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Chemistry A-level > Organic Chemistry And Analysis (6) > Flashcards

Organic Chemistry And Analysis (6) Flashcards

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1
Q

What is the formula for benzene?

A

C6H6

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2
Q

What family is benzene apart of?

A

Aromatic

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3
Q

How many electrons from each carbon are delocalised?

A

1

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4
Q

Where is benzene found?

A

Crude oil

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5
Q

Describe the kekule model.

A

A six membered ring of carbon joined by alternate single and double bonds

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6
Q

What are the three reasons against the kekule model of benzene?

A
  • Lack of reactivity of benzene
  • Lengths of the carbon carbon bonds
  • Less energy produced than expected
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7
Q

What evidence concerning the reactivity of benzene disproves the kekule model?

A

If benzene had a double bond, it should decolourise bromine in an electrophilic addition reaction but it does not undergo that reaction due to resonance stabilisation nor decolourises bromine.

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8
Q

What evidence concerning the lengths of the carbon-carbon bonds disproves the kekule model?

A

In X-ray diffraction, its possible to measure bond lengths, and the bond length in benzene should alternate between that in alkane and alkene but remains in the middle.

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9
Q

What evidence concerning enthalpy change of hydrogenation disproves the kekule structure?

A

The kekule structure suggests benzene would have an enthalpy change of hydrogenation that is 3X more than cyclohexene (-120) but benzene (-208) is more stable than expected due to resonance stabilisation so is less exothermic than expected. (-152 less energy produced than expected)

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10
Q

What is resonance stabilisation?

A

The stabilisation offered due to the delocalisation of electrons

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11
Q

Describe the delocalised model of benzene?

A

Forms when the p-orbitals overlap sideways forming a pi bond above and below the plane of carbon atoms. The six electrons occupying this system of pi bonds are delocalised.

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12
Q

What bond angle does benzene have?

A

120 as it is trigonal planar.

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13
Q

Describe phenol.

A

Derivative of benzene.
Has a hydroxyl group attached to a phenyl group on a benzene ring

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14
Q

Describe phenyl

A

Six carbon atoms in a hexagonal planar, five bonded to hydrogen atoms. One hydrogen has been substituted by a functional group (not OH) or another element.

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15
Q

What conditions are required for nitration of benzene?

A

50 Degrees Celsius.
Concentrated sulphuric acid catalyst.

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16
Q

What conditions are required for the halogenation of benzene?

A

Halogen carrier catalyst

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17
Q

Why is phenol less soluble in water than alcohols?

A

Due to its non-polar ring

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18
Q

Is phenol a strong or weak acid?

A

Weak acid as it partially dissociates to provide a proton.

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19
Q

What is the electrophile in acylation of benzene?

A

A carbocation
RCO+

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20
Q

What conditions are required for the acylation of benzene?

A

Halogen carrier catalyst

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21
Q

What is required for the alkylation of benzene?

A

Halogen carrier catalyst

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22
Q

Compare electron density of alkene and arene.

A

Alkene= High electron density
Arenes= Low electron density

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23
Q

Compare type of reaction mechanism involved for alkene and arene.

A

Alkene= Electrophilic addition
Arenes= Electrophilic subsitution

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24
Q

Compare chemical test with bromine for alkene and arenes.

A

Alkenes= decolourise bromine
Arenes= Don’t do anything

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25
Q

Compare the description of the pi bond electrons for alkenes and arenes?

A

Alkenes= Sideways overlap of p-orbitals where there are localised electrons
Arenes= Delocalised electron ring above and below the plane creating a pi system

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26
Q

Compare the alkenes and arenes ability to polarise.

A

Alkenes= Can
Arenes= Cannot

27
Q

Do alkenes or arenes require a catalyst?

A

Arenes

28
Q

Why are alkenes more reactive than arenes?

A

Alkenes have localised electrons in the double bond so can polarise molecules and so are more susceptible to electrophilic attack.

29
Q

Name the two activating groups

A
  • NH2
  • OH
30
Q

Name the deactivating group

A
  • NO2
31
Q

What position do activating groups direct the substituent to?

