Module 4 - [ch12,13] (alkanes alkenes)
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what are alkanes
crude oil among the most stable organic compounds unreactive used as fuels saturated hydrocarbons
what does the bonding look like in alkanes
saturated hydrocarbons only single covalent bonds each carbon makes 4 bonds
what is the shape of alkanes
each carbon atom is surrounded by 4 bonded pairs tetrahedral 109.5
What is meant by the term fractional distillation? [1]
separation by (differences in) boiling point
why does boiling point of alkanes increase with chain length
as chain length increases, molecules have larger surface area more surface contact possible between molecules london forces between molecules are greater so more energy is required to overcome these forces
what is the effect of branching on boiling point of alkanes
branched isomers have lower boiling points
what are the reasons for the lack of reactivity of alkanes
C - C C - H sigma bonds are strong C - C are non-polar barely any difference in electronegativity of C and H so can also be described as non-polar
combustion of alkanes
alkane + oxygen -> carbon dioxide + water gives out heat
incomplete combustion of alkanes
limited supply of oxygen toxic gases such a carbon monoxide (CO) soot
when do alkanes react with halogens
in the presence of sunlight UV radiation provides initial energy for reaction to take place substitution reaction methane + bromine -> bromomethane
mechanism from bromination of alkanes
initiation Br - Br -> Br. + Br. homolytic fission propagation CH4 + Br. -> CH3. + HBr CH3. + Br2 -> CH3Br + Br. termination Br. + Br. -> Br2 CH3 + CH3 -> C2H6 CH3. + Br. -> CH3Br
what are the limitations to radical substitution
further substitution substitution at different positions on the carbon chain
what are alkenes
unsaturated hydrocarbons contain a C=C double bond
how is a pi bond formed
overlap of adjacent p-orbitals below and above the Carbon atoms can only be made after a sigma bond is formed
describe the shape around a double bond
three regions of electron density around each carbon atom trigonal planar 120
what are stereoisomers what are the two types
they have the same structural formula but different arrangement of the atoms in space EZ isomerism (only in compounds with C=C) optical isomerism
what are the conditions for EZ isomerism
. C=C double bond . different groups attached to each carbon atom of the double bond
what is cis-trans isomerism
a special case of EZ isomerism where one of the attached groups on each carbon atom of the double bond must be the same
how do you assign priority
examine the atom directly attached to the carbon atoms of the double atom greater atomic number = higher priority if there is no difference, continue down chains till point of difference and compare those atoms
cahn-ingold
if highest priority groups are on the same side (Z) on opposite sides (E)
why are alkenes more reactive than alkanes
the pi-bond it is outside of the double bond, therefore more exposed a pi bond readily breaks and alkenes undergo addition reactions easily
hydrogenation of alkenes
alkene + H2 -> alkane
halogenation of alkenes
alkene + halogen -> something
how do you test for an alkene
bromine water tests for unsaturation (the double bond) orange -> colourless