Last Minute Cram - Organic Flashcards
when atomic orbitals overlap…
they combine to form molecular orbitals
why is a hydrogen molecule more stable than hydrogen atoms
bonding molecular orbital has lower energy than the seperate atomic orbitals
overlap of two hydrogen atomic orbitals
the formation of a σ bond
more stable
σ bonds
covalent bonds formed between atoms when end-on overlap of orbitals occurs
How can bonding in hydrocarbons be explained?
using hybridisation
2s and 2p orbitals combine to form four degenerate orbitals
hybrid formed from 1 s orbital and 3 p orbitals is called sp3
sp3 orbitals in hydrocarbons
all half filled with the electron far more likely to be found in the larger lobe
(shape is like sideways bowling pin)
electrons repel so the 4 sp3 orbitals surrounding the carbon will form a tetrahedral shape max angle 109.5 degrees
alkanes (hybridisation)
carbon to carbon single bonds in alkanes result from overlapping sp3 orbitals forming σ bonds
4 σ bonds in methane
7 σ bonds in ethane
free rotation of orbitals in alkanes
σ bonds must be lying along the line joining both atoms so there will be free rotation around these orbtials.
alkenes (hybridisation)
one electron from 2s shell is promoted to empty 2p orbital
this results in three hybrid orbitals and one remaining unhybridised 2p orbital
sp2 orbitals
formed from one s orbital and 2 p orbitals
sp2 orbitals in alkenes
repel each other resulting in bond angle of 120 degrees between them
overlap to form σ bonds
orbitlas in a molecule of ethene
σ bonds
unhybridised p orbitals perpendicular to the plane of the molecule
p orbitals of carbon parallel and close enough to overlap sideways
π molecular orbitals
covalent bond formed by the sideways overlap of two parallel atomic orbitals
hybridisation summary alkanes
longer C-C bond free rotation tetrahedral sp3 hybridisation all σ
hybridisation summary alkenes
short C=C bonds **bonds NOT twice as strong** planar no rotation sp2 hybridisation σ and π
hybridisation summary alkynes
sp hybridised
bond angles of 180
σ and π
comparing σ and π
double bonds are stronger but not twice as strong
this is because sideways overlap is weaker than end-on overlap
hybridisation in benzene
sp2
each C has 3 filled sp2 orbitals
1 makes a sigma bond with H and other two make sigma bonds with neighbouring 2 Cs
this leaves an electron occupying a p-orbital on each of the carbons
each p orbital overlaps side on creating a pi bond above and below the plane of the molecule
orbitial diagram of increasing energy
top
pi anti-bonding sigma anti-bonding non-bonding (contains lone pairs) pi bonding sigma bonding
bottom
what compounds are colourless
organic compounds that only contain sigma bonds
HOMO is sigma bonding orbital
LUMO is sigma anti-bonding orbital
energy gap to promote an electron is very large
these absorptions correspond to UV part of the specturm
excitations of electrons in compounds containing simple pi bonds
still involve large transition from HOMO (pi bonding) to LUMO (sigma anti-bonding)
still absorb in UV region
conjugated systems
prganic molecules that are coloured contain delocalised electrons within alternating carbon to carbon single and double bonds
the greater the number of atoms spanned by the delocalised electrons…
the smaller the energy gap will be between the delocalised orbital and the next unoccupied orbital
will require less energy to excite electrons (coloured)
chromophores
group of atoms within a molecule that is responsible for its colour
why do coloured compounds arise?
visible light is absorbed by the delocalised electrons in the chromophore which are then promoted to a higher energy molecular orbital
if the chromophore absorbs light of one colour..
the complementary colour is observed
greater degree of conjugation…
smaller energy gap
less energy required
appear orange or red
what can stereoisomerism be split into?
