C12 Alkanes Flashcards
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.
How many bonds can a Carbon form in an alkane?
4 covalent bonds
Are alkanes saturated or unsaturated?
saturated hydrocarbons (single bonds only)
What is the general formula of an alkane?
CnH2n+2
What’s is the shape of an alkane?
tetrahedral
bond angle = 109.5*
caused by = each c atom surrounded by 4 e pairs in 4 sigma bonds, repulsion between these e pairs cause this shape around each C atom
Lone pairs repel more than bonding electrons
4 electron pairs in 4 sigma bonds (4 bonding pairs)
Why does BP of alkanes increase as chain length increases?
because of weak LFs
What is the effect of chain length on boiling point?
- chain length increases
- increases surface area
- Greater (stronger) London forces
- more energy required to overcome the forces
What’s is the effect of branching on boiling point?
(Example may include 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 is an alkane?
A saturated hydrocarbons containing C-H bonds only.
Are alkane bonds polar? Why/why not?
Non-polar because carbon and hydrogen have similar electronegativities.
Describe the sigma bond in alkane?
The sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms.
What type of intermolecular forces do alkanes have? Why?
London forces -> induced dipole-dipole interaction, because the bonds are non-polar
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
Does branched have a lower/higher BP compared straight chain?
lower BP
have fewer surface area
less LFs
Are alkanes insoluble/soluble in water? Why?
Insoluble
bonds in H2O stronger than LFs in alkanes
How reactive are alkanes?
Very unreactive
What reactions will alkanes undergo?
Combustion and reaction with halogens
What is complete combustion?
Combustion in plentiful supply of oxygen
What are the products of complete combustion when alkanes are used?
Carbon dioxide and water
What is the colour of the Bunsen burner flame during complete combustion?
Blue flame
What is incomplete combustion and what products are formed in the case of alkanes?
Combustion in limited supply of O2
Products = H2O, CO2 and CO
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
Write equations for the reaction of CH4 with Cl2 to form CH3Cl
Initiation: Cl2 —> 2Cl* (in presence of UV light)
Propagation: Cl* + CH4 —>HCl + *CH3
CH3 + Cl2 —> CH3Cl + Cl
Termination:
CH3 + Cl —> CH3Cl
2Cl* —> Cl2
*CH3 + *CH3 —> CH3CH3
What are limitations of free radical substitution
Further substitution
Substitution at different positions in carbon chain
What are the 3 steps In free radical substitution
Initiation
Propagation
Termination
What happens in initiation UV radiation
Homolytic fission
Form radicals
Energy provided by
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, stopping reaction.
How can you separate crude oil
Separate into fractions by fractional distillation in distillation tower
What happens to boiling point of alkanes as chain length increases
BP increases as chain length increases
The effect of chain length on BP
Chain length increases
Larger SA
More surface contact possible between molecules
LFs between molecules will be greater
More energy required to overcome stronger LFs
Why do branched isomers have lower BP compared to straight chain isomers
Fewer surface points of contact
Fewer LFs
Branches get in the way + prevent branched molecules getting as close together as straight chain molecules
Decreasing IMFs further
Simple explanation of effect of branching on BP
More branching
Less surfed points of contact
Weaker/fewer LFs
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
What does the incomplete combustion of alkanes produce
Limited supply of oxygen, form toxic gas CO or C(s)
E.g. C7H16 + 7 1/2 O2 —> 7CO + 8H2O
C7H16 + 4O2 —> 7C + 8H2O
What is an alkane
Saturated HC containing C-H bonds only
What’s the general formula of an alkane
CnH2n+2
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*
Describe the sigma bond in alkane
Sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms
What type of IMFs do alkanes have
LFs
Induced dipole-dipole interaction
Because bonds are non-polar
What happens to BP as alkane chain length increases? Why?
BP increases
There’s more SA and so more number of induced dipole-dipole interaction
More energy required to overcome attraction
Does a branched molecule have a lower or higher BP compared to equivalent straight chain? Why?
Branched molecule has lower BP because they have fewer SA and hence less induced dipole - dipole interactions
Are alkanes soluble in water
Insoluble because HBs in water are stronger than alkanes’ LFs
How reactive are alkanes
Very unreactive
What reactions will alkanes undergo
Combustion and reaction with halogens
What type of reaction is combustion
Oxidation reaction
What is complete combustion
Combustion that occurs with plentiful supply of air
What are the products of complete combustion when alkanes are used
CO2 & H2O
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)
What type of HC are most likely to undergo incomplete combustion
Longer chains
What is the environmental impact of CO
Toxic/poisonous
What’s the environmental impact of soot (carbon)
Asthma, cancer, global dimming
How are halogenoalkanes formed from alkanes
Radical substitution
In the presence of what does alkane react with halogens
UV light
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 remain \
Termination - free radicals removed, stable products formed
