Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Aqa a level chemistry > Aromatic chemistry > Flashcards

Aromatic chemistry Flashcards

1
Q

notes from spec

A

The nature of the bonding in a benzene ring, limited to planar structure and bond length intermediate between single and double.

Delocalisation of p electrons makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

electrophilic subsitution

A

Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions.

Nitration is an important step in synthesis, including the manufacture of explosives and formation of amines.

Friedel–Crafts acylation reactions are also important steps in synthesis.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

benzene

A

-Benzene doesn’t undergo addition reactions like expected

-Benzene – planar, bond angles of 120 degrees

C6H6

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

overlap of orbtials

A

-carbon can form 2 bonds as it promotes electrons from 2s orbital to 2p

-overlap of orbitals = delocalised electrons

=in beneze each carbon forms 3 sigma bonds

-no delocalised electrons = no alternating double and single bonds (carbon-carbon) (p orbtials involving in forming pi bond can overlap)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

aromatic compound

A

-when compounds contain 6 carbon ring and overlapping p orbital (benzene) then they are known as an aromatic compound

-when a compound has delocalised electrons it is more stable

-more stable = electron density decreases = electrons spread out

-benzene has a lower electron density than alkenes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

explain the bonding in benzene

A

You should say that each carbon atom forms three covalent bonds.

You should say that the remaining p orbitals overlap above and below the ring to form a pi system.

You should say that the electrons in these p orbitals are delocalise

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

explain the shape of benzene

A

You should say that benzene is planar.

You should say that the bond angles in benzene are 120°.

You should say that the C-C bonds are equal in length.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

naming aromatic compounds

A

-other atoms bonded to benzene ring = called phenyl group

-alkene has higher priortiy than benzene

-suffix = benzene when highest priority

-prefix = phenyl when not high priority

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

nitro group

A

Nitro group:

NO2 = 1,2,3trinitrobenzene

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

benzene and bromine electrophilic subsitution

A

-Benzene and bromine don’t undergo addition reactions because benzene has a lower electron density than alkenes

-this means it doesn’t induce a large dipole in bromine

-aluminium bromide (halogen carrier) + benzene –> bromobenzne + H+ in electrophillic subsitution reaction

Br+ is stronger electrophille than Br2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

reagents for the nitration of benzene

A

-concentrated HNO3

-concentrated H2SO4

needs to be concentrated bc otherwise would react with water instead of benzene as electrophiles are very reactice

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

classes of organic molecules

A

There are two major classes of organic chemicals
aliphatic : straight or branched chain organic substances

aromatic or arene: includes one or more ring of six carbon
atoms with delocalised bonding.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

single and double bonds

A

Benzene is a planar molecule. The
evidence suggests all the C-C bonds
are the same and have a length and
bond energy between a C-C single
and C=C double bond.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

bond angle in benzene

A

The H-C-C bond angle is 120o
in
benzene

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

thermodynamically stable

A

This when represented on an energy level
diagram shows that the delocalised benzene
is more thermodynamically stable than the
theoretical structure.

Theoretically because there are 3
double bonds in the theoretical
cyclohexa-1,3,5-triene one might
expect the amount of energy to be
3 times as much as cyclohexene.
However, in actual benzene the
amount of energy is less. The 6 pi
electrons are delocalised and not
arranged in 3 double bonds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

toxicity of benzene

A

Benzene is a carcinogen (cancers causing
molecule) and is banned for use in
schools.
Methylbenzene is less toxic and also
reacts more readily than benzene as the
methyl side group releases electrons into
the delocalised system making it more
attractive to electrophiles.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

nitration of benzene

A

Change in functional group: benzene  nitrobenzene
Reagents: concentrated nitric acid in the presence of
concentrated sulfuric acid (catalyst)
Mechanism: Electrophilic substitution
Electrophile: +NO2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

overall equation for formation of electrophile

A

Overall equation for formation of the electrophile:
HNO3 + 2H2SO4  +NO2 + 2HSO4
- + H3O+

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Fridel crafts acylation

A

Change in functional group: benzene  phenyl ketone
Reagents: acyl chloride in the presence of anhydrous
aluminium chloride catalyst
Conditions: heat under reflux (50OC)
Mechanism: Electrophilic substitution

Formation of electrophile:
AlCl3 + CH3COCl  [CH3CO]+ + [AlCl4]-

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

benzene = attract electrophiles

A

-electrophile = electron pair acceptor
-benzene = high electron density = delocalised ring of electrons

benzene undergoes electrophilic subsitution rather than addition as it would disrupt the stability of the molecule as the delocalised ring of electrons would have to break

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

AlCl3 = halogen carrier equation

A

RCOCl + AlCl3 –> RC+O + AlCl4-

AlCl3 accepts pair of electrons from acyl group
carbocation is formed due to increased polarisation
stronger electrophile is produced which can now react with benzene

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

nitration of benzene = x2 equations

A

HNO3 + H2SO4 –> H2NO3+ + HSO4-

H2NO3+ –> NO2+ + H2O

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

use these data to show that benzene is 152Kjmol-1 more stable than the hypothetical compound cyclohexa-1,3,5-tirene

