o chem– fundamentals + isomerism + alkanes Flashcards
what is the empirical formula? (e.g. C3H6 => CH2)
it is the simplest formula that shows the ratio of all the different atoms in a molecule
what is the molecular formula?
it is the actual number of atoms of each element in a molecule
why is the structural formula useful?
it clearly shows how the atoms are bonded to each other
give me two types of aliphatic (open chains) hydrocarbons
- straight-chain
- branched
what class of compounds has the structure –C☰C–
alkyne
what structure do arenes have?
benzene ring
what structure do ethers have
R–O–R
what’s the structural difference between ketones and aldehydes?
- ketone structure is R–C–H
〢
O - aldehyde structure is R–C–R
〢
O
is this an amide or an amine: –NH2
amine
what class of compounds is this structure?R–C–NR2
〢
O
amide
what defines a homologous series?
- same functional group
- each successive member increases by one –CH2 unit
homologous series have the following characteristics:
- same _______ formula
- show a gradual change in _______ properties as molar mass increases
- have similar ________ properties
general, physical, chemical
why is there resonance in benzene?
benzene has a delocalised π electron system
benzene is stable because…
it is resonance stabilised
what hybridisation does C in benzene undergo
sp2
why is benzene so stable? (4)
- each C atom is sp2 hybridised
- each C atom contains one unhybridised p orbital that lies perpendicular to the plane of the benzene ring
- the p orbitals overlap sideways with one another -> continuous overlap of unhybridised p orbitals
- π e- can delocalise across benzene ring -> creates delocalised π e- cloud -> resonance
what are the effects of resonance stability on benzene
1. ???
2. planar molecule
3. can undergo substitution rxns
identical C-C bond lengths
why does benzene have identical C-C bond lengths?
- due to the partial double bond character of C–C bonds
- as a result of delocalisation of π e- across the ring
why does benzene undergo substitution but not addition rxns?
- sub rxns preserve the resonance stability of benzene
- add rxns disrupt resonance stability -> energetically unfavourable
what are the two reactions benzene can undergo?
- nitration
- reduction
chain isomers have the same ______ formula and functional group, but different ________ skeleton
molecular, carbon
positional isomers have different positions of ______________
functional group
aldehydes are functional grp isomers with …
ketones
carboxylic acids are functional grp isomers with…
esters
alcohols are functional grp isomers with…
ethers
what are stereoisomers?
molecules w same molecular formula and same structural formula, but different spatial arrangement of its atoms
compounds that can have E/Z isomerism must:
1. have a covalent bond with ____________
2. the two different grps bonded to ea C atom on the end of the restricted bond must be _________
restricted rotation, different
in determining whether an isomer is E/Z, we need to determine which atom/molecule bonded directly to the carbon has higher ______ and thus higher _______
Mr, priority
what are enantiomers?
stereoisomers that are mirror images of ea other and non-superimposable on ea other
do enantiomers have identical physical, chemical and biological properties?
- yes for physical
- yes for chemical except for reaction with other chiral molecules
- no for bio as diff spatial arrangement makes them rxt w diff enzymes
are alkanes polar?
no, they r non polar
alkanes have _______ _____ boiling points
relatively low
what is LDF in secondary school terms?
intermolecular FOA/ Van Der Waals’ FOA
what is the trend of boiling points down the homologous series of alkanes?
- electron cloud size ↑
- more polarisable
- LDF ↑ -> energy required ↑
- ↑ b.p.
do straight chain or branched alkanes have a higher boiling point? (idea: ‘packing’ of the molecules)
- straight chain
- larger surface area
- greater extent of LDF
- requires more energy to break
why are alkanes soluble in non-polar solvents? (hint: energy)
energy released from LDF btw alkanes + non-polar solvent releases enough energy to overcome LDF in alkanes and LDF in solvent
why are alkanes insoluble in water? (hint: energy)
energy released from LDF btw alkanes + water is insufficient to overcome H bonding in water
why are alkanes so unreactive? (3)
- they are non-polar
- do not attract any species for rxn
- they are saturated molecules
- no triple + double bonds to undergo addition
- strong C-H and C-C bonds
- rxns have high activation energy
what are the products of complete combustion of alkanes?
CO2 and H2O
what are the products of incomplete combustion of alkanes?
CO, unburnt hydrocarbons, soot (C)
what is the reaction of alkanes with halogens called?
free radical substituition
in free radical sub, what reagents and conditions are needed?
Cl2 (g) or Br2 (l), UV light/heat
why are free radicals so reactive? (short)
free radicals have an unpaired electron
UV light is required in free radical sub as it provides ________ for the ______________ of _______ to form Cl* radicals
energy, homolytic fisson, Cl-Cl bond
how is homolytic fission of the Cl-Cl bonds achieved?
UV light provides energy for it
the steps of free radical sub are:
1. initiation
2. ???
3. termination
propagation
in which step of FRS does the regeneration of the free radical take place?
propagation
what are the products of FRS?
- HX
- X halogenoalkane
- trace amounts of alkane w double the no. of C atoms
(X being the halogen)
what happens during initiation of FRS? (1)
UV light provides energy for the homolytic fission of Cl-Cl to form Cl radicals
what happens during propagation of FRS? (4)
- highly reactive Cl* radical reacts with alkane CH4, removing a H atom to produce HCl
- as a result, another radical *CH3 is produced
- methyl radical *CH3 reacts further w another Cl2 molecule to produce e final product -> CH3Cl
- at the same time another Cl* is regenerated
what happens during termination of FRS?
- when two radicals collide -> unpaired electrons pair up to form a stable product
- this terminates the chain rxn
- trace amt of ethane formed
why is only a flash of UV light necessary to initiate the rxn?
- UV is only required for initiation
- radicals can be regenerated -> allows for a chain rxn
write out the mechanism w eqns ONLY for FRS on paper
-
why is FRS NOT used to synthesise halogenalkanes?
it is difficult to control the
- extent of substitution
- position of subtitution
what is a tertiary radical?
a radical with where the C atom w the unpaired electron is attached to three other alkyl grps
why are tertiary radicals the most stable?
- has largest no. of e- donating alkyl groups
- can disperse the e- deficiency of the radical to the largest extent
how does the stability of radicals affect the actual ratio of products in FRS? (compare to expected ratio)
actual ratio may be different than expected
how does the rate of FRS change down the halogen group?
it decreases
why does the rate of FRS decrease down the halogen grp?
rate of FRS is dependent on the ∆H of the propagation step
why is incomplete combustion harmful
- releases CO
- CO binds irreversibly w haemoglobin in blood -> O2 deficiency
what components of incomplete combustion can result in photochemical smog
- unburnt hydrocarbons rxt w O2, O3, oxides of nitrogen -> in e presence of sunlight
NO2 can form _______ (environmentally harmful) and catalyses the formation of ____ to ____, forming _______ (also env harmful)
acid rain, SO2, SO3, acid rain
