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Flashcards in Standard Questions Deck (37):
1

Outline a simple chemical test to detect the presence of a carbonyl and determine its identity

Add 2,4-Dinitrophenylhydrazine (2,4-DNP)

 Orange Precipitate hydrazone will form if a carbonyl is present (Aldehyde or Ketone)

 To determine the identity purify the hydrazone precipitate by recrystallization

 Measure the melting point of the purified precipitate

 Compare the melting point with data table values to identify the aldehyde or ketone

2

Outline a simple chemical test to distinguish between a Ketone and an aldehyde

 Add Tollens Reagent (Ammoniacal Silver Nitrate) Warm in water bath to 60°C

 Aldehyde – A silver mirror will form as Silver reduced Ag+(aq) + e-  Ag(s)

 Ketone – No visible observation

3

Why does Sodium Borohydride have to be aqueous to act as a reducing agent?

 It dissolves in water so the ions dissociate NaBH4 -> Na+ + BH4

 Dative bond breaks releasing Hydride ions (H-)in solution

4

What type of reaction takes place in reduction of a carbonyl and what type of bond fission occurs?

 Nucleophilic addition; Heterolytic fission as both electrons go to one atom

5

Describe in words the mechanism of reduction of a carbonyl

 Nucleophile is attracted to δ+ C

 The lone pair of electrons is donated to form a dative bond

 The π electron pair goes to the oxygen atom

 Causing the π bond to break

6

Why are short chain (

 Highly polar C=O bond and O-H bonds in carboxylic acid molecules for H bonds with polar water molecules

 As the number of carbons increases a greater proportion of the molecule is non polar which is hydrophobic. This does not attract H2O and cannot form H bonds with H2O molecules.

 The more O’s with lone pairs on a compound the more soluble

7

Describe a simple chemical test to differentiate a carboxylic acid from phenol and an alcohol

 Add a carbonate, the carboxylic acid will react the others wont

8

Why are saturated fats solid at room temperature?

 Higher melting point

 More intermolecular forces between molecules  More energy required to overcome intermolecular forces

9

Why do unsaturated fats have lower melting points than saturated fats?

 Lower Boiling point

 Less intermolecular forces between molecules #

 Less energy to overcome

10

Explain why triglycerides are soluble in non-polar solvents and not in water

 There are Van Der Waals forces between triglycerides

 There is Van Der Waals forces between triglycerides and non-polar solvent

 Triglycerides cannot hydrogen bond to water (enough)

 Because there are not enough suitable oxygen atoms/Long Hydrocarbons chains

11

Why do bromine atoms substitute the H atoms on positions 2, 4 and 6 in a reaction with phenol?

 Most reactive hydrogens due to the position of the highly electronegative O atom

12

Uses of Phenols

 Plastics, Antiseptics, disinfectants & Resins for Plants

13

What can be deduced from an a 13C NMR spectrum

 The different types of carbon present, from chemical shift values

 The possible structures of the molecule

14

What can be deduced from a High resolution Proton NMR spectrum

 TMS is a standard for chemical shift measurements and produces a peak at δ=0 (ppm)

15

What information can be deduced from an integration trace

 Relative number of protons in each environment

16

Why are deuterated solvents used? State an example

 So they do not produce a chemical shift/peak an example is CDCl3

17

What is the use of MRI and what is its relation to NMR?

 MRI uses the same technology as NMR to obtain diagnostic information about internal structures in body scanners

18

Describe a proton exchange using D2O

 A proton NMR spectrum is run on the sample of the compound under investigation

 A small amount of D2O is added to another sample; and another scan run

 You can now compare the two spectra produced, to identify the –NH, -OH by identifying which peak/s are not present in the second spectra as these will have been removed

19

Explain how D2O can be used to confirm which protons are responsible for peak ‘x’

 D replaces OH (or NH) proton/Protons are labile

 The Peak for OH (or NH) protons disappears

20

Describe and explain the different ways that a high resolution NMR spectrum can give information of a molecule

