation

CFCs destroy the ozone by forming chlorine free radicals when C-Cl bonds break which react with ozone

chlorine free radicals are catalysts

they react with ozone to form an intermediate ClO• and an oxygen molecules

CFCs

• unreactive

• non fmamable

• non toxic

CFCs used as

• fire extinguishers

• propellants in aresols

• coolant gas in fridges

• added to form plastics to make insulation and packaging materials

HCFCs created as a safer alternative to CFCs

halogenoalkanes

alkanes with at least one halogen aotm in place of a hydrogen atom

halogenoalkanes are polar

because halogens are electronegative

δ+ carbon doesnt have much electrons

so it can be attacked by nucleophiles

nucleophiles

electron pair donors, attracted to positive regions

nucleophilic substitution

the mechanism for halogenoalkanes

nucleophilic substitution step by step:

• LPE on nucleophile attacks the slightly positive carbon attached to the halogen

• the carbon can only form 4 bonds so the addition of the nucleophile breaks the bond between carbon and the halogen, the electron is attracted to the halogen as is electronegativity is high

halogenoalkane + OH- (via warm aqueous sodium/potassium hydroxide)

=ALCOHOL

halogenoalkane + CN- (via warming halogenoalkane with ethanolic potassium cyanide/ potassium cyanide dissolved in ethanol)

=NITRILE

halogenoalkane + potassium cyanide dissolved in water

= the OH- could act as a competing nucleophile and you’d get some alcohol product

R3N, where R could be Hydrogens or another group

amine

halogenoalkane + EXCESS ethanolic ammonia (ammonia dissolved in ethanol) in a sealed tube

= AMINE

going down group 7, carbon-halogen bond strength DECREASES

therefore fluoroalkanes undergo nucleophilic substitution slower than iodoalkanes, which undergo nucleophilic substitution the fastest

drawing mechanisms for nuceophilic substitution reactions

iys important to draw the curly arrows coming from the electrons (which come from either a bone/a lone pair on an atom or ion) and going to the atoms

make sure the charges are balanced at every stage of a mechanism

if you start with a negative charge you should end with a negative charge

halogenoalkane + OH- (warmed together with OH- from dissolved ethanol NOT water)

halogen elimination from a halogenoalkane produces ALKENES

halogenoalkane + water under reflux

= ALCOHOL via nucleophilic substitution

halogenoalkane + ethanol under reflux

= ALKENE via elimination

halogenoalkane + water + ethanol

= ALCOHOL + ALKENE