A-Level Chemistry (WJEC - Eduqas) > Organic Chemistry: Aromatic Compounds > Flashcards

Organic Chemistry: Aromatic Compounds Flashcards

Explain benzene structure and electrophilic substitution reactions. (29 cards)

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1
Q

What type of hydrocarbon is benzene classified as?

A

Aromatic hydrocarbon

Aromatic compounds contain a benzene ring with delocalised electrons.

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2
Q

What is the shape of the benzene molecule?

A

Planar hexagon

All carbon atoms lie in the same plane with bond angles of about 120°.

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3
Q

Fill in the blank:

In benzene, all carbon–carbon bond lengths are _______ between single and double bonds.

A

intermediate

The bonds are all the same length due to electron delocalisation.

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4
Q

True or False:

Benzene contains alternating single and double bonds that remain fixed in position.

A

False

The π electrons are delocalised around the ring.

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5
Q

What type of electrons are delocalised in the benzene ring?

A

p electrons

These electrons form a delocalised π system above and below the ring.

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6
Q

True or False:

Delocalisation of electrons increases the stability of benzene.

A

True

Delocalisation lowers the overall energy of the molecule.

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7
Q

Fill in the blank:

The hypothetical molecule with alternating double bonds used for comparison with benzene is ______-______.

A

cyclohexa-1,3,5-triene

Benzene is more stable than this theoretical structure.

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8
Q

What thermochemical measurement provides evidence for benzene’s extra stability?

A

Enthalpy of hydrogenation

Benzene releases less energy than expected when hydrogenated.

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9
Q

True or False:

Benzene undergoes addition reactions easily like alkenes.

A

False

Addition would break the delocalised electron system.

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10
Q

Fill in the blank:

Benzene usually reacts by _______ reactions rather than addition reactions.

A

substitution

Substitution preserves the aromatic ring.

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11
Q

Why are substitution reactions preferred over addition reactions in benzene?

A

Preserve delocalisation

Addition would destroy the stable aromatic system.

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12
Q

What feature of benzene bonding explains its unusual stability?

A

Electron delocalisation

The π electrons are spread evenly over the entire ring.

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13
Q

What type of reaction commonly occurs when benzene reacts with electrophiles?

A

Electrophilic substitution

A hydrogen atom on the benzene ring is replaced by another group.

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14
Q

Why does benzene prefer substitution reactions rather than addition reactions?

A

Preserve aromatic stability

Addition would disrupt the delocalised π electron system.

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15
Q

Fill in the blank:

In electrophilic substitution, the benzene ring acts as a source of _______.

A

electrons

The π electron cloud attacks the electrophile.

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16
Q

True or False:

Electrophiles are electron pair acceptors.

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A

True

They are attracted to regions of high electron density.

17
Q

What reaction introduces a nitro group into a benzene ring?

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A

Nitration

This reaction forms nitrobenzene.

18
Q

True or False:

Nitration of benzene requires a mixture of concentrated nitric and sulfuric acids.

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A

True

Sulfuric acid acts as a catalyst and generates the nitronium ion electrophile.

19
Q

Fill in the blank:

The electrophile in benzene nitration is the _______ ion.

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A

nitronium

The ion has the formula NO2+.

20
Q

What role does concentrated sulfuric acid play in nitration?

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A

Catalyst

It helps generate the nitronium ion from nitric acid.

21
Q

True or False:

Friedel–Crafts acylation introduces an acyl group onto a benzene ring.

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A

True

The reaction forms a ketone attached to the ring.

22
Q

Fill in the blank:

In Friedel–Crafts acylation, the catalyst commonly used is _______.

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A

AlCl3

Aluminium chloride helps generate the electrophile.

23
Q

What type of compound is produced when benzene undergoes acylation?

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A

Phenyl-ketone

The benzene ring becomes bonded to a carbonyl-containing group.

24
Q

Why are nitration and acylation important in industrial chemistry?

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A

Synthetic intermediates

They produce compounds used in explosives, dyes and pharmaceuticals.

25
What is the **equation for the generation of the nitronium ion** from nitric acid?
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
26
What is the **equation for formation of the acylium ion**?
RCOCl + AlCl₃ → RCO⁺ + AlCl₄⁻
27
What happens to **AlCl₄⁻ after substitution**?
Reacts with H⁺ to regenerate AlCl₃ and form HCl
28
Why is **AlCl₃** considered a **catalyst**?
It is regenerated at the end of the reaction
29
How is **sulfuric acid regenerated** at the end of the nitration of benzene?
HSO₄⁻ + H⁺ → H₂SO₄ ## Footnote Reacts with the H+ released from the benzene ring.
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