 Chemical Shift values give the type of proton environment

 The number of peaks gives the number of different proton environments

 Relative peak areas give the number of protons in each environment

 Splitting gives the number of adjacent protons o Using n+1 rule so e.g. Doublet means one proton on adjacent carbon

 D2O can be used to identify –OH and –NH protons

21

In TLC how can you use the chromatogram to identify the compounds

 Measure how far each spot travels relative to the solvent front

 Calculate the Rf value Using Equation 𝑅𝑓 = 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑 𝑓𝑟𝑜𝑚 𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑏𝑦 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡/ 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑚𝑜𝑣𝑒𝑑 𝑓𝑟𝑜𝑚 𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑏𝑦 𝑆𝑜𝑙𝑣𝑒𝑛𝑡 𝑓𝑟𝑜𝑛𝑡

 Compare the Rf value found with data table values for the type of compound

22

State disadvantages of chromatography

 Similar compounds have similar retention times/Rf values due to similar structutes

 Unknown compounds have no reference retention times/Rf values

 It can be difficult to find a solvent that separates all the components in a mixture (too soluble will not retain on the stationary phase and if not soluble enough then components will hardly move)

23

Discuss the advantages of GC-MS (Gas Chromatography-Mass Spectroscopy)

 Provides a far more analytical tool than chromatography alone

 Gas chromatography separates but cannot conclusively identify components

 Gas Chromatography can give the relative proportions from the relative peak areas

 Mass Spectroscopy identifies structures by a computer comparing fragment ions with a spectral database

24

Describe (Solid) Thin Layer Chromatography (TLC)

 Stationary Phase: Solid, Separation by adsorption

 Mobile Phase: Liquid e.g. Solvent

25

Describe (Liquid or Gas) Gas Chromatography

 Stationary Phase: Liquid or Solid (on a solid support) separation by relative solubility into liquid or adsorption onto solid

26

What are disadvantages of nitration of hagenoalkane; what is a way to minimize this?

 Further substation can occur forming secondary and tertiary amines
 Use an excess if NH3 to ensure this acts as nucleophile

27

Why do Amines act as bases?

 Lone pair of electrons on N accepts a proton/H+

 Lone pair of electrons on N can be donated to H+;H+ accepts lone pair

 Forms Dative coordinate bond

28

Why must a diazonium salt/ion be formed at a temperature

 Diazonium salt/ion is very unstable and will readily hydrolyses to form phenol

29

Why must HNO2 be created in Situ using HCl and NaNO2 when forming a diazonium salt/ion?

 Nitrous acid decomposes very readily and so is always made in situ

30

Suggest how the carboxylic acid group and the primary amine group are able to react with each

 -COOH donates H+ ion to amine group

 N of amine group donate lone pair of electrons to H+

 H+ from acid accepts lone pair

 Dative coordinate bond is formed

31

What are the conditions for there to be e/z isomerism?

 C=C which cannot rotate

 Two different groups attached to each C of the C=C

 Two of the substituent groups on either side of the C=C are the same

32

What are the conditions for there to be a chiral centre?

 Four different groups attached

33

Advantages of a Single Optically pure isomer

 Fewer Side effects

 Increases the Pharmacological activity

 Reduces the need/cost/difficulty in separating stereoisomers

 Drug Doses are reduced

34

Method to prepare a single isomer

 Using enzymes as biological catalysts – As Single isomers are made naturally in living systems, enzymes are being employed to mimic these natural processes

 Chiral Synthesis – Starting with naturally occurring chiral molecules such as amino acids and sugars as starting materials can lead to production of one of a desired optical isomer

 Chiral Catalyst – Such as a transition metal complex resulting in the formation of one optical isomer

 Chiral Synthesis

35

Why are biodegradable polymers increasing in importance?

 Minimize environmental waste and Reduce Landfill Waste

 Polymers produces toxic gases when burnt

36

Why are condensation polymers biodegradable?

 May be hydrolysed at the ester or amide group

 May be photodegradable due to the C=O bond which absorbs radiation

37

What are uses of Kevlar?

 Kevlar is very strong/fire resistant so can be used as protective clothing for firefighters or bullet proof vests