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Questions and Answers
What is the main purpose of Huckel's rule?
What is the condition for a molecule to be aromatic according to Huckel's rule?
Which of the following compounds is NOT aromatic?
What is the structural criterion that requires a molecule to be planar?
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Why does benzene not readily undergo addition reactions?
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What is the carbon-to-hydrogen ratio in benzene?
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What is the parent hydrocarbon of aromatic compounds?
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What is the consequence of not fulfilling the fourth criterion for aromaticity?
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What type of reaction does benzene mainly undergo with bromine in the presence of FeBr3?
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What is the reason behind benzene's lack of reactivity towards electrophilic addition?
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What is the characteristic feature of the bond between any two carbon atoms in the Kekulé structure of benzene?
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What is the reason behind the naming of certain compounds having properties similar to those of benzene as 'aromatic compounds'?
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What is the number of additional degrees of unsaturation in benzene?
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What is the shape of the benzene molecule?
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What is the characteristic feature of the C—C bond lengths in benzene?
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What is the purpose of the resonance description of benzene?
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What is the actual bond length in benzene?
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What is the approximate energy released per double bond in hydrogenation?
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What is the heat of hydrogenation of cyclohexene?
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How many kcal/mol is benzene more stable than the hypothetical 1,3,5-cyclohexatriene?
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What is the heat of hydrogenation of benzene?
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What is the expected heat of hydrogenation of 1,3-cyclohexadiene?
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What type of reaction is the hydrogenation of a carbon-carbon double bond?
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What is the resonance energy of a substance?
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What is the origin of the word 'aromatic' in organic chemistry?
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What is the characteristic of aromatic compounds in terms of their molecular formula?
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What is the common feature of molecules of aromatic chemicals?
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Who proposed the theory to determine if a planar ring molecule would have aromatic properties?
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What is the name of the rule that states that a cyclic, planar molecule has 4n+2π electrons to be considered aromatic?
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What is the characteristic of aromatic compounds in terms of their bonding?
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What is the simplest and original member of the class of aromatic hydrocarbons?
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What is another term used to describe aromatic compounds?
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What is the primary reason for benzene and other aromatic compounds to react in a specific way?
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What is the largest class of aromatic compounds?
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What is the purpose of using prefixes like ortho-, meta-, and para- in di substituted aromatic compounds?
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What is the common name for the benzene ring when it is treated as a substituent?
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Why are some mono substituted benzenes named using benzene as the parent?
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What is the abbreviation for the prefix meta-?
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What is the name of the organization that accepts common names for aromatic compounds?
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What is the purpose of using common names for aromatic compounds?
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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.
