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A levels > Chemistry - Rings, Polymers and Analysis > Flashcards

Chemistry - Rings, Polymers and Analysis Flashcards

0
Q

Define an Arene

A

A hydrocarbon with a ring of 6 carbon atoms and a delocalised π system of electrons e.g. Benzene

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1
Q

Define an Aromatic Compound

A

A compound containing a benzene ring (an arene)

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2
Q

What evidence is there to suggest that the Kekule formula of Benzene is not correct?

A

1) Benzene does NOT DECOLOURISE BROMINE IN THE DARK
2) ALL THE CARBON-CARBON BONDS ARE THE SAME LENGTH c=c bonds are shorter than c-c bonds
3) HYDROGENATION of benzene is LESS EXOTHERMIC than expected (hydrogenation of trienes have ∆H = -360, benzene has ∆H of -208

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2
Q

What does benzene’s ∆H show about its bonding?

A

MORE STABLE π BONDS than expected - due to DELOCALISED ELECTRONS

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3
Q

How reactive is benzene compared to alkenes, and why is this?

A

1) LESS REACTIVE
2) π-electrons DELOCALISED - spread out more - LOW ELECTRON DENSITY - LESS ATTRACTIVE THE ELECTROPHILES

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5
Q

Why are all of benzenes carbon-carbon bonds the same length?

A

LOW ELECTRON DENSITY - DELOCALISED electrons are spread out over the ring

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6
Q

Why does benzene have a lower ∆H than expected?

A

DELOCALISATION LOWERS ENERGY OF ∏-ELECTRONS

more stable state than isolated π-bonds - more energy is needed to break them - less favourable

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7
Q

What is the name of the mechanism where electrophiles react with aromatic rings?

A

ELECTROPHILIC SUBSTITUTION

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8
Q

Why don’t Arenes react with electrophiles by addition?

A

1) LESS REACTIVE than alkenes - delocalisation lowers electron density - electrophiles are less attracted 2) SUBSTITUTION MAINTAINS THE DELOCALISED ∏ - SYSTEM - more favourable

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9
Q

What is the overall equation for the Nitration of Benzene?

A

C6H6 + HNO3 -> C6H5NO2 + H2O

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10
Q

What conditions are necessary for the nitration of benzene?

A

50-60ºC CONC NITRIC AND CONC SULPHURIC ACID

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11
Q

How is the electrophile formed for the nitration of benzene?

A

HNO3 + H2SO4 (NO2)+ + H2O + HSO4-

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12
Q

Describe benzene’s structure:

A

1) PLANAR - all atoms on same plane, all c-c bond lengths the same 2) Each C forms 3 SIGMA BONDS - 2 to adjacent carbons, 1 to H 3) each carbon has 1 electron in a P ORBITAL above and below plane 4) P orbitals overlap and form a DELOCALISED ∏ - ELECTRON SYSTEM (cloud)

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13
Q

Describe the mechanism of the reaction of Nitric acid with benzene:

A

1) Curly arrow from inner circle edge of benzene ring to N on (NO2)+ 2) a) H and NO2 both bonded to one carbon b) inner circle now semicircle, +ve charge in circle opposite bonded c c) Curly arrow from H bond to where the circle edge was below bonded c 3) products: C6H5NO2 + H+

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14
Q

How is the catalyst regenerated in the Nitration of Benzene?

A

H+ formed in reaction combines with HSO4- formed in formation of electrophile HSO4- + H+ -> H2SO4

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15
Q

What conditions are necessary for the Halogenation of Benzene?

A

HALOGEN CARRIER catalyst AlBr3 or FeBr3

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16
Q

How does the halogen carrier catalyst aid halogenation of benzene?

