Aromaticity hydrocarbons aliphatic alkanes alkenes aromatic alkynes Aliphatic

Aromaticity hydrocarbons aliphatic alkanes alkenes aromatic alkynes

Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc. Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.

Benzene. This aromatic hydrocarbon was first discovered in 1825 but its structure was not generally agreed upon until 1946. Facts about benzene: a) Formula = C 6 H 6 b) Isomer number: c) one monosubstituted isomer C 6 H 5 Y known d) three disubstituted isomers C 6 H 4 Y 2 known c) Benzene resists addition reaction, undergoes substitution reactions. d) Heats of hydrogenation and combustion are far lower than they should be. e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.

a) Formula = C 6 H 6 Max. number of H’s for 6 -carbons = 14. Benzene only has 6 hydrogens. Given one degree of unsaturation (double bond or ring) for every two missing hydrogens less than the maximum, benzene has 4 degrees of unsaturation; that is four combinations of pi-bonds and rings. CH 3 C C-C CCH 3 HC C-CH 2 CH 3 HC CCH 2 C CCH 3 HC CCH 2 C CH

CH 3 HC CCHC CH CH 2=CHC CCH=CH 2=CHCH=CHC CH CH 2=C=CHCH 2 C CH CH 2=C=CHC CCH 3 CH 2=C=CC CH CH 2=C=CHCH=C=CH 2 CH 3 CH=C=CHC CH CH=CH 2=C C CH

CH 2 H 2 C =C=C=CH 2 =C=CH 2 HC 2= =CH 2

=CH 2 Which of these structures is benzene?

b) Isomer number. There is only one monosubstituted benzene of any type: only one bromobenzene C 6 H 5 Br, only one nitrobenzene C 6 H 5 NO 2, etc. CH 3 C C-C CCH 3 one possible HC CCH 2 C CCH 3 three possible HC C-CH 2 CH 3 three possibles HC CCH 2 C CH two possible

CH 3 HC CCHC CH three CH 2=CHCH=CHC CH five + CH 2=C=CHC CCH 3 three + CH 2=C=CHCH=C=CH 2 four + CH 2=CHC CCH=CH 2 two + CH 2=C=CHCH 2 C CH four + CH 3 CH 2=C=CC CH three + CH=CH 2=C C CH four + CH 3 CH=C=CHC CH four +

CH 2 H 2 C CH 2 one possible =C=C=CH 2 two CH 2 =C=CH 2 three + HC 2= =CH 2 two +

=CH 2 one three two

There are three disubstituted benzenes of any type: three dibromobenzenes C 6 H 4 Br 2, etc. CH 2 CH 3 C C-C CCH 3 two possible CH 2 four

No classical valence bond structure for C 6 H 6 correctly explains the existence of only one monosubstituted benzene and three disubstituted benzenes. Kekulé (1890) proposed that the following were in rapid equilibrium:

If benzene is 1, 3, 5 -cyclohexatriene as Kekulé proposed, what should its chemistry be? Alkenes, dienes, cyclcoalkenes, etc. typically give addition reactions with electrophiles. But benzene doesn’t undergo the reactions typical of unsaturated hydrocarbons!

Reagent Cyclohexene Benzene KMn. O 4 oxidation no reaction Br 2/CCl 4 addition no reaction HI addition no reaction H 2/Ni reduction no reaction

Benzene + 3 H 2, Ni, room temp. NR Benzene + 3 H 2, Ni, 200 o. C, 1500 psi cyclohexane Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.

Reactions of benzene: 1. Nitration 2. C 6 H 6 + HNO 3, H 2 SO 4 C 6 H 5 NO 2 + H 2 O 2. Sulfonation 3. C 6 H 6 + H 2 SO 4, SO 3 C 6 H 5 SO 3 H + H 2 O 3. Halogenation 4. C 6 H 6 + X 2, Fe C 6 H 5 X + HX 4. Freidel-Crafts alkylation 5. C 6 H 6 + RX, Al. Cl 3 C 6 H 5 R + HX 6. substitutions

d) Heats of hydrogenation and combustion are far lower than they should be. cyclohexene + H 2, Ni cyclohexane + 28. 6 Kcal/mole 1, 3 -cyclohexadiene + 2 H 2, Ni cyclohexane + 55. 4 Kcal/mole (predicted value = 2 X 28. 6 = 57. 2 Kcal/mole) benzene + 3 H 2, Ni, heat, pressure cyclohexane + 49. 8 Kcal/mole (predicted value = 3 X 28. 6 = 85. 8 Kcal/mole) Heat of hydrogenation for benzene is 36 Kcal/mole lower than predicted!

e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds. C—C single bonds 1. 50 Å C = C double bonds 1. 34 Å The bonds in benzene are all equal and 1. 39 Å but 1, 3, 5 -cyclohexatriene has three double bonds and three single bonds!

Resonance! We can draw more than one classic structure that differ only in where the electrons are. The two structures are of the same energy, so resonance is important. The molecule cannot be adequately represented by classic structures but must be thought of as a hybrid of the contributing structures. Additionally, the hybrid is more stable than any of the contributing structures (resonance stabilization energy).

Facts about benzene: a) Formula = C 6 H 6 b) Isomer number: c) one monosubstituted isomer C 6 H 5 Y known d) three disubstituted isomers C 6 H 4 Y 2 known c) Benzene resists addition reaction, undergoes substitution reactions. d) Heats of hydrogenation and combustion are far lower than they should be. e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.

Aliphatic hydrocarbons are open-chain and ring compounds that react like open chain compounds: saturated: alkanes and cycloalkanes (typical reaction = substitution) unsaturated: alkenes, alkynes, dienes, cycloalkenes (typical reaction = addition). Aromatic hydrocarbons are unsaturated ring compounds that resist the typical addition reactions of aliphatic unsaturated compounds, instead undergoing substitution reactions. They are also much more stable than they should be.

Can we predict which compounds will be aliphatic and which ones will be aromatic like benzene? Yes. In order to be aromatic, the compound must be: 1) cyclic with p-atomic orbitals on all members of the ring. 2) and 2) have 4 i + 2 electrons in the p-orbitals of the ring (where i = 0, 1, 2, 3, …). 3) [ eg. = 2 or 6 or 10 or 14 or 18 or 22 or 26… ]