Alkanes Flashcards
How is crude oil separated into its components?
Fractional distillation separates the components of crude oil based on their different boiling points.
Describe alkanes.
Alkanes are saturated hydrocarbons. Each C atom is sp^3 hybridised, tetrahedrally surrounded by H and other C atoms with a bond angle of 109.5.
Why do cyclopropane and cyclobutane have much higher enthalpy of combustion?
For cyclopropane, the C-C-C bond angles are 60, almost half of the expected tetrahedral angle of 109.5, resulting in ring strain and hence high strain energy.
For cyclobutane, is it non planar and hence has a C-C-C bond angle of 88 as it adopts a slightly folded form to minimise repulsion
Describe the boiling point/volatility of alkanes.
Alkanes have relatively low boiling points as they have weak intermolecular dipole-induced dipole interactions.
Why does the melting and boiling point of alkanes generally increase from one member to the next?
This is due to the additional -CH2- group which leads to a larger number of electrons and therefore a larger and polarisable electron cloud, and hence stronger id-id interactions.
Why does straight chain alkanes have a larger boiling and melting point?
This is because they have larger area of contact between each other and hence stronger id-id interactions.
Explain why alkanes are generally unreactive.
C-C bonds and C-H bonds are strong covalent bonds which are difficult to break under normal conditions. Alkanes do not possess any electrophilic sites to attract nucleophiles or nucleophilic sites to attract electrophiles since C-H and C-C sigma bonds are non-polar.
What is the general equation fo complete combustion of alkanes?
CnH2n+2(g) + (3n+1)/2O2(g) –> nCO2(g) + (n+1)H2O(l)
What are the requirements for complete combustion of alkanes?
Solid and liquid alkanes must be converted into the gaseous state and the reaction must occur in excess oxygen or normal air.
What are the reagents and condition required for free radical substitution in alkanes?
Presence of UV light and limiting amounts of Cl2(g)/Br2(l)
Describe the initiation stage.
Under ultraviolet light or heat, the homolytic fission of Cl-Cl bond takes place, and chlorine free radicals are formed.
Describe the propagation stage.
The highly reactive chlorine free radical on colliding with alkanes such as methane abstracts a hydrogen atom to produce HCL and a methyl radical.
The methyl radical reacts further with another chlorine molecule to form the product and a chlorine free radical is regenerated.
Describe the termination stage.
When any two free radicals collide, the unpaired electrons pair up to form a stable product. This terminates the chain reaction
+this reaction is exothermic and the energy released can help speed up the reaction.
Why is free radical substitution not a good method to prepare halogenoalkanes?
It is not a good method as a mixture containing many different halogen-substituted alkanes is produced.
To improve: limit the amount of halogen/use a large excess of alkane used. This decreases the probability that the chlorine radical will collide with a monosubstituted alkane molecule, which will lead to the formation of the polysubstituted product.
WhY does the reactivity of halogens decrease down the group from fluorine to iodine?
As the size of the halogen atom increases down the group, orbitals become more diffuse and overlap between the H and halogen atom is less effective, hence te enthalpy change of reaction for the first propagation stage becomes more endothermic down the group.
How is the stability of the alkyl radicals determined and what is the order of their stability?
Radicals with a higher number of electron-donating alkyl groups help increase electron density in the election deficient carbon, stabilising it, and forming faster.
Order of stability:
3 radical > 2 radical > 1 radical > methyl radical
What are the gases that contribute to the greenhouse effect?
CO2, CH4, N2O and water vapour
