Organic Chemistry II: Aromatic Compounds
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Questions and Answers

What is the main purpose of Huckel's rule?

  • To calculate the molecular weight of a compound
  • To determine the physical structure of the ring system
  • To determine the number of π electrons in a molecule
  • To determine whether a molecule is aromatic or nonaromatic (correct)

    • What is the condition for a molecule to be aromatic according to Huckel's rule?

  • The number of π electrons equals 3n+1
  • The number of π electrons equals 2n+1
  • The number of π electrons equals 5n+1
  • The number of π electrons equals 4n+2 (correct)

    • Which of the following compounds is NOT aromatic?

  • Furan
  • Benzene
  • Pyrrole
  • 1,3,5-hexatriene (correct)

    • What is the structural criterion that requires a molecule to be planar?

    <p>Criterion 2</p> Signup and view all the answers

    Why does benzene not readily undergo addition reactions?

    <p>Because it is an unsaturated hydrocarbon with a special structure</p> Signup and view all the answers

    What is the carbon-to-hydrogen ratio in benzene?

    <p>1:6</p> Signup and view all the answers

    What is the parent hydrocarbon of aromatic compounds?

    <p>Benzene</p> Signup and view all the answers

    What is the consequence of not fulfilling the fourth criterion for aromaticity?

    <p>The molecule is not aromatic</p> Signup and view all the answers

    What type of reaction does benzene mainly undergo with bromine in the presence of FeBr3?

    <p>Electrophilic aromatic substitution</p> Signup and view all the answers

    What is the reason behind benzene's lack of reactivity towards electrophilic addition?

    <p>The resonance description of benzene</p> Signup and view all the answers

    What is the characteristic feature of the bond between any two carbon atoms in the Kekulé structure of benzene?

    <p>It is sometimes a single bond and sometimes a double bond</p> Signup and view all the answers

    What is the reason behind the naming of certain compounds having properties similar to those of benzene as 'aromatic compounds'?

    <p>Their chemical properties</p> Signup and view all the answers

    What is the number of additional degrees of unsaturation in benzene?

    <p>Three</p> Signup and view all the answers

    What is the shape of the benzene molecule?

    <p>Planar</p> Signup and view all the answers

    What is the characteristic feature of the C—C bond lengths in benzene?

    <p>They are all equal</p> Signup and view all the answers

    What is the purpose of the resonance description of benzene?

    <p>To account for its chemical properties</p> Signup and view all the answers

    What is the actual bond length in benzene?

    <p>1.39 Å</p> Signup and view all the answers

    What is the approximate energy released per double bond in hydrogenation?

    <p>26-30 kcal/mol</p> Signup and view all the answers

    What is the heat of hydrogenation of cyclohexene?

    <p>28.6 kcal/mol</p> Signup and view all the answers

    How many kcal/mol is benzene more stable than the hypothetical 1,3,5-cyclohexatriene?

    <p>36 kcal/mol</p> Signup and view all the answers

    What is the heat of hydrogenation of benzene?

    <p>49.8 kcal/mol</p> Signup and view all the answers

    What is the expected heat of hydrogenation of 1,3-cyclohexadiene?

    <p>57.2 kcal/mol</p> Signup and view all the answers

    What type of reaction is the hydrogenation of a carbon-carbon double bond?

    <p>Exothermic reaction</p> Signup and view all the answers

    What is the resonance energy of a substance?

    <p>The difference between the energy of the real molecule and the calculated energy of the most stable contributing structure</p> Signup and view all the answers

    What is the origin of the word 'aromatic' in organic chemistry?

    <p>From the Greek word 'aroma', meaning 'fragrant'</p> Signup and view all the answers

    What is the characteristic of aromatic compounds in terms of their molecular formula?

    <p>Low hydrogen to carbon ratio</p> Signup and view all the answers

    What is the common feature of molecules of aromatic chemicals?

    <p>A six-carbon unit</p> Signup and view all the answers

    Who proposed the theory to determine if a planar ring molecule would have aromatic properties?

    <p>German chemist and physicist Erich Hückel</p> Signup and view all the answers

    What is the name of the rule that states that a cyclic, planar molecule has 4n+2π electrons to be considered aromatic?

    <p>Hückel's Rule</p> Signup and view all the answers

    What is the characteristic of aromatic compounds in terms of their bonding?

