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OCR A-Level Chemistry A > Chapter 25 - Aromatic Compounds > Flashcards

Chapter 25 - Aromatic Compounds Flashcards

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1
Q

What is benzene

A

aromatic hydrocarbon C6H6

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2
Q

Aromatic

A

Compound containing benzene ring(s)

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3
Q

Aliphatic

A

Chain of compounds without benzene

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4
Q

Alicyclic

A

Organic compound in any ring structure without benzene

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5
Q

Properties of benzene

A
  • colourless with a sweet smell
  • found naturally in crude oil
  • carcinogenic
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6
Q

Disproofs of Kekule’s model

A
  • Benzene does not undergo electrophilic addition (does not decolourise bromine water)
  • X ray diffraction shows that all the C-C bond lengths in benzene were the same (length of double bond is longer than that of single bond)
  • Hydrogenation enthalpy of benzene is less exothermic than expected, benzene is more stable than expected
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7
Q

Describe the structure of benzene (the delocalised model of benzene)

A
  • The p-orbitals are above and below the plane of the benzene ring
  • Sideways overlap of p-orbitals creates a delocalised ring of electron density above and below the plane of the benzene ring
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8
Q

Benzoic acid

A
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9
Q

phenylamine

A
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10
Q

benzaldehyde

A
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11
Q

Why does benzene not undergo electrophilic addition (decolourise bromine water)?

A
  • An alkene can polarise the Br - Br molecule to make Br (delta plus) - Br (delta minus)
  • Benzene cannot polarise Br - Br as the electrons are delocalised and spread across the ring
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12
Q

What are the conditions for the nitration of benzene?

A

Sulfuric acid catalyst and heat to 50 degrees Celsius with a water bath (if temp goes above 50 further substitution occurs)

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13
Q

Mechanism for nitration of benzene and equation for formation of electrophile

A
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14
Q

conditions for halogenation of benzene

A

halogens react with benzene in the presence of a halogen carrier catalyst (FeBr3 or AlCl3 or AlBr3 ect..)

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15
Q

Mechanism for halogenation of benzene and equation for formation of electrophile

A
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16
Q

conditions for the alkylation of benzene

A

react benzene with haloalkane in the presence of a halogen carrier catalyst (AlCl3)

17
Q

equations for the alkylation of benzene

A
18
Q

Conditions for the Acylation of benzene

A

React benzene with an acyl chloride in the presence of a halogen carrier catalyst (AlCl3)

19
Q

equations and mechanisms for acylation of benzene

A
20
Q

What is a phenol

A

an OH group directly bonded to a benzene ring (if Oh group is bonded to a side chain it will be an alcohol)

21
Q

Why is phenol a weak acid

A

It partially disassociates with its H+ ion to form a phenoxide ion
add img

22
Q

solubility of phenol

A

less soluble than alcohol as benzene ring is non polar so OH group has less significance

23
Q

order of acidity

A

alcohols < phenols < carboxylic acids

24
Q

formation of phenol salt (neutralisation reaction)

A

Phenol is a weak acid so reacts with a strong base in a neutralisation reaction

25
Q

conditions for the electrophilic substitution of phenol with bromine

A

room temp with bromine water and NO halogen carrier catalyst

26
Q

formula for the reaction of phenol with bromine

A
27
Q

conditions for the nitration of phenol

A

room temperature and dilute nitric acid

28
Q

formula for the nitration of phenol

A
29
Q

Why does phenol react more readily than benzene

A
  • Lone pair of electrons from the oxygen donated to the delocalised pi system
  • electron density of benzene ring in phenol is increased so polarises electrophiles more strongly than benzene
30
Q

Reasoning behind directing groups

A

substituted benzene rings distort the electron density of the ring and affect the reactivity of each carbon atom

31
Q

electron withdrawing groups

A
  • electronegative groups (like NO2) withdraw electron density from carbon 2,4,6
  • electrophiles are more likely to attack carbons 3 and 5
32
Q

electron donating groups

A
  • donating groups (like OH and NH2) donate electrons to carbons 2,4,6
  • so electrophiles are more likely to attack carbons 2,4,6
33
Q

What is filtration under reduced pressure used for?

A

A technique for separating a solid product from a solvent or liquid reaction mixture

34
Q

How to set up filtration under reduced pressure?

A
  • Connect the büchner flask to the tap and to the büchner funnel ensuring the tubing is sealed
  • Place filter paper in the funnel and dampen with the same solvent used when preparing the solid
  • rinse the crystals with the solvent and leave them under suction for a few minutes so they dry
35
Q

Why is recrystallisation used?

A

The solid product obtained after filtering still contains impurities, which can be removed by recrystallisation

36
Q

How is recrystallisation carried out?

A
  • Pour the chosen solvent into a conical flask. heat with water bath to warm the solvent
  • tip the impure sample into a second conical flask or beaker
  • slowly add the warm solvent to the sample until it dissolves, you should add the minimum amount of solvent needed to dissolve the sample to make a saturated sollution
  • allow the sollution to cool, crystals should form.
  • filter under reduced pressure
37
Q

Why is melting point determination used?

A

To determine whether a solid compound is pure. A pure organic substance has a berm sharp melting range of one or two degrees.
- Impurities cause the compound to have a large melting range, they also lower the melting point

38
Q

What does thin layer chromatography show?

A

Shows if the samples are different compounds

39
Q

How to do thin layer chromatography?

A
  • Draw thin pencil line at bottom of TLC plate
  • spot the samples on the line
  • place in a beaker with 10cm3 solvent and cover with lid to prevent solvent evaporating
  • when solvent is about 1cm from top of plate remove the plate and mark the solvent front with pencil
    -place plate in beaker with iodine crystals and wait for spots to form then calculate Rf values.

OCR A-Level Chemistry A (36 decks)

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