Topic 14 - Hydrocarbons Flashcards
Define
Alkane
Alkanes are saturated hydrocarbons with a general fomula of C2n H2n+2
How to identify
Alkanes
- suffix - ane
- molecular formula should comply with general formula
- Stuctural fomula - only sigma bonds
- Practically - immiscible in water, highly flammable, do not decolourise bromine water in the dark (no UV)
What types of isomers can alkanes form
Chain isomers
Optical isomers
Sources of alkanes
Obtained from crude oil as fractions
fractions are mixture of hydocarbons with very close boiling points
Why are alkanes inert at rtp?
strong C-C and C-H bonds that require high activation energies and no bond polarities to attract nucleophiles and electrophiles
Reactions
Combustion of alkanes
complete and incomplete
- Reagents - alkane(petrol) and oxygen(air)
- Conditions - Flame or Electric spark
- Precautions - No naked flames and a small amount of petrol
Advantages of alkanes as fuels
- highly flammable
- If burned completely, they do not pollute the environment
- Easy to store and transport
- High energy density
Disadvantages of alkanes as fuels
- if mishandled, high risk of fire accident
- If buned incompletley, produces poisonous pollutant CO
- Crude oil is finite
Reactions with Halogens
- Reagents - Alkane and Halogen
- Conditions - UV light
- Precaution - Flame cupboard(toxic gases), no naked flame, small amount of alkane
- Mechanism - Free Radical Substitution
Describe
Free radical Substitution
Step 1: Initiation
In the presence of UV light the halogen molecule forms 2 free radicals 2Cl.
Step 2: Propagation
1st - Alkane + halogen free radical -> alkyl free radical + hydrogen halogen
CH4 + .Cl -> .CH3 + HCl
Why is cracking advantageous?
cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier
crude oil fractions
