2.3 - Alkanes

Define hydrocarbon

compound containing only C and H

Define saturated hydrocarbon

hydrocarbon containing no double/triple C bonds

Define substitution

replacing one atom/group with another atom/group

Define radical

particle with unpaired e-

Define homolytic fission

bond breaking where one shared e- go to each atom forming 2 free radicals

Define heterolytic fission

bond breaking where both shared e- go to single atom forming oppositely charged ions

Alkanes that are gas at room temperature

C4 and below

Alkanes that are liquid at room temperature

C5 to C16

Alkanes that are solid at room temp

C17 and above

Are alkanes immiscible or miscible with water?

immiscible

Why are branched isomers more volatile?

molecules pack less closely together as less VDW forces

How does RMM affect mpt and bpt of alkanes?

only VDW forces between molecules, bpt increase with RMM as increasing strength of VDW forces

chemical properties of alkanes

stable, almost inert

What is combustion?

reaction of fuels with O2 producing heat/oxides

General formula for complete combustion of a hydrocarbon

CxHy + (x + y/4)O2 = xCO2 + (y/2)H2O

Complete combustion

unlimited supply of O creating CO2 + steam

What colour does carbon burn with?

sooty orange flame

What colour does hydrogen burn with

clean blue flame

What do colour and smokiness of a flame indicate?

C chain length and series fuel belongs to

What is eudiometry?

general combustion equation to identify unknown hydrocarbons by measuring gas volume changes after reaction

What to assume in eudiometry?

water liquid at room temp, bubbling through NaOH/KOH removes acidic CO2, residual gases are unused O2/CO2 formed

What is incomplete combustion?

in limited supply of O2, C/CO formed

What does CO cause death by?

inhalation

What does burning fossil fuels produce?

heat, CO2, CO, SO2, oxides of nitrogen and carbon(soot)

Carbon monoxide

toxic gas, in high traffic areas

Soot (carbon)

small C particles, enters lungs/bloodstream causing respiratory damage

Carbon dioxide

0.03/0.04% of atmosphere, greenhouse gas trapping IR causing global warming

Sulfur dioxide and oxides of nitrogen

acidic gases that dissolves in water to form acid rain - kills fish/defoliates trees

Catalytic converters role

converts unburnt hydrocarbon/CO/nitrogen oxides into H2O/CO2/N2 using platinum catalyst

CO - car engine

binds irreversibly to haemoglobin preventing O2 transport

Nitrogen oxides - car engine

high temps let N/O in air to react

What are nitrogen oxides responsible for?

photochemical smog, low-level ozone and acid rain

Catalytic converter structure

honeycomb structure coated with thin platinum layer providing large SA for reaction

What does leaded petrol do to catalytic converters?

catalyst poisoning as binds permanently to platinum surface blocking active sites

What is halogenation of alkanes?

saturated alkanes undergo substitution reaction and react with halogens

What other reaction is halogenation of alkanes to as because it occurs in presence of light?

photochemical reaction

methane + chlorine - 1

CH4 + Cl2 = CH3Cl + HCl

methane + chlorine - 2

CH3Cl + Cl2 = CH2Cl2 + HCl

methane + chlorine - 3

CH2Cl2 + Cl2 = CHCl3 + HCl

methane + chlorine - 4

CHCl3 + Cl2 = CCl4 + HCl

Where does halogenation of an alkane take place?

series of simple reactions - mechanism

What is the reaction between alkanes and halogen molecules?

UV light splits halogen molecules into 2 free radicals

What substitution is halogenation of alkanes?

free radical substitution

Monohalogenation free radical substitution mechanism of methane by chlorine

CH4 + Cl2 = CH3Cl + HCl

Initiation - general equation

1 molecule = 2 free radicals

Chain propagation - general equation

molecule + radical = molecule + radical

Chain termination - general equation

2 radicals = 1 molecule

initiation - homolytic fission of single covalent bond in Cl2 molecule

Cl2 =(UV light)= 2Cl•

chain propagation 1 - Cl free radicals reactive and react with methane

CH4 + Cl• = •CH3 + HCl

chain propagation 2 - methyl radicals react with Cl

•CH3 + Cl2 = CH3Cl + Cl•

chain termination 1 - methane + chlorine

2Cl• = Cl2

chain termination 2 - methane + chlorine

•CH3 + Cl• = CH3Cl

chain termination 3 - methane + chlorine

2 •CH3 = CH3CH3

What happens in fission if bonded atoms have similar EN?

each get a bonding e- forming free radicals by homolytic fission

What is fission?

breaking of chemical bonds

What happens in fission if one atom more electronegative than the other?

bonding e- go to more EN atom by heterolytic fission

Why does heterolytic fission occur?

attack by electrophile/nucleophile

What is an electrophile?

e- deficient species/e- pair acceptor

What is a nucleophile?

e- rich species/e- pair donor

Electrophilic addition of a hydrogen halide to an alkene

HBr = H+ + :Br-