4.1 basic concepts and hydrocarbons Flashcards
general formula
Simplest algebraic formula of a member of a homologous series
Alkanes- CnH2n+2 Alkenes- CnH2n
homologous series
Series of organic compounds having the same functional group but with each successive member different by CH2
functional group
Group of atoms responsible for the characteristic reactions of a
compound
E.g.- -Cl, -OH
functional group isomerism
Position of atoms cause a different functional group
structural isomerism
Compounds with the same molecular formula but a different structural formula
chain isomerism
Isomers have different chain length caused by branching
positional isomerism
The position of a functional group differs along a chain
isomerism
Aliphatic isomers –
compound arranged in a non-aromatic rings. With or without branching chains
Aromatic isomers-
a compound containing a benzene ring
Structural isomers- Compounds with the same molecular formula but a different structural formula
saturated
single carbon–carbon bonds only) and unsaturated (the presence of multiple carbon–carbon bonds, including C=C, C multiple / and aromatc rings
saturated
single carbon–carbon bonds only) and unsaturated (the presence of multiple carbon–carbon bonds, including C=C, C multiple / and aromatc rings
covalent bond fission- homolytic
in terms of each bonding atom receiving one electron from the bonded pair, forming two radicals
covalent bond fission- heterolytic fission
in terms of one bonding atom receiving both electrons from the bonded pair
radicals
a species with an unpaired electron
‘dots’ represent species that are radicals in mechanisms
Dots, •, are required in all instances where there is a single unpaired electron (e.g. Cl• and CH3•).
Dots are not required for species that are diradicals (e.g. O). (h)
curly arrow
Is described as the movement of an electron pair, showing either heterolytic fission or formation of a covalent bond
alkanes
saturated hydrocarbons containing single C–C and C–H bonds as σ-bonds (overlap of orbitals directly between the bonding atoms) which allow free rotation of the σ-bond
Is non-polar, so has only London dispersion forces
Is only soluble in non-polar solvents.
The properties depend on chain length and branching
More points of contact mean there are more intermolecular forces
boiling point of alkanes
Increases with the length of chain
Packing describes how closely they can get together and branching in alkanes cause them to not pack as close together, which lowers the boiling point
alkanes and reactivity
Low because of:
Low polarity
High bond enthalpy
A very slight dipole but still non-polar
combustion of alkanes
Is exothermic- addition of oxygen Complete when there is excess oxygen CH4 + 2O2 → CO2 + 2H2O Incomplete when there is limited oxygen CH4 + O2 → C + 2H2O CH4 + 2O2 → CO + 2H2O Complete calculations are often balanced so that the fuel is given as 1 mol, allowing oxygen to have halves
oxidation of alkanes
Combustion of methane CH4 + 1 ½ O2 → CO + 2H2O CH4 + O2 → C + 2H2O CH4 + 2O2 → CO2 + 2H2O Carbon is reduced and hydrogen is oxidised
radical substitution
An unbonded electron in an atom/molecule is highly reactive
Formed by homolytic fission ( homolysis)
Homolytic- each atom in a bond takes 1 of the shared electron
Fission- splitting of the covalent bond .
UV radiation has enough energy to split a covalent bond within bonds to form radicals.
∙ represents an unpaired electron
what are the 3 stages to radical substitution
Initiation- radical formation
Propagation- formation of product and new radicals
Termination- reaction ends as radical is removed
alkenes
unsaturated hydrocarbons containing a C=C bond comprising a π-bond (sideways overlap of adjacent p-orbitals above and below the bonding C atoms) and a σ-bond (overlap of orbitals directly between the bonding atoms)
stereoisomerism
Occurs in compounds with the same structural formula but different arrangement in space
The pi- bond in alkenes restrict the rotation of the molecule potentially causing E/Z isomerism
Sigma-bonds can rotate so stereoisomerism cant occur in alkanes
E/Z stereoisomerism
Can only occur if:
There’s a C=C bond
Each carbon is bonded to a different atom/group
E-isomerism- priority groups are on opposite sides
Z-isomerism- priority groups are on the same side
Cahn, Ingold and Prelog
assign stereoisomers as either E or Z, using the atomic number of the atom that is directly bonded to C=C
The rules:
- Highest atomic numbers are diagonally opposite
- Highest atomic numbers aren’t diagonally opposite ( Z- isomers)
- Is atomic numbers are equal, the adjacent atoms are taken into account.
- Longer alkyl chains take priority
cis/trans isomerism
A type of E/Z isomers where 2 of the substituent groups on the C=C bond are the same.
Cis- not diagonally opposite
Trans- diagonally opposite
electrophile
an electron pair acceptor so there are often positive ions or have a 𝛿+ dipole
alkene reactions
The lower bond enthalpy of a pi-bond compared to a sigma-bond makes alkenes more reactive than alkanes
The position of the pi-bond above and below the molecule also makes it easier to ‘attack’ the electron pair
All alkenes undergo addition reactions whereby 2 reactants combine to form a single product
With species bonding either side of the
C=C bond
Markownikoff’s rule
When an asymmetrical alkene reacts with a hydrogen halide or water molecule, there are 2 possible isomers formed
But 1 isomer is more commonly produced than the other.
States that the hydrogen being added bonds to the carbon with the most hydrogen present
addition polymerisation
Alkenes can undergo addition reactions in which 1 alkene molecule ( a monomer) joins to another until a long molecular chain is built up ( a polymer).
Polymerisation can be initiated in a number of ways.
Often, the initiator is incorporated at the start of the long molecular chain.
However, if the imitator is disregarded, the empirical formulae of the monomer and polymer is the same
problems with polymers
The problem with addition polymers is that they don’t biodegrade ( rot)
They persist in the environment and this can cause problems such as turtles eating plastic bags thinking they are jellyfish
photo-degradation
They break down into smaller fragments as a result of sunlight but each fragment is the same addition polymer and this can cause different problems, such as ingestion by filter-feeding organisms
recycling
Usually means melting down and re-forming into new products
Is expensive because there are many different polymers in domestic waste and they must be separated by hand before melting.
This leads to contamination- so recycled polymers are always lower quality than freshly manufactured ones.
Some polymers are thermoset- don’t melt when heated ( although they could still burn.)
chemical-feedstock recycling
Breaks down polymers without separating them, forming simple gases that can then be used to manufacture pure, fresh polymers.
This system is yet to be shown to work.
incineration
Burning them removes the polymers and can generate power as an additional benefit- thereby saving fossil fuels.
Is a worry about toxic products of some polymers and those polymers can contain chlorine ( such as PVC) would create HCl as a waste gas.
Adds expense as it would be necessary to neutralise the waste gases to avoid acid rain.
