Hydrocarbons

Why are alkanes generally unreactive?

• C-C and C-H bonds have a fairly high bond enthalpy and therefore require a lot of energy to overcome

• The difference between electronegative of C and H small so it is resistant to reactions with polar reagents

What is chemical equation for the complete combustion of ethane

C2H6 +3 1/2O2 → 2CO2 + 3H2O

Write an overall for the substitution reaction of ethane and chlorine

C2H6 + CL2 →(UV light) C2H5Cl + HCl

What are the three steps of free radical substitution?

1. Initiation

2. Propagation

3. Elimination

What is crude oil a source of?

Aliphatic and aromatic alkanes

What is cracking?

The process which large alkanes are broken down into smaller more useful molecuels

Conditions for cracking

• Hydrogen

• Nickel/platinum catalyst

• 150°C

Write the equation for the hydration of ethene

C2H4 + H2O → C2H5OH

H3PO4 Catalyst

Conditions for the hydration of alkenes

• Presence of steam

• H3PO4 (phosphoric acid) catalyst

• 300°C

• 60-70 atm

What is Markovnikov’s law?

In electrophilic addition of hydrogen halide an asymmetric alkene, they hydrogen atom bond to the carbon that is already bonded to the most hydrogens

State the equation for the hydration of ethene

C2H4 + H2O → C2H5OH

How are carbocations classified?

Primary

Secondary

Tertiary

What is the trend in stability of carbocations?

Stability increases

Tertiary carbocation is the most stable

In terms of carbocation stability, what will be the major product in the addition reaction between hydrogen halide and an unsymmetrical alkene?

The major product will be the one whose carbocation intermediate is the most stable

How can alkenes be oxidised to give a diol?

Manganate (VII) ions are a strong oxidising agent

When cold, dilute and acidified, the ions change from purple to colourless

Give the equation for the oxidation of ethene

C2H4 + H2O + \[O\] → (CH2OH)2

What happens when an alkene is oxidised with, hot concentrated acidified potassium manganate?

The C=C double bond is broken and replaced with 2 X C=O bonds

What happens next is based on whether the R groups are alkyl groups or hydrogen atoms

After an alkene has been oxidised using hot concentrated H+/Cr2O72-, both R groups of one the products are alkyl groups. What happens to this product?

The alkene was split into a ketone (both R groups are alkyl groups and another carbonyl)

Ketones cannot be oxidised further so no further reaction occurs

After an alkene has been oxidised using hot concentrated H+/Cr2O72-, both R groups of one the products has the following R groups: an alkyl chain and a hydrogen chain. What happens to this product?

The alkene was split into an aldehyde ( with an alkyl group and a hydrogen atom as the R groups) and another carbonyl

The aldehyde is further oxidised to form a carboxylic acid

After an alkene has been oxidised using hot concentrated H+/Cr2O72-, both R groups of one the products are hydrogen atoms. What happens to this product?

The alkene was oxidised to form methanal (with 2 hydrogen groups as R groups) and another carbonyl

Methanal will be completely oxidised form CO2 and H2O

What is addition polymerisation?

The joining together of unsaturated alkene monomers to form a polymer (atom economy is 100%)

How do alkenes undergo addition polymerisation?

The π bond breaks and each electron goes towards forming a σ bond with an adjacent monomer unit

Deduce the repeat unit of the addition polymer formed from the monomer below

Turn the C=C bond into C-C

Add and extend the bonds out from the side of the carbon atoms

Add in square brackets around the molecule

Why polymers difficult to dispose of?

They are non-biodegradable

Their combustion releases harmful compounds

Why are alkanes suitable as fuels?

High standard enthalpy change of combustion ( releases a lot of energy)

Problems with CO

Poisonous

Irreversibly binds to haemoglobin

Cells may become deoxygenated - leading to death

Why are amphoteric oxides of nitrogen pollutants?

Nitrogen monoxide reacts with chemicals that can cause acid rain

Nitrogen dioxide can also contribute to photochemical smog

Describe how a catalytic converter works

• Nitrogen monoxide is formed in car engines: N2 + O2 → 2NO

• Catalytic converter removes majority of NO so it isn’t released into the atmosphere: 2NO + 2CO → N2 + CO2

• Ceramic honeycomb structure is coated in a thin layer of metal catalysts like platinum and rhodium

What are the environmental consequences of unburnt hydrocarbons?

They absorb infrared radiation, causing the greenhouse effect

What the environmental effects of greenhouse gases?

• Electromagnetic radiation from the sun passes through the atmosphere, some radiation is absorbed and the earths temperature rises

• Heat is radiated from the earth as infrared radiation which is absorbed by greenhouse gases in the atmosphere causing bonds in the molecules to vibrate

• As a result the atmosphere warms up more

How is infrared spectroscopy used to monitor air pollution?

IR spectroscopy identifies particular bonds in a molecule

Different pollutants will have different patterns of absorption as they contain different bonds

It is also possible to measure the concentrations of the different pollutants

How does the halogen carrier generate the electrophile when benzene reacts with a halogenoalkane?

Halogen carrier reacts with halogenoalkane to generate electrophile: AlCl3 + C2H5Cl → AlCl4- + C2H5+

AlCl4- reacts with proton expelled from the intermediate to regenerate the halogen carrier:

AlCl4- + H+ → AlCl3 + HCl