Alkanes Flashcards
Explain the relation between melting points and boiling points with carbon chain length
Boiling point increases with chain length as the strength of London dispersion forces increase. However, the more branched isomers have a lower boiling point than their respective straight chain isomer as there is less surface contact with another molecule and so less points of contact for London dispersion forced to act
Give examples of complete and incomplete combustion of alkanes
Alkanes burn readily in excess air / oxygen. Complete combustion only occurs with an excess supply of oxygen, to release CO2 and H2O instead of CO and H2O as CO is an odourless, colourless and toxic gas. This is an exothermic reaction as the bonds on the RHS are stronger than the LHS so energy is released
Explain the relatively low reactivity of alkanes
The sigma bonds in alkanes have a low polarity (since the Eneg of Carbon and Hydrogen are very similar). They also have very high bond enthalpies and so are relatively quite stable - making them have a relatively low stability
Give the mechanism for free radical substitution of halogen (X) with an alkane
Alkanes react with halogens under UV light as it breaks down halogens through Homolytic fission and forms free radicals.
Cl2 —> 2Cl • (initiation)
CH4 + Cl • —> CH3 • + HCl (propagation)
CH3• + Cl • —> CH3Cl (termination)
Explain the limitations for free radical substitution
Cl • radicals could also remove a H from CH3Cl and keep reacting to form CCl4 - this is called polysubtitution. The formation of further substituted products is a limitation as:
1) it is a random and uncontrollable process, forming multiple different (possibly unwanted) products
2) products have to be separated by fractional distillation
Explain the tetrahedral shape and bond angle around each carbon atom in alkanes in terms of electron pair repulsion
The bonding pairs repel each other equally due to the positive inductive effect of alkyl groups. Due to the 4 bonding pairs and 0 lone pairs, the maximum equal repulsion of bonding pairs results in a bond angle of 109.5 degrees.
Explain what it means for alkanes to be saturated hydrocarbons
Alkanes are saturated hydrocarbons, meaning they can not undergo addition reactions to add more hydrogen since the contain the maximum amount of hydrogen
Explain the formation of sigma bonds
In alkanes, only sigma bonds are present, which are formed by an overlap of (SP3 hybridised) orbitals directly between bonding atoms which allows bonds to be freely rotatable.