    <p>Conjugated planar ring systems with delocalized π-electrons clouds</p> Signup and view all the answers

    What is the simplest and original member of the class of aromatic hydrocarbons?

    <p>Benzene</p> Signup and view all the answers

    What is another term used to describe aromatic compounds?

    <p>Arenes</p> Signup and view all the answers

    What is the primary reason for benzene and other aromatic compounds to react in a specific way?

    <p>To preserve their aromatic structure and retain their resonance energy</p> Signup and view all the answers

    What is the largest class of aromatic compounds?

    <p>Substituted derivatives of benzene</p> Signup and view all the answers

    What is the purpose of using prefixes like ortho-, meta-, and para- in di substituted aromatic compounds?

    <p>To indicate the position of the substituent</p> Signup and view all the answers

    What is the common name for the benzene ring when it is treated as a substituent?

    <p>Phenyl group</p> Signup and view all the answers

    Why are some mono substituted benzenes named using benzene as the parent?

    <p>Because the substituent is larger than the benzene ring</p> Signup and view all the answers

    What is the abbreviation for the prefix meta-?

    <p>m-</p> Signup and view all the answers

    What is the name of the organization that accepts common names for aromatic compounds?

    <p>International Union of Pure and Applied Chemistry (IUPAC)</p> Signup and view all the answers

    What is the purpose of using common names for aromatic compounds?

    <p>To provide a historic and widely accepted name</p> Signup and view all the answers

    Study Notes

    Aromaticity

    • Aromatic compounds have a distinctive odor and a low hydrogen to carbon ratio in their molecular formula.
    • The word "aromatic" comes from the Greek word "aroma," which means "fragrant."
    • Aromatic compounds have conjugated planar ring systems with delocalized π-electrons clouds rather than distinct alternating single and double bonds.

    Hückel's Rule

    • Hückel's Rule states that a cyclic, planar molecule with 4n+2 π electrons is considered aromatic.
    • The rule combines the number of π electrons and the physical structure of the ring system to determine whether the molecule is aromatic or nonaromatic.

    Criteria for Aromaticity

    • Four structural criteria must be satisfied for a compound to be aromatic:
      • A molecule must be cyclic.
      • A molecule must be planar (all atoms in the molecule lie in the same plane).
      • A molecule must be completely conjugated.
      • The molecule has 4n+2 π electrons (Hückel's Rule).

    Benzene

    • Benzene (C6H6) is the parent hydrocarbon of aromatic compounds.
    • The carbon-to-hydrogen ratio in benzene points to a structure that is extremely unsaturated.
    • Benzene does not readily undergo addition reactions, unlike unsaturated hydrocarbons.
    • Benzene reacts mainly by substitution (Electrophilic Aromatic Substitution reactions).

    Structure of Benzene

    • August Kekulé proposed that benzene was a rapidly equilibrating mixture of two compounds, each containing a six-membered ring with three alternating π bonds.
    • The true structure of benzene is a resonance hybrid of the two Lewis structures, with the dashed lines of the hybrid indicating the position of the π bonds.
    • The actual bond length in benzene is intermediate between the carbon—carbon single bond and the carbon—carbon double bond.

    Stability and Resonance Energy of Benzene

    • The Heat of hydrogenation is the quantity of heat evolved when one mole of an unsaturated compound is hydrogenated.
    • The heat of hydrogenation of benzene is lower than expected, indicating that benzene is more stable than the hypothetical molecule 1,3,5-cyclohexatriene.
    • The resonance energy of benzene is about 36 kcal/mol.

    Nomenclature of Benzene Derivatives

    • All compounds that contain a benzene ring are aromatic, and substituted derivatives of benzene make up the largest class of aromatic compounds.
    • Mono substituted benzenes can be named systematically using benzene as the parent and listing the substituent as a prefix.
    • If the substituent is larger than the benzene ring, the benzene ring is treated as a substituent and is called a phenyl group.

    Di substituted Derivatives of Benzene

    • Some di substituted aromatic compounds have common names which have acquired historic respectability and are accepted by IUPAC.
    • When there are two substituents present, the prefixes ortho-, meta-, and para- are used to indicate the position of the substituents.

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    Description

    Learn about aromaticity and Erich Hückel's rule for determining aromatic properties in planar ring molecules.

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