Alkanes Flashcards
Alkanes molecular formula
C(n)H(2n+2)
Cycloalkanes molecular formula
CnH2n
What is the relative boiling and melting point of alkanes and why
Relatively low melting and boiling points, due to non polar nature hence only id-id interactions need to be overcome
Trend in melting and boiling point of alkanes with Mr
1.) As Mr increases, the melting and boiling points increase.
2.) This is due to the increase in size of the electron cloud,
3.) which allows for greater polarisation of the electron cloud. 4.) Hence, there is stronger and more extensive id-id interactions 5.) which require more energy to be overcome. Hence boiling and melting point increase.
Trend of melting and boiling points of alkanes for straight or branched chain
1.) Melting and boiling point for branched chain is lower than straight chain.
2.) This is since branching makes it more spherical in shape.
3.) This reduces the surface area of contact.
4.) Thus there is less extensive id-id interactions, and
5.) less energy is required to overcome them as extent of branching increases
Density trend with Mr for alkanes
As Mr increases, density increases, due to the stronger and more extensive id-id interactions which hold molecules more closely together
Alkanes are less dense than water
What is the reactivity of alkanes
Alkanes are generally unreactive, due to
1.) high bond energies of C-C and C-H bonds so a large amount of energy is required to overcome them
2.) they are unreactive towards polar reagents as C-C and C-H bonds are non polar due to similarity in electronegativity
What is the solubility of alkanes
Alkanes are non polar so there are soluble in non polar solvents and insoluble in polar solvents. Liquid alkanes can act as non polar solvents
Combustion of alkanes (equation and products)
CxHy + (X+Y/4) O2 —> X CO2 + Y/2 H2O
If excess oxygen, products are CO2 and H2O
If limited oxygen, products are C/CO and H2O
Reagents and conditions for FRS of alkanes
Limited Cl2(g), uv light
(or Br2(g))
How to find relative amount of monosubstituted products after FRS of alkanes?
Draw out the thing and find the number of different H (Ha, Hb) then find the ratio
Why is the actual ratio of monosubsituted products from FRS of alkanes different from the theoretical ratio?
The ratio assumes that all H have the same reactivity, however the different H have different reactivities based on the stabilities of the radicals formed.
The stabilities of the radicals are:
Methyl radical < primary < secondary < tertiary
Due to the presence of more electron donating alkyl groups which stabilise the radical to the greatest extent
The more stable, the faster it is formed and hence the ratio is increased for the more stable product
How can alkanes be prepared plus reagents and conditions
Through reduction of alkenes.
CnH2n + H2 —> CnH(2n+2)
Reagents and conditions: H2(g) with Pd or Pt catalyst at rtp OR H2(g) with Ni catalyst at elevated temperature and pressure
