Ch. 12.2: Chemical Reactions Of The Alkanes Flashcards
Why don’t alkanes react with most common reagents?
- C-C and C-H sigma bonds are strong
- C-C bonds are non - polar
- the electronegativity of C and H is so similar that their bind can be considered non- polar
What is combustion?
When alkanes react with plentiful supply of oxygen to produce carbon dioxide and water.
Why are alkanes used as fuels?
- readily available
- easy to transport
- burn in plentiful supply of oxygen without releasing toxic products
Formula for balancing any combustion of alkane
C(x)H(y) + (x+y/4)O2 -> (x)CO2 + (y/2) H2O
What happens during incomplete combustion of alkanes?
- hydrogen atoms are always oxidised to water
- forms toxic gas carbon monoxide (CO) or even carbon itself ( soot)
Describe the reactions between alkanes and halogens
- They react in the presence of sunlight
- UV radiation provides initial energy needed
- it is a substitution reaction, as the hydrogen atom gets replaced by a halogen atom
What are the three stages of radical substitution?
Initiation
Propagation
Termination
What happens in initiation?
The covalent bond in the halogen is broken by homolytic fission. Each bonded atom takes one of the electrons, resulting into two radicals ( highly reactive species with unpaired electron ).
What happens in propagation?
1st Step: radical reacts with alkane and takes a hydrogen atom from it. This results in an alkane radical.
2nd Step : alkane radical reacts with halogen ( diatomic ) and takes one of its atoms, and leaves a halogen radical
What happens in termination?
Two radicals collide, forming a molecule with all electrons paired. There are usually three possible termination reactions.
What are the limitations of radical substitution in organic synthesis?
Further substitution: another halogen radical can collide with the halo-alkane made, substituting a further hydrogen atom to form another halo-alkane.
Substitution at different positions in a carbon chain: if a carbon chain is long, it produces a mixture of monosubstituted isomers at different positions in the carbon chain.