A

Polarises the halogen -> stronger electrophile

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17
Q

Give the reaction for the formation of the electrophile in the halogenation of benzene:

A

1) Br2 + AlBr3 Br+ + (AlBr4)-

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18
Q

Describe the mechanism by which the Bromination of Benzene occurs:

A

1) Curly arrow from circle in Benzene ring to Br+ 2) a) H and Br bonded to same carbon in ring b) semicircle formed with + inside ring opposite bonded c c) curly arrow from H bond to where ring would be by bonded c 3) C6H5Br and H+ formed

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19
Q

How is the Halogen Carrier Catalyst reformed?

A

H+ released in reaction reacts with AlBr4- formed in generation of electrophile H+ + (AlBr4)- -> AlBr3 + HBr

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20
Q

What state is phenol at rtp, and what is the reason for this?

A

Solid - large surface area= lots of van der waals forces

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21
Q

How soluble is phenol in water?

A

Slightly soluble - OH group can form H bonds, but large non-polar surface area cannot

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22
Q

Why is phenol more acidic than water or ethanol?

A

The negative charge on the O of the Phenoxide ion is delocalised into the π - system of the aromatic ring, making the ion more stable - e.g. more favourable to the dissociation of H+. Ethanol and water do not have a π-system, and so the charge cannot be delocalised

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23
Q

Give an equation for the reaction of Phenol with Sodium Hydroxide:

A

C6H5OH(aq) + NaOH(aq) -> C6H5O-Na(aq)+ + H2O(l)

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24
Q

What are the properties of Phenoxide Salts?

A

1) VERY SOLUBLE in water - ion-dipole associations -> phenol more soluble in NaOH than H2O 2) HIGH MPT - strong ionic bonds have to be broken -> phenol only has to break intermolecular forces

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25
Q

What bases will phenol not react with?

A

WEAK BASES -Carbonate (CO3)2- or Hydrogen carbonate (HCO3)- ions too weak an acid to react

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26
Q

In what reaction does phenol resemble alcohols?

A

redox REACTION WITH Na C6H5OH + Na -> C6H5O-Na+ + 1/2 H2

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27
Q

Why does phenol undergo electrophilic substitution much more easily than bezene?

A

Ring is ACTIVATED by OH group: One of oxygens LONE PAIRS is in a P-orbital -> Overlaps with delocalised π system of the ring INCREASED ELECTRON DENSITY -> ring more attractive to electrophile

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28
Q

What is observed when phenol reacts with bromine

A

1) BROMINE DECOLOURISED 2) WHITE PPT FORMED

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29
Q

What is the electrophile in the Nitration of an arene?

A

NO2+

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30
Q

Where on the phenol ring does electrophilic substitution normally occur, and why?

A

Carbons 2, 4, and 6 - INCREASED ELECTRON DENSITY

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31
Q

What are the uses of phenols?

A

1) ANTISEPTICS/ DISINFECTANT - not used so widely anymore - corrosive 2) Making plastics 3) making resins for paints

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32
Q

Give an equation for the electrophilic substitution of phenol with bromine:

A

C6H5OH (aq) + 3Br2 (aq) -> C6H5OHBr3 (s) + 3HBr (aq)

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33
Q

What is meant by Carbonyl Compound?

A

Aldehyde or Ketone

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34
Q

What is meant by carbonyl group?

A

C=O

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35
Q

How do the melting points of carbonyls compared to hydrocarbons of a similar size, and what is the reason for this?

A

Higher mpt/bpt as C=O allows for Permanent Dipole: Permanent Dipole interactions -> stronger than VDWs

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36
Q

How do the boiling points of carbonyls compare to alcohols of similar sizes?

A

Lower Bpt - Permanent Dipole: Permanent Dipole interactions are weaker than Hydrogen Bonds

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37
Q

How soluble are Carbonyls in water?

A

small non - polar carbonyls - quite soluble Delta negative oxygen can form H bonds with water

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38
Q

How would you prepare an Aldehyde?

A

Oxidation of a primary alcohol CH3CH2OH + [O] -> CH3CHO + H2O

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39
Q

How would you produce an aldehyde experimentally?

A

Heat with Aqueous Acidic Potassium Dichromate Distillation

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40
Q

How would you prepare a ketone?

