module 4: alkenes and alkanes Flashcards
what is homolytic fission
when a bond’s electrons are split equally, each atom receives one of the electrons, forming free radical fragments - electron movement is shown with a single- headed curly arrow
what are free radicals
a species with an unpaired electron, they are unstable, short lived and highly reactive
what is heterolytic bond breaking
when one atom takes both electrons from a covalent bond, leading to the formation of a positive and negative ion
how is it decided which molecule takes the electrons in heterolytic bond breaking
the more electronegative substance takes the electrons
what is an electrophile
an electron deficient ion/molecule that seeks an atom/molecule with an electron pair for bonding
what is a nucleophile
an atom or functional group with a pair of electrons that can be shared to another species (usually an electrophile) to form a chemical bond in a reaction
define stereoisomer
compounds with the same structural formulae but with a different arrangement in space
• E/Z isomers
• optical isomers
what is E/Z isomerism
it’s a form of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for 2 different groups to be attached to each carbon atom of the C=C group
when are the CIP rules used
when naming stereoisomers
what are the CIP rules
- compare the atomic number of the atoms attached directly to the stereocentre (double bond); the group with a higher atomic number gains priority
- if there’s a tie, consider the atoms at the 2nd distance from the stereocentre
what does the complete combustion of an alkane result in
the production carbon dioxide and water it occurs when the alkane is reacted with a plentiful oxygen supply
what does the incomplete combustion of an alkane result in
the formation of carbon monoxide/soot and water, it occurs when there is not enough oxygen supplied
how and under what conditions do alkanes react with halogens
condition = when exposed to UV light
via a free radical substitution reaction, where a hydrogen is replaced by a halogen
eg. CH4 + Cl2 –> CH3Cl + HCl
what are the three stages of the mechanism of a free radical substitution reaction
- initiation
- propagation
- termination
explain the initiation, propagation and termination steps in a free radical reaction mechanism
initiation - the UV light and high temperature provides energy to break the halogen bond and form two free radicals by homolytic fission
propagation - the free radical from the initiation stage reacts with the neutral methane molecule, a chain reaction occurs
termination - the reaction terminates when two free radicals collide to form products
why do alkenes have a double bond and what are the characteristics of it
the double bond is due to the sideways overlap of a spare, unbonded, singly filled p-orbital on each C atom
the overlap produces a cloud of electrons above and below the molecule, known as a pi bond
Pi bonds are much weaker than sigma bonds
1. the electron density gets spread out above and below the nuclei
2. the electrostatic attraction between the nuclei and shared electron pair is weaker so they have relatively low bond enthalpy
the C=C bond has both a sigma and pi bond
what are the conditions for hydrogenation and what does it do
alkenes react with hydrogen at 150-180˚C, on the surface of a nickel catalyst
an alkene is turned into an alkane
when is hydrogenation used in life
it’s used in transforming vegetable oils into solid fats, eg. margarine
softer margin needs less hydrogen to react with the polyunsaturated oil, as that leaves some double bonds intact
what is the bond angle around double bonds
120˚, trigonal planar
what is the bond angle and name around a fully saturated carbon
109.5˚, tetrahedral
why do alkenes form stereoisomers
atoms can’t rotate around C=C double bonds because of how p-orbitals overlap sideways to form a pi bond
what happens when an alkene reacts with a halogen
halogenation, it’s an electrophilic addition reaction where the double bond is broken to fully saturate the carbon atoms
what does a tertiary carbocation look like
what does a primary carbocation look like
what does a secondary carbocation look like
order the carbocations from most to least stable
tertiary
secondary
primary
why is the tertiary carbocation the most stable
it has the most alkyl groups which can stabilise the positive charge
state Markovnikov’s rule
when a hydrogen halide reacts with an asymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the most hydrogen atoms
because that forms a more stable carbocation
how can ethene be converted into ethanol
ethene is reacted with steam and phosphoric acid (H3PO4) catalyst
why is steam used rather than water in the hydration of ethene
- reactions work better at higher temperature
- ethene is a gas
how can synthetic polymers be disposed of
- buried
- reused/recycled
- burned
why are waste plastics buried
landfill is generally used when the plastic is difficult to separate from other waste, quantities too small to make separation financially worthwhile
- too technically difficult to recycle
- we need to reduce use of landfill because we produce so much waste
how are waste plastics reused
many plastics are made from non-renewable oil fractions so we should try to reuse them as much as possible
- they can be sorted, then melted and remoulded
- they can be cracked into monomers and then used as organic feedstock to make more plastics/other chemicals
when and how are waste plastics burned
is recycling isn’t possible, they can be burned and the heat can generate electricity
the process has to be carefully controlled to reduce toxic gases
waste gases from combustion are passed through scrubbers which can neutralise gases allowing them to react with a base
what are biodegradable polymers and how can they decompose
can be made from renewable materials like starch/oil fractions, but currently more ££ than non-biodegradable equivalents
used in plastic sheets for plants in frost, poly(ethene) is embedded with starch which microorganisms nom and the plastic turns to dust
can decompose quickly as organisms can digest them but they need moisture and oxygen to decompose, e.g. a compost bin
they need to be separated from non-biodegradable plastics
what is the general formula of an alkane
CnH2n+2
what determines an alkanes boiling point
- chain length
- alkanes have covalent bonds inside the molecules; there are induced dipole-dipole interactions that hold the molecules together
- the interactions get stronger as the length of the chain increases because there’s more surface contact and electrons to interact
- longer molecules need more energy to overcome the interactions and the boiling point rises
- branched-chain vs straight chain
- branched alkane has a lower boiling point than straight chain as there is less surface interaction between molecules, weakening induced dipole-dipole interactions
what are the issues with free radical substitution reactions
the substitution can take part at any point along the carbon chain, leading to the formation of a mixture of isomers
you can also form a mixture of products rather than solely the one you want; not only the thing you want to get substituted will be substituted, you then have to separate the product you’re looking for from the mixture
what is the general formula of an alkene
CnH2n
how does a sigma bond form and what are it’s characteristics
two s orbitals overlap in a straight line to give the highest possible electron density between the nuclei and form a single covalent bond
the high electron density means there is strong electrostatic attraction between the nuclei and shared electron pair so sigma bonds have high bond enthalpy
why are alkanes not very reactive
sigma bonds have high bond enthalpy
the bonds are also non-polar bc hydrogen and carbon have similar electronegativity so nucleophiles and electrophiles aren’t attracted
why are alkenes more reactive than alkanes
the C=C double bond contains and a sigma and pi bond
it contains 4 electrons, giving it a high electron density and the pi bond sticks out above and below the molecule so it makes the bond likely to be attacked by electrophiles
the low bond enthalpy also increases reactivity
what conditions are needed to turn an alkene into an alkane
hydrogen gas and a nickel catalyst
how can bromine water be used to test for C=C
shaking an alkene with bromine water will cause to to decolourise as bromine is added across the double bond to form a colourless dibromoalkane
what conditions are needed to form halogen free radicals
- uv light
- high temperature
what part of an alkene is high electron density
the C=C bond
what’s the difference between a major and minor product
major - forms from the more stable carbocation
minor - forms from the less stable carbocation
how are photodegradable polymers broken down
they absorb light to weaken chemical bonds in the polymer
they are oil based
what is addition polymerisation
the reaction where many monomers with at least one C=C double bond form long polymer chains
the pi bonds break and the monomers link together to form single C-C bonds
what are optical isomers
molecules that are non-superimposable and non-identical mirror images of each other
shown by molecules with a chiral
carbon
what is a chiral centre
a carbon atom bonded to 4 different functional groups
aka asymmetric carbons
they exhibit optical
isomerism (a type of stereoisomerism)
what is E/Z vs cis/trans isomerism
cis/trans: used only when the alkene has two different groups on each C of the C=C and each C has one of the same group (looks at identical groups)
E/Z: based on the two highest priority groups
what are the types of stereoisomers
E/Z and optical only!!!!!!
define fission
the breaking of a covalent bond
is cis/trans a stereoisomer
no loser!!!
what is repulsion in an orbital
having 2 electrons in the orbital vs just one
how do you work out the formula of a hydrocarbon based on the CO2 formed when given mass of both
find moles of CO2, this is equal to the moles of carbon
use to get mass
subtract from total hydrocarbon mass to get mass of H, find moles
use to get formula
name the method of purifying an organic solid
recrystallisation
how is an organic liquid purified
- distill
- shake and let settle in spectating funnel and tap off each layer
- add anhydrous salt until stop clumping
- redistill