A

Positions 2, and 4

32
Q

What position do deactivating groups direct their substituent to?

A

3

33
Q

What number is ortho?

A

2

34
Q

What number is meta?

A

3

35
Q

What number is para?

A

4

36
Q

Explain -OH directing ability in terms of the oxygen lone pair of electrons

A

Oxygen has a lone pair of electrons in the p-orbital. This p-orbital can overlap with the pi electron cloud becoming delocalised so increases the electron density of the pi system at positions 2,4 and 6

37
Q

Summarise the effect of the activating groups

A

They direct substituents to positions 2,4 and 6 by increasing the electron density when they donate a pair of electrons

38
Q

What effect does -NO2 have on the pi electrons?

A

Withdraws electron density from the electron cloud at positions 2,4 and 6

39
Q

Summarise the effect of NH2 and OH groups

A

Electron donating groups so increases electron density at 2,4 and 6 so electrophiles are directed to these positions

40
Q

Summarise the effect of NO2 groups

A

NO2 is electron withdrawing so lowers electron density at 2,4 and 6 so electrophiles are attracted to position 3.

41
Q

Define a directing group

A

bonded substituents that influence electrophiles to yield towards positions ortho, meta, para

42
Q

What type of acid is CN?

A

A weak acid
HCN-> H+ +CN-

43
Q

What does CN- do?

A

Cyanide ions act as a nucleophile due to its lone pair of electrons on its carbon atom. It attacks the positively charged carbon.

44
Q

What are the steps of nucleophilic addition using HCN?

A
  1. Covalent bond forms between cyanide and carbon
  2. Covalent bond forms between oxygen and hydrogen ion from water
  3. Product is formed
45
Q

What happens to the intermediate of nucleophilic addition using HCN?

A

It is protonated

46
Q

What is the product formed in nucleophilic addition using HCN?

A

Hydroxynitrile

47
Q

What are the conditions and reagent needed for nucleophilic addition using HCN?

A

NaCN
H2SO4

48
Q

Is the C-C double bond polar or non-polar?

A

Non-polar

49
Q

Is the C–O double bond polar or non-polar?

A

Polar

50
Q

What is NaBH4?

A

Reducing agent

51
Q

What is aldehyde reduced to?

A

Primary alcohol

52
Q

What is ketone reduced to?

A

Secondary alcohol

53
Q

What conditions and reagents are needed for reduction?

A

NaBH4
H20

54
Q

What are the steps for the mechanism of reduction (nucleophilic addition)?

A
  1. Lone pair of electrons from hydride ion is attracted to and donated to the delta positive carbon forming a dative covalent bond
  2. Pi bond of the CO breaks by heterolytic fission forming the intermediate
  3. Oxygen atoms donates a pair of electrons to a hydrogen in the water (protonation)
55
Q

Are carboxylic acids soluble in water?

A

Yes

56
Q

Explain the trend in solubility in terms of chain length of a carboxylic acid

A

Smaller carboxylic acids are soluble but bigger ones have limited solubility because of the increasing hydrophobic nature of the non-polar alkyl chain.

57
Q

Do carboxylic acids fully or partially dissociate?

A

Partially

58
Q

Describe the test using 2,4-DNP (Bradys reagent)

A

Starts pale orange and turns into a yellow/orange precipitate in the presence of a carbonyl group.

59
Q

How do you work out the specific carbonyl group after using 2,4-DNP?

A

Filter precipitate using a Buchner funnel, recrystallise to purify and measure mpt to compare to known values.

60
Q

Describe Tollens reagent

A

Ammoniacal silver nitrate. Only aldehydes react to produce a silver mirror. This is due to the formation of a silver precipitate.

61
Q

What is the colour change when using acidified potassium dichromate?

A

Orange to green in the presence of alcohols and aldehydes

62
Q

Describe Fehlings solution

A

Used to test aldehydes and gives a brick red precipitate

63
Q

What are the equations explaining tollens reagent?

A

Contains silver ions reduced to form silver
Ag+ +e- -> Ag
Acts as a mild oxidising agent which can only oxidise aldehydes
RCHO +[O] ->RCOOH

64
Q
A

Chemistry A-level (8 decks)

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