A

Oxidise a secondary alcohol under reflux with aqueous acidic potassium dichromate CH3CHOHCH3 + [O] -> CH3COCH3 + H2O

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41
Q

Why is the C=O Bond polar?

A

Oxygen is much more electronegative than Carbon

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43
Q

What substance is used in the test for the presence of a carbonyl compound?

A

2,4 - DINITROPHENYLHYDRAZINE YELLOW/ORANGE PRECIPITATE

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44
Q

What substance is used to identify the presence of an aldehyde?

A

TOLLENS REAGENT (AgNO3 in aqueous NH3) SILVER MIRROR

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45
Q

Give the equations to represent the reaction of an aldehyde with Tollens’ Reagent:

A

RCHO + [O] -> RCOOH } Aldehyde OXIDISED to carboxylic acid Ag+ + e- -> Ag } Ag+ reduced

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46
Q

What substance is used to form alcohols from carbonyl compounds?

A

SODIUM TETRAHYDRIDOBORATE, NaBH4

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47
Q

Give an equation for the formation of an alcohol from an aldehyde:

A

RCHO + 2[H] -> RCH2OH

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48
Q

What properties must a nucleophile have in order to undergo nucleophilic substitution with a carbonyl?

A

LONE PAIR and (ideally) negative charge to be attracted to the delta positive carbon

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49
Q

Describe the mechanism of the 1st stage of the nucleophilic substitution of Ethanal with NaBH4:

A

1) Curly arrow from lone pair on :H- to delta +ve charged C (nucleophile donates lone pair of electrons, forming a bond) 2) Curly arrow from double bond to O with delta -ve charge ( electronegative O takes both shared electrons from π bond 3) CH3CH2O-

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50
Q

Describe the mechanism for the second part of the nucleophilic substitution of ethanal with NaBH4:

A

1) curly arrow from lone pair on :O- to H on H-O-H 2) curly arrow from H-O bond to O on Water 3) CH3CH2OH + H-O- formed

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51
Q

What is the overall reaction for the nucleophilic substitution of Ethanal?

A

CH3CHO + H- + H2O -> CH3CH2OH + OH-

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52
Q

Are carbonyls reactive with electrophiles, or nucleophiles?

A

Nucleophiles (nucleophilic substitution)

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53
Q

How does the boiling point of esters compare to: 1) Carboxylic acids 2) Hydrocarbons of a similar size?

A

1) LOWER BPT - CANNOT FORM H Bonds 2) HIGHER BOILING POINT - C=O bond allows PERMANENT DIPOLE:PERMANENT DIPOLE INTERACTIONS

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56
Q

What is the general formula of an ester?

A

n-O-C(=O)-m

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58
Q

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

A

the C=C double bonds make the fatty chain less flexible and so are less able to pack together efficiently in a solid

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59
Q

What are the conditions for margarine production?

A

Bubble hydrogen gas through oil high temp nickel catalyst

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60
Q

what is a side effect of partial hydrogenation?

A

some of the naturally occurring cis alkene groups isomerise to form trans groups

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61
Q

What are amines?

A

Organic compounds related to ammonia, with one or more of the hydrogens replaced by an organic group

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62
Q

How would you prepare simple Amines?

A

HEAT a HALOGENOALKANE under PRESSURE with EXCESS ETHANOLIC AMMONIA (NH3 dissolved in Ethanol)

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63
Q

What is the name of the mechanism of the reaction that forms simple amines?

A

Nucleophilic substitution - :NH3 acts as a Nucleophile

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64
Q

How can you form Secondary and Tertiary Amines from the reaction that forms non-aromatic primary amines?

