Organic Chemistry And Analysis

Question 1

What does infrared spectroscopy allow us to identify?

Answer 1

IR spec allows for identification of bonds and therefore functional groups.

This is due to different bonds absorbing the IR energy and shake but cause different wavelengths of IR to pass through which can be seen on a graph.

Question 2

What is the fingerprint region on IR spec
+ what can it be used for?

Answer 2

Peaks between 1400 cm-1 - 800cm-1 are the fingerprint region

They an be matched to a know database of spectra to identify the known substance

Question 3

What does the greenhouse effects depend on ?

Answer 3

The atmospheric concentration of different gases
The ability to absorb infrared radiation
Residency time

Question 4

The different types of vibrations possible when electro-magnetic radiation is absorbed? + draw out each one

Answer 4

stretching and bending:
Symmetric stretching
Bending
Asymmetric stretching
(See notes for diagrams)

Question 5

What does the position of the IR spec peak depend on?

Answer 5

• the strength of the bonds
• masses of the atoms joined by the bonds

(Strong bonds & light atoms = absorb longer wave numbers)
(Weak bonds & heavy atoms = absorb higher wave numbers)

Question 6

Uses of infrared spec?

Answer 6

• identification of different functional groups to identify unknown molecules
• monitoring air pollution
• in breathalysers to monitor amount of ethanol in breath

Question 7

What does mass spec show?

Answer 7

The mass spectrometer detects the mass-to-charge ratio (m/z) of the molecular ions which gives the molecular mass of compound

Question 8

How do you find molecular mass from mass spec? + why might you have a peak that is one above this?

Answer 8

The highest peak on the right is the molecular ion of the compound (M+ peak).

+ a very small peak one unit after the M+ peak exists because some of the molecules will naturally contain carbon 13 isotope.

Question 9

Describe the process of fragmentation in mass spectrometer?

Answer 9

When electrons bombard molecule in mass spec energy transferred into molecule causing loss of electron.

This molecular ion can fragment “break” further forming smaller pieces (fragments) of ions, molecules, radicals. These fragment ions are what are seen as the other peaks on mass spec graph and can be used to determine the structure of the compound

Question 10

With haloalkanes why might there be two peaks for the molecular ion in mass spec?

Answer 10

One of the molecules contains the isotope 79 Br and the other contains 81 Br. Becasue the two isotopes are of similar abundance, the peaks are of similar height.
(Examples with Br but also done with Cl)

Question 11

In mass spec why are some peaks higher in abundance that others?

Answer 11

Taller peaks come from the more stable species which is why they are formed more Ortern during fragmentation

Question 12

Equations to show how NO radicals catalyse the breakdown of ozone in the stratosphere

Answer 12

Propagation:
NO* + O3 —> NO2* + O2
NO2* + O —> NO* + O2

Question 13

How do infrared radiation lead to global warming?

Answer 13

Radiation passes through the atmosphere to the earth surface, where most of it is absorbed .
Some is re-emitted in form of long-wavelength IR radiation.

Greenhouse gases absorb this long-wavelength IR radiation because they have same frequency as natural frequency of their bonds.
These molecules re-emit energy as radiation which increases the temp of atmosphere close to earth - global warming

Question 14

Sequence for identification (spectroscopy)

Answer 14

1) elemental analysis
Use percentage composition data to determine the empirical formula

2) mass spectrometry
Use molecule ion peak from a mass spectrum to determine the molecular mass; use fragments ions to identify sections of a molecule

3) infrared spectroscopy
Use absorption peaks from an infrared spectrum to identify bonds and function groups present in the molecule

Question 15

How is proton NMR calibrated/
what is the standard reference chemical in which all chemical shifts are measured?

+why is it used?

Answer 15

Tetramethylsilane (TMS) —— (CH3)4Si
Standard reference as chemical shift value of O ppm.

+ only gives one signal, non-toxic, inert, low boiling point so easily removed from sample afterwards, signal further right than most of the signals from organic compounds

Question 16

What do proton NRM chemical shifts depend on?

Answer 16

• shift depends on what other atoms/ groups are near the H
  The close the H is to electronegative atoms ( O, Cl) the greater the shift.
  The data sheet can be used to find values for chemical shifts

Question 17

Draw and describe a proton NMR spectrum?

Answer 17

• contains a series of signals (peaks) showing where the compound absorbs radiowaves.
• horizontal scale is chemical shit- this indicates how far each signal is shifted away from TMS signal
• vertical axis represents the intensity of the absorption
  (E.G if more O-H groups present then higher peak.)

Question 18

In 1H NMR (proton) what is the area of signal proportional to?

Answer 18

Signal is proportional to the number of H atoms it represents

Note: it is not the height of signal that matters but the overall area that the signal covers. For example signals with different heights and in some cases made from different number of individual peaks can still represent the same number of H atoms

Question 19

What are splitting patterns of 1H NMR?
(Also called coupling or multiplicity)

Answer 19

The number of lines a signal is split into give info about neighbouring H atoms- where they are inequivalent, and if they are, how many H atoms there are?

Question 20

How do you work out H NMR splitting pattern signal?

Answer 20

The number of lines = 1 + number of H atoms on adjacent C atoms

Question 21

Draw the splitting patterns of 1H NMR
+ name the signal
+ number of lines
+ number of H atoms/ group adjacent

Answer 21

See notes
Singlet - 1 line, no neighbouring H atoms
Doublet - 2 lines, (CH) 1 H atoms
Triplet- 3 lines, (CH2) 2 H atoms
Quartet - 4 lines, (CH3) 3 H atoms

Can also be multiples

Question 22

Differences and similarities betwee 1H NMR and 13C NMR
1) number of finals
2) position of signals
3) relative size of signals
4) splitting of signals

Answer 22

1) similar
- one single for each set of equivalent 1H or 13C atoms
2) similar
- the closer the atom to a very electronegative atom and/ or double bond, the greater the chemical shift
3) different
1H NMR - relative area of signal related to relative number of 1H atoms
13 C NMR - no link between area o signal to number of 13 C atoms
4) different
1H NMR- signal split by 1H atoms on adjacent atoms
13 C NMR - no splitting

Question 23

What happens in 13C NMR if molecule is symmetrical?

Answer 23

If molecules is symmetrical or 2 carbon atoms are equivalent
They will have the same chemical environment so absorb radiation at the same chemical shift and contribute to the same peak

Question 24

Molecular formula of Benzene

Answer 24

C6H6

Question 25

Describe structure of benzene

Answer 25

Hexagonal ring of 6 carbon atoms with each carbon atom joined to 2 other carbon atoms and to 1 hydrogen atom

Question 26

Kekule model of Benzene

Answer 26

6 membered carbon ring joined by alternate single and double bond.

Question 27

Evidence to disprove Kekule’s model of benzene

Answer 27

1) lack of reactivity of benzene with bromine
- benzene does not undergo electrophilic addition reactions
- benzene does not decolourise bromine so not double bond

2) all C-C bonds were equal length
- if there were single and double bonds bond lengths would be slighlrly different

3) Hydrogenation enthalpies
If kekule strutre enthalpy change of hydrogenation would be 3 time that of cyclohexene but benzene is 152 KJmol-1 is less energy than expected

Question 28

Features of delocalised model of benzene

Answer 28

• benzene s planar, cyclic hexagonal hydrocarbon containing 6 C and 6 H
• each carbon atom used 3 of 4 electrons in bonding to 2 other Cs and one H
• each carbon atom has 1 electron in P-orbital at right angles to plane of bonded C-H atoms
• ## -

Question 29

Draw the pi system formed in benzene

Answer 29

Rings above and below the plane of the ring (see notes)

Question 30

Draw a graph to show the enthalpy of hydogenation to show benzene isn’t kekules model

Answer 30

See notes but remember hexagonal double bond enthalpy = 120
2 double bonds in the ring - 240
3 double bonds in the ring = 360

Benzene = 208 (much below expected value)

Question 31

Draw the mechanism for nitration of benzene

Answer 31

See mechanism sheets
Reagents =
Mechanism =

Question 32

Why is nitration of benzene so useful?

Answer 32

Nitrobenzene is the start of important chain reactions to produce most benzene derivatives

Question 33

Draw the mechanisms for halogenation of benzene

Answer 33

See sheets
Reagents =
Conditions =
Equation =
Mechanism

Question 34

What compound is used as a halogen carrier for benzene reactions

Answer 34

Anhydrous Aluminium Chloride (AlCl3)

Question 35

Why does AlCl3 act as a halogen carrier

Answer 35

It increase the polarisation of the halogen (e.g Cl— Cl) bond
This makes the change on C more positive and the following occurs to form an electrophile

Question 36

Why is the halogenation of benzene more difficult than of alkenes

Answer 36

The conditions are much tougher than with alkenes because the delocalised system makes benzene more stable and less reactive

Question 37

What is alkylation

Answer 37

Substitute with an alkyl (methyl, ethyl) group

Question 38

Draw the mechanism for the alkylation of benzene

Answer 38

See cards

Question 39

What is acylation

Answer 39

Substitutes with an acyl (e.g. ethanoyl) (RCOCl) group

Question 40

Draw the mechanism for the acylation of benzene

Answer 40

See notes

Question 41

Compare the reactivity of benzene and alkenes

Answer 41

ALKENE
1) electrophilic addition
2) between C-C bond, 2 e- form a sigma bond and 2 e- form a localised pi bond
3) the pi bond has higher electron density than benzene
4) H polarises electrophiles more
5) alkenes do not need a halogen carrier

Benzene
1) electrophilic substitution
2) between each C bond 3 e- form C-C and C-H sigma bonds, 1 e- forms the delocalised ring has a lower e- density than alkenes
4) polarises electrophiles less than alkenes
5) requires halogen carrier

Question 42

What is a phenol?

Answer 42

An organic compound containing a benzene ring with an oh group attached directly

Question 43

Chemical properties of phenols

Answer 43

-They are weak acids
-They can be neutralised in a reaction with NaOH but not react with carbonates.
- they are more reactive than benzene

Question 44

Order of acidity with phenols, alcohols, carboxylic acids

Answer 44

Carboxylic acids > phenols > alcohols

Question 45

Explain why phenol is more reactive than benzene?

Answer 45

The lone pair of electrons from the oxygen (hydroxyl group attached)donates its electrons into the delocalised ring

This increases the electron density around the benzene ring because there are more electrons present
This allows H to dissociate more easily/ attracts other compound more readily increasing reactivity

Question 46

Bromination test with phenols?

Answer 46

Phenols can react with bromine water during electrophilic substitution.
- this causes decolouration of Br unlike with benzene.
- if 2,4,6-tribromophenol is made then a white precipitate will also form this separates it from other alkenes

Question 47

Nitration of benzene compared with phenols

Answer 47

Benzene reacts very slowly with HNO3 so electrophile (NO2+) is formed by resting with H2SO4

Phenol due to its increased reactivity reacts readily with HNO3 without need of catalyst.
In the nitration of phenol 2 produces produced = 2-nitrophenol and 4-nitrophenol

Question 48

The 2 types of directing groups on benzene?

Answer 48

2 and 4 positions = electron donating groups such as OH and NH2

3- directing = electron withdrawing groups such as NO2

Question 49

Why is directing effects useful in aromatic compounds

Answer 49

Directing effects can be used to predict substitution products. This is very important in organic synthesis as it allows you to control the structure of products

Question 50

Reaction of the reduction of nitrobenzene?
+conditions
+reagents
+equation

Answer 50

Reagents
> tin and conc hydrochloric acid

Conditions
> under reflux

Equation
> C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O

Question 51

Draw a diagram to show the bonding of phenol compared to the bonding of benzene

Answer 51

Benzene=
Delocalised ring above and below the benzene ring

Phenol=
Larger delocalised ring with a stretched arm over the oxygen group attached
(See notes for diagram)

Question 52

explain what is meant by the term delocalised pi-bond electrons?
(2 marks)

Answer 52

Delocalised electrons are electrons spread over more than 2 atoms .
Pi-bond is formed by the overlap of P-orbitals

Question 53

Name two commercially important materials whose manufacture involves the nitration of benzene?
(4 points)

Answer 53

-fibres
- dyes
- explosives
- pharmaceuticals

Question 54

the benzene ring and he intermediate ring have different structures. Both structures have pi-bonds.
deduce how many electrons are involved in the pi-bonding in each structure and describe how their arrangements are different?
(5 marks)

Answer 54

1. Pi-bonding electrons are delocalised
2. 6 pi-electrons in benzene
3. 4 pi-bond electrons in the intermediate
4. Pi-electrons are not over one carbon/over five carbon atoms/ p-orbitals in the intermediate
5. Pi-electrons are over the complete ring/ all around the ring all 6 carbon atoms/ p-orbitals overlapping

Question 55

in contrast to benzene, the reaction of an alkene with bromine does not need a halogen carrier
compare the different reactivities of benzene and alkenes toward chlorine
(3 marks)

Answer 55

1) In benzene, pi-electrons are delocalised

2) In alkenes, pi-electrons are concentrated between 2 carbons

3) Electrophiles attracted to more electron dense area in alkenes

Question 56

compare the reagents and conditions for nitration of phenol with those used for the nitration of benzene
state and explain the effect of the -OH group on the reactivity of the benzene ring in phenol
(7 marks)

Answer 56

conditions for nitration of benzene
1) HNO3 is conc
2) Conc H2SO4 is present
3) heated or stated temp above 50oc

explanation for greater reactivity of phenol
4) lone pair from O atom is delocalised into the ring
5) greater electron density around the ring
6) the benzene ring in phenol is activated
7) attracts electrophiles/ makes it more susceptible to electrophiles

Question 57

Why does benzene not undergo electrophilic addition reactions and instead undergoes electrophilic substitution?

Answer 57

Benzene resists electrophilic addition reactions since this would involve breaking up the stable delocalised ring of electrons.