Organic Chemistry Aromatic Compounds Flashcards
“Aromatic” now describes an unusually stable ring system with several unique characteristics. ___ are cyclic, conjugated molecules that possess 4n+2 pi electrons and adopt planar conformations to allow maximum overlap of the conjugated pi orbitals. Aliphatic describes all compounds that are not aromatic
aromatic compounds
All ___ are conjugated, but not all conjugated molecules are aromatic. A conjugated system consists of alternative single and double bonds, creating a delocalized pi electron network. This allows one or more resonance structures to exist; resonance indicates stability, which is a key characteristic of aromatics. Because every other bond is a double bond, every carbon in the conjugated system is sp2 hybridized, and thus is trigonal planar in shape. When confined to a ring system, all of the sp2 hybridized carbons must lie flat relative to each other, giving another characteristic of aromatics: they are cyclic and planar
aromatic compounds
The ___ of possessing 4n+2 pi electrons is known as Huckel’s rule, and is an important indicator of aromaticity. If a cyclic conjugate molecule follows Huckel’s rule, then it is an aromatic compound. The integer n can be any nonnegative whole number, including 0. It is often easier to remember and recognize these allowable values for Huckel’s rule than to calculate them. To determine the number of pi electrons, determine the number of pi bonds and multiply by two. Unbonded p orbitals that contain lone pairs of electrons sometimes take part in the conjugation of an aromatic compound. Resonance forms are possible that include these electrons and usually have formal charges; when this is possible, the lone pairs are counted toward Huckel’s rule
criterion
___, anions, and cations may all be aromatic, and many compounds must be in their ionic form in order to be aromatic. A cyclic, conjugated molecule that possesses 4n pi electrons is said to be anti-aromatic (a cyclic conjugated molecule that is unstable and highly reactive).
neutral compounds
___ compounds are referred to as aryl compounds, or arenes, and are represented by the symbol Ar. Aliphatic compounds are called alkyl and are represented by the symbol R. Common names exist for many mono- and disubstituted aromatic compounds.
The benzene group is called a phenyl group (Ph) when named as a substituent. the term benzyl refers to a toluene molecule substituted at the methyl position
aromatic
___ are named as alkyl benzenes, with the substituents numbered to produce the lowest sequence. A 1,2-disubstituted compound is called ortho- or o-; a 1,3-disubstituted compound is called meta-, or m-; and a 1,4-disubstituted compound is called para- or p-
substituted benzene rings
The ___ of aromatic compounds are generally similar to those of other hydrocarbons. In contrast, chemical properties are significantly affected by aromaticity. The characteristic planar shape of benzene permits the ring’s six pi orbitals to overlap, delocalizing the electron density. All six carbon atoms are sp2 hybridized, and each of the six orbitals overlaps equally with its two neighbors. As a result, the delocalized electrons form two pi electron clouds, one above an done below the plane of the ring. this delocalization stabilize the molecule, making it fairly unreactive: in particular, benzene does not undergo addition reactions as alkenes do. The same stability holds true for other aromatic compounds, since the definition of an aromatic compound includes the presence of a delocalized pi electron system
physical properties
The most important reaction of ___ is electrophilic aromatic substitution. In this reaction an electrophile replaces a proton on an aromatic ring, producing a substituted aromatic compound. Because benzene is nonreactive, a form of acidic catalyst is required for the substitution reaction to proceed
aromatic compounds
___ react with bromine or chlorine in the presence of a Lewis acid, such as FeCl3, FeBr3, or AlCl3, to produce monosubstituted product in good yield. Reaction of fluorine and iodine with aromatic rings is less useful, as fluorine tends to produce multisubstituted products and iodine’s lack of reactivity requires special conditions for the reaction to proceed
aromatic rings
___ rings react with fuming sulfuric acid (a mixture of sulfuric acid and sulfur trioxide) to form sulfonic acids (sulfonation)
aromatic
The ___ of aromatic rings is another synthetically useful reaction. A mixture of nitric and sulfuric acids is used to create the nitronium ion, NO2+, a strong electrophile. This reacts with aromatic rings to produce nitro compounds
nitration
In a Friedel-Crafts acylation reaction, a ___, usually an acyl group, is incorporated into the aromatic ring. These reactions are usually catalyzed by Lewis acids such as AlCl3. This reaction can also be used to add alkyl groups, but the reaction is difficult to control and can lead to multiple products
carbocation electrophile
The first ___ on an aromatic ring strongly influences the susceptibility of the ring to further electrophilic aromatic substitution, and also strongly affects what position on the ring another electrophile is most likely to attack. Substituents can be grouped into three different classes according to whether they are activating or deactivating, and where on the ring the next reaction is likely to take place
substituent
___ cause the next reaction to occur more quickly and more easily; deactivators cause the next reaction to occur more slowly and with greater difficulty. These effects depend on whether the group tends to donate or withdraw electron density
The three classes of substituents, each in decreasing strength of effect, are:
a. Activating, ortho/para-directing substituents (electron-donating): NH2 > NR2 > OH > NHCOR > OR > OCOR > R
b. Deactivating, ortho/para-directing substituents (weakly electron-withdrawing): F> Cl > Br > I
c. Deactivating, meta-directing substituents (electron-withdrawing): NO2 > SO3H > carbonyl compounds, including COOH, COOR, COR, and CHO
activators
___ rings can be reduced by catalytic hydrogenation under vigorous conditions (elevated temperature and pressure) to yield cyclohexane. Ruthenium or rhodium on carbon are the most common catalysts; platinum or palladium may also be used
benzene
