Alkanes Flashcards
What is the hybridisation of carbon in alkanes?
sp3 hybridised
What is the general formula for alkanes?
CnH(2n+2)
for non-cyclic alkanes
Why aren’t alkanes very reactive?
They are saturated hydrocarbons and are stable
What reactions do alkanes undergo?
- Oxidation
CH4 (g) + O2 (g) → CO2 (g) + H2O (g) - Substitution
What is the name for the substitution reactions in alkanes?
Free radical substitution reactions
These are necessary because free radials are very reactive, instigating a reaction in alkanes
What can be said about the physical state of alkanes?
The first 3 exist as volatile gases but their state increases with an increase in ‘n’ due to increases van De Waal’s forces
How many ‘umbrella’ steps are there in a free radical substitution reaction?
Three: Initiation, Propagation and Termination
What happens in the initiation step?
The first radicals are formed
What happens in the propagation step?
The first radicals react with compounds and more radicals are formed
What happens in the termination step?
Radicals combine with each other to form compounds
Similar/dissimilar radicals can combine
What is the catalyst in the reaction of chlorine gas with an alkane?
UV light/radiation (from the sun)
Be sure to specify
What is the catalyst in the reaction of chlorine gas with an alkane?
UV light/radiation (from the sun)
Be sure to specify
What does the UV catalyst do?
It causes the homolytic fission of the Cl-Cl bond forming two radicals ( . Cl)
What does the . Cl radical take each time it attacks the alkane?
A hydrogen atom
This C-H bond is also broken homolytically, and an alkyl radical is formed, meanwhile, HCl is also formed/
What does this alkyl radical attack?
Note the difference, carbocations are electrophiles and are not used here
A neutral chlorine molecule, forming another (single) chlorine radical
For methane,
.CH3 + Cl2 (g) → CH3Cl (g) + . Cl
A similar mechanism to this continues until all the hydrogen atoms are substituted
Do alkanes decolourise bromine water?
No
Will there be any change to the reacting system
if the mixture containing the alkane and bromine water si exposed to UV light?
Yes
but it will be SLOW
Free radicals would be formed and a substitution reaction will occur, decolourising the solutiion
Which has a higher boiling point?
branched or straight-chain alkanes
with the same molecular mass
Straight-chain alkanes
Why do straight-chain alkanes have a higher boiling point?
There is a larger surface area
What happens when incomplete combustion of alkanes occurs?
Carbon monoxide is formed
with water
as the carbon is not fully oxidised to carbon (IV)
What kind of relationship can be see between the no. of carbons in an alkane and the energy released from combustion?
A direct relationship
The higher the molecular number, the more Σ released on combustion