A

EXCESS HALOGENOALKANE Primary amines formed in the initial reaction still have a lone pair and so can also react with leftover halogenalkane the secondary amine formed also has a lone pair, and can also undergo nucleophilic substitution to form a tertiary amino

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65
Q

How would you prepare an aromatic amine? give any necessary conditions

A

Reduction of nitroaromatic compounds - e.g. Nitrobenzene + 6[H] -> Phenylamine + 2H2O TIN and CONCENTRATED HCl = reducing agent heat under reflux

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66
Q

Describe and explain the difference in boiling points between primary amines and alkanes of a similar size:

A

Boiling points of Amines = HIGHER than alkanes Amines can form HYDROGEN BONDS which are STRONGER than Van der Waals

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67
Q

Describe and explain amines solubility in water:

A

1) SMALL AMINES = VERY SOLUBLE - H BONDS 2) Larger amines - slightly soluble - LARGE NON-POLAR AREA

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68
Q

Why are amines weak bases?

A

can use the lone pair of electrons on N to accept a proton (accept a dative covalent bond to a hydrogen ion) weak as reaction is reversible

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69
Q

Give an equation for Amines dissolving in water:

A

CH3NH2 + H2O CH3NH3+ + OH- Forms alkaline solution in water

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70
Q

Give a reaction for an amine reacting with an acid:

A

CH3NH2 + HCl -> CH3NH3+Cl-

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71
Q

What are Azo dyes?

A

Aromatic compounds with rings linked by the AZO GROUP, -N=N-

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72
Q

What is the first step in the formation of Azo Dyes?

A

Aromatic amine and NITROUS ACID react together BELOW 10*c

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73
Q

How is the Nitrous Acid formed for the formation of Azo Dyes?

A

SODIUM NITRATE and DILUTE HYDROCHLORIC ACID react 2H2O and a diazonium ion is formed (- N2+)

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74
Q

What happens in the Second step of forming an Azo Dye?

A

Diazonium ions in solution are added to PHENOL IN AQUEOUS ALKALI -usually NaOH- (C6H5O-) conditions: still temp<10*c Forming an azo dye: a phenol ring and another aromatic ring joined by -N=N-

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75
Q

What name is given to the useful products of azo dyes?

A

DYESTUFFS

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76
Q

What is the General Formula of an alpha-amino acid?

A

H2NC(R)(H)COOH

77
Q

What does the R represent in the general formula of an alpha amino acid?

A

An organic group, of which there are 20 variants in the amino acids used in cellular protein synthesis.

78
Q

Why can enzymes selectively catalyse the formation of a particular optical isomer?

A

All* amino acids have a chiral centre, and so have optical isomers. In biology, only ONE OF THE OPTICAL ISOMERS IS USED IN PROTEIN SYNTHESIS, so proteins (e.g. enzymes) have a built in handedness, an therefore are able to catalyse the formation of a specific isomer

79
Q

What is a Zwitterion, and how is it formed?

A

Amino Acid With COO- and NH3+ group (2 ionic parts with opposite charges in the same species) The H+ on the COOH is donated to NH2

80
Q

How does the melting points of amino acids compare to that of amines, and what is the reason for this?

A

MUCH HIGHER MELTING POINT Solid amino acids exist ONLY AS ZWITTERIONS GIANT IONIC STRUCTURE FORMED - very strong electrostatic attractions must be broken

81
Q

How soluble are amino acids in water?

A

Very soluble - ionic groups can form ion-dipole interactions with water

82
Q

Is an amino acid acidic or basic?

A

Weakly Acidic - COOH AND Weak Base - NH2

83
Q

What charge does an amino acid have at a Low pH? (acidic)

A

+H3N-C(R)(H)-COOH O- accepts a proton to form COOH

84
Q

What ion is formed of an amino acid when it is at intermediate pH?

A

+H3N-C(R)(H)-COO-

85
Q

What ion is formed when an amino acid is at High pH? (alkali)

A

H2N-C(H)(R)-COO- (Extra H+ donated to solution to neutralise it)

86
Q

What is an Isoelectric Point?

A

The pH at which the AVERAGE OVERALL CHARGE on a molecule of the amino acid is ZERO

87
Q

What effect does extra Amine groups on a amino acid have on Isoelectric Point?

