Alkanes

What are the two types of alkanes?

• Aliphatic — straight chain and branched

• Cyclic — ring

What is the chemical formula of alicyclic alkanes?

CnH2n+2

What is the trend of an alkane’s boiling point as the no. of C increases?

increases (as such with the other alkanes in the homologous series)

• The bigger the molecule, the bigger the electron cloud

• Greater the likelihood of distorting electron cloud

• Giving rise to stronger id-id attractions

Why do alkanes have relatively low boiling points?

• They are non-polar (electro-negativity of C and H are very similar)

• Boiling and melting involves overcoming the instantaneous dipole-induced dipole forces of attraction that exist between the molecules.

Why does branching of alkanes reduce the boiling point of alkanes?

• Branching makes alkanes more spherical

• Results in smaller surface area of contact with neighbouring molecules.

• Results in less extensive id-id forces of attraction

Why does density of alkanes increase with molecular mass?

The increase in the no. of electrons lead to stronger id-id holding the molecules together; which even made them less dense than water.

Chemical properties of alkanes

• nonpolar — C and H have similar electronegativities

• chemically inert

What are the three reactions that alkanes can undergo?

• combustion

• Free radical substitution with halogens

• Cracking

What are the products formed from complete combustion of alkanes?

CO2 and H2O

What are the products formed from incomplete combustion?

Carbon (soot), CO2 and H2O

What is the condition for free radical substitution to occur?

UV light; to make the highly inert alkane undergo homolytic fission

What is free radical substitution?

Reaction of alkanes with halogens undergo free radical substitution where one or more of the hydrogen atoms in. Alkanes are being replaced by halogen atoms

What is the disadvantage of free radical substitution?

Highly uncontrollable; produce a large amount of products via the chain reaction sequence, which can be difficult to separate

Why is it that iodoalkanes cannot be formed through free radical substitution?

Bonds between H-I are very low in energy; can easily dissociate and recombine with ethyl radical formed in propagation step

Is there a direct relationship between the bond strength of halogens and their reactivity with alkanes?

No; Lower bond strength would make it more reactive to alkanes as it involves the breaking of halogen-halogen bonds, making it easier for free radicals to form

How do we ensure a particular major product is formed (CH3Cl)? 

Use excess CH4 such that there will not be enough Cl2 for further substitution and mono-substituted methane predominates

Uses of catalytic converters

To convert harmful pollutant gases into harmless gases

Eqn: Oxides of nitrogen removed by decomposition to N2 & O2 or reduction by CO to N2

2NO + 2CO —> N2 + 2CO2

Eqn: Complete combustion of unburnt hydrocarbons

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

Oxidation of CO to CO2

2CO + O2 —> 2CO2

What is the greenhouse effect?

An energy trapping process where infra-red radiation reflected from the Earth’s is prevented from escaping completely into space and is absorbed by greenhouse gases and re-radiated back to Earth.