Introducing Benzene Flashcards

1
Q

Benzene structure

A

C6H6

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2
Q

Two ways benzene can be shown as

A
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3
Q

What model did Friedrich August Kekulé propose for the structure of benzene?

A

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

Kekulé claimed to have thought of this structure while day-dreaming about a snake seizing its own tail.

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4
Q

What is one piece of evidence that disproves Kekulé’s model regarding benzene’s reactivity?

A

Benzene does not undergo electrophilic addition reactions

If benzene contained C-C bonds, it should decolourise bromine in an electrophilic addition reaction.

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5
Q

What did scientists observe about benzene’s ability to decolourise bromine?

A

Benzene does not decolourise bromine under normal conditions

This observation led scientists to suggest that benzene cannot have any C-C bonds in its structure.

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6
Q

What technique was used to measure the bond lengths in benzene?

A

X-ray diffraction

This technique allows for the precise measurement of bond lengths in a molecule.

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7
Q

True or False: The bond lengths in benzene support the existence of distinct C-C bonds as proposed by Kekulé.

A

False

The uniform bond length suggests that benzene does not have distinct single and double bonds.

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8
Q

What is the expected enthalpy change for reacting three double bonds with hydrogen according to the Kekulé structure?

A

-360 kJ mol

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9
Q

What is the actual enthalpy change of hydrogenation of benzene?

A

-208 kJ mol

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10
Q

How much less energy is produced in the actual enthalpy change compared to the expected value?

A

152 kJ mol

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11
Q

What does the comparison between theoretical and actual enthalpy changes suggest about the structure of benzene?

A

The actual structure of benzene is more stable than the theoretical Kekulé model

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12
Q

What model of benzene was proposed based on the stability observations?

A

The delocalised model of benzene

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13
Q

Fill in the blank: The expected enthalpy change for the Kekulé structure is _______.

A

-360 kJ mol

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14
Q

True or False: The actual enthalpy change of hydrogenation of benzene is greater than the expected change.

A

False

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15
Q

What is the delocalised model of benzene?

A

A model developed to disprove the Kekulé structure of benzene.

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16
Q

What is the molecular structure of benzene?

A

Planar, cyclic, hexagonal hydrocarbon containing six carbon atoms and six hydrogen atoms.

17
Q

How many electrons does each carbon atom use in bonding?

A

Three of its available four electrons.

18
Q

What is the orientation of the p-orbital electrons in benzene?

A

At right angles to the plane of the bonded carbon and hydrogen atoms.

19
Q

What occurs when adjacent p-orbital electrons overlap?

A

They overlap sideways to form a ring of electron density.

20
Q

What type of bonds are created by the overlapping of p-orbitals in benzene?

A

π-bonds (pi-bonds).

21
Q

Over how many carbon atoms do the pi-bonds in benzene spread?

A

Over all six carbon atoms in the ring structure.

22
Q

What is the characteristic of the six electrons in the pi-bond system of benzene?

A

They are said to be delocalised.

23
Q

Draw the p orbitals on a benzene ring

24
Q

Show what happens to the p orbitals and the formation of the ring

25
Q

What are the common substituent groups shown as prefixes to benzene?

A

Short alkyl chains, halogens, nitro groups

Alkyl groups include CH₃, C₂H₅, etc. Halogens refer to F, Cl, Br, I.

26
Q

What are aromatic compounds with one substituent group called?

A

Monosubstituted

The benzene ring is often considered the parent chain in these compounds.

27
Q

True or False: In aromatic compounds, the benzene ring is always the parent chain.

28
Q

What is the chemical name for benzoic acid?

A

benzenecarboxylic acid

29
Q

What is the chemical name for benzaldehyde?

A

benzenecarbaldehyde

30
Q

What type of compounds have two substituent groups on the benzene ring?

A

disubstituted compounds

31
Q

How is the benzene ring numbered when there are substituent groups?

A

Starting with one of the substituent groups

32
Q

In naming substituents on the benzene ring, how are they arranged?

A

In alphabetical order using the smallest numbers possible

33
Q

True or False: Compounds with more than one substituent on the benzene ring are referred to as monosubstituted compounds.

34
Q

Fill in the blank: Compounds with more than one substituent group on the benzene ring are called _______.

A

disubstituted compounds

35
Q

Name the exceptions to benzene naming phenyl rule

A

Benzoic Acid
Phenylamine
Benzaldehyde