A

MORE NH3+ -> HIGHER ISOELECTRIC POINT more alkaline conditions needed to neutralise to NH2

88
Q

What are proteins?

A

Polymers formed by CONDENSATION reactions of amino-acids, linking COOH with NH2 of a different amino acid (Peptide linkage) eliminating water

89
Q

What name is given to 2 amino acids joined together?

A

DIPEPTIDE

90
Q

What is the general repeat unit of a protein?

A

H -(-N-C-C=O) ) \n

91
Q

Give a general formula for the Acid Hydrolysis of a protein:

A

H2NCHRCONHCHRCOOH + H2O -> H2NCHRCOOH + H2NCHRCOOH

92
Q

What is the ionic equation for the acid-catalysed hydrolysis of a protein?

A

+H3N…COOH + H2O + H+ -> +H3NC(R)(H)COOH + +H3C(R)(H)COOH (Amide group is charged, Carboxylic group is not)

93
Q

What conditions are necessary for the acid-catalysed hydrolysis of a protein?

A

HEAT UNDER REFLUX 6Mol/dm3 AQUEOUS HCl

94
Q

How do proteins maintain an 3D structure?

A

Held together by non-covalent interactions (e.g H bonds)

95
Q

What names can be given to this Group? | C=O | H-N |

A

1) AMIDE GROUP - for general molecules 2) PEPTIDE LINK - for linking 2 amino acids

96
Q

What is the ionic equation for the Hydrolysis of a protein under alkaline conditions?

A

H2N…COO- + OH- -> H2N..COO- + H2N..COO-

97
Q

How soluble are esters in water?

A

slightly soluble - cannot form h bonds, but can form weaker permanent dipole:permanent dipole interactions

98
Q

Give an equation for the formation of an ester from a carboxylic acid:

A

CH3COOH + CH3OH -> CH3COOCH3 + H2O

99
Q

What type of reaction is the formation of esters from carboxylic acids, and what conditions are necessary?

A

1) CONDENSATION 2) CONC SULPHURIC ACID - HEAT under REFLUX

100
Q

What is the general formula of an Acid Anhydride?

A

R1COOCOR2

101
Q

How is an acid anhydride formed?

A

DEHYDRATION of carboxylic acid CH3COOH + CH3OOH -> CH3COOCOCH3 + H2O

102
Q

Give an equation for the formation of an ester from an acid anhydride:

A

R1COOCOR1 + R2-OH -> R1COOR2 + R1COOH

103
Q

What conditions are necessary for the esterification of an acid anhydride

A

Gentle heat- no catalyst

104
Q

What are the advantages of using acid anhydrides for the formation of esters, compared to carboxylic acids?

A

1) MILDER CONDITIONS - the sulphuric acid catalyst could react with other functional groups in the molecule as well 2) HIGHER YIELD - anhydride reaction is irreversible, carboxylic acid reaction is irreversible

105
Q

Give a reaction for the alkali-catalysed hydrolysis of an ester:

A

R1-COO-R2 + HO- -> R1COO- + R2-OH ester + base -> Carboxylic acid SALT + Alcohol

106
Q

What conditions are necessary for the alkali-catalysed hydrolysis of an alkali?

A

REFLUX (heat) AQUEOUS NaOH

107
Q

How would you convert the salt formed in the alkali-catalysed Hydrolysis of an ester into its parent carboxylic acid?

A

1) NEUTRALISE with aqueous acid 2) DISTILL of carboxylic acid

108
Q

Give an equation for the acid-catalysed hydrolysis of an ester:

A

R1-COO-R2 + H2O -> R1-COOH + R2-OH

109
Q

What conditions are necessary for the acid-catalysed hydrolysis of an ester?

A

REFLUX AQUEOUS HCl

110
Q

Why is base catalysed hydrolysis generally preferred over acid-catalysed hydrolysis?

A

1) Alkali catalysed hydrolysis (under given conditions) is IRREVERSIBLE - HIGH YIELD acid catalysed is reversible

111
Q

What are esters used for?

