Brainscape's Knowledge GenomeTM

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


See full index

Organic Chemistry Year 13 > 6.1.1 Aromatic Compounds > Flashcards

6.1.1 Aromatic Compounds Flashcards

You may prefer our related Brainscape-certified flashcards:
AP® Chemistry flashcards Periodic Table flashcards Chemistry 101 flashcards Organic Chemistry 101 flashcards
1
Q

what is the formula of benzene

A

C6H6

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

what are characteristics of benzene

A
  • colourless, sweet smelling and highly flammable
  • found in crude oil, cigarette smoke and petrol
  • classified as a carcinogen
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what is the structure of benzene

A
  • hexagon with circle in middle
  • hexagon with double bond on ever other bond
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what is the Kekule model of benzene

A
  • hexagon, with alternating double and single bonds
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what are the 3 evidence points disproving the Kekule model

A

1) lack of reactivity
2) length of the C=C bond
3) hydrogenation enthalpies

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

how does lack of reactivity disprove Kekule’s model

A
  • if benzene did contain C=C
  • it would discolour bromine
    HOWEVER
  • benzene does not undergo electrophilic addition
  • so it does not discolour bromine under normal conditions
  • so must not contain C=C
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

how does the C=C bond length disprove Kekule

A

you can measure bond lengths using x-ray diffraction:
- length of C-C is 1.53 nm
- length of C=C is 1.34 nm
HOWEVER
- bromine contains only one bond length
- equal to 1.39 nm in between both
- so must not contain those two bonds
- (evidence examined by crystallographer Kathleen Lonsdale)

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

how does hydrogenation enthalpies disapprove Kekules theory

A
  • Kekule’s benzene would be called cyclohexatriene
  • so would expect its enthalpy to be three times that of cyclohexene
    HOWEVER:
    1) for cyclohexene, enthalpy change is -120kJmol
    2) for cyclohexadiene, its -240kJmol, as expected
    3) HOWEVER, for benzene it is -208kJmol, which is 152 kJ mol less energy than expected
  • so actual structure of benzene must be more stable than expected
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what is the actual model of benzene called

A

the delocalised model

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

what is the structure of the delocalised model of benzene

A
  • two rings connecting the top and bottom p-orbitals of all the C atoms in benzene
  • called the pi-bond system
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

explain the delocalised model of benzene

A
  • each C-atom uses 3/4 of its available electrons
  • each carbon has 1 electron left in a p-orbital perpendicular to the plane of carbon and hydrogen atoms
  • adjacent p-orbital electrons overlap sideways in both directions above and below plane of C-atoms
  • forms a ring of e-density, creating a pi-bond system spread across all C-atoms
  • electrons in pi-bond are delocalised
  • DELOCALISED RING OF E- DENSITY ABOVE AND BELOW PLANE OF CARBON ATOMS
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

how do you name aromatic compounds

A
  • need to use benzene
  • and then add prefixes
    1) for short alkyl chains (1,4-dimethylbenzene)
    2) halogens (bromobenzene)
    3) nitro groups NO2 (1,3-dinitrobenzene)
  • all mono-substituted
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

when is benzene not the main element of a compound

A
  • when attached to an alkyl chain with a functional group
  • when attached to an alkyl group with more than 7 carbon atoms
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is the prefix used for benzene

A

phenyl-

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

what is benzene attached to COOH called

A

benzoic acid

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

what is benzene attached to NH2 called

A

phenylamine

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

what is benzene attached to CHO called

A

benzaldehyde

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

what is benzene attached to HC=CH2

A

phenylethene

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

what is benzene attached to OH called

A

phenol

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

what is benzene attached to CH2COCH3 called

A

phenylpropanone

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

what do you need to remember when drawing out anything with benzene

A

which molecule is attached to the benzene
- eg for phenol IT MUST BE THE O ATTACHED TO THE LINE, AND THEN H AFTER, NOT JUST IN THE MIDDLE

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

what mechanism does benzene undergo

A

electrophilic substitution

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

what is the typical overall equation of electrophilic substitution of benzene

A

benzene + E+ ===> Ebenzene + H+

  • where E+ represents an electrophile
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

what is the typical mechanism for electrophilic substitution of benzene

A

1) the E+ attracts an electron pair from the dense pi-bond (shown via curly arrow going from circle in benzene to the E+)
2) the H bonded to the carbon that has gained the E gives its e- pair to repair the pi-bond (shown by curly arrow going from C-H bond to the broken horseshoe bond in benzene, with a + towards the bottom of it)
3) the H+ leaves the molecule and the pi-bond is repaired, leaving the product

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

what are the uses of nitrobenzene

A

used in:
- dyes
- pesticides
- starting material of paracetamol

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

what is the overall equation of the nitration of benzene

A

benzene + nitric acid ===> nitrobenzene + water

( H2SO4 and 50C on the arrow)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

why is benzene heated to 50C in nitration

A
  • obtains a good reaction rate
  • not too high that further substitution could occur, resulting in dinitrobenzene from forming
  • can put in water bath to maintain temperature
28
Q

what are the 3 step mechanisms of nitration

A

1) HNO3 + H2SO4 ===> NO2+ + HSO4- + H2O

  • where the nitronium NO2+ ion is used as the electrophile
  • both HNO3 and H2SO4 are concentrated

2) the mechanism (check overall for example)

3) HSO4- + H+ ===> H2SO4

29
Q

why can H2SO4 be described as a catalyst when talking about it in nitration

A

it is regenerated in the final step, so isn’t used up

30
Q

what does the halogenation of benzene require

A

a halogen carrier catalyst, which is generated in situ (in the reaction vessel)
- e.g. AgCl3 and FeBr3

31
Q

what are the reaction conditions of the halogenation of benzene

A

room temperature and pressure

32
Q

what is the overall equation of the bromination of benzene

A

benzene + Br2 ===> bromobenzene + HBr

  • FeBr3 or AlBr3 goes on the arrow
33
Q

what is the 3 step mechanism of the bromination of benzene

A

1) Br2 + AlBr3 ===> AlBr4- + Br+

  • where the bromonium ion is used as the electrophile ( generated via a catalyst as benzene is too stable to react with non-polar benzene)

2) same mechanism for electrophilic substitution as before

3) FeBr4- + H+ ===> FeBr3 + HBr

  • showing that FeBr3 acts as a catalyst
34
Q

what is the alkylation of benzene

A

where the H off one of one of the carbon is substituted with an alkyl group
- could also be called Friedel-Crafts

35
Q

what is the reaction conditions required for the alkylation of benzene

A

presence of AlCl3
- acts as a halogen-carrier catalyst
- generated the electrophile needed

36
Q

what is the overall equation for the alkylation of benzene

A

benzene + R-Cl ===> Rbenzene + HCl

  • where R refers to the alkyl chain
  • and AlCl3 is on the arrow
37
Q

what is the acylation of benzene

A

the reaction of benzene with an acyl chloride and AlCl3 catalyst
- forms an aromatic ketone

38
Q

what is the structure of acylchloride

A

alkyl-C=O
-Cl

39
Q

what is the overall reaction of the acylation of benzene

A

benzene + R-C=O(-C) ===> benzene-C=O(R) + HCl

  • where the R is the alkyl chain
  • and AgCl3 goes on the arrow
40
Q

which mechanisms do alkenes use to react with bromine

A

electrophilic addition

41
Q

what is the overall equation of cyclohexene and bromine

A

cyclohexene + Br2 ===> 1,2-dibromocyclohexane

42
Q

why can alkenes react with bromine

A
  • have a localised pi-bond above and below the plane of the 2 carbon atoms
  • creating an area of high e-density
  • which is able to polarise a dipole onto Br2
43
Q

explain the mechanisms of electrophilic addition

A

1) arrow goes from double bond to the slightly positive bromine, and arrow goes from bromine bond to the slightly negative bromine
2) one bromine bonds to the alkane, and a carbocation is formed at the other carbon. curly arrow goes from the Br- to the +
3) the final product is formed, with both bromines

44
Q

why can benzene not react via electrophilic addition

A
  • it contains delocalised pi-electrons
  • so the electron density between any 2 carbon atoms is less than in a C=C
  • so when non-polar Br2 approaches, there is insufficient pi-electron density around any 2 carbon atoms to polarise it
  • so no reaction
45
Q

what is important to remember when naming phenols

A
  • need to consider which groups take priority
    -e.g. NO2 attached to phenol is 2-nitrophenol
  • BUT a COOH group attached takes priority, so is called 2-hydroxybenzoic acid
46
Q

why is phenol an acid

A
47
Q

what is the solubility of phenol as compared to benzene

A

benzene: cannot dissolve, as only contains L-forces, and is liquid
phenol: can dissolve as has L-forces, but also H-bonding on the OH group, and is a solid as has stronger IMF
(phenol is less soluble than OH tho as it has the presence of the non-polar benzene ring)

48
Q

explain why phenol is a weak acid

A

it only partially dissociates in water, forming a phenoxide ion

49
Q

what bases can phenol, alcohol and carboxylic react with

A

alcohol= none
phenol= only strong bases
carboxylic acids= weak and strong bases

50
Q

how does phenol react with a base

A

phenol + base ===> salt + water

51
Q

what would you get with ohenol and sodium hydroxide

A

phenol + NaOH ===> sodium phenoxide salt + H2O

52
Q

how would you draw a phenoxide salt

A
  • benzene
  • ATTACHED to the O-
  • with the metal+ next to it
  • NEVER do a bond between metal and O tho, as is not a covalent bond
  • for more than monobasic bases, would just do the metal sandwiched between multiple O- benzenes
53
Q

how would phenol react with a metal

A

phenol + metal ===> metal phenoxide + 1/2H2

54
Q

is benzene or phenol more reactive

A

phenol

55
Q

why is phenol more reactive than benzene

A
  • lone pair on the oxygen of OH
  • partially delocalises into the pi-bond system
  • increasing the electron density (8e- with 7 atoms vs 6e- with 6 atoms)
  • more reactive as is able to induce dipoles on non polar molecules
  • more susceptible to electrophilic attack
  • shown by phenol with lone pair on the OH with curly arrow going to the circle of electrons
56
Q

how would u show the overall pi bond in phenol

A
  • draw out phenol skeletally
  • add pi orbitals above and below the Cs and the O
  • attach them together in a circle with a bit bumping out
57
Q

what can be said about the mechanisms in which phenol reacts

A
  • via electrophilic substitution
  • with more milder and readily conditions than with benzene
58
Q

equation for the bromination of phenol

A

phenol + 3Br2 ===> 2,4,6-tribromophenol + 3HBr

59
Q

what are the observations of the bromination of phenol

A

1) would turn from orange to colourless
2) form white precipitate

  • at room temp, and NO catalyst needed
60
Q

what is the equation of the nitration of phenol

A

phenol + nitric acid ===> either 2-nitrophenol or 4-nitrophenol + H2O

  • no 6-nitrophenol needed to be stated as the same as 2- (why you form more of 2 than 4, as 6 is included)
  • at room temp
61
Q

what is disubstitution

A
  • when substituted aromatic compounds go through a second substitution
  • e.g. 2-nitrophenol can react again with a nitro group
62
Q

what are directing groups

A

groups which have an impact on the second substitution of benzene

63
Q

what are the activating groups

A
  • increase the electron density, electron donating and are more reactive than benzene with electrophiles
  • direct at the 2,4(,6) positions (except halogens which can react here)
  • NH2 and OH
64
Q

what are deactivating groups

A
  • decrease electron density
  • electron withdrawing
  • less reactive than benzene with electrophiles
  • direct at 3 an 5
  • NO2
65
Q

why are directing factors useful

A

useful for organic synthesis, where can be used to decide which steps are taken for the products to end up on that position

Organic Chemistry Year 13 (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions