12.2 - Chemical reactions of the alkanes Flashcards Preview

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Why don't alkanes react with most common reagents?

C-C and C-H sigma bonds are strong
- C-C bonds are non-polar (same electronegativity)
Carbon and hydrogen have similar electronegativity so C-H is considered non polar


Describe the general reaction of alkanes with halogens

In the presence of sunlight (UV radiation to be precise)
A substitution reaction takes place

e.g. CH4 + Br2 > CH3Br + HBr


What is the mechanism for the reaction of halogens with alkanes called and state the 3 steps?

Free radical substitution
1. Initiation
2. Propagation 1, Propagation 2
3. Termination


What occurs during initiation? (for these examples use Cl2)

Using UV radiation
The Cl2 covalent bond is broken by homolytic fission.
Forming 2 chlorine radicals - very reactive and have an unpaired electron


What occurs during propagation 1?

The Cl. (radical) reacts with the methane C-H bond forming a methyl radical and a HCl molecule.


What occurs during propagation 2?

The CH3. (radical) reacts with another Cl2 molecule, it forms another Cl. and CH3Cl


Why are the propagation steps known as chain reactions?

New radicals are constantly released which continue to react.


What occurs during the termination step?

Where any 2 radicals react
Possible combinations

Cl. + Cl. > Cl2
Cl. + CH3. > CH3Cl
CH3. + CH3. > C2H6


What are the limitations of radical substitution in organic synthesis?

Makes the desired haloalkane but also makes other unwanted products


What are the issues with further substitution?

CH3Cl may have formed in Propagation 2 but it may react with the Cl., forming CH3Cl2, and this can happen again and again, this means to get your desired product it requires further separation


If the carbon chain is long what could happen?

Substitution can occur at any point along the chain, meaning with further substitution you would get even more products