A

FLAVOURINGS PERFUMES

112
Q

What are triglycerides?

A

esters of GLYCEROL

113
Q

What is glycerol’s systematic name, and what is its displayed formula?

A

1) PROPAN-1,2,3-TRIOL 2) OH OH OH | | | H2C—-CH—-CH2

114
Q

What are Fatty Acids?

A

Carboxylic Acids that react with glycerol

115
Q

What are Unsaturated compounds?

A

Compounds with a C=C double bond

116
Q

What are Polyunsaturated compounds?

A

compounds with many c=c double bonds

117
Q

What is the formula for reading the shorthand names of fatty acids?

A

c,d(p1…p(d)) c = no. of carbons d = no. of double bonds p1..p(d) = carbon position of double bonds

118
Q

What is the shorthand name for Cis,cis - 3,7 octadienoic acid?

A

8, 2 (3,7)

119
Q

What is the longhand name for 10, 3 (2,5,8)?

A

2,5,8 Decatrianoic acid

120
Q

What health issues are linked to saturated and unsaturated fats?

A

1) CIRCULATORY, HEART and OBESITY problems - too much sat. fat 2) excess BAD CHOLESTEROL produced by high intake of sat. fat - deposited onto artery walls - increased risk of HEART DISEASE 3) TRANS FATS - high bad cholesterol

121
Q

What is the name of the reaction that forms biodiesel?

A

TRANSESTERIFICATION

122
Q

What is the equation that forms biodiesel?

A

fatty acid + 3(short alcohol) -> Biodiesel (ester) + Glycerol

123
Q

Give an example of a catalyst used in the formation of biodiesel:

A

NaOH (or other bases)

124
Q

What are advantages of biodiesel?

A

1) burning BIOFUELS is CARBON - NEUTRAL - no overall increase of CO2 in the atmosphere 2) Decreases consumption of fossil fuels - combats climate change 3) Can be used on its own (usually blended with normal diesel) 4) RENEWABLE - can always grow more plants for oil/alcohol - can be formed form leftover cooking oil

125
Q

What is a disadvantage of using Biodiesel?

A

1) Land is taken up - deforestation - lack of land for food production

126
Q

What is an addition polymer?

A

A polymer formed from the addition of many alkene molecules

127
Q

give the equation for the addition polymerisation of CH2=CHX:

A

n(CH2=CHX) -> -[-C(H)2-C(H)(X)-]- n

128
Q

What are the advantages of using addition polymers?

A

Many applications -e.g. polythene Inexpensive - formed from - alkenes - byproduct of cracking

129
Q

What is the main disadvantage of Addition polymers?

A

NON-BIODEGRADABLE - will remain in landfill for a very long time

130
Q

What type of polymer is one with the linkage: O || -C-O-

A

POLYESTER, which is a type of condensation polymer

131
Q

What normally are the monomers of a polyester?

A

DIOL and DICARBOXYLIC ACID HO-n-OH and HOOC-m-COOH

132
Q

Give a general equation for the formation of a polyester:

A

n(HO-x-OH) + n(HOOC-y-COOH) -> -[-O-x-OOC-y-COO-]-n + 2n(H2O)

133
Q

What are polyesters used for?

A

FIBRES FOR CLOTHING

134
Q

What is a disadvantage of using polyesters made from diols and dicarboxylic acids?

A

NON-BIODEGRADABLE - long lifetime in landfill

135
Q

What methods are used to dispose of Addition polymers, and what are their drawbacks?

A

1) BURNING -produce TOXIC FUMES 2) RECYCLING - sorting needed - expensive and labour intensive 3) CRACKING for use as feedstocks and fuels

136
Q

What is an example of a biodegradable polyester?

A

POLYLACTIC ACID -[-O-C(CH3)(H)-C(=O)-]-n

137
Q

What makes polyesters, such as polylactic acid, different to those formed from diols and dicarboxylic acids?

