6. Organic Molecules (ALKANES) Flashcards
What is the general formula for alkanes
Cn H2n+2
Are alkanes and cycloalkanes saturated or unsaturated
Saturated - all carbon bonds are single
What are the 2 types of bond fission that break a covalent bond
Heterolytic fission
Homolytic fission
What is heterolytic fission
When the covalent bond breaks unevenly so electrons are distributed unevenly to form a cation and ion. The cation receives both of the electrons in the bonding pair
What is homolytic fission
When the covalent bond breaks evenly so electrons are distributed equally to form 2 radicals
Define what is meant by the term radicals
Radicals are particles that have an unpaired electron
(represented by a dot)
–> due to unpaired electron, radicals are highly reactive
What is free radical substitution
When a halogen reacts with alkanes in a photochemical reaction ( a reaction that requires UV light)
What are the three stages of free radical substitution
- Initiation - Creation of radicals
- Propagation (2 stages) - reactions that produce new radicals
- Termination - Removal of radicals
(This is a chain reaction)
Describe the first stage 1 of free radical substitution (making chloromethane from reaction of chlorine and methane)
Initiation - sunlight / uv light provides enough energy to break the Cl-Cl bond. This is called photodissociation. The bond equally breaks forming 2 highly reactive radicals through process of HOMOLYTIC fission
Describe stage 2 of free radical substitution
Propagation -
1) Chlorine radical react with methane. Chlorine radical removes a hydrogen to produce a methyl radical and the second product - HCL
2) This methyl radical can then react with a chlorine molecule (not a chlorine a radical) to produce the desired product of chloromethane and a chlorine radical
3) New chlorine radical can react with methane and so on…… causes a chain reaction
Describe stage 3 of free radical substitution
Termination - 2 radicals react together to form a stable non - radical
What are the problems with free radical substitution? How to combat this when making chloromethane
You create a mixture of products
–> Excess Cl2 promotes further substitution as its a chain reaction, creating more products and therefore issues for separating the mixture as di,tri and tetrachloromethane can be produced)
–> Add excess methane to combat as there is more chance of a chlorine radical colliding with a methyl molecule and not a chloromethane molecule
Define what is meant by an aliphatic alkane
Straight chain organic compounds e.g. C2H5OH
Define what is aromatic alkane
Organic compounds containing the benzene ring
