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The alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups of different sizes. They are a subset of the aromatic hydrocarbons. The simplest member is toluene, in which a hydrogen atom of the benzene was replaced by a methyl group.
| Examples of alkylbenzenes | |||
|---|---|---|---|
| parent compound: benzene |
alkylbenzenes | ||
 ≡ |
 | ||
| Comment: The structural formulas of aromatic compounds are usually shown only in one mesomeric form. | |||
Due to the chemical structure of alkylbenzenes, chemical reactions that can happen to this kind of molecules could either target the alkyl groups on the side chains or the benzyl group itself.
Reactions on benzyl groups
[edit]Hydrogenation
[edit]Benzyl groups are unsaturated functional groups, leading to the result that hydrogenation reaction can happen on benzyl groups using hydrogen with the catalysis of nickel, platinum and other transition medals [2]. Benzene group can be reduced to cyclohexane and alkylbenzenes can be reduced to their corresponding cyclohexane derivatives.
Substitution
[edit]Depending on the effect of side chains, the aromatic substitution could be either nucleophilic and electrophilic . When it comes to alkylbenzenes, the side chains of alkylbenzenes are alkyl groups, which are electron donating groups due to the hyperconjugation of alkyl groups to benzyl group. Thus, electrophilic substitution is more inclined to happen compared to nucleophilic aromatic substitution . Thus, the substitution reactions that happen on the benzyl group of an alkylbenzene are called electrophilic substitution of alkylbenzenes.
Substitution reactions on benzyl groups can happen on three positions: ortho, para and meta. However, electrophilic substitution reactions of alkylbenzenes on benzyl groups are more inclined to happen on the ortho and para positions. The benzyl ring becomes more nucleophilic because of the electron donated by the alkyl group on the side chains. Thus electrophilic molecules can react with alkylbenzenes more easily.
Take the mononitration of alkylbenzene as an example, the substitution can selectively happen on the para or ortho site by controlling the size of alkyl groups and reacting conditions [3]. However, because of the electronic nature of alkyl groups, it’s harder to have meta-substitution on the benzyl ring.
Reactions on alkyl groups
[edit]Radical Substitution
[edit]If a chemical structure has more resonance structures, they usually tend to be stable. Since benzylic radical has many resonance structures, benzylic radicals have high stability compared to other kinds of radicals like alkyl radicals, etc.. What’s more, because of the hyperconjugation between benzyl groups and its alkyl side chains, alkyl groups on the side chains help to further stabilize the radical formed on the α-position on the benzyl group [4].
The radical substitution reaction can be induced under various ways. Let’s take the bromination of alkylbenzenes under the catalysis of manganese oxide as an example [5]. First of all, the catalyst in the reaction generates a free radical to initiate the radical reaction trigger by heat, light or other initiator . Then the initiated radical, bromine radical, will react with the hydrogen on the α-position of the alkylbenzene to form a new benzylic radical. Finally, the halogen radical will react with the benzylic radical, thus this will form a new α-bromine substituted product.
Alkyl groups have hydrogens on the carbon atom, so they have the possibility of being oxidized by oxidant like potassium permanganate , hydrogen peroxide , etc. [6] , [7]. Depending on the oxidation ability of different oxidant, alkylbenzenes can be oxidized into different phases.
Under relatively mild conditions, alkyl groups can be oxidized into hydroxyl groups or ketones. If the reaction happens with the existence of strong oxidant reagent like potassium permanganate, the alkyl group will usually transform into a carboxylic group8. Since the transformation from an alkyl group to a carboxylic group will break the carbon-carbon bond, every alkyl side chain on the benzyl ring will essentially become the same group, which is a carboxylic group [8]. .
If the α or β position of the alkylbenzene is substituted by a halogen atom, like chloride or bromide, it is possible to form substituted olefins at the α-position of the alkylbenzene. This kind of elimination reaction can be called olefination or dehydrohalogenation . Olefination can happen under various conditions. By using Wittig -Heck reaction, scientists can achieve dehydrohalogenation using an one-pot reaction [9]. The elimination can also be induced by some catalysts like arsonium ylides [10]. The mechanism of the elimination is identical to halogenated alkane. However, it’s much easier to happen due to the activation and stabilization of the benzyl group.
References
[edit]- ^ Smith, M. B.; March, J. March’s Advanced Organic Chemistry; 2007.
- ^ Bancroft, W. D.; George, A. B. Hydrogenation of Benzene with Nickel and Platinum. J. Phys. Chem. 1930, 35 (8), 2219–2225.
- ^ Smith, K.; Pry, K. Para-Selective Mononitration of Alkylbenzenes under Mild Conditions by use of Benzoyl Nitrate in the presence of a Zeolite Catalyst. Tetrahedron Lett. 1989, 30 (39), 5333–5336.
- ^ Lagercrantz, C. Spin Trapping of Some Short-Lived Radicals by the Nitroxide Method. J. Phys. Chem. 1