Alkanes (12) Flashcards
What are alkanes?
They are saturated hydrocarbons and the most stable organic compounds.
What is the general formula of alkanes?
C(n)H(2n+2)
What shape do alkanes have?
They have a tetrahedral arrangement with bond angles of 109.5 degrees.
Sigma Bonds
Sigma bonds are a special type of covalent bond and are defined as the direct overlap of two s orbitals so share 2 electrons. They make up the C-C and C-H single bonds.
Why are alkanes not rigid?
The sigma bonds allow the atoms to rotate freely around them which is why alkanes can rotate into different shapes
What is the trend of boiling points like in alkanes?
All alkanes have different boiling points but the bigger the alkane is, the higher its boiling point will be this is why fractional distillation is possible.
What is the effect of chain length on the boiling point?
London forces will act on molecules that are in close surface contact. This means the larger the surface area, the more London forces are formed so the more energy will be required to overcome them so the boiling point will get higher.
What is the effect of branching on the boiling point?
The branched alkanes will have a lower boiling point than the straight-chain alkanes because there are fewer surface points of contact so not as many London forces will be formed. Also, the branches prevent the molecules from coming as close together as the straight-chain molecules which decreases the strength and number of intermolecular forces. This means it takes less energy to overcome so the boiling point is lower.
What is the solubility of alkanes like?
Alkanes are non-polar because C and H have very similar electronegativities. Therefore they don’t dissolve in water or polar solvents but they can dissolve in non-polar solvents
What reaction can alkanes take part in?
Even though they are fairly unreactive, they can take part in combustion which is where they react with oxygen and form CO2 and water. They are used for fuels
Complete Combustion Equation
When there is plenty of oxygen to be burned
CxHy + (balanced) O2 -> xCO2 + 0.5y H2O
Incomplete Combustion Equation
This is when there isn’t enough oxygen so you produce CO and C instead
CxHy + (balanced) O2 -> xCO + 0.5y H20
CxHy + (balanced) O2 -> xC + 0.5y H20
What is required for the reaction of alkane + halogen
You need UV light because alkanes are reactive so you need homolytic fission to occur to create reactive free radicals
What are the steps of radical substitution?
Initiation - forms free radicals
Propagation - add the radicals to each of the molecules, you should get two equations
Termination - adding all combos of radicals
What are the limitations of radical substitution?
A mixture of products are formed because of reactions happening anywhere on the carbon chain.