A

has ONE MONOMER, which contains BOTH OH and COOH GROUPS

138
Q

What are the uses of polylactic acid?

A

1) SOLUBLE STITCHES 2) BIODEGRADABLE WASTE SACKS

139
Q

How are polyamides formed?

A

creating AMIDE LINKAGES between monomers - reaction of amine groups with carboxylic acid groups n(HOOC-x-COOH) + n(H2N-y-NH2) -> -[-C(=O)-x-C(=O)-NH-y-(H)N-]-n + 2n (H2O)

140
Q

What are polyamides used for?

A

Making fibres - e.g. nylon

141
Q

What is the difference between normal polyamides and proteins?

A

Proteins have ONE MONOMER - amino acids

142
Q

How may you chemically degrade condensation polymers? -e.g. polyesters and polyamides-

A

HEAT under REFLUX with CONCENTRATED AQUEOUS ACID/ALKALI (reactions same as hydrolysis for amines/esters)

143
Q

Why is Poly(lactic acid) useful?

A

MORE SUSTAINABLE - it is BIODEGRADABLE, and RENEWABLE as monomer is produced easily form corn starch

144
Q

What word is give to describe the breakdown of condensation polymers due to UV light, and why does this happen?

A

PHOTODEGRADATION C=O bond absorbs radiation, causing a reactive excited state in which the ester or amide linkage is easily broken

145
Q

what is a tertiary amine?

A

3 other groups bonded to the N - no H -N

146
Q

What is a secondary Amine?

A

Amine which has 2 carbons bonded to the Nitrogen -NH

147
Q

What is a primary Amine?

A

An amine which has one carbon bonded to the nitrogen -NH2 group

148
Q

How are amines classified?

A

according to how many carbons are bonded to the nitrogen in the molecule

152
Q

What is the stationary and mobile phase in Thin Layer Chromatography?

A

1) Stationary = Solid - silica 2) mobile = liquid - organic solvent

152
Q

What is the purpose of carrying out Chromatography on a small scale?

A

Analysis

152
Q

What is the purpose of carrying out Chromatography on a large scale?

A

Purification

153
Q

what is the formula for Rf value?

A

distance travelled by the component ——————————————- distance travelled by solvent

153
Q

what factors affect the rate at which the component moves up the tlc? (and therefore Rf)

A

1) How strongly the component is adsorbed 2) Solubility - more soluble = faster moving

153
Q

How are the components separated in TLC?

A

adsorbption

153
Q

what are the advantages of TLC?

A

1) quick 2) inexpensive

153
Q

Describe the method of gas chromatography:

A

1) sample injected into GC machine and VAPORISED 2) Sample carried into column by INERT gas carrier - passes over S.P. 3) the sooner the sample is ADSORBED(solid) or DISSOLVED(liquid) onto the stationary phase the longer the retention time - depends on: a) volatility of compound b) how strongly compound interacts with stationary phase

153
Q

what is a limitation of TLC?

A

molecules of a similar size and functional groups poorly separated

154
Q

What is the stationary and mobile phase in Gas Chromatography?

A

1) Stationary = liquid or solid 2) mobile = gas

155
Q

Why may compounds have similar retention times?

A

Similar functional groups - similar interactions with column/mobile phase

156
Q

What does the area under a peak in a gas chromatogram indicate?

A

The amount of each compound

157
Q

What is GC-MS?

A

Gas Chromatography combined with Mass Spectrometry

158
Q

Describe the process of GC-MS:

A

1) Substance sent through a gas chromatography machine 2) Peaks are detected in the chromatogram 3) Components sent to a mass spectrometer 4) Generated Mass Spectra compared with Spectral databases using a computer

159
Q

What are the advantages of GC-MS?

A

1) HIGH RESOLUTION - possible to resolve complex mixtures 2) Can operate with SMALL AMOUNTS of sample 3) Due to large database, very likely positive indication of individual components

160
Q

What are the limitations of GS-MS?

A

1) Difficult to identify similar compounds - similar retention times 2) Unknown/uncharacterised compounds have to reference retention times/spectra for identifying 3) EXPENSIVE equipment

161
Q

What are the applications of GC-MS?

A

1) Forensics - drug testing - small traces can be analysed in detail 2) testing for organic pollutants - pesticides 3) Airport security - detecting explosives 4) Space missions - finding substances to indicate traces of life

162
Q

How does Infra-red spectroscopy work?

A

Molecules absorb IR radiation, the energy causes the covalent bonds in the molecule to VIBRATE at different frequencies dependent on the functional groups. this different frequencies are then picked up on the spectra

163
Q

What peak indicates the Mr of a substance on a Mass Spectra?

A

MOLECULAR ION PEAK (furthest peak to the right)

164
Q

What name is given to all other peaks in mass spectra?

A

FRAGMENT ION PEAKS

165
Q

What must be remembered about mass spectra fragments?

A

MUST HAVE +VE CHARGE

166
Q

What are the 2 types of NMR spectroscopy?

A

13C and 1H

167
Q

How do 13C nuclei give NMR peaks?

A

Different 13C nuclei in DIFFERENT CHEMICAL ENVRIONMENTS absorb at different frequencies, giving different peaks

168
Q

What term is given to 13C or 1H nuclei in identical chemical environments, and what does this mean?

A

EQUIVALENT - give the same peak

169
Q

What does the number of peaks in a 13C NMR spectrum indicate?

A

The number of carbons with different environments in the molecule

170
Q

What gives the reference peak in 1H NMR, and where on the spectrum is this shown?

A

TETRAMETHYLSILANE (TMS) - Si(CH3)4 (4 equivalent carbons, 12 equivalent protons) 0ppm

171
Q

Why does ‘Splitting’ occur in 1H NMR?

A

The energy gap between the spin states of one proton is slightly affected by the spin states of protons attached to NEIGHBOURING carbons - the protons are COUPLED

172
Q

What factor affects the splitting pattern?

A

How many protons are attached to the ADJACENT carbons in the molecule

173
Q

What rule can be used to describe the splitting pattern?

A

N+1 RULE - an extra peak is added for every proton on an adjacent carbon

174
Q

What names are given to the first 4 types of splitting?

A

Singlet - 1 peak - 0 adjacent protons Doublet - 2 peaks - 1 adjacent protons Triplet - 3 peaks - 2 adjacent protons Quartet - 4 peaks - 3 adjacent protons

175
Q

What are the exceptions to the n+1 rule?

A

1) OH PEAKS NEVER SPLIT other protons and never are split themselves 2) EQUIVALENT PROTONS NEVER SPLIT EACH OTHER e.g. CH3-CH3 only gives one peak, with no splitting, as all the protons are equivalent

176
Q

How would you identify a -OH or -NH peak on an NMR spectrum?

A

1) Run the spectrum normally for reference 2) Add D2O to the sample - O-H -> OD, -NH -> -ND 3) Run spectrum again - Peaks that have disappeared indicate the presence of -OH or -NH

177
Q

Why can’t pure phenol be run through an NMR machine, and how is this resolved?

A

NMR spectra can only be obtained for liquid samples Solvent needed

178
Q

What factors must be considered when determining a suitable solvent for NMR?

A

1) If a compound is in SOLUTION, the solvent must not give rise to peaks that will interfere with the spectrum 2) For proton NMR, the solvent cannot contain protons, or all protons must be deuterated - wont give an NMR signal

179
Q

Give 2 examples of common deuterated solvents:

A

1) D2O 2)CDCL3

180
Q

Give a practical example of use of NMR spectroscopy:

A

1) MRI - Magnetic Resonance Imaging - diagnosing location of tumours A person is placed in a strong magnetic field - NMR signals are detected from different regions of space within the body, usually from magnetic 1H nuclei in water molecules, which give different signals dependent on the envrionment

A levels (12 decks)

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