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

Organic Chemistry And Analysis (6) Flashcards

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
Compare the description of the pi bond electrons for alkenes and arenes?
Alkenes= Sideways overlap of p-orbitals where there are localised electrons Arenes= Delocalised electron ring above and below the plane creating a pi system
26
Compare the alkenes and arenes ability to polarise.
Alkenes= Can Arenes= Cannot
27
Do alkenes or arenes require a catalyst?
Arenes
28
Why are alkenes more reactive than arenes?
Alkenes have localised electrons in the double bond so can polarise molecules and so are more susceptible to electrophilic attack.
29
Name the two activating groups
- NH2 - OH
30
Name the deactivating group
- NO2
31
What position do activating groups direct the substituent to?
Positions 2, and 4
32
What position do deactivating groups direct their substituent to?
3
33
What number is ortho?
2
34
What number is meta?
3
35
What number is para?
4
36
Explain -OH directing ability in terms of the oxygen lone pair of electrons
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
Summarise the effect of the activating groups
They direct substituents to positions 2,4 and 6 by increasing the electron density when they donate a pair of electrons
38
What effect does -NO2 have on the pi electrons?
Withdraws electron density from the electron cloud at positions 2,4 and 6
39
Summarise the effect of NH2 and OH groups
Electron donating groups so increases electron density at 2,4 and 6 so electrophiles are directed to these positions
40
Summarise the effect of NO2 groups
NO2 is electron withdrawing so lowers electron density at 2,4 and 6 so electrophiles are attracted to position 3.
41
Define a directing group
bonded substituents that influence electrophiles to yield towards positions ortho, meta, para
42
What type of acid is CN?
A weak acid HCN-> H+ +CN-
43
What does CN- do?
Cyanide ions act as a nucleophile due to its lone pair of electrons on its carbon atom. It attacks the positively charged carbon.
44
What are the steps of nucleophilic addition using HCN?
1. Covalent bond forms between cyanide and carbon 2. Covalent bond forms between oxygen and hydrogen ion from water 3. Product is formed
45
What happens to the intermediate of nucleophilic addition using HCN?
It is protonated
46
What is the product formed in nucleophilic addition using HCN?
Hydroxynitrile
47
What are the conditions and reagent needed for nucleophilic addition using HCN?
NaCN H2SO4
48
Is the C-C double bond polar or non-polar?
Non-polar
49
Is the C--O double bond polar or non-polar?
Polar
50
What is NaBH4?
Reducing agent
51
What is aldehyde reduced to?
Primary alcohol
52
What is ketone reduced to?
Secondary alcohol
53
What conditions and reagents are needed for reduction?
NaBH4 H20
54
What are the steps for the mechanism of reduction (nucleophilic addition)?
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
Are carboxylic acids soluble in water?
Yes
56
Explain the trend in solubility in terms of chain length of a carboxylic acid
Smaller carboxylic acids are soluble but bigger ones have limited solubility because of the increasing hydrophobic nature of the non-polar alkyl chain.
57
Do carboxylic acids fully or partially dissociate?
Partially
58
Describe the test using 2,4-DNP (Bradys reagent)
Starts pale orange and turns into a yellow/orange precipitate in the presence of a carbonyl group.
59
How do you work out the specific carbonyl group after using 2,4-DNP?
Filter precipitate using a Buchner funnel, recrystallise to purify and measure mpt to compare to known values.
60
Describe Tollens reagent
Ammoniacal silver nitrate. Only aldehydes react to produce a silver mirror. This is due to the formation of a silver precipitate.
61
What is the colour change when using acidified potassium dichromate?
Orange to green in the presence of alcohols and aldehydes
62
Describe Fehlings solution
Used to test aldehydes and gives a brick red precipitate
63
What are the equations explaining tollens reagent?
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

Chemistry A-level (8 decks)

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