Reduction: ydrogenation 1 Addition of one molar equivalent of hydrogen (-) will generate a diene and addition of two molar equivalents of -) will give an alkene. Complete reduction of benzene to cyclohexane (addition of three molar equivalents of - ) typically requires the use of an excess of hydrogen. The character of the benzene ring makes the reduction more difficult, so a hydrogenation reaction requires more vigorous conditions when compared to hydrogenation of an alkene. This means that higher temperatures and/or higher pressures of hydrogen gas are necessary. When benzene reacts with three molar equivalents of hydrogen gas in the presence of a Raney nickel catalyst, abbreviated Ni(R), reduction to cyclohexane occurs if the reaction is heated. ydrogenation of benzene is also possible using a palladium catalyst or a rhodium (Rh) catalyst. 3 2, Ni(R) 1 eq. 2, Ni(R) 3
Reduction: Birch Reduction When benzene is treated with sodium and ethanol in liquid ammonia, the product is the non-conjugated 1,4- cyclohexadiene, 89. 2 Na 86A 86B 86C Et Et Na 87 88 89
Reduction: Birch Reduction Although there are three resonance contributors to 86, the lowest energy resonance contributor will arise by separation of the two of the carbanion and the single electron of the radical in order to diminish electronic repulsion. This means that 86B represents the lowest energy electronic distribution in the intermediate, and it will lead to the final product. 3 Na 86A 86B 86C Et Et Na 87 88 89
Reduction: Birch Reduction 4 In 91, the negative charge on the ipso carbon leads to electron repulsion that greatly destabilizes that resonance contributor. Electronic repulsion is minimized and the intermediate is more stable (it will form faster) when the negative charge does not reside on the ipso carbon, as in 90. Me!" Me!" Me Me Na or 21 90 91 92 In 94 the negative charge is on the ipso carbon adjacent to the positively charged nitrogen atom of the nitro group. The attraction of positive and negative charges is stabilizing and this intermediate is more stable and formed faster than the alternative 93, where the negative and positive charges are never adjacent to each other. N N N N 2 Na or 40 93 94 95
Reduction: Birch Reduction An electron releasing group (including alkyl groups such as methyl or ethyl) will be attached to an sp 2 carbon in the final product, whereas an electron withdrawing group will be attached to a sp 3 carbon in the final product. 5 Me!" Me!" Me Me Na or 21 90 91 92 N N N N 2 Na or 40 93 94 95
Reduction: Substituents 6 2 heat Pd-C excess 2, Ni(R) 6 15 96 C 2, Pt 2 29 97 N 2 2, Pd-C N 2 1. LiAl 4, TF 2. dil. aq. + N N 23 40 C!N 2, Pd-C N 2 or 1. LiAl 4, TF 2. dil. aq. + 28 99 98
Reduction: Substituents 7 C 2 1. LiAl 4, TF 2. dil. aq. + 27 97 C!N N 1. LiAl 2 4, TF P 2 5, heat 2. dil. aq. + N 2 28 104 99 49 Cl Clemmensen Zn(g), Cl 50 N 2 N 2, K 105 Wolff-Kishner
Reactions 8 Na, N 3, Et 1. NaB 4, Et 2. aq. N 4 Cl 1. 1-bromopropane AlCl 3 2. Br 2, FeBr 3 Br Br 1. propanoyl chloride AlCl 3 2. N 2 N 2, K 1. S 3, 2 S 4 2. 3 2, Ni(R) Et, heat S 3
Aromaticity 9 ückel's rule states that for planar, monocyclic hydrocarbons containing completely conjugated sp 2 hybridized atoms, the presence of (4n+2) π- leads to ity (n is an integer in the series 0, 1, 2, 3, etc.). For ity there must be a ring, so there is a requirement for cyclic polyenes. If the number of π- does not equal 4n+2, then ity does not exist and the system is rather unstable (this is sometimes called an anti- system). If ückel's rule or the "4n+2 rule" as it is called is used, the presence of 2, 6, 10, 14, 18, etc. π- in a ring where every atom is sp 2 hybridized is characteristic of an compound. 6! 14! 18! 1 111 112 anti- 4! 8! 113 114
Anti-Aromaticity 10 There are another set of cyclic polyenes that have (4n) π- such as cyclobutadiene (113; 4 π-) and cyclooctatetraene (114; 8 π-), so they do not adhere to the ückel rule. Both 113 and 114 do not satisfy the ückel rule it is not. These compounds are not, and it is also true that they are also very unstable and difficult to prepare. Cyclic compounds such as this with 4n π- are called anti- compounds. Assume that such compounds cannot be prepared (in fact, they can if extremely low temperatures and specialized conditions are used). 6! 14! 18! 1 111 112 anti- 4! 8! 113 114
Intermediates 11 2! 6! 4! not 6! 8! not 115 116 117 118 119 The cyclopentadienyl anion (118) has six π- and it is, very easy to form and is quite stable. Formation of 118 from cyclopentadiene (120) is an acid-base reaction. The pk a of cyclopentadiene is 14-15 (compare that with a pk a of 15.8 for water). The cycloheptatrienyl anion (119), which has 4n π-, is not and is particularly unstable and difficult to form. The pk a of 121 is about 36, which reflects the great difficulty in forming the anti- conjugate base. base base pk a = 14-15 pk a = 36 120 118 121 119
Are These Aromatic? Look for a monocyclic ring. All atoms sp 2, with no sp 3 atoms in the ring. 4n+2 π- 12 n = 4 not n = 6 but sp 3 break not n = 8 not n = 12 not 18 20 n = 18 n = 20 not
Aromatic Anions and Cations Cations and anions can be. They must meet the same criteria: ring, all sp 2 atoms so that each atom has a p orbital, 4n+2 π-. Remember that a cation is treated as an empty p orbital so that atom will be sp 2, and an anion is a filled p orbital Cyclopropenyl cation 2π-. Aromatic Cyclopropenyl anion 4π-. Not Allyl cation - no ring resonance 2π-. Not Cyclopentadienyl cation 4π-. Not Cyclopentadienyl anion 6π-. Aromatic Cycloheptatrienyl cation 6π-. Aromatic Cycloheptatrienyl anion 8π-. Not Cyclohexadienyl cation Resonance -sp 3 break 4π-. Not
Polycyclic Aromatic Compounds For n = 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 4n+2 = 2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50, 54, 58, 62 5 π-bonds 10 π - azulene 5 π-bonds 10 π - 7 π-bonds 14 π - 9 π-bonds 18 π - Note sp 3 carbon not Benz[α]pyrene 21 π-bonds 42 π -
Polynuclear Aromatic ydrocarbons 15 3 7 6 8 5 1 8 9 1 4 2 7 2 5 6 3 6 3 4 5 10 4 7 122 123 124 8 9 10 2 1 Naphthalene is a bicyclic compound with the formula C 10 8 and structure 122. Naphthalene is planar, with 10 π- in a π-cloud above and below the plane of the ten carbon atoms and, like benzene, it is and particularly stable. Another polycyclic compound has three rings are fused together as in 123, and this molecule (14 π-) is called anthracene (formula, C 14 10 ). There is an isomer of anthracene called phenanthrene, in which the point of attachment of the "third ring" on the "middle ring" is different. Phenanthrene has the same empirical formula but its structure is 124.