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Module 6 Chemistry > Chapter 25 - Benzene > Flashcards

Chapter 25 - Benzene Flashcards

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1
Q

What is the molecular formula of Benzene and what are its key properties

A

C6H6

  • colourless
  • sweet smelling
  • highly flammable
  • carcinogen
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2
Q

what was Kekule’s Model for Benzene

A

cyclohex 1,3,5 tri-ene

effectively cyclohexane with alternating single bond, double bond

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3
Q

what are the three key pieces of evidence to disprove Kekule’s model

A

1) lack of reactivity
2) length of C-C bonds in Benzene
3) energies of hydrogenation of cycloalkenes

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4
Q

explain the evidence for the lack of reactivity of Benzene

A
  • if benzene did have a C=C double bond it would be very reactive e.g. decolourise bromine

However:
- it doesn’t undergo electrophilic addition and therefore doesn’t decolourise bromine water

This suggests it doesn’t have a C=C double bond

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5
Q

explain the evidence for the bond lengths in benzene

A

with X-ray diffraction you can measure bond lengths

  • If Kekule’s model was correct you would expect alternating short C=C bond and long C-C bond lengths
  • It was actually a constant intermediate bond length
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6
Q

explain the evidence for the hydrogenation of cycloalkenes

A

1) cyclohexene + H2 –> cyclohexane,
enthalpy change = -120kjmol^-1

2) cyclohex 1,3, diene + 2H2 –> cyclohexane
enthalpy change = -240kjmol^-1

so therefore you would expect kekule’s model to have an enthalpy of -360kjmol^-1 but when Benzene was reacted with it, the enthalpy change was only -208kjmol^-1

  • this is lower than expected so the molecule is more stable than expected
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7
Q

what are the 6 main points of the delocalised model

A
  • Benzene is a planar, hexagonal hydrocarbon, C6H6
  • Each carbon has 3 shared pairs of electrons/3 bonds
  • Each carbon has an electron in a P orbital perpendicular to the plane of the molecule
  • each adjacent P-orbital overlaps sideways to form rings of electron density above and below the plane of the molecule
  • this creates a system of Pi bonds, all 6 electrons in it are delocalised and can move freely over all the carbons
  • the bond angle is 120 degrees
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8
Q

points on naming aromatic compounds

what are the exceptions

A
  • where there are groups attached to the benzene you name it as such e.g. chlorobenzene, nitrobenzene etc.
  • where benzene is attached to a carbon chain either of 7 or more carbons or with another functional group, Benzene becomes the substituent e.g. phenyl

the exceptions are
Benzoic acid
Phenylamine
Benzaldehyde

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9
Q

what is the type of reaction that benzene typically undergoes, give a generalised equation

A

Electrophilic substitution
C6H6 + E+ —> C6H5E + H+

benzene doesn’t undergo addition

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10
Q

what occurs in the nitration of Benzene, give an overall equation and name the conditions

A
  • benzene will react slowly with concentrated nitric acid to form nitrobenzene
  • a catalyst of concentrated H2SO4 is used at 50 celcius

C6H6 + HNO3 —(H2SO4)(50 degrees)–> C6H5NO2 +H2O

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11
Q

what is the first step in the nitration of Benzene, name the electrophile

A

1) Formation of the electrophile from the catalyst
H2SO4 + HNO3 —> HSO4(-) + NO2(+) + H2O
the electrophile is NO2+

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12
Q

what is the second step in the nitration of benzene, draw out the mechanism, explain what happens

A

2) attack from the electrophile
- the nitronium ion accepts a pair of electrons from the Pi system of the benzene ring forming a daitive covalent bond
- the organic intermediate formed is positively charged and is unstable, the hydrogen donates an electron pair to the Pi ring to form a hydrogen ion and the product

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13
Q

what is the third step in the nitration of benzene, give an equation

A

3) reforming of catalyst
HSO4- + H+ —> H2SO4
- catalyst reforms using the H+ ion from step 2

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14
Q

what occurs in the Halogenation of Benzene, give an overall equation

A
  • benzene reacts with a halogen e.g. Cl2
  • they react at RTP with a catalyst halogen carrier of AlCl3

C6H6 + Cl2 —(RTP)(AlCl3)–> C6H5Cl + HCl

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15
Q

what occurs in the first step of the halogenation of Benzene, give an equation

A

1) formation of electrophile

Cl2 + AlCl3 —> Cl(+) + AlCl4(-)

this can occur with any halogen

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16
Q

what occurs in the second step of the halogenation of benzene, draw the mechanism, explain what occurs

A

2) electrophilic attack
- the positive halogen ion accepts an electron pair from the pi system forming a daitive covalent bond
- the positive intermediate is unstable and breaks down to form a hydrogen ion and a halogenobenzene as the carbon- hydrogen bond breaks by heterolyic fission

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17
Q

what occurs in the third step of the Halogenation of Benzene, give and equation

A

3) reforming of catalyst

AlCl4(-) + H(+) —> AlCl3 + HCl

18
Q

what occurs in the Alkylation of Benzene, give the conditions and the equation, state the type of reaction

A
  • an Alkyl group substitutes for a hydrogen on a benzene ring through electrophilic substitution
  • react Benzene and a haloalkane with a halogen carrier catalyst of AlCl3 at RTP
  • don’t need to know mechanism but likely to work similarly to before with AlCl4(-) being formed

overall equation
C6H6 + C2H5Cl —(AlCl3)–> C6H5(C2H5) + HCl

19
Q

what occurs in the Acylation of Benzene, give the conditions, type of reaction and equation

A
  • An acyl group substitutes for one of the ring’s hydrogens
  • electrophilic substitution
  • AlCl3 acts as a catalyst, RTP

equation
Benzene + acyl Chloride —> Phenyl ketone + HCl
C6H6 + CH3(CH2)nCOCl —-> C6H5(CO(CH2)nCH3) + HCl

20
Q

explain why Alkenes are more reactive than arenes

A

Alkenes will undergo electrophilic addition because:

  • Pi-bond contains localised electrons above or below the C=C bond
  • this gives a higher electron density so they can induce greater dipoles on molecules, and attract more electrophiles
  • the induced dipoles allow molecules to act as electrophiles

Arenes won’t undergo electrophilic addition because:

  • the electron density in the delocalised ring is too low
  • so it doesn’t attract or induce big enough dipoles on electrophiles for addition to take place
21
Q

Define a Phenol

A

“Phenols are a type of organic compound containing an -OH hydroxyl group directly attached to a Benzene ring”

22
Q

what is the solubility of Phenol

A
  • it is more soluble than Benzene due to the presence of the -OH group to hydrogen bond
  • but it is less soluble than Alcohols
23
Q

write the equation for what happens when Phenol dissolves and state the effect of this

A

IN DYNAMIC EQUILIBRIUM
Phenol —> Phenoxide ion + H+

C6H5OH —> C6H5O- + H+

this makes phenol a weak acid

24
Q

explain why phenol is a stronger acid than an alcohol

A
  • there is a positive inductive (electron pushing) effect of an Alkyl group in an alcohol, increasing the dipole on the OH
  • but in phenol, a lone pair on the oxygen is partially donated to the pi ring reducing the dipole on the OH
  • this means there is a greater dipole on the O-H bond in an alcohol
  • so the H+ ion is lost less easily in an alcohol or more easily in Phenol
25
Q

state the order of acidity of Alcohols, Caboxylic acids, and Phenols and state the reactions that show it

A

in increasing acidity
Alcohol - doesn’t react with NaOH or Na2CO3
Phenol - only reacts with NaOH
Carboxylic acid - reacts with both

26
Q

state the equation for the reaction of Phenol with NaOH

A

Phenol + Sodium Hydroxide —> Sodium Phenoxide + water

C6H5OH + NaOH —-> C6H5O-Na+ + H2O

27
Q

generally how do phenols react and how is it different to arenes

A
  • they generally undergo electrophilic substitution reactions
  • but they occur at milder conditions than those for Benzene, e.g. at RTP and without a Halogen Carrier
28
Q

state the equation and conditions for the reaction of Phenol with an excess of Bromine and state what you would see

A

Phenol + Aqueous Bromine —> 2,4,6 Tribromophenol + hydrogen Bromide

C6H5OH + 3Br2 —> C6H2Br3OH + 3HBr

  • it decolourises the bromine water then forms a white precipitate of the 2,4,6 tribromophenol
  • occurs at RTP with no catalyst
29
Q

state the equation of the nitration of Phenol (major and minor products) and the conditions

A

Phenol + Nitric acid —> 2 or 4 Nitrophenol + water

  • occurs at RTP with no catalyst

C6H5OH + HNO3 —> C6H4(NO2)OH + H2O

30
Q

explain the differences in reactivity of Benzene and Phenol

A
  • Phenol reacts more readily than Benzene
  • This is because the lone pair of electrons on the oxygen P-orbital are PARTIALLY donated to the Pi-System
  • this gives Phenol a greater electron density than benzene
  • so it is more able to POLARISE molecules and attract electrophiles, making it more susceptible to attack
31
Q

what occurs when a Benzene ring is activated

A
  • some groups (on Benzene) activate the ring
  • this is where a lone pair on the group is donated to the ring
  • this increases its electron density and reactivity with electrophiles
  • these reactions usually occur at lower temperatures and milder conditions
32
Q

where are groups directed to when the ring is activated

A

2 and 4

33
Q

what occurs when a ring is deactivated

A
  • some groups deactivate the ring
  • this is where they attract or ‘withdraw’ electrons from the ring
  • so its reactivity to electrophiles is decreased
  • this means higher temperatures and a halogen carrier catalyst are usually required
34
Q

where are groups directed when the ring is deactivated

A

position 3

35
Q

which groups are 2 and 4 directing

A 
NH2 or NHR
OH
OR (Not or)
R or C6H5
F, Cl, Br, I
36
Q

which groups are 3 directing

A 
RCOR
COOR
SO3H
CHO
COOH
CN
NO2
NR3+
37
Q

what 3 things should you always mention when talking about the bonding in benzene/comparing to others

A
  • DELOCALISED electrons, which can move to ALL 6 CARBONS
  • ABOVE AND BELOW the plane of the molecule
  • C-C bond length is an intermediate length
38
Q

what to ALWAYS mention when writing about the conditions for the nitration of benzene (or comparing to phenol/cyclohexene etc.)

A

CONCENTRATED HNO3

CONCENTRATED H2SO4

39
Q

when Phenol forms a phenoxide ion, what can we say about the thing it reacts with

A
  • it will form a phenoxide ion when in solution or reacting with a base such as NaOH
  • the phenol loses a proton
  • therefore whatever it reacts with acts as a base
  • this means water/OH can act as a base (despite the mechanism looking like its a nucleophile)
40
Q

how can we use the isomers of a di-substituted benzene compound to provide evidence for the delocalised model

A
  • for 1,2 substituted molecule, there is only 1 isomer possible with the delocalised model
  • however there are two possible with the Kekulé model
  • there has only been the one for the delocalised model found
  • thus providing evidence for the delocalised model

Module 6 Chemistry (6 decks)

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