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A-Level Chemistry > Chemistry 6 - Organic Chemistry and Analysis > Flashcards

Chemistry 6 - Organic Chemistry and Analysis Flashcards

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1
Q

What is the structural formula of benzene?

A

C₆H₆

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2
Q

What is the empirical formula of benzene?

A

CH

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3
Q

What is the state of benzene at room temperature and pressure?

A

Liquid

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4
Q

What is the bond angle of benzene?

A

120°

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5
Q

What are the three features of benzene that don’t support Kekulé’s model?

A
  • Benzene is resistant to addition reactions
  • The enthalpy change of hydrogenation of benzene is more stable than predicted.
  • All the carbon-carbon bonds are the same length.
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6
Q

What technique was used to find the bond lengths of benzene?

A

X-ray diffraction.

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7
Q

What happens to the 4th electron in the p-orbital of each carbon atom in benzene?

A

It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.

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8
Q

How do the rings of electron density affect the stability of benzene?

A

They make benzene very stable, even though it is unsaturated (aromatic stability).

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9
Q

What is the difference between cyclohexa-1,3,5-triene (Kekulé model) and benzene

A

Benzene’s C atom’s are all held together by single covalent bonds and the delocalised π-electron system, whereas cyclohexa-1,3,5-triene’s C atoms are held together by alternating single and double covalent bonds.

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10
Q

Why does benzene have a relatively high melting point?

A

There is a close pacing of the flat hexagonal molecules when solid.

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11
Q

Is benzene soluble in water? Explain why?

A

No, because it is non-polar.

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12
Q

What are the dangers of benzene? Why is it not used in schools?

A

It is a carcinogen.

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13
Q

How do you name compounds containing a benzene ring?

A

-benzene, or phenyl-; it can designate position on ring using numbers if there is one more than one substituent.

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14
Q

Why is benzene attacked by electrophiles?

A

The high electron density above/bellow the benzene ring due to the delocalised π-electrons.

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15
Q

Nitration of benzene is what type of reaction?

A

Electrophilic substitution reaction.

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16
Q

What ion is used to nitrate benzene?

A

NO₂⁺

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17
Q

What is the catalyst in the nitration of benzene?

A

Sulfuric acid.

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18
Q

How can NO₂⁺ ions be generated for the nitration of benzene?

A

With concentrated H₂SO₄ and HNO₃.
H₂SO₄+HNO₃→H₂NO₃⁺+HSO₄⁻
H₂NO₃⁺→H₂O+NO₂⁺
Overall: H₂SO₄+HNO₃→HSO₄⁻+H₂O+NO₂⁺

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19
Q

How is the H₂SO₄ catalyst regenerated in the nitration of benzene?

A

HSO₄⁻+H⁺→H₂SO₄ (the H⁺ is from the benzene ring)

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20
Q

What type of catalyst is used for a Friedel-Crafts reaction?

A

A halogen carrier (such as AlCl₃)

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21
Q

Why does benzene not react directly with halogens?

A

The aromatic ring is too stable.

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22
Q

What is happening when AlCl₄⁻ is formed in terms of electrons?

A

The lone pair of electrons on the chlorine atom forms a co-ordinate bond to Al.

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23
Q

How is the AlCl₃ catalyst reformed?

A

AlCl₄⁻+H⁺→HCl+AlCl₃ (The H⁺ is from the benzene)

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24
Q

How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

A

Use a halogenoalkane and AlCl₃ to create an electrophile that can attack benzene.

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25
Q

What is phenol?

A

A benzene ring where one of the H atoms has been substituted with an -OH group.

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26
Q

What reactions can you carry out to show the weak acidity of phenol?

A

A neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates.

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27
Q

Write the equation of the reaction between phenol and bromine to form 2,4,6-tribomophenol?

A

C₆H₅OH+3Br₂→C₆H₂Br₃OH+3HBr

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28
Q

Write the equation for the reaction between phenol and dilute nitric acid.

A

C₆H₅OH+HNO₃→C₆H₄NO₂OH+H₂O

C₆H₄NO₂OH can be as either 2-nitrophenol (o-nitrophenol) or as 4-nitrophenol (p-nitrophenol).

