Alkanes Flashcards
What is an alkane?
A saturated hydrocarbon containing C and H bonds only
What is the general formula for an alkane?
CnH2n+2
Are alkanes bonds polar? Why?
They are non polar because hydrogen and carbon have similar electronegativities
What is the shape and angle of an alkane?
Tetrahedral 109.5 degrees
Describe the sigma bond in an alkane.
A covalent bond which has a direct overlap of the electron clouds of the bonding atoms
What type of intermolecular forces do alkanes have?
London forces because they are non polar
What happens to the boiling point as alkane chain length increases? Why?
The bp increases because there is more surface area and so more induced dipole-dipole interaction
Does a branched molecule have a lower or higher boiling point compared to equivalent straight chain? Why?
Lower bp because of the fewer surface area so less induced dipole-dipole interactions
Are alkanes soluble in water? Why?
Insoluble because hydrogen bonds in water are stronger than alkanes’ London forces
How reactive are alkanes?
Very unreactive
What reactions will alkanes undergo?
Combustion and reaction with halogens
What is complete combustion?
Combustion that occurs with plentiful of oxygen
What are the products of complete combustion?
CO2 and H2O
What is incomplete combsution?
Combustion in a limited supply of oxygen
What are the products formed in incomplete combustion?
H2O, CO2 and CO
What type of hydrocarbon are most likely to undergo incomplete combustion?
Longer chains
How are halogenoalkanes formed from alkanes?
Radical substitution
In the presence of what does alkanes react with halogens?
UV light
What is homolytic fission?
Each of the bonded atoms takes one of the shared pair of electrons, each atom has a single unpaired electron (radical)
What is heterolytic fission?
One of the bonded atoms takes both of the electrons from the bond
What is an addition reaction?
A molecule is added to the unsaturated alkene
What is a substitution reaction?
An atom or group of atoms is replaced by a different atom
How would you calculate how much CO2 is produced during complete combustion of 1kg of C7H16?
- Calculate the amount of moles of C7H16 in 1kg of heptane (1kg=1000g)
- Write the balanced equation for the complete combustion of heptane and calculate the moles of CO2 produced
- Use the n=v/24 equation to find the volume of CO2 produced
What are the steps for the bromination of methane?
- Initiation Br2 -> 2Br.
- Propagation CH4 + Br. -> CH3. +HBr
CH3. + Br2 -> CH3Br + Br. - Termination 2Br. -> Br2
What are some limitations of radical substitution?
- Further substitution e.g another bromine radical can collide with bromomethane molecule substituting a further hydrogen atom
- Monosubstitution, if the carbon chain is longer we will get a mixture of monosubstituted isomers by substitution at different positions
Name the process used to obtain kerosene from crude oil and explain why the process
works.
- Fractional distillation
- Fractions have different boiling points
Explain why the straight chain isomer of C10H22 has a higher boiling point than any of its branched chain structural isomers
- Branched chains have less surface contact
- Branched chains have fewer van der Waals’ forces
Explain why the straight chain isomer of C10H22 is converted by the petroleum industry into its branched chain isomers.
Branched chains have more efficient combustion
Write an equation for the reaction between methane and bromine to make bromomethane and hydrogen bromide.
CH4+ Br2 → CH3Br + HBr
Name one other bromine-containing organic product which is formed when methane reacts with bromine
Dibromomethane
What is the name of the process used to convert long chain alkanes into more useful shorter chain alkenes?
Cracking
What is the name of the process used to convert long chain alkanes into more useful shorter chain alkenes?
Cracking
Why is ethane made in the radical substitution of methane and bromine?
Two CH3 radicals
