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Chemistry > Unit 3: Section 6 - Aromatic Carbons and Amines > Flashcards

Unit 3: Section 6 - Aromatic Carbons and Amines Flashcards

1
Q

What is the most common type of reaction of benzene?

A

Substitution

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2
Q

What is an arene?
(aromatic compound)

A

Compounds containing a benzene ring

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3
Q

What is the shape of benzene?

A

Flat, regular hexagon, bond angle is 120°

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4
Q

What is the bond length between adjacent C atoms?

A

Intermediate between C-C and C=C

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5
Q

What happens to the 4th e;ectron in the p-orbital of each C 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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6
Q

What is the effect on benzene’s stability of the rings of eectron density?

A

Makes benzene very stable, even though it is unsaturated

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7
Q

What is the thermochemical evidence that benzene is more stable than cyclohexa-1,3,5-triene?

A

Hydrogenation of cyclohexene - enthalpy of hydrogenation : 120kJmol^-1
Cyclohexa-1,3,5-triene - 360kJmol^-1
Benzene - 208kJmol^-1 so benzene is more stable

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8
Q

Other than enthalpy of hydrogenation why else is cyclohexa-1,3,5-triene not a suitable model for benzene?

A
  • Wouldn’t be symmetrical (C=C shorter than C-C), but benzene is
  • Would easily undergo addition reactions across the double bonds - benzene doesn’t
  • Would form 2 isomers on the addition of Br2 or similar - benzene doesn’t
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9
Q

What is the appearance of benzene at 298K?

A

Colourless liquid

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10
Q

Why does benzene have a relatively high melting point?

A

CLose packing of flat hexagonal molecules when solid

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11
Q

Is benzene soluble in water? Why?

A

No - non polar

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12
Q

Why is benzene not used in schools?

A

Carcinogen

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13
Q

How do you name compounds containing a benzene ring?

A
  • benzene, or -phenyl ; can designate position on ring using numbers if there is more than one substituent
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14
Q

Why is benzene attacked by electrophiles?

A

High electron density above/below ring due to delocalised electrons

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15
Q

What is delocalisation energy and what is the effect of this on benzene’s reactions?

A

The large amount of energy that is needed to break the aromatic ring apart
Results in the aromatic ring almost always staying intact

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16
Q

What is seen when benzene is combusted? Why?

A

Smoky flames due to soot from unburnt carbon
High Carbon:Hydrogen ratio

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17
Q

Which ion is used to nitrate benzene?

A

+NO2
Nitronium ion or nitryl cation

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18
Q

How is the +NO2 ion generated? (conditions and equations)

A

Concentrated H2SO4, and concentrated HNO3
H2SO4 + HNO3 -> HSO4- + +NO2 + H2O

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19
Q

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

A

HSO4- + H+ -> H2SO4 (H+ from benzene ring)

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20
Q

How do you form only one NO2 group (mononitration)?

A

Keep the temperature below 55°C otherwise you’ll get lots of substitutions

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21
Q

What are the uses of nitrated arenes?

A

Production of explosives e.g. TNT - (1-methyl-2,4,6-trinitrobenzene) - releases lots of heat and gas on explosion
To make aromatic amines htat are used for industrial dyes

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22
Q

How do substituents with a negative inductive effect (e.g. NH2) affect further substitution?

A

Remove electrons from the delocalised electron ring, decreasing the electron density and making further substitution reactions less likely/quick

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23
Q

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

A

Halogen carrier ( e.g. AlCl3)

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24
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

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25
Q

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

A

Chlorine atom’s lone pair of electrons is forming a coordinate bond to Al

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26
Q

How is the AlCl3 catalyst reformed?

A

AlCl4- + H+ -> HCl + AlCl3 (H+ from benzene)

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27
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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28
Q

Why is Friedel-Craft acylation used?

A

Once an acyl group has been added to benzene, the side chains can be modified using further reactions to make useful products

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29
Q

If you are considering cyclic compounds, what might happen if 2 double bonds are next to each other?

A

C=C bonds are in close proximity, so electrons in pi cloud/p orbitals can partially delocalise and move between the 2 C=C double bonds

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30
Q

What effect would electrons in p orbitals moving betweeen the 2 C=C doubl bonds have on the stability of the molecule and its enthalpy of hydrogenation?

A

Makes the molecule more stable; makes enthalpy of hydrogenation more positive

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31
Q

How do you name amines?

A

-amine or amino-

32
Q

Why are amines so reactive?

A

The lone pair of electrons on the nitrogen - due to polar N-H bond

33
Q

Wha shape are amines around the N? Bond angle?

A

Trigonal pyramindal, 107° due to lone pair on N

34
Q

What kind of intermolecular forces do they have? Why?

A

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

35
Q

Do amines have intermolecular forces which are stronger than or weaker than alcohols? Why?

A

Weaker, as N has a lower electronegativity than O -> weaker hydrogen bonding

36
Q

What state are amines at 298K?

A

Short chains are gases, longer chains are volatile liquids

37
Q

What do amines smell of? Why?

A

Fishy smell - rotting fish/flesh as releases di- and tri- amines

38
Q

Which primary amines are soluble in water/alcohols? Why?

A

Up to 4 carbon atoms, as they can hydrogen bond to water molecules
After this, non-polarity of hydrocarbon chain makes them insoluble

39
Q

What kind of solvents are most other amines soluble in?

A

Less or non-polar solvents

40
Q

How do you name amines?

A

-amine or amino-

41
Q

Why are amines so reactive?

A

The lone pair of electrons on the nitrogen - due to polar N-H bond

42
Q

What shape are amines around the N? Bond angle?

A

Trigonal pyramidal, 107° due to lone pair on N

43
Q

What kind of intermolecular forces do amines have? Why?

A

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

44
Q

Do amines have intermolecular forces which are stronger than or weaker than alcohols? Why?

A

Weaker, as N has a lower electronegativity than O -> weaker hydrogen bonding

45
Q

What state are amines at 298K?

A

Short chains are gases, longer chains are volatile liquids

46
Q

What do amines smell of? Why?

A

Fishy smell - rotting fish/flesh releases di- and tri- amines

47
Q

Which primary amines are soluble in water/alcohols? Why?

A

Up to 4 carbon atoms, as they can hydrogen bond to water molecules
After this, non-polarity of hydrocarbon chain makes them insoluble

48
Q

What kind of solvents are most other amines soluble in?

A

Less or non-polar solvents

49
Q

Solubility of phenylamine? Why?

A

Not very soluble, due to the non-polarity of the benzene ring - C6H5 cannot form hydrogen bonds

50
Q

How can/when do amines act as bases?

A

When they bond with a H+ ion

51
Q

How can/when do amines act as nucleophiles?

A

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

52
Q

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

A

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

53
Q

Is the product (ammonium ion) soluble in water? Why?

A

Yes, as it is ionic so is attracted to the polar bonds in H2O

54
Q

How could you regenerate the soluble amine from the ammonium salt?

A

Add a strong base (NaOH) -> removes H+ ions from ammonium ion

55
Q

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

A

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

56
Q

What does positive/negative inductive effect mean?

A

Positive inductive effect = donate electrons, increase density around N
Negative means remove electrons, decrease density around N

57
Q

What effect do alkyl groups have (on electron density and base strenght)?

A

Positive inductive effect - increase electron density around N -> stronger base

58
Q

What effect do aryl groups have (on electron density and base strength)?

A

Negative inductive effect - decrease electron density around N -> weaker base

59
Q

Why are 3° amines never good bases?

A

They are insoluble in water

60
Q

Place these in order of base strength (in general): NH3, 1° amine, 2° amine, phenylamine

A

2° amine > 1° amine > NH3 > phenylamine

61
Q

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

A

Multiple substitutions; primary amine is a nucleophile that attacks the original haloalkane etc

62
Q

What are the problems with this method?

A

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

63
Q

How would you maximise the yield of the primary amine?

A

Use excess ammonia

64
Q

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

A

Nucleophilic substitution

65
Q

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

A

Ethanol as a solvent
A nitrile is formed

66
Q

How do you get from a nitrile to a primary amine? (name of reaction type and reagents/catalysts)

A

Reduction using Ni catalyst and H2

67
Q

Why is this a purer method of synthesising amines?

A

Only the primary amine can be formed

68
Q

What conditions are needed to form nitrobenzene from benzene?

A

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

69
Q

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

A

Reduce the nitrile using Tin/HCl -> forms an ammonium salt with Cl- ions
Room temperature

70
Q

Equation for the reaction of nitrobenzene -> penylamine?

A

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

71
Q

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

A

Nucleophilic addition/elimination

72
Q

In which industries/products are amines used?

A

Dyes, nylon, drugs, synthesis of new molecules

73
Q

What are cationic surfactants (used in fabric/hair conditioners)?

A

Quaternary ammonium salts, with a cation that is charged at one end (N+) and non-polar at the other

74
Q

How do cationic surfactants work in fabric/hair conditioner?

A

Negative charges on the surface of the fabric/hair are attracted to the cation, removing them from the surface; prevents build-up of static electricity and keeps hair flat and fabric smooth

75
Q

How do cationic surfactants sit when placed in water?

A

Charged end in the water, non-polar end sticking out of the water/at the surface

Chemistry (21 decks)

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