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29
Q

What is the relative ease of electrophilic substitution of phenol compared to benzene and why?

A

It is easier for electrophilic substitution to occur with phenol because oxygen lone pair of electrons from the -OH group are partially delocalised into the ring of the delocalised π-electron system. Therefore, this increases the electron density of the ring, thus electrophiles are more attracted to phenol.

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30
Q

What is the directing effect of electron donating groups OH and NH₂?

A

They direct the group to the 2 and 4 position of the ring in electrophilic substitution of aromatic compounds.

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31
Q

What is the directing effect of the electron withdrawing group NO₂?

A

NO₂ directs atoms to the 3 position of the ring in electrophilic substitution of aromatic compounds.

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32
Q

What is a carbonyl group?

A

C=O

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33
Q

What is the functional group and general formula for an aldehyde?

A

RCHO (The C is double bonded to O and single bonded to the H and R)

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34
Q

What is the functional group for a ketone?

A

RCOR’ (The C is double bonded to O and single bonded to the R and R’)

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35
Q

How do you name aldehydes?

A

-al suffix (C=O is on the end of a chain).

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36
Q

How do you name ketones?

A

-one suffix (designate the number for which carbon C=O is on).

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37
Q

What kind of intermolecular forces do molecules with the carbonyl group have? Why?

A

Permanent dipole-dipole due to the polar C=O bond (O is δ-).

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38
Q

How soluble are molecules with the carbonyl group? What influences their solubility?

A

Yes, they are soluble, they form hydrogen bonds between the molecules of water and the oxygen of the C=O group. As the C chain length increases, however, solubility decreases.

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39
Q

Which bond in carbonyl compounds is usually involved in reactions? Why?

A

C=O, due to the polarity of the bond (there is a large difference in electronegativity between C and O)

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40
Q

What is the strongest bond in carbonyl compounds?

A

C=O

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41
Q

Why is the addition of HCN to a carbonyl important?

A

It increases the length of the carbon chain by one atom, allowing more useful molecules to be made.

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42
Q

Will the product of HCN added to a carbonyl compound have optical isomers? Why?

A

Yes they will. In the aldehyde/ketone, the carbonyl carbon is planar, so the :CN⁻ can attack from either above or below, forming two enantiomers.

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43
Q

What is the name of the product when HCN is added to a carbonyl compound?

A

Hydroxynitriles (they have OH and CN groups).

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44
Q

What is Fehling’s solution? What colour is it?

A

Copper complex ions, blue.

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45
Q

What happens when an aldehyde is added to Fehling’s solution?

A

The Fehling’s solution is reduced to Cu⁺ ions and there is a colour change to a brick red precipitate.

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46
Q

What happens when a ketone is added to Fehling’s solution?

A

There is no visible change, so the solution stays blue.

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47
Q

How do you test for a carbonyl compound?

A

Use Brady’s reagent- 2,4-dinitrophenylhydrazine.

If a carbonyl compound is present an orange precipitate is formed.

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48
Q

What is in Tollens’ reagent?

A

Silver complex ions in colourless solution.

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49
Q

What happens when an aldehyde is added to Tollens’ reagent?

A

A silver mirror forms as Ag⁺ reduced to Ag(s).

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50
Q

What happens when a ketone is added to Tollens’ reagent?

A

No visible change.

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51
Q

What is another oxidising agent for alcohols and aldehydes? What change in colour does this undergo?

A

Acidified potassium dichromate (VI)- H₂SO₄ and K₂Cr₂O₇. Colour change from orange to green.

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52
Q

What is a reducing agent for aldehydes and ketones? What ions does this release in solution?

A

NaBH₄ (sodium tetrahydridoborate(III)), it releases an H⁻ ion.

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53
Q

What type of reaction is the reduction of an aldehyde?

A

Nucleophilic addition.

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54
Q

Write an equation for the reduction of pentan-2-one and for 3-methylbutanal.

A

CH₃COCH₂CH₂CH₃+2[H]→CH₃CH(OH)CH₂CH₂CH₃

CH₃CH₂CH₂(CH₃)CHO+2[H]→CH₃CH₂CH₂(CH₃)CH₂OH

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55
Q

How do you convert an aldehyde to a carboxylic acid?

A

Through oxidising the aldehyde using acidified C₂O₇²⁻.

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56
Q

What is the functional group of a carboxylic acid?

A

-COOH (C=O and C-OH)

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57
Q

How do you name carboxylic acids?

A

-oic acid suffix.

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58
Q

Are carboxylic acids soluble in water? Why? What influences their solubility?

A

Yes they are soluble. The acid group can form hydrogen bonds with water molecules. However, as the carbon chain length increases, the solubility decreases.

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59
Q

What are the intermolecular forces in carboxylic acids?

A

There are hydrogen bonds in solid state- which are very strong.

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60
Q

What are esters (what are they formed from)? Their functional group, general formula?

A

They are formed from carboxylic acids and alcohols. RCOOR’ (C=O, C-O-C).

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61
Q

Write an equation for the reaction of ethanoic acid with propan-1-ol.

A

CH₃COOH+CH₃CH₂CH₂OH→CH₃COOCH₂CH₂CH₃+H₂O

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62
Q

How do you name esters?

A

Start with the alcohol group that has replaced the hydrogen, then the acid part.
For example propyl (from the alcohol part- propanol) ethanoate (from the carboxilic acid part- ethanoic acid).

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63
Q

What characteristic physical properties do esters have?

A

Volatile, pleasant fruity smells.

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64
Q

What are some uses of esters?

A

Flavourings, perfumes (both for longer chains), solvents (short chains), plasticisers.

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65
Q

How could you distinguish carboxylic acids from other -OH containing compounds?

A

Add NaHCO₃, acids will produce sodium salt, water and carbon dioxide.

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66
Q

Write an equation for the reaction of ethanoic acid with NaOH.

A

CH₃COOH+NaOH→H₂O+CH₃COO⁻Na⁺

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67
Q

Write an equation for the reaction of ethanoic acid with Na₂CO₃

A

2CH₃COOH+Na₂CO₃→2CH₃COO⁻Na⁺+H₂O+CO₂

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68
Q

Write an equation for the reaction of ethanoic acid with calcium oxide.

A

2CH₃COOH+CaO→(CH₃COO)₂Ca+H₂O

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69
Q

What catalyst is needed for the formation of esters from alcohols and carboxylic acids?

A

Concentrated strong acid, such as H₂SO₄.

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70
Q

What catalyst is needed for the hydrolysis of esters?

A

Dilute strong acid, such as H₂SO₄.

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71
Q

What is an alternative method of hydrolysis?

A

Base hydrolysis.

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72
Q

What are the advantages of base hydrolysis?

A

Reaction goes to completion due to neutralisation by base- more product in the mixture than acid catalysed hydrolysis.

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73
Q

What are carboxylic acid derivatives?

A

Molecules that have the acyl group as part of their structure, formed from carboxylic acids.

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74
Q

Name two acid derivatives and give their functional groups.

A

Acyl chlorides: RCOCl

Acid anhydrides: RCOOCER/(RCO)₂O

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75
Q

How do you form an acyl chloride?

A

React a carboxylic acid with SOCl₂.

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76
Q

If the nucleophile is ammonia for the acylation of acyl chlorides or acid anhydrides, what are the products of the reaction?

A

An amide.

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77
Q

Write an equation for the reaction of ethanoyl chloride and ammonia.

A

CH₃COCl+2NH₃→CH₃CONH₂+NH₄Cl

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78
Q

If the nucleophile is a primary amine, what are the products of the acylation of acyl chlorides or acid anhydrides?

A

N-substituted amide.

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79
Q

Write an equation for the reaction of ethanoyl chloride and methylamine.

A

CH₃COCl+CH₃CONHCH₃+CH₃NH₃Cl

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80
Q

If the nucleophile is an alcohol, what are the products of the acylation of acyl chlorides or acid anhydrides?

A

An ester.

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81
Q

Write an equation for the reaction of ethanoyl chloride and ethanol.

A

CH₃COCl+CH₃CH₂OH→CH₃COOCH₂CH₃+HCl

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82
Q

If the nucleophile is water, what are the products of the acylation of acyl chlorides or acid anhydrides? What is the name of this reaction?

A 
Carboxylic acid (hydrolysis produces an ester linkage).
This is a hydrolysis reaction.
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83
Q

Write an equation for the reaction of ethanoyl chloride and water.

A

CH₃COCl+H₂O→CH₃COOH+HCl

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84
Q 
Describe the structures of :
Ammonia
A primary amine
A secondary amine
A tertiary amine
A quaternary ammonium ion
A

Ammonia has zero carbons, three hydrogen atoms covalently bonded to the nitrogen, which has a lone pair of electrons.
Primary amines have a single carbon group and two hydrogen atoms covalently bonded to the nitrogen, which has a lone pair of electrons.
Secondary amines have two carbon groups and a hydrogen atom covalently bonded to the nitrogen, which has a lone pair of electrons.
Tertiary amines have three carbon groups and no hydrogen atoms covalently bonded to the nitrogen, which has a lone pair of electrons.
Quaternary amines have a four carbon groups and no hydrogen atoms covalently bonded, one of the bonds is dative, to the nitrogen, which is positively charged.

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85
Q

How do you name amines?

A

-amine suffix or amino- prefix.

86
Q

Why are amines so reactive?

A

The lone pair of electrons on the nitrogen, and the highly polar N-H bonds present in many amines.

87
Q

What shape are amines around the N? What is the bond angle?

A

Trigonal pyramidal, 107° due to the lone pair on N.

88
Q

What kind of intermolecular forces do amines have? Why?

A

Hydrogen bonding due to the polar N-H bond and the lone pair of electrons on the N atom.

89
Q

Are the intermolecular forces of amines stronger or weaker than alcohols? Why?

A

Weaker, since N has a lower electronegativity than O, which leads to weaker hydrogen bonding.

90
Q

How can/when do amines act as bases?

A

The lone pair on the nitrogen atom accepts a proton.

91
Q

How can/when do amines act as nucleophiles?

A

When they bond with an electron-deficient C atom (they donate the lone pair from the nitrogen).

92
Q

What is the product from the basic action of an amine with water?

A

RNH₃⁺- an ammonium ion, which forms a salt with an anion.

93
Q

Write an equation for methylamine reacting with HCl.

A

H₃C-NH₂+HCl→H₃C-NH₃+Cl⁻

94
Q

In order to be the strongest base, what must a particular amine gave (out of a set of amines)?

A

The greatest electron density around the N atom, making it a better electron pair donor (so it attracts protons more)

95
Q

What effect do alkyl groups on electron density and base strength of an amine?

A

They have a positive inductive effect, which increases the electron density around N, which makes it a stronger base.

96
Q

Place the following in order if increasing base strength (in general): NH₃, 1° amine, 2° amine, phenylamine.

A

Phenylamine

97
Q

How can primary amines then 2°, 3° amines and 4° ammonium ions?

A

Through multiple substitutions; the primary amine is a nucleophile that attacks the original haloalkane, etcetera.

98
Q

What are the problems with substituting to form a primary amine?

A

Not efficient as low yield of 1° amine due to multiple substitution.

99
Q

How would you maximise the yield of the primary amine in a substitution?

A

Use excess ammonia.

100
Q

What type of mechanism is the reaction of a haloalkane with a cyanide ion?

A

Nucleophilic substitution.

101
Q

What conditions does the electrophilic substitution of a haloalkane with a cyanide ion require? What is the product formed?

A

Ethanol as a solvent

A nitrile is formed

102
Q

How do you get from a nitrile to a primary amine?

A

Reduction using nickel/hydrogen catalyst.

103
Q

Why is the reduction of nitriles to amines a purer method of synthesising primary amines?

A

Only the primary amine can be formed.

104
Q

What conditions are needed to form nitrobenzene from benzene?

A

Concentrated H₂SO₄ and HNO₃ to form the NO₂⁺ ion for the electrophilic attack.

105
Q

How do you form an ammonium chloride salt from nitrobenzene? What conditions are needed?

A

Reduce the nitrile using Tin/HCl, which forms an ammonium salt with Cl⁻ ions.
This is done at room temperature.

106
Q

Give the equation for the reaction of nitrobenzene to phenylamine.

A

C₆H₅NO₂+6[H]→C₆H₅NH₂+2H₂O

107
Q

What mechanism is used for forming amides from acyl chlorides and amines?

A

Nucleophilic addition/elimination

108
Q

What are the two functional groups of amino acids?

A

NH₂ and COOH (amino and carboxylic acid).

109
Q

What is the general formula for α-amino acids?

A

RCH(NH₂)COOH

110
Q

Are α-amino acids chiral? Why?

A

Yes, one carbon has 4 different substituents. Except glycine, where R=H.

111
Q

Define a zwitterion.

A

Ions which have both a permanent positive and negative charge, but are overall neutral.

112
Q

How do zwitterions occur in amino acids? Give the general formula of one.

A

COOH is deprotonated to COO⁻
NH₂ is protonated to NH₃⁺
RCH(N⁺H₃)COO⁻

113
Q

What happens to amino acids in acidic conditions?

A

They gain a proton on the NH₂ group (NH₂ group is protonated to NH₃⁺).

114
Q

What happens to amino acids in alkaline conditions?

A

They lose a proton from the COOH group (COOH is deprotonated to COO⁻).

115
Q

What is the peptide linkage?

A

-CONH-

The C of the COOH group bonds to the N of the NH₂ group, releasing a H₂O molecule.

116
Q

What are the structures of primary and secondary amides?

A

Primary: RCONH₂
Secondary: RCONHCH₃
(The O is part of a C=O group)

117
Q

What property must a carbon atom have for the molecule to display optical isomerism about that carbon atom?

A

4 different substituents attached to one carbon atom.

118
Q

What are the similarities and differences between two optical isomers?

A

They have the same atoms and bonds present, but they are non-superimposable mirror images of one another. As such they are not necessarily identical in chemical properties.
They differ in the way they rotate plane polarised light- they rotate the plane of polarisation by the same angle but in different directions.

119
Q

What word is used to describe optically active molecules?

A

Chiral.

120
Q

Give two examples of chiral molecules.

A

All alpha amino acids, except glycine.

Lactic acid/ 2-hydroxypropanoic acid.

121
Q

What is condensation in terms of polymers?

A

Where a small molecule is eliminated (usually H₂O) to form a larger molecule.

122
Q

How many monomers are condensation polymers usually formed from?

A

Two.

123
Q

What properties do the monomers forming condensation polymers have?

A

Each has two functional groups.

124
Q

What are some examples of condensation polymers?

A

Polyesters
Polyamides
Polypeptides

125
Q

What is the linkage in a polyester?

A

The ester linkage; RCOOR’

126
Q

What molecule is eliminated in the formation of a polyester?

A

H₂O

127
Q

What are the two monomers which form a polyester (generic names and structures)?

A

Diol and dicarboxylic acid or a molecule with both alcohol and carboxylic acid functional groups.

128
Q

What monomers is Terylene made from?

A

Benzene-1,4-dicarboxylic acid and ethane-1,2-diol

129
Q

What is Terylene used for?

A

As a fibre for making clothes.

130
Q

Which molecule is eliminated when a polyamide is formed?

A

H₂O

131
Q

What are the two monomers used to form a polyamide?

A

Diamine and dicarboxylic acid.

132
Q

What are some examples of polyamides?

A

Nylon, Kevlar

133
Q

Which monomers is Nylon-6,6 made from?

A

1,6-diaminohexane and hexanedoic acid.

134
Q

If you are making Nylon in the lap, what monomers would you use and why? What molecule is eliminated?

A

Use hexane-1,6-diacul chloride as the rate of reaction is much faster. HCl is eliminated.

135
Q

What is Kevlar used for?

A

In body armour (bullet-resistant vests, stab vests) helmets, oven gloves

136
Q

Which monomers make up Kevlar?

A

1,4-diaminobenzene and benzene-1,4-dicarboxylic acid.

137
Q

Why are poly(alkenes) not biodegradable?

A

Non-polar C-H and C-C bonds, which makes them hard to break down.

138
Q

Why is it bad to burn poly(alkenes)?

A

It releases CO₂, CO, C (soot) and other toxic chemicals from monomers.

139
Q

Where do most poly(alkenes) end up?

A

Landfill sites.

140
Q

Why can condensation polymers be broken down?

A

They have polar bonds.

141
Q

How are condensation polymers broken down?

A

Hydrolysis (the opposite of condensation).

142
Q

What is the difference between addition and condensation polymerisation?

A

Condensation makes the polymer and eliminates a small molecule; addition polymerisation breaks a C=C bond to form only one product (just the polymer).

143
Q

Explain hydrogen bonding between polyamides.

A

Both C=O and N-H are polar bonds, as nitrogen’s electronegativity is greater than hydrogen’s and oxygen’s electronegativity is greater than carbon’s.
Hydrogen bonding between Hδ+ and Oδ- in different molecules.
This uses the lone pair of electrons on the Oxygen atom.

144
Q

Why do polyesters not show hydrogen bonding?

A

All O-H bonds are removed during polymerisation.

145
Q

What type of mechanism is involved in the reaction between haloalkanes and cyanide ions?

A

Nucleophilic substitution.

146
Q

What type of reaction is involved in the reaction between carbonyl compounds and cyanide ions?

A

Nucleophilic addition.

147
Q

Write an equation for the reaction of the reduction to butanenitrile.

A

[H]=H₂/Ni

C₃H₇CN+4[H]→C₃H₇CH₂N₂

148
Q

How can you form a carboxylic acid from a nitrile?

A

Acid hydrolysis.

149
Q

What type of catalyst is used for a Friedel-Crafts reaction?

A

A halogen carrier (such as AlCl₃).

150
Q

Write an equation to form an electrophile that could be used to acylate benzene, starting with AlCl₃ and RCOCl.

A

AlCl₃+RCOCl→AlCl₄⁻+RCO⁺ (+ on C)

RCO⁺ can attack benzene.

151
Q

How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

A

Use a halogenoalkane and AlCl₃ to create an electrophile that can attack benzene.

152
Q

When purifying by recrystallisation, why is the minimum volume of hot solvent used?

A

So that a saturated solution is created, so that as many crystals will fall out of solution as possible when it is cooled.

153
Q

Why is the solution filtered hot when purifying by recrystallisation?

A

To remove insoluble impurities and ensure that the crystals do not form in the filter paper.

154
Q

Why is the solution cooled in an ice bath when purifying by recrystallisation?

A

To ensure that as many crystals as possible fall out of solution, so its yield is higher.

155
Q

Why are the crystals washed with cold water when purifying by recrystallisation?

A

To remove soluble impurities.

156
Q

How would you separate the crystals from the reaction mixture when purifying by recrystallisation?

A

By filtering under pressure using a Buchner funnel.

157
Q

Why might percentage yield for recrystallisation be below 100% (practical reasons)?

A

Product is lost during filtration, drying and weighting- spills may also occur and not all the product is transferred from one piece of apparatus to another.
Product may also be left dissolved in the solution, some doesn’t crystallise. Some may be left on the filter paper and a sample may not be completely dry.

158
Q

Describe how Quickfit apparatus is connected.

A

Grease the joints using some petroleum jelly on the inside of the joints before connecting the pieces together.

159
Q

In a distillation setup, why is it necessary to have a continuous water flow around the condenser?

A

So that the water remains cool in order for the mixture to be distilled.

160
Q

Describe a method that can be used to separate immiscible liquids.

A
  • Pour the mixture into a separating funnel and some diluted water.
  • Add the stopper and invert the flask to mix the mixture.
  • Equalise the pressure by opening the stopper as required.
  • Continue shaking until there is no “whistle” sound.
  • To collect the water in the lower layer, open the stopper and place a beaker under the spout.
  • Use another beaker to collect the desired organic layer.
  • Shake the liquid with some drying agent.
161
Q

Name two drying agents.

A
  • Magnesium sulphate

* Calcium chloride

162
Q

How is a drying agent used?

A
  • Add a selected drying agent to the organic product.
  • If the drying agent forms clumps add some more until they are moving freely.
  • Use gravity filtration to collect the dry product.
  • The filtrate is the product.
163
Q

What does re-distillation mean?

A

When a liquid is purified using multiple distillations.

164
Q

What are the three key steps to purify an organic solid?

A

1) Filtration under reduced pressure
2) Recrystallisation
3) Measurement of melting points

165
Q

What are the basic principles of all kinds of chromatography?

A

A family of separation techniques that depend on the principle that a mixture is separated if it is dissolved in a solvent and this mobile phase is passed over a solid (the stationary phase).

166
Q

What is the mobile phase?

A

Carries the soluble components of the mixture in chromatography.

167
Q

What relationship between a sample and mobile phase makes the sample move faster?

A

The more soluble the components are/the more affinity the components have with the solvent, the faster it will move.

168
Q

What does the stationary phase do?

A

It holds back components of a mixture that are attracted to it/have an affinity with it.

169
Q

What relationship between a sample and the stationary phase makes the sample move slower?

A

The more affinity a component has with the stationary phase, the slower it will move; this often involves attraction by hydrogen bonding.

170
Q

How are substances separated by chromatography?

A

If suitable stationary and mobile phases are chosen, the balance between affinity for the mobile phase and affinity for the stationary phase is different for each compound of the mixture. Thus they move at different rates and are separated over time.

171
Q

Why will different substance show different Rբ values?

A

They are bonded differently and have different polarities; more polar bonds mean longer retention times or smaller Rբ values, solvents since hydrogen bonding/dipoles are attracted more strongly to the stationary phase.

172
Q

What does TLC stand for?

A

Thin Layer Chromatography.

173
Q

What is the stationary phase in TLC?

A

Plastic/glass/metal sheet or “plate” coated in silica (SiO₂) or Alumina (Al₂O₃).

174
Q

What are the advantages of TLC over paper chromatography?

A

It runs faster.
Smaller amounts of a mixture can be separated.
TLC plates are more robust than paper.

175
Q

How can you observe colourless spots in TLC?

A

Shine a UV light on them, or spray them with a developing agent.

176
Q

Give an example of a developing agent.

A

Ninhydrin turns amino acid spots from colourless to purple so they can be seen, but it requires heating.

177
Q

How do you calculate Rբ values?

A

Rբ=distance moved by spot/distance moved by solvent front

178
Q

What does Rբ value stand for?

A

Retention factor; a measure of the rate of movement of a component through the chromatography apparatus; a ratio between the rate of movement of the solvent and that component.

179
Q

How could you confirm the identity of a substance from its Rբ value?

A

Compare your Rբ value to accepted values of Rբ for that substance run in the same solvent and set-up; if they match then the substance’s identity is confirmed.

180
Q

What is the stationary phase in gas-liquid chromatography?

A

Powder, coated with oil. Packed into a long, thin capillary tube (100m long, 0.5mm diameter).
Coiled and placed in an oven, the temperature of which can be varied.

181
Q

What is the mobile phase in gas-liquid chromatography?

A

A carrier gas, usually inert, like N₂ or He.

182
Q

What do you measure in gas-liquid chromatography?

A

Retention time; different components of the mixture take different amounts of time to move through the chromatography apparatus.

183
Q

What are the advantages of GLC?

A

It is very sensitive; GLC can detect minute traces of substances in foodstuffs and link oil pollution on beaches to the specific tanker the oil came from.

184
Q

What are GLC’s uses?

A

Testing blood and urine for drugs.

185
Q

How can you use GC or GCMS to identify substances?

A

Match gas Chromatograph to that of a known substance under the same conditions; retention times should exactly match. The substance’s identity can be confirmed by mass spectrometry, NMR or infrared spectroscopy.

186
Q

How does GCMS work?

A

Gas chromatography is run and the retention time is recorded, then the mixture is run through a mass spectrometer. The fragmentation pattern/molecular ion peak confirms the substance’s identity.

187
Q

How do you test for alkenes? What is the result?

A

Shake it with bromine water, the result is bromine water is decolourised (from orange to colourless).

188
Q

How do you test for haloalkanes? What is the result?

A

Add NaOH(aq) and warm the mixture, acidify with HNO₃, add AgNO₃(aq)
The result is:
for a chloroalkane; A white precipitate of AgCl that is soluble in dilute and concentrated aqueous ammonia.
for a bromoalkane; A cream precipitate of AgBr that is soluble in concentrated aqueous ammonia.
for a iodoalkane; a yellow precipitate that is insoluble in aqueous ammonia.

189
Q

How do you test for alcohols? What is the result?

A

Add acidified K₂Cr₂O₇ (potassium dichromate(VI)) and heat.
The result is a colour change from orange to green for primary and secondary alcohols, there is no change for a tertiary alcohol.

190
Q

How do you test for aldehydes? What is the result?

A

The first method is to warm it with Fehling’s solution, the result is a brick red precipitate forms (from the blue solution).
The second method is to warm with Tollens reagent, the result is a “silver mirror” (Ag(s) precipitate) forms.

191
Q

How do you test for carboxylic acids? What is the result?

A

Add Na₂CO₃(aq), the result is that CO₂(g) is effervesced (given off).

192
Q

How do you test for phenols?

A

By weak acidity, there is a neutralisation reaction reacted with NaOH but no reaction with CO₃²⁻.

193
Q

How do you test for carbonyl compounds?

A

React it with 2,4-DNP and an orange precipitate should form.

194
Q

What does NMR stand for?

A

Nuclear magnetic resonance.

195
Q

What are the basic principles of NMR?

A

You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them. If radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel. This energy change can be monitored and recorded. It uses the resonance of nuclei with spin.

196
Q

Give one use of NMR?

A

MRI scans.

197
Q

What kind of nuclei does NMR work with?

A

Those with an uneven number of nucleons, meaning they will spin ¹H and ¹³C.

198
Q 
Summarise what these mean for ¹³C NMR:
•Number of signals
•Chemical shift
•Area under peak
•Splitting
A
  • Number of signals: One signal for each carbon environment (each set of inequivalent ¹³C atoms)
  • Chemical shift: grater shift from atoms closer to electronegative atoms or C=C
  • Area under peak: no meaning
  • Splitting: there is no splitting for ¹³C NMR.
199
Q

Why is it easier to get a spectrum of ¹H NMR than ¹³C NMR?

A

Most H atoms are ¹H- it is much more abundant than ¹³C . This means that almost all H atoms have spin so they show up.

200
Q

On a low resolution spectrum, what peaks would you expect to see for H NMR?

A

One peak for each set of inequivalent H atoms (each chemical environment shows 1 peak).

201
Q

What does the area under the peak represent for H NMR?

A

The area under the peak is proportional to the number of ¹H atoms represented by the peak.

202
Q

What is the integration trace?

A

A stepped line that makes it easier to measure the area under the curve (the height of the line=area under that peak).

203
Q

What is TMS (name and structure)?

A

Tetramethylsilane

Si(CH₃)₄

204
Q

Why is TMS used?

A

It can be added to a sample to calibrate the NMR equipment. It provides a peak at exactly δ=0ppm. It is the reference point against which all δ are measured.

205
Q

What are the other advantages of using TMS?

A

Inert, non-toxic, easy to remove from the sample (as it is relatively volatile).

206
Q

When does splitting/spin-spin coupling occur?

A

Neighbouring hydrogen atoms (3 or fewer bonds away, or on the adjacent carbon) affect the magnetic field of ¹H atoms and causes their peaks to split.

207
Q

What is the n+1 rule?

A

If there are n inequivalent ¹H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks.

208
Q

Why must solvents used for ¹H NMR not contain any hydrogen atoms?

A

Signals from the solvent would swamp signals from the sample, as there is much more solvent than sample.

209
Q

Which solvents are used for ¹H NMR?

A

Deuterated solvents CDCl₃, D₂O, C₆D₆

CCl₄ - tetrachloromethane

210
Q 
Summarise what these indicate for ¹H NMR:
•Number of signals
•Chemical shift
•Splitting
•Area under peak
A
  • Number of signals: One main signal for each set of inequivalent ¹H atoms (for each hydrogen environment)
  • Chemical shift: larger δ for ¹H atoms closer to electronegative atoms or C=C
  • Splitting: number of smaller peaks = 1+number of inequivalent hydrogen atoms 3 bonds away
  • Area under peak: proportional to the number of atoms represented by that peak
211
Q

Why does the peak from O-H bonds disappear if D₂O is used as a solvent?

A

An O-D bond is formed in preference to O-H due to labile protons that move/swap from one molecule to another.

A-Level Chemistry (6 decks)

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