hydrocarbons

what are the properties of alkanes?

• saturated hydrocarbons

• insoluble in water

• generally unreactive

why does the boiling point of alkanes increase as the length of the chain increases?

• longer chain alkanes have more electrons than shorter chain alkanes which increases the strength of the london forces

• longer chain alkanes have a greater surface area so there are more points along the chains where london forces can be formed

why do branched alkanes have a lower boiling point than straight chain alkanes?

• the branches prevent alkane molecules from getting closer together - and london forces are strongest over short distances

what is the cracking of alkanes?

converts long chain hydrocarbons into short chain hydrocarbons and produces alkenes which are highly reactive material

how is thermal cracking carried out?

• requires a high temperature and high pressure

• 450-900^celcius

• 70 atm

• makes short chain alkanes, alkenes and water

what is a free radical?

a species with an unpaired pair of electrons

how is thermal cracking carried out?

• 450^celcius

• 1-2 atm

• aluminium oxide catalyst

• products are often branched chain alkanes - useful for petrol as they combust efficiently

• also produces cyclic alkanes and aromatic compounds

describe the process of free radical substitution of alkanes

step 1: initiation

• Cl2 (UV)→(heat) 2Cl*

step 2: propagation

• Cl* + C2H6 → *C2H5 + HCl

• C2H5 + Cl2 → C2H5Cl + Cl*

step 3: termination step

• Cl* + Cl* → Cl2

• C2H5 + Cl* → C2H5Cl

• *C2H5 + *C2H5 → C4H10

outline the mechanism for the electrophilic addition of alkenes (ethene and hydrogen bromide)

why is 1-bromobutane the minor product and 2-bromobutane the major product?

• the carbocation intermediate is an unstable molecule

• carbon two is more stable as it forms a secondary carbocation (bonded to two alkyl groups) and exists for a longer period of time

how to test for an unsaturated molecule?

mix with bromine water and shake - if unsaturated molecule is present bromine will add across the double bond and the solution will go from orange to colourless