Module 4 - [ch12,13] (alkanes alkenes) Flashcards
alkanes alkenes
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
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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
alkene + hydrogen halide
haloalkanes
hydration of alkenes
alkene + water -> alcohol
what does the double bond in an alkene represent
a region of high electron density
because of the presence of the pi-electrons
this high electron density attracts electrophiles
what is the mechanism for the addition of a hydrogen halide
Hydrogen halide is polar
has dipoles
positive dipole attracted to double bond (high electron density)
double bond break open and H-Br bond also broken by heterolytic fission, electron pair goes to bromine atom
bromide ion and carbocation formed
carbocation has a positively charged carbon atom which attracts the ion
product formed!
what makes the mechanism of the electrophile addition of Br2 different to the addition of HBr
Br2 is non-polar
when it approaches the double bond
pi-electrons interact with the electrons in the Br2
induced dipole
What is Markovnikoff’s rule
when a hydrogen halide reacts with an unsymmetrical alkene the hydrogen attaches itself to the carbon with the greater number of hydrogen atoms and least carbon atoms
carbocation stability
number of ‘alkyl’ groups attached to the positively charged carbon, more alkyl groups -> more stable
(how many carbons are attached to it)
primary carbocations are the least stable, tertiary the most
why are tertiary carbocations the most stable
more alkyl groups
the charge is more spread out
making the ion more stable
what are polymers
extremely large molecules made up from repeating units called monomers
why am i waffling about polymerisation in this topic
alkene molecules undergo addition polymerisation
open the double bond
square brackets
slap an ‘n’ at the bottom right corner
!polymer!
uses of polyethene
most commonly used polymers
supermarket bags
toys
shampoo bottles
uses of polychloroethene
PVC
pipes, flooring, bottles, insulation
polytetrafluoroethene (teflon)
coating non-stick pans
permeable membrane for clothing
polystyrene (polyphenylethene)
packaging material
thermally insulating
environmental concerns regarding polymers
disposal -> non-biodegradable
recycling PVC
hazardous due to high chlorine content
when burnt releases hydrogen chloride - a corrosive gas
feedstock recycling - reclaiming monomers from waste polymers
bioplastics
produced from plant starch, cellulose
biodegradable polymers
broken down by microorganisms into water, cO2 and biological compounds
compostable polymers
photodegradable polymers
weakened by absorbing light
what are the conditions for halogenation of alkenes
rapid addition reaction
room temp
addition reaction
what are the conditions for hydrogenation of alkenes
nickel catalyst 150 degrees, 423K
addition reaction
double bond breaks, addition of hydrogen
what are the conditions for hydration of alkenes
alkenes react with steam (H2O)
presences of phosphoric acid catalyst (H3PO4)
addition reaction
electrophilic addition
alkene + hydrogen halide
what are the conditions
room temp
addition reaction
(bubbles through if the alkene is liquid)
are alkane bonds polar
no
carbon and hydrogen have similar electronegativites
describe the sigma bond in an alkane
covalent bond
direct overlap of s-orbitals
what type of intermolecular force do alkanes have and why
london forces
because bonds are non-polar
are alkanes soluble in water
why?
insoluble
hydrogen bonds in water are stronger than the london forces in the alkane
no polar bonds, so no attraction between water and alkane molecules
what reactions will alkanes undergo
combustion
reaction with halogens
what is the colour of the flame on a bunsen burner during complete combustion
blue
how are halogenoalkanes formed from alkanes
radical substitution
what intermolecular forces do alkenes have and why
london forces
non-polar bonds
what will major products be formed from
the most stable carbocation
tertiary
what are 5 ways in which plastic is disposed of
landfill
combustion
organic feedstock
recycling
reuse
what is organic feedstock
plastics are separated and broken down into small organic molecules
Outline two ways that waste polymers are processed usefully, rather than just dumped
in landfill sites [2]
combustion for energy generation
feedstock for chemicals, or plastics
why do branched alkanes have lower boiling points
have fewer surface points of contact between molecules
weaker london forces
branches mean the molecules can’t get close together
decreasing intermolecular forces
