Chapter 12: Alkanes

• main components of natural gas and crude oil • most stable organic compounds • General formula CnH2n+2 • Common uses: Gas (c1-c4); petrol (c5-c9); kerosene (c10-c16); diesel (c12-c20)

• saturated hydrocarbons, so only single covalent bonds • each carbon atom is joined by 4 other atoms by single covalent bonds - sigma bond

3D tetrahedral arrangement around each carbon atom, ~109.5 degrees - 4 electron pairs with 4 sigma bonds - repulsion results in this shape - sigma bonds act as axes around which atoms can rotate freely (not rigid)W

result of direct overlap of the bonding orbitals in each bonding atom

From crude oil, alkanes are separated into fractions by fractional distillation. This is because they have different boiling points.

Chain length - as this increases, BP increases Branched chain/straight chain - branched chain isomers have lower boiling points

As chain length increases, boiling point of alkanes increase because - molecules have a larger surface areas, so more surface contact between molecules - the greater the intermolecular (London) forces, the higher the boiling point

Branched-chain isomers have lower boiling points, because - fewer surface points of contact on branched chains - fewer London forces, so lower BP - branches prevent the branched molecules getting as close together as straight-chain molecules

C-C and C-H sigma bonds are strong C-C bonds are non-polar (electronegativity of C and H are so similar that C-H bonds are considered nonpolar)

two reactants join together to form one product

Where an atom or group of atoms are replaced by a different atom or group of atoms

Removal of a molecule from a saturated molecule to make an unsaturated molecule

They are readily available Easy to transport Burn in plentiful supply of oxygen without releasing toxic products. All alkanes react with oxygen to combust despite its low reactivity.

There is a limited supply of oxygen - with each CH2 molecule, 1.5O2 extra is needed for complete combustion. Toxic carbon monoxide may be formed, or carbon as soot

CO is toxic because it’s colourless, odourless, and combines irreversibly with haemoglobin to produce carboxyhaemoglobin (cherry-pink compound). The danger lies in the lack of any odour (i.e. people often don’t know they’ve got CO poisoning until it’s too late)

• Presence of sunlight (UV radiation) The high energy UV radiation provides initial energy for the reaction to take place.

Substitution reaction

Sequence of bond-breaking and bond-making steps that show the path taken by electrons during a reaction.

Radical substitution

1- Initiation 2- Propagation 3- Termination

1 - Initiation: - covalent bond in a halogen molecule is broken by homolytic fission - each halogen atom takes 1 electron forming 2 highly reactive halogen radicals - energy provide by UV radiation for bond fission

2 - Propagation - Pair of propagation steps as this is a chain reaction - halogen radicals react with alkanes in a chain reaction where the halogen radical acts as a catalyst - for each H replaced, there is a pair of propagation steps

Reaction in which propagation steps release new radicals that continue the reaction

3- termination - When any 2 radicals collide, they react to form a molecule - this removes both free radicals, stopping the chain reaction (radicals are far more likely to meet a molecule than radical)