aromatic compounds, amines, NMR

Question 1

what is benzene

Answer 1

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

Question 2

what are the bonds in benzene like

Answer 2

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

Question 3

the outer electrons in benzene

Answer 3

outer electrons from the p orbital of each carbon are delocalised forming the central ring. this ring makes benzene very stable compared to other molecules of a similar size

Question 4

what was the prediction of benzenes enthalpy structure

Answer 4

• predicated it had similar structure to cyclohexatriene (3 double bonds & 3 single)
• expected the enthalpy change of hydrogenation for benzene was 360kjmol-1 (3x cyclohexene)

Question 5

what is the actual enthalpy change of hydrogenation of benzene

Answer 5

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

Question 6

benzene reacting with bromine

Answer 6

only at high temperature or in presence of a halogen carrier

Question 7

what reactions do benzene undergo

Answer 7

• no addition a it would involve breaking up the delocalised system
• normally substituting H for another atom - electrophilic addition

Question 8

what are arenes/aromatic compounds

Answer 8

compounds that contain benzene as part of their structure

Question 9

features of arenes/aromatic compounds

Answer 9

• 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

Question 10

nitration
generation of an electrophile

Answer 10

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

Question 11

nitration
generation of an electrophile
intermediate

Answer 11

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

Question 12

nitration
generation of an electrophile
overall

Answer 12

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

Question 13

what is Friedel-crafts acylation

Answer 13

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

Question 14

what do we need for acylation

Answer 14

• catalyst - AlCl3
• acyl chloride
• anhydrous conditions

Question 15

acylation
C2H3OCl + AlCl3 ->

Answer 15

C2H3OCl + AlCl3 -> C2H3O+ + AlCl4-

Question 16

acylation - mechanism
H+ + AlCl4- ->

Answer 16

AlCl3 + HCL
AlCl3 - catalyst

Question 17

nucleophilic substitutions of an amine

Answer 17

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

Question 18

nucleophilic substitution producing a quarternary ammonium salt

Answer 18

substitution can continue until all the hydrogen atoms have been replaced

Question 19

what are the problems with nucleophilic substitution with amines

Answer 19

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

Question 20

how to produce a primary amine

Answer 20

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

Question 21

reduction of nitriles to produce amines

Answer 21

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

Question 22

how to produce aromatic amines

Answer 22

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

Question 23

cationic surfactants

Answer 23

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

Question 24

amines as a base

Answer 24

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

Question 25

what does the base strength depend on

Answer 25

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

Question 26

what is the inductive affect

Answer 26

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

Question 27

inductive effect benzene rings

Answer 27

draw electron density away from the nitrogen making it less available

Question 28

inductive effect alkyl groups

Answer 28

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

Question 29

nucleophilic addition-elimination

Answer 29

amines can undergo this reaction with acyl chlorides to produce amides

Question 30

aromatic compounds undergoing electrophilic substitution

Answer 30

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

Question 31

strongest to weakest base

Answer 31

- tertiary amine - secondary amine - primary amine - ammonia - phenyl amine - diphenyl amine - triphenyl amine

Question 32

suitable isotopes for nuclear magnetic resonance

Answer 32

- 1H NMR - 13C NMR - 31P NMR - 19F NMR

Question 33

1H NMR

Answer 33

- main type of NMR - give lots of structural information - quick (1 minute)

Question 34

13C NMR

Answer 34

- not as common as "it takes too long" 5 minutes - there are only time amounts of 13C in sample most of its 12C

Question 35

Tetramethlysilane (TMS)

Answer 35

- 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

Question 36

what is NMR spectroscopy

Answer 36

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

Question 37

how does NMR spectroscopy work

Answer 37

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

Question 38

what are bond environment peaks measured against

Answer 38

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

Question 39

carbon environments near an oxygen

Answer 39

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

Question 40

carbon environment molecules that have symmetry

Answer 40

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

Question 41

what is H NMR - proton NMR

Answer 41

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

Question 42

features needed for proton NMR

Answer 42

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

Question 43

What does the height on peaks on H NMR spectra show

Answer 43

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

Question 44

spin-spin coupling in NMR

Answer 44

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

Question 45

how to find the number of peaks

Answer 45

adjacent hydrogens + 1

Question 46

what does each peak of H NMR show

Answer 46

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

Question 47

smaller peaks in H NMR

Answer 47

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