What are alkanes?
- The main components of natural gas and crude oil
- Most stable organic compounds and lack of reactivity had allowed crude oil deposits remain in the earth for millions of years- they are unreactive due to the high bond enthalpies of C-C and C-H bonds and low polarity of sigma bonds
- Alkanes are mainly used as fuels, reactions with oxygen to generate heat
- They are hydrocarbons because they contain hydrogen and carbon atoms only
- They have the general formula CnH2n+2
What is the bonding in alkanes?
- they are saturated hydrocarbons containing carbon and hydrogen atoms joined together by single covalent bonds
- Each carbon atom in an alkane is joined to four other atoms by single covalent bonds called a sigma bond
- Covalent bond is defined as a shared pair of electrons and a sigma bond is the result if the overlap between 2 orbitals, one from each bonding atom
- Each overlapping orbital contains one electron so the sigma bond has 2 electrons that are shared between the bonding atoms. The sigma bond is positioned on a line directly between bonding atoms
- Each carbon atom in an alkane has four sigma bond either C-C or C-H
What is the shape of alkanes?
- each carbon atom is surrounded by 4 electron pairs/bonding electrons in four sigma bonds that repel each other equally resulting in a tetrahedral arrangement and a bond angle of 109.5 degrees
- Electron pair repulsion theory state that regions of negative charge about an atom will repel each other as far apart as possible
How can crude oil be separated?
Crude oil contains hundreds of different alkanes that can be separated by fractional distillation because different alkanes have different boiling points, increasing as their carbon length increases
What is the effect of chain length on boiling point?
- London forces act between molecules that are in close surface contact
- As chain length increases the molecules have a larger surface area so more surface contact is possible
- Therefore there is more London forces/induced dipole dipole forces between the molecules and so more energy is required to overcome the forces
What is the effect of branches on the boiling point?
Branched isomers have a lower boiling point than straight chain isomers as there is less surface contact since they can’t pack as closely together and fewer London forces/induced dipole-dipole interactions therefore less energy is needed to break the bonds.
Why are alkanes unreactive?
- C-C and C-H sigma bonds are strong
- C-C bonds are non polar
- The electronegativity of carbon and hydrogen are so similar the C-H bond can be considered non polar
What is the complete combustion of alkanes?
- burn completely to produce CO2 and H20
- Highly exothermic reaction
- Gives blue flame
What is the incomplete combustion of alkanes? What are the consequences of CO buildup?
- not enough oxygen for complete combustion due to limited oxygen
- Flame is orange due to carbon particles glowing red
- happens when combustion is attempted in a closed system like a car engine
- carbon monoxide is produced which is highly toxic and can lead to death if it builds up in faulty heating appliances
- Carbon monoxide is a better competitor than oxygen and can bind to haemoglobin preventing oxygen transport around the body.
What are the problems with free radical substitution?
- Radicals are extremely reactive so further substitution can happen and also reactions at different positions along the carbon chain
- Can produce compounds that are not necessarily required
- You can also produce longer alkane chains which need to be separated from desired haloalkane (mixture of organic products)
What is general formula?
The simplest algebraic formula for a member of the homologous series
What is displayed formula?
The relative positioning of atoms and the bonds between them
What is the homologous series?
A series of organic compounds having the same functional group but each successive member differing by CH2
What is the functional group?
Group of atoms responsible for the characteristic reactions of a compound
What is the formula for an alkyl group?
CnH2n+1
What is aliphatic?
Refers to straight chain hydrocarbons, branched chains and non aromatic rings
What is alicyclic?
An aliphatic compound arranged in non aromatic rings with or without side chains
What is an aromatic compound?
Contains a benzene ring
What are saturated and unsaturated hydrocarbons?
Saturated- carbon-carbon single bonds
Unsaturated- carbon to carbon double bonds
What are structural isomers?
Compounds with the same molecular formula but a different structural formulae
What is homolytic fission?
Each of the bonding atoms receiving an electron from the bonding pair forming 2 free radicals
What is heterolytic fission?
One bonding atom receives both electrons from the bonded pair
