C12 Alkanes Flashcards
(24 cards)
What is the bonding like in an alkane?
2 p-orbitals, 1 from each bonding atom
overlap and contain 1 electron each
sigma bond has 2 electrons that are shared between bonding atoms
each carbon in an alkane has 4 sigma bonds
What is a sigma bond?
The head on overlap of 2 atomic orbitals 1 from each bonding atom, with electron density centred around a line directly between the bonding atoms nuclei.
What’s is the effect of branching on boiling point?
(isomers of alkanes- branched isomers has lower boiling point than straight chain isomers)
- more branching
- fewer surface points of contact (decreased surface area)
- weaker (fewer) London forces
Also another reason is to do with the shape.
- branches get in the way and prevent branched molecules getting as close together as straight chain molecules, decreasing IMFs further
What happens to the boiling point as alkane chain length increases? Why?
BP increases
Increased surface area
Stronger LFs
more energy required to overcome LFs
What type of hydrocarbon are most likely to undergo incomplete combustion?
Long chain Hydrocarbons
What is the environmental impact of carbon monoxide?
It is toxic/poisonous
What is the environmental impact of soot (carbon)?
Asthma, cancer, global dimming
What is the name of the mechanism for the reaction of alkanes with halogens
Free radical substitution
In the presence of what does alkane react with halogens?
UV light
(Sunlight)
What are the 3 stages of free radical substitution?
Initiation - breaking halogen bond to form free radicals
Propagation - chain part of the reaction where products are formed but free radicals remains.
Termination - free radicals removed, stable products formed
What are limitations of free radical substitution
Further substitution
Substitution at different positions in carbon chain
What happens in initiation
Homolytic fission
Form radicals
Energy provided by UV radiation
What happens in propagation
Chain reaction
Step terminated when 2 radicals collide
What happens in termination
2 radicals collide
form molecule with all electrons paired
both radicals removed from reaction mixture
reaction stops
How can you separate crude oil
Separate into fractions by fractional distillation in distillation tower
Why do alkanes lack reactivity
C-C and C-H sigma bonds are strong
C-C are non-polar
EN of C + H, so similar C-H considered non-polar
Why are alkanes used as fuels
Readily available
Easily transport
Burn in plentiful supply of oxygen without releasing toxic products
What does combustion give out
Heat
Are alkane bonds polar? Why
Non polar
C and H have similar electronegativities
What’s the shape and bond angle of an alkane
Tetrahedral
109.5*
Are alkanes soluble in water
No
HBs in H2O stronger than LFs in alkane
What is complete combustion
Combustion that occurs with plentiful supply of air
What’s the colour of the Bunsen burner flames during complete combustion
Blue flame
What is incomplete combustion and what products are formed in the case of alkanes
Combustion in a limited supply of oxygen
Products = water, CO(g) or C(s)
