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chemistry A-level 2 > aromatic compounds, amines, NMR > Flashcards

aromatic compounds, amines, NMR Flashcards

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1
Q

what is benzene

A

aromatic compound consisting of a ring of 6 carbon atoms with 6 hydrogen atoms and a ring of delocalized electrons

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2
Q

what are the bonds in benzene like

A

each bond has an intermediate length in-between a double and single bond

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3
Q

the outer electrons in benzene

A

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

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4
Q

what was the prediction of benzenes enthalpy structure

A
  • 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)
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5
Q

what is the actual enthalpy change of hydrogenation of benzene

A

-208kjmol-1
so it’s a different, unusual structure

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6
Q

what are arenes/aromatic compounds

A

compounds that contain benzene as part of their structure

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7
Q

features of arenes/aromatic compounds

A
  • 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
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8
Q

nitration
generation of an electrophile

A

H2SO4 + HNO3 -> HSO4- + H2NO3+
concentrated acid and base

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9
Q

nitration
generation of an electrophile
intermediate

A

H2NO3+ -> NO2+ + H2O
nitronium ion 0 electrophile
H2SO4 + H2O -> HSO4- + H3O+

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10
Q

nitration
generation of an electrophile
overall

A

HNO3 + 2H2SO4 -> NO2+ 2HSO4- + H3O+

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11
Q

what is Friedel-crafts acylation

A

the delocalised electron ring in benzene can also act as a nucleophile , leading to the attack on acyl chlorides

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12
Q

what do we need for acylation

A
  • catalyst - AlCl3
  • acyl chloride
  • anhydrous conditions
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13
Q

acylation
C2H3OCl + AlCl3 ->

A

C2H3OCl + AlCl3 -> C2H3O+ + AlCl4-

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14
Q

acylation - mechanism
H+ + AlCl4- ->

A

AlCl3 + HCL
AlCl3 - catalyst

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15
Q

nucleophilic substitutions of an amine

A

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

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16
Q

nucleophilic substitution producing a quarternary ammonium salt

A

substitution can continue until all the hydrogen atoms have been replaced

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17
Q

what are the problems with nucleophilic substitution with amines

A

mixture of products are produced. the reaction has low efficiency and the conditions have to be changed so only one substitution occurs

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18
Q

how to produce a primary amine

A
  • ammonia added in excess
  • or mixture of products can be separated using fractional distillation
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19
Q

reduction of nitriles to produce amines

A

hydrogenation. requires LiAlH4, a reducing agent and acidic conditions or a combination of hydrogen and nickel (catalytic hydrogenation)

20
Q

how to produce aromatic amines

A

produced from the reduction of nitrobenzene using conc. HCL and tin catalyst, room temp

21
Q

cationic surfactants

A

moelcuesl with +ve & -ve end. good conditioners as two ends attract different substances, preventing static from building up on surfaces

22
Q

amines as a base

A

weak bases due to the lone electron pair on the nitrogen atom can accept protons

23
Q

what does the base strength depend on

A

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

24
Q

what is the inductive affect

A

different functional groups can affect how available a lone electron pair is by changing electron density around the bond

25
Q

inductive effect
benzene rings

A

draw electron density away from the nitrogen making it less available

26
Q

inductive effect
alkyl groups

A

push electron density towards the nitrogen making it more available.
more alkyl groups mean more pushing

27
Q

nucleophilic addition-elimination

A

amines can undergo this reaction with acyl chlorides to produce amides

28
Q

aromatic compounds undergoing electrophilic substitution

A

delocalised ring in benzene is an area of high electron density. so amines and nitrobenzene can be produced via electrophilic substitution

29
Q

strongest to weakest base

A
  • tertiary amine
  • secondary amine
  • primary amine
  • ammonia
  • phenyl amine
  • diphenyl amine
  • triphenyl amine
30
Q

suitable isotopes for nuclear magnetic resonance

A
  • 1H NMR
  • 13C NMR
  • 31P NMR
  • 19F NMR
31
Q

1H NMR

A
  • main type of NMR
  • give lots of structural information
  • quick (1 minute)
32
Q

13C NMR

A
  • not as common as “it takes too long” 5 minutes
  • there are only time amounts of 13C in sample most of its 12C
33
Q

Tetramethlysilane (TMS)

A
  • 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
34
Q

what is NMR spectroscopy

A

analytical technique that allows the structure of a molecule to be determined by analysing the energy of each bond environment.

35
Q

how does NMR spectroscopy work

A

different bond environment within molecule absorb different amounts of energy meaning they show as different peak on a spectra print out

36
Q

what are bond environment peaks measured against

A

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

37
Q

carbon environments
near an oxygen

A

carbon environments will shift to the rights as oxygen is electronegativity and changes the bond environment and how it absorbs energy

38
Q

carbon environment
molecules that have symmetry

A

may display fewer peaks than the number of carbons in the molecule. so you have to check the MR to determine its structure

39
Q

what is H NMR - proton NMR

A

the different hydrogen environments in a molecule are analysed and displayed as peaks on spectra. these peaks are measured against the TMS standard

40
Q

features needed for proton NMR

A

samples being analysed must be dissolved in a non-hydrogen-containing solvent so it doesn’t produce andy peaks on the spectrum.

41
Q

What does the height on peaks on H NMR spectra show

A

show the relative intensity of each value. the relative intensities correspond to the number of hydrogen in that certain environment within a molecule

42
Q

spin-spin coupling in NMR

A

hydrogen bonded to the adjacent carbon causes splitting of the peak.
this is for hydrogens that are not in the same environment

43
Q

how to find the number of peaks

A

adjacent hydrogens + 1

44
Q

what does each peak of H NMR show

A

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

45
Q

smaller peaks in H NMR

A

these are splitting patterns and follow an N+1 rule when n is the number of hydrogens on the adjacent carbon

chemistry A-level 2 (8 decks)

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