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A - Chemistry > Unit 3: Section 6 - aromatic compounds and amines MDY * > Flashcards

Unit 3: Section 6 - aromatic compounds and amines MDY * Flashcards

aromatic compounds amines and amides

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1
Q

what is benzene’s formula?

A

C6H6

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2
Q

what is benzene’s structure?

A

planar cyclic
- 6 carbons carbon atoms joined together in a flat ring
bond angle = 120*

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3
Q

what is the bonding like in benzene?

A

each carbon forms a single covalent bond to the carbons on either side of it and to 1 hydrogen atom

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4
Q

what happens to the fourth electron on the carbon atoms in benzene?

A

it’s located in a p orbital that sticks out above and below the plane of the ring,
each p orbital overlaps with the neighbouring p orbitals to form a pi bond
neighbouring pi bonds overlap
they form a ring of delocalised electrons above and below the plane - a pi system

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5
Q

how long are the bonds in benzene?

A

they are all the same length and are between the lengths of a single C-C bond and a double C=C bond

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6
Q

why is benzene so stable?

A

the enthalpy of hydrogenation of cyclohexene (with 1 double bond) is -120 kJ/mol
you would then expect the energy to be -360kJ/mol for 3 double bonds
but in benzene it is -208kJ/mol
meaning more energy was put into breaking the bonds in benzene than making them, so it is more stable than it should be
due to the delocalised ring of electrons (delocalisation stability)

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7
Q

what are aromatic compounds?

A

a.k.a. arenes

compounds containing a benzene ring

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8
Q

how are arenes named?

A 
chlorobenzene (has Cl)
nitrobenzene (NO2)
methylbenzene (methyl group)
phenol (OH)
phenylamine (NH2)
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9
Q

why do arenes undergo electrophilic substitution?

A

the benzene ring has a high electron density, so it attracts electrophiles
the benzene ring is stable so it doesn’t undergo addition
it swaps a hydrogen for another group or atom to keep the delocalised ring of electrons intact

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10
Q

what are friedel-crafts acylation reactions?

A

they are used to add an acyl group to the benzene ring

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11
Q

how can benzene be made into useful chemicals?

A

using friedel-crafts acylation

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

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12
Q

how are electrophiles made to attack benzene?

A

catalysts called halogen carriers can be used to make stronger electrophiles that have a positive enough charge o attack the stable benzene ring

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13
Q

what halogen carrier is used in Friedel-crafts acylation?

A

AlCl3

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14
Q

how does AlCl3 make an acyl chloride electrophile stronger?

A

AlCl3 accepts a lone pair of electrons from the acyl chloride, increasing it’s polarisation and forming a carbocation
it is now a strong enough electrophile

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15
Q

what is the mechanism for friedel-crafts acylation?

A

electrons in benzene ring attracted to carbocation
2 electrons from benzene bond with carbocation
delocalised ring partially breaks and has positive charge
AlCl4 - ion attracted to positively charged ring
1 chloride breaks away and bonds with hydrogen ion
this removes hydrogen from the ring forming HCl and allows catalyst to reform

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16
Q

how is the electrophile made for the nitration of benzene?

A

HNO3 + H2SO4 –> NO2 + + HSO4 - + H2O

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17
Q

what conditions are needed to only add one NO2 group?

A

below 55*C

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18
Q

why are nitration reactions useful?

A

nitro compounds can be reduced to form aromatic amines (used to manufacture dyes and pharmaceuticals
2,4,6-trinitromethylbenzene (trinitrotoluene - TNT)

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19
Q

what are amines?

A

ammonia (NH3) with one or more H’s replaced by an organic group

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20
Q

what are the different types of amines?

A 
primary amine (RNH2)
secondary amine (R2NH)
tertiary amine (NR3)
quaternary ammonium ion (NR4 +)
21
Q

what is phenylamine?

A

benzene-NH2

a primary amine

22
Q

what is a quaternary ammonium salt?

A

quaternary ammonium ions are positively charged so they hang around with negative ions, forming salts

23
Q

what are cationic surfactants?

A

quaternary ammonium salts with at least 1 long hydrocarbon chain

24
Q

what do cationic surfactants do?

A

long hydrocarbon tail binds to nonpolar substances like grease
cationic head dissolves in water

25
Q

what are cationic surfactants used for?

A

fabric cleaners and hair products

fabric conditioners

26
Q

why can cationic surfactants be used as fabric conditioners?

A

the positively charged part will bind to negatively charged substances like hair and fibre to get rid of static

27
Q

how can amines act as weak bases?

A

they accept protons

the lone pair of electrons on the nitrogen can form a dative covalent bond with an H+ ion

28
Q

what determines the strength of amine bases?

A

the more available the lone pair is, the more likely the amine is to accept a proton, the stronger the base
a lone pair is more available if it’s electron density is higher

29
Q

which amines are the strongest bases?

A 
tertiary amine - strongest
secondary amine
primary amine
ammonia
phenylamine - weakest
30
Q

why are tertiary amines the strongest bases?

A

alkyl groups push electrons onto attached groups, so electron density on nitrogen increases.
more alkyl groups = lone pair on nitrogen more available

31
Q

why are aromatic amines the weakest bases?

A

the benzene ring draws electrons towards itself and the nitrogen lone pair gets partially delocalised onto the ring.
electron density on the nitrogen decreases, making lone pair less available

32
Q

how can aliphatic amines be produced?

A

from halogenoalkanes

by reducing nitriles

33
Q

how can amines be made from halogenoalkanes?

A

by heating halogenoalkanes with excess ammonia

the ammonia acts as a nucleophile, replacing the halogen

34
Q

what if there is an excess of halogenoalkanes when making amines?

A

the primary amine has a lone pair so can act as a nucleophile to join to another halogenoalkane and become a secondary amine
this can keep happening until a quaternary ammonium ion is formed
excess amines can also become ammonium salts with the halogen

35
Q

how are nitriles reduced to form amines?

A

using LiAlH4 in dry ether with dilute acid

catalytic hydrogenation

36
Q

what is catalytic hydrogenation?

A

reducing a nitrile using hydrogen gas with a metal catalyst e.g. platinum at high temperature and pressure

37
Q

what is the equation for the reduction of a nitrile?

A

R-CN + 4[H] –> R-CH2-NH2

38
Q

how are aromatic amines produced?

A

by reducing a nitro compound like nitrobenzene

39
Q

how to reduce nitrobenzene to phenylamine?

A

heat a mixture of a nitro compound, tin metal and concentrated hydrochloric acid under reflux
add sodium hydroxide
nitrobenzene + 6[H] – tin, HCl, NaOH –> phenylamine + 2H2O

40
Q

how are aromatic amines useful?

A

used as starting molecules for dyes and pharmaceuticals

41
Q

what are amides?

A

they have functional group CONH2

carbonyl group pulls electrons away from NH2, so amides behave differently to amines

42
Q

what is kekule’s structure of benzene?

A

a triene, every other bond is double

if this were true benzene would behave like a alkene

43
Q

what are the properties if the pi system in benzene?

A

electrons are delocalised, they don’t belong to any 1 carbon

delocalised electrons repel each other and spread out, making benzene more stable

44
Q

what does aliphatic mean?

A

a substance made up of carbon chains

45
Q

what is a pi bond?

A

a bond caused by the overlap of p orbitals, found in double C=C bonds and aromatic compounds

46
Q

what is a pi system?

A

an area where close pi bonds overlap to create extra stability in a molecule

47
Q

why does benzene react more slowly than alkenes?

A

pi bond is delocalised in benzene but it is localised in alkenes
benzene has lower electron density than C=C
so it polarises other molecules less (to create dipoles), so doesn’t react with electrophiles as strongly

48
Q

how to name an aromatic if it has a carboxylic acid and a hydroxyl group?

A

carboxylic acids always take priority so it is -hydroxybenzenecarboxylic acid
the carboxylic acid is on carbon 1

A - Chemistry (26 decks)

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