geometric isomerism
optical isomerism
structural isomerism
organic compounds which have the same molecular formula but different structural formula
have different physical and chemical properties and can belong to different homologous series
stereoisomerism
arises whenever there is more than one way to organise a given number of atoms
molecules have the same molecular and structural formula
have a different three-dimensional arrangement in space which makes them non-superimposable
geometric isomerism - cis
both groups are on the same side of the double bond
geometric isomerism - trans
the groups are on opposite sides of the double bond
naming cis and trans moleules
same systematic name as before but add cis or trans at start of name
physical properties of geometric isomers
trans isomer - closer packing in the solid state
increase in London dispersion forces
increases mp
eg: in C-Cl bonds are polar
trans - polarities cacel out and overall non-polar
cis - extra pdp.pdp interactions between molecules gives a higher bp
major kinds of fats in food
saturated
polyunsaturated
monounsaturated
trans fatty acids
what fats raise blood cholesterol
saturated and trans
risk of coronary heart disease which leads to heart attack and increased risk of stroke
unsaturated fats (oils)
kinks in hydrocarbon which makes close packing more difficult
ldf weaker
lower mp
triglycerides
made from tans unsaturated fatty acids
pack together closely
ldf stronger
higher mp
aromatics
organic molecules containing a benzene ring
benzene ring
3 valence electrons form sigma bonds
fourth delocalised over entire ring
six sp2 orbitals
planar molecule with the unfilled p orbital above and below
2p orbitals combine to form a set of delocalised pi molecular orbitlas
C6H6
why does benzene resist bromine addition reaction?
does not have C=C double dins
does not discolour bromine solution
does not undergo addition reactions
substituted benzene ring
phenyl group
C6H5
reactions of benzene
1. with bromine or chlorine
requires a catalyst
eg: aluminium chloride or iron(III) bromide
catalyst polarises bromine molecule
partially +ve bromine can then attack one of the C on benzene
creates carbonation which is stabilised by delocalisation on the ring
reactions of benzene
2. nitration
heat HNO3 or H2SO4 under reflux
electrophilic substitution by nitronium cation NO2+
reactions of benzene
3. sulfonation
heat under reflux for several hours with conc. H2SO4
used in manufacture of detergents
reactions of benzene
4. alkylation
halogenoalkane and ALCl3 catalyst
ACl3 increases polarity of C-halogen
partially positive R group attacks benzene ring
some halogenoalkanes can form a carbocation
what is a drug?
natural or synthetic substances which affect the biochemical systems in the body
what is a medicine?
a drug that has a beneficial effect on the body
normally contains one or more drugs amongst other ingredients
hwo a medicine functions
most drugs work by binding to a receptor (usually protein)
effective medicines work by binding to the receptor site
either mimic response of active molecule or block the effect of an active molecule
this is reversible and involves weak forces i.e. H bonding
lock and key analogy
key=molecule
lock=receptor
agonist
medicine that interacts with a receptor to produce a response similar to the body’s natural active compound
in analogy: a good copy of the original key. Will fit into lock and will turn
antagonist
interacts with a receptor to produce no response because it prevents the action of the body’s natural active compound
stops cell activity and prevents other substances occupying receptor
analogy: badly copied key that fits into lock but wont turn
pharmacophore
minimum structural feature that gives pharmacological activity
shape compliments receptor site
designing a drug
needs to be absrobed in blood stream, reach target efficiently, stable enough to survive journey and be eliminated in a sensible timeframe.
functional groups involved in drug-receptor sites
generally contained in amino acids
enantiomers
substances which show optical isomerism
R-form
if a solution of one enantiomer rotates the plane of polarised light in a clockwise direction it is known as the R-form
S-form
if a solution of the other enantiomer rotates the plane of polarised light in an anti-clockwise direction it is known as the S-form
if the solutions are equally concentrated…
the amount of rotation caused by the two isomers is exactly the same but in opposite directions
racemic mixture
50/50 mixture of the two enantiomers in equal concentrations
optically inactive and has no effect on plane polarised light
chiral carbon
contains four different groups
a mass spectrometer does 3 things:
- vapourises a minute sample of compound
- ionises the vaporised molecules
- separates and analyses the ions produced when the molecules fall apart according to their mass/cahrge ratio
this gives a mass spectrum
how is a radical cation formed?
a high-energy electron can dislodge an electron from a bond, creating a radical cation
(+ve ion with an unpaired electron)
fragmentation then occurs
m or m/z
mass or mass to charge ratio
x-axis on mass spectrum
main use of mass spectrum
identify the molecular ion (biggest one)
table used in empirical formula
mass
gfm
moles
divide by smallest
empirical formula
XnYmZr
simplest ratio
what does infrared cause?
molecules to vibrate
different bonds vibrate differently
explain the effect infrared radiation has on the bonds within molecules and how this allows different functional groups to be identified
infrared causes molecules to vibrate. Different bonds absorb at different wavenumber so they can be identified
the greater the mass of the atoms…
the lower the frequency of vibrations
the stonger the bond…
the higher the frequency of vibrations
fingerprint region
area of the spectrum below 1500cm^-1
it has very complicated series of absorptions due to lots of different bonding vibrations within the molecule
what nuclei have nuclear spin?
odd mass or odd atomic number
magnetic field of nucleus
since the nucleus is a charged particle in motion, its will develop a magnetic field
when a magnetic field is applied the nuclei…
line up parallel to the applied field, either spin aligned or spin opposed
nuclei aligned opposed to magnetic field
high energy state
nuclei aligned with magnetic field
low energy state
as nuclei relax back to low-energy alignment, energy in the radio wave frequency is released…
this energy is detected and recorded as peaks on a spectrum
absolute values are difficult to obtain so
values often obtained by refernce to a standard assigned with delta value 0
values mearued in the ppm
standard used in NMR
tetramethylsilane (TMS)
has an NMR signal well away from those found in most organic molecules
protons emitting radiation of the same frequency are said to be…
in the same proton environment
NMR
the area under the peak gives information about
how many protons are in each environment
High resolution NMR
run using a higher radio frequency and the peaks have more detail
n+1 rule
the number of peaks in a multiplet can give additional information about the structure
n is the number or hydrogen atoms attached to the adjacent carbon
plotting on an nmr spectra
chemical shift ppm found in d.b.
how high the line goes up depends on how many of the same proton environment
amines
organic compounds based on ammonia
functional group -NH2
one or more H atoms replaced by alkyl groups or aryl groups (based on benzene)
classification of amines
similar to alcohols
primary - one R group and 2 H atoms
sec. - 2 R groups and 1 H
tert. - 3 R groups
R groups can be the same or different
naming amines (basic)
add suffix -amine to the name of the R group
eg: ethylamine
naming amines with multiple groups
prefix amino is used
eg: 2-aminopropanoic acid
groups named alphabetically
eg: butyldimethylamine
amines and intermolecular forces
primary and secondary have N-H bond i.e. hydrogen bonding
tertiary amines have no H atoms so no H bonding
amines boiling points
tertiary amines lower due to lack of H bodning
solubility of amines
H bonding between amine and water
solubility decreases as the number of carbon atoms increases
reaction of amines
amines can accept protons from water or an acid
since amines are bases they react with acids in neutralisation reactions
what is gravimetric analysis
a quantitative determination of an analyte based on the mass of a solid
the mass of the analyte present in a substance is determined by
changing that chemical substance into a solid by precipitation, with an appropriate reagent, of known chemical composition and formula
heating to a constant mass
final product has to be dried completely
heating substance in a crucible with the blue frame of a bunsen burner, allowing it to cool in a desiccator and then reweighing
repeat until constant mass obtained
HEAT
DRY
WEIGH
REPEAT
weighing by difference
mass of crucible is measured before adding substance
final mass of substance determined by subtracting mass of crucible from the mass of crucible and dried substance
steps in gravimetric analysis calculation
use mass of empty cruicible and cruicuble+ substance to work out mass of hydrated substance
same with anhydrous substance
mass of water removed
moles of water removed
moles of substance
ratio of moles
needs to be whole number