A

-120 x 3 = -360

-360 – 208 = 152

24
Q

state in terms of bonding why benzene is more stable than cyclohexa-1,3,5-triene

A

-each carbon has 3 covalent bonds
-electrons in p orbital form pi system (delocalised electrons)

25
Q

write the letters in increaseing bond length

A

X Y W

26
Q

explain your answer the bonding of these 2 dienes

A

proximity for 1,3 C=C bonds are close together

delocalisation = overlap of electrons in p orbital = extra stability

27
Q

closer bonds =

A

partial delocalsation

C=C bonds are closer = repel so less stable

they become stabilised by delocalisation

28
Q

pentanedioyl chloride

A

O=C-Cl – (CH2)3 - Cl-C=O

29
Q

electrophile in electrophilic subsitution

A

NO2+

30
Q

methylbenzene + HNO3 + H2SO4

A

2-nitromethylbenzene

31
Q

methylbenzene + ethanoic anhydride + AlCl3 to make

A

2-methylphenylethanone

32
Q

write an equation for the reaction of concentrated HNO3 and concentrated H2SO4

A

HNO3 + 2H2SO4 –> NO2+ + H3O+ + 2HSO4-

33
Q

name and outline the mechanism for the reaction of this species with nitrobenzene to form 1,3-dinitrobenzene

A

electrophilic subsitution

34
Q

equation for ethanoic anhydride and propan-2-ol

A

(CH3)2CHOH + (CH3CO)2O –> (CH3)CH3COOH

ester = methylethylethanonate

35
Q

give the formula of the biodisel molecule with the highest mr that can be produced from the reaction of this ester with methanol

A

C17H33COOCH3

36
Q

biodiesel formula

A

RCOOCH3

37
Q

arenes

A

benzene = high melting points due to stability of delocalised ring

but low boiling points as they are non-polar

38
Q

benzene

A

-C6H6 = alternating double and single bonds

-Kekule’s structure = cyclohex-1,3,5-triene

-Benzene has a delocalised electron within the pi system meaning it is more stable than kekule’s model

39
Q

bonding in benzene

A

Bonding in benzene –> planar structure and bond length intermediates between double and single bonds

40
Q

HNO3 + H2SO4

A

HNO3 + H2SO4 –> NO2+ + HSO4- + H2O

41
Q

aromatic vs aliphatic

A

Aromatic:

-carbon and hydrogen arranged in ring structures with delocalised pi electrons

Aliphatic:

-organic compounds arranged in straight chains = branched and non-aromatic

42
Q

kekule structure

A

-C=C bond length in cyclohexane is shorter than C-C bond length in benzene

-C=C bond in cyclohexane is larger than benzne

Ethalphy of hydrogenation for kekule’s model is more exothermic than cyclohexene suggesting benzene is more stable and the bonding is stronger

-benzene = less reactive than kekule’s model

43
Q

delocalisation of benzene

A

-pi delocalised system in benzene formed by overlap of p orbitals spreads electron density throughout the structure

-benzene = planar structure = each carbon has 3 covalent bonds

44
Q

bonding / shape/ stability of benzene

A

Bonding –> each carbon has 3 covalent bonds, spare electrons in p orbital overlap, delocalisation

Shape –> Planar (120 degree), 6 carbon ring, C-C bonds equal in length

Stability = benzene = more stable

45
Q

electrophilic ring

A

-electrophilic ring = area of high electron density so is attacked by electrophiles

Benzene doesn undergo addition reactions in the same way as ethene. It would take a lot of energy to break the delocalisation

46
Q

nitrated arene

A

Nitrated arene are useful for making explosives such as TNT and industrial dyes

The NO2+ ion is produced by a 2 step reaction. Both acids are concentrated

Sulfuric acid is stronger acid than nitric acid

H2SO4 + HNO3 –> H2NO3+ + HSO4-

HNO3 + 2H2SO4 –> NO2+ + 2HSO4- + H3O+

47
Q

phenyl

A

Benzene which has lost 1 H is called phenyl

48
Q

general mechanism for electrophilic subsitution of benzene

A

-two of the electrons in the delocalised system attracted towards the X+ and form a bond with it

-a lone pair of electrons on Y- forms a bond with the hydrogen atom at the topic of the ring

49
Q

2 types of electrophilic subsitution

A

1) nitration = concentrated nitric acid and concentrated sulfuric acid

2) Friedel craft acylation of benzene

50
Q

electrophiles

A

X = NO2+

Y = HSO4- (has lone pair of electrons)

51
Q

equation for nitration of benzene

A

-benzene + HNO3 –> nitrobenzene + H2O

-generation of electrophile = HNO3 + 2H2SO4 –> NO2+ + 2HSO4- + H3O+

52
Q

friedel crfats acylation

A

-reagent = acyl chloride or acid anhydride

-conditions = anhydrous = H atom on ring is replaced by acyl group

-products = aromatic ketones

53
Q

acyl chloride equation

A

benzene + acyl chloride –> aromatic ketone + HCL

54
Q

acid anhydride equation

A

benzene + acid anhydride –> atomatic ketone + carboxylic acid

55
Q

Equation for regeneration of catalyst in acyl chloride:

A

-AlCl4- + H+ –> AlCl3 + HCl

-electrophile = RCO+ (acylium ion)

Aqa a level chemistry (23 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions