aromatic compounds, amines, NMR Flashcards
what is benzene
aromatic compound consisting of a ring of 6 carbon atoms with 6 hydrogen atoms and a ring of delocalized electrons
what are the bonds in benzene like
each bond has an intermediate length in-between a double and single bond
the outer electrons in benzene
outer electrons from the p orbital of each carbon atom are delocalised to form the central ring. this ring makes benzene very stable compared to other molecules of a similar size
what was the prediction of benzenes enthalpy structure
- predicated it had similar structure to cyclohexatriene (3 double bonds & 3 single)
- expected the enthalpy change of hydrogenation for benzene was to be 360kjmol-1 (3x cyclohexene)
what is the actual enthalpy change of hydrogenation of benzene
-208kjmol-1
so it’s a different, unusual structure
what are arenes/aromatic compounds
compounds that contain benzene as part of their structure
features of arenes/aromatic compounds
- high melting points due to high stability of the delocalized ring
- low boiling points as the are non-polar molecules and often cannot be dissolved in water
nitration
generation of an electrophile
H2SO4 + HNO3 -> HSO4- + H2NO3+
concentrated acid and base
nitration
generation of an electrophile
intermediate
H2NO3+ -> NO2+ + H2O
nitronium ion 0 electrophile
H2SO4 + H2O -> HSO4- + H3O+
nitration
generation of an electrophile
overall
HNO3 + 2H2SO4 -> NO2+ 2HSO4- + H3O+
what is Friedel-crafts acylation
the delocalised electron ring in benzene can also act as a nucleophile , leading to the attack on acyl chlorides
what do we need for acylation
- catalyst - AlCl3
- acyl chloride
- anhydrous conditions
acylation
C2H3OCl + AlCl3 ->
C2H3OCl + AlCl3 -> C2H3O+ + AlCl4-
acylation - mechanism
H+ + AlCl4- ->
AlCl3 + HCL
AlCl3 - catalyst
nucleophilic substitutions of an amine
the reaction of halogenoalkane with ammonia sealed tube. 1 mole of halogenoalkane reacts with two moles of ammonia producing a primary amine and an ammonium salt
nucleophilic substitution producing a quarternary ammonium salt
substitution can continue until all the hydrogen atoms have been replaced
what are the problems with nucleophilic substitution with amines
mixture of products are produced. the reaction has low efficiency and the conditions have to be changed so only one substitution occurs
how to produce a primary amine
- ammonia added in excess
- or mixture of products can be separated using fractional distillation
reduction of nitriles to produce amines
hydrogenation. requires LiAlH4, a reducing agent and acidic conditions or a combination of hydrogen and nickel (catalytic hydrogenation)
how to produce aromatic amines
produced from the reduction of nitrobenzene using conc. HCL and tin catalyst, room temp
cationic surfactants
moelcuesl with +ve & -ve end. good conditioners as two ends attract different substances, preventing static from building up on surfaces
amines as a base
weak bases due to the lone electron pair on the nitrogen atom can accept protons
what does the base strength depend on
depends on how available the electron pair is on the molecules. the more available the electrons, the more likely it is to accept a proton meaning a stronger base
what is the inductive affect
different functional groups can affect how available a lone electron pair is by changing electron density around the bond
inductive effect
benzene rings
draw electron density away from the nitrogen making it less available
inductive effect
alkyl groups
push electron density towards the nitrogen making it more available.
more alkyl groups mean more pushing
nucleophilic addition-elimination
amines can undergo this reaction with acyl chlorides to produce amides
aromatic compounds undergoing electrophilic substitution
delocalised ring in benzene is an area of high electron density. so amines and nitrobenzene can be produced via electrophilic substitution
strongest to weakest base
- tertiary amine
- secondary amine
- primary amine
- ammonia
- phenyl amine
- diphenyl amine
- triphenyl amine
suitable isotopes for nuclear magnetic resonance
- 1H NMR
- 13C NMR
- 31P NMR
- 19F NMR
1H NMR
- main type of NMR
- give lots of structural information
- quick (1 minute)
13C NMR
- not as common as “it takes too long” 5 minutes
- there are only time amounts of 13C in sample most of its 12C
Tetramethlysilane (TMS)
- added to a sample to calibrate the spectrum
- the TMS chemical to 0
- it is used because the signal is away from everything else
- only gives a strong signal
- non-toxic and inert (not reactive)
- low bp so can be removed from a sample easily
what is NMR spectroscopy
analytical technique that allows the structure of a molecule to be determined by analysing the energy of each bond environment.
how does NMR spectroscopy work
different bond environment within molecule absorb different amounts of energy meaning they show as different peak on a spectra print out
what are bond environment peaks measured against
a standard molecule TMS Si(CH3)4. This is standard as it contains 4 identical carbon and hydrogen environments. it is seen as a peak at 0 on the x-axis
carbon environments
near an oxygen
carbon environments will shift to the rights as oxygen is electronegativity and changes the bond environment and how it absorbs energy
carbon environment
molecules that have symmetry
may display fewer peaks than the number of carbons in the molecule. so you have to check the MR to determine its structure
what is H NMR - proton NMR
the different hydrogen environments in a molecule are analysed and displayed as peaks on spectra. these peaks are measured against the TMS standard
features needed for proton NMR
samples being analysed must be dissolved in a non-hydrogen-containing solvent so it doesn’t produce andy peaks on the spectrum.
What does the height on peaks on H NMR spectra show
show the relative intensity of each value. the relative intensities correspond to the number of hydrogen in that certain environment within a molecule
spin-spin coupling in NMR
hydrogen bonded to the adjacent carbon causes splitting of the peak.
this is for hydrogens that are not in the same environment
how to find the number of peaks
adjacent hydrogens + 1
what does each peak of H NMR show
where each environment is positioned within the molecule. peaks are split into smaller clusters with smaller peaks indicating how many hydrogens are on the adjacent carbon atom
smaller peaks in H NMR
these are splitting patterns and follow an N+1 rule when n is the number of hydrogens on the adjacent carbon
