4.1.2 alkanes
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how does a sigma bond form
one sp2 orbital from each carbon overlap to form a single c-c bond
give the shape and bond angle around a carbon atom in saturated hydrocarbons
tetrahedral 109.5°
explain the trend in boiling points as the number of carbons in alkanes increases
b.p increases as there number of electrons increase as strength of london forces increase between molecules
explain the difference in boiling points between straight chain and branch chain alkanes
straight chain alkanes have a higher b.p as they have a larger surface area of contact between molecules for London forces to form so have stronger forces
free radical definition
a reactive species which has an unpaired electron
suggest why the petroleum industry processes hexane into cyclohexane
cyclohexane has more efficient combustion
why do alkanes have low reactivity
• they have strong C-C bonds and C-H bonds that require lots of energy to break in order to form new bonds • they have little difference in electronegativity (low polarity) so electrophiles and nucleophiles won’t be attracted to them
why are hydrocarbons used as fuel
• readily burn with oxygen - combustion is highly exothermic
describe complete combustion in alkanes
• occurs in excess oxygen - forms CO2 and H2O
describe incomplete combustion in alkanes
• occurs when there is a limited amount of oxygen - forms CO and H2O
describe CO and its effects
• toxic, colourless gas • carbon can cause global dimming • can cause death in enclosed spaces
describe the steps of free radical substitution
initiation- UV light gives energy to break the bond by homolytic fission which forms free radicals eg Cl2->2Cl propagation-as the free radical is regenerated it can react with several more alkane molecules in a chain termination- collision of two free radicals does not generate further free radicals and the chain is terminated