Chapter 12 Flashcards
Describe what is meant by alkanes
saturated hydrocarbons containing single C–C and C–H bonds as σ-bonds (overlap of orbitals directly between the bonding atoms),
with free rotation of the σ-bond
What is a σ-bond ( sigma bond ) ?
A sigma bond is the overlap of atomic orbitals directly between bonding atoms.
Each sigma bond has two electrons shared between the bonding atoms.
( each carbon atom is joined to 4 other atoms by single covalent bonds - sigma bonds )
A sigma bond is positioned on a line directly between the bonding atoms
Explain the tetrahedral shape and bond angle around each carbon atom in alkanes in terms of electron pair repulsion
Each carbon atom is surrounded by 4 electron pairs in 4 sigma bonds
These electron pairs repel one another so that they are arranged as far as possible
This results in a 3D tetrahedral arrangement around each carbon atom, with all bond angles at 109.5 degrees
The sigma bonds act as axes around which the atoms can rotate freely
Explain the variations in boiling points of alkanes with different carbon-chain length and branching, in terms of induced dipole–dipole interactions (London forces)
As chain length increases, molecules have a larger surface area
More surface contact is possible between molecules
More electrons so london forces will be greater
More energy is required to overcome them so boiling point increases
As branching increases, there are fewer surface points of contact, leading to fewer london forces
As branching increases, molecules can’t get close together as straight chains, leading to weaker london forces
Less energy required to overcome them so boiling point decreases
Describe the complete combustion of alkanes
Alkanes will react with oxygen to produce carbon dioxide and water. Heat is produced also, which is why alkanes are used as fuels. They are readily available (from crude oil and natural gas) and easily transported. If they are burnt in excess oxygen, there are no toxic
products.
Describe the incomplete combustion of alkanes
If the alkane is in excess, there is not enough oxygen to react with
all the fuel. The hydrogen atoms are always oxidised to water. Some of the carbon atoms, however, are oxidised to carbon monoxide and some to carbon itself ( soot ) - toxic to humans, causes respiratory issues
How can alkanes be made to react and explain this process
Via a free radical mechanism.
The first step is the formation of a radical that can react with the alkane.
The radical is formed by homolytic fission of a covalent bond.
What is homolytic fission ? ( and exam tip )
One of the two shared electrons goes to each atom.
Must say that the bond breaks and one electron from the bond goes to each atom.
eg
Each Br atom has an unpaired electron (often shown as a dot).
An atom or group of atoms with an unpaired electron is called a radical.
The radicals formed by homolytic fission are uncharged.
Br2 → 2Br*
What happens in the reaction of alkanes with halogens ?
Alkanes will react with halogens in the presence of sunlight. The high-energy UV radiation in sunlight provides the energy required for the reaction to take place.
e.g. CH 4(g) + Br2(l) → CH3Br (g) + HBr (g)
This is a substitution reaction – a hydrogen in the alkane molecule has been substituted by a halogen atom.
Form haloalkanes
What are the the main steps in the reaction mechanism ( eg Mechanism for the bromination of alkanes ) ?
Reaction happens in a series of steps (reaction mechanism). This is called radical substitution, and it takes place in three stages:
Initiation, propagation, termination
What is meant by initiation in the reaction mechanism ?
This is when radicals form from a molecule.
eg the bromine molecule is broken by homolytic fission:
Br2 → 2Br*
covalent bond breaks
one electron from the bond goes to each atom
the radicals formed each have an unpaired electron, and are very reactive.
What is meant by propagation in the reaction mechanism ?
In a propagation step, a reactant is a radical, and one of the products is a different radical.
Propagation has two steps:
1) CH 4 + Br* → CH 3 + HBr (Br pinches a H)
2) CH 3 + Br 2 → CH 3 Br + Br
The new bromine radical formed reacts with another CH 4 molecule, and the two steps are repeated.
Up to 1 million propagation steps take place before two radicals collide in a termination step.
What is meant by termination in the reaction mechanism ?
In this step, two radicals collide, and form a molecule in which all electrons are paired.
e.g.
*Br + *Br → Br 2
*CH 3 + *CH 3 →C 2 H 6
*CH 3 + *Br → CH 3 Br
Describe the limitations of radical substitution in synthesis by the formation of a mixture of organic products, in terms of further substitution and reactions at different positions in a carbon chain
Further substitution – another bromine radical could collide with the bromomethane formed, substituting a second hydrogen atom to produce dibromomethane – which could further react until tetrabromomethane is formed.
CH 4 CH 3 Br then CH 3 Br CH 2 Br 2 and so on…
Limitations of radical substitution: if there are more than two carbon atoms in the molecule, a mixture of isomers will be formed.
Why does radical substitution produce a mixture of organic products and why is this a problem ?
further substitution
OR
produces different termination products
OR
More than one termination step✔
substitution at different positions along chain
