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hydrocarbons > benzene > Flashcards

benzene Flashcards

1
Q

What is the molecular formula of benzene?

A

C6H6

This formula indicates a high degree of unsaturation.

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2
Q

Who isolated benzene?

A

Michael Faraday in 1825

Faraday’s work led to the identification of benzene as a stable molecule.

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3
Q

What is unique about the structure of benzene?

A

Cyclic arrangement of six carbon atoms with alternating single and double bonds

This structure contributes to benzene’s unusual stability.

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4
Q

Who proposed the structure of benzene in 1865?

A

August Kekulé

Kekulé’s observation led to the understanding of benzene’s identical carbon and hydrogen atom arrangement.

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5
Q

What indicates the presence of benzene in chemical reactions?

A

Formation of a triozonide

This reaction confirms the presence of benzene within a compound.

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6
Q

What are the names of the isomers of dimethylbenzene?

A
  • 1,2-Dimethylbenzene (o-Xylene)
  • 1,3-Dimethylbenzene (m-Xylene)
  • 1,4-Dimethylbenzene (p-Xylene)

The position of the methyl groups determines the isomer name.

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7
Q

True or False: All hydrogen atoms in benzene are equivalent.

A

True

Each hydrogen atom is attached to a carbon atom in a symmetrical cyclic structure.

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8
Q

Fill in the blank: The molecular formula of benzene, C6H6, indicates a high degree of _______.

A

unsaturation

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9
Q

What did the observation of 1,2-dibromobenzenes indicate?

A

The presence of different isomers

This observation supports the concept of structural isomerism in benzene derivatives.

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10
Q

What type of bonding is found in benzene?

A

Alternating single and double bonds

This bonding pattern contributes to the resonance stability of benzene.

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11
Q

How many carbon and hydrogen atoms are in a monosubstituted derivative of benzene?

A

One fewer hydrogen atom than in benzene

This results in a derivative that retains the cyclic structure of benzene.

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12
Q

What type of hybridization occurs in benzene?

A

sp² hybridization

Each carbon atom in benzene is sp² hybridized, allowing for the formation of sigma bonds.

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13
Q

How many sigma bonds does each carbon atom in benzene form?

A

Six C-H sigma bonds

Each carbon atom overlaps with the s orbital of a hydrogen atom to form these bonds.

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14
Q

What do the sp² hybrid orbitals of carbon atoms in benzene contribute to?

A

Formation of C-C sigma bonds

The sp² hybrid orbitals overlap with those of adjacent carbon atoms.

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15
Q

What is the structure of benzene often represented by?

A

A hexagon with a circle inside

The circle represents the delocalized electrons among the six carbon atoms.

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16
Q

According to Valence Bond Theory, what concept explains the stability of benzene?

A

Resonance

The resonance theory explains the oscillating nature of double bonds in benzene.

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17
Q

True or False: Benzene primarily undergoes addition reactions.

A

False

Benzene prefers substitution reactions due to its stability.

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18
Q

What did Kekulé suggest to explain the structure of benzene?

A

A concept of oscillating double bonds

This was an attempt to address the limitations of earlier models of benzene.

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19
Q

How many main contributing structures are there for benzene according to Kekulé?

A

Two

The two structures are often represented as alternating single and double bonds.

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20
Q

Fill in the blank: The unhybridized p orbital of carbon atoms in benzene forms ______.

A

π bonds

These p orbitals overlap laterally to form π bonds, contributing to benzene’s stability.

21
Q

What is the significance of the internuclear distance in benzene?

A

It corresponds to two Kekulé structures

This distance indicates the nature of bonding in the benzene ring.

22
Q

What problem did Kekulé’s structure face regarding substitution products?

A

Only one ortho disubstituted product forms

This observation contradicted expectations based on the structure proposed by Kekulé.

23
Q

What type of bond forms between carbon atoms in benzene?

A

Sigma bonds

These bonds result from the overlap of hybridized orbitals.

24
Q

What are the two equal possibilities in benzene’s bonding?

A

Lateral overlap of p orbitals

This allows for the formation of π bonds between adjacent carbon atoms.

25
Q

What is the probability distribution of p orbitals in benzene?

A

Equal probability for the p orbital of each carbon atom to overlap with adjacent carbon atoms

This results in a delocalized electron cloud over the carbon atoms.

26
Q

What is the characteristic feature of the electron cloud in benzene?

A

Delocalized π electrons can move freely about the six carbon nuclei

This contributes to the stability of benzene compared to cyclohexatriene.

27
Q

What does X-ray diffraction reveal about the bond lengths in benzene?

A

All six C-C bond lengths are the same (139 pm)

This bond length is intermediate between a single bond (154 pm) and a double bond (133 pm).

28
Q

Why does benzene show reluctance to undergo addition reactions?

A

Absence of pure double bonds in benzene

This unusual behavior is due to the delocalization of electrons.

29
Q

What are the three essential characteristics of aromatic compounds?

A
  • Planarity
  • Complete delocalization of π electrons in the ring
  • Presence of (4n + 2) π electrons in the ring

The integer n can take values 0, 1, 2, etc., known as Hückel’s Rule.

30
Q

Which compound is considered the parent ‘aromatic’ compound?

A

Benzene

The term ‘aromatic’ is now applied to all compounds with similar characteristics.

31
Q

List examples of aromatic compounds.

A
  • Benzene
  • Naphthalene
  • Anthracene
  • Cyclopentadienyl anion
  • Cycloheptatrienyl cation

These compounds vary in the number of π electrons based on Hückel’s Rule.

32
Q

How can benzene be commercially isolated?

A

From coal tar

This is the primary industrial method for obtaining benzene.

33
Q

What is one laboratory method to prepare benzene?

A

Cyclic polymerization of ethyne

This is one of the methods used in laboratory settings.

34
Q

What is the result of decarboxylation of aromatic acids?

A

Sodium salt of benzoic acid heated with sodalime gives benzene

This method is another way to synthesize benzene in the lab.

35
Q

What are aromatic hydrocarbons also known as?

A

Arenes

The term ‘aromatic’ is derived from the Greek word ‘aroma’, meaning pleasant smelling.

36
Q

What is a common characteristic of most aromatic compounds?

A

Contain a benzene ring

Benzene rings are highly unsaturated but retain their unsaturation in most reactions.

37
Q

What are aromatic hydrocarbons containing a benzene ring called?

A

Benzenoids

38
Q

What are aromatic hydrocarbons not containing a benzene ring called?

A

Non-benzenoids

39
Q

Give three examples of arenes.

A
  • Benzene
  • Toluene
  • Biphenyl
40
Q

What is the systematic name for methylbenzene?

A

Toluene

41
Q

What is the name of the compound with the formula C10H8?

A

Naphthalene

42
Q

What is the nomenclature for disubstituted benzene compounds when two hydrogen atoms are replaced?

A

Ortho, Meta, Para

The positions are indicated as follows: ortho (1,2), meta (1,3), and para (1,4).

43
Q

How many equivalent hydrogen atoms are there in benzene?

A

Six

44
Q

What type of product does monosubstituted benzene form?

A

One type

45
Q

Fill in the blank: The 1,2 or 1,6 position is known as ______.

A

Ortho (o-)

46
Q

Fill in the blank: The 1,3 or 1,5 position is known as ______.

A

Meta (m-)

47
Q

Fill in the blank: The 1,4 position is known as ______.

A

Para (p-)

48
Q

What is an example of a disubstituted compound derived from benzene?

A

1,2-Dimethylbenzene

hydrocarbons (2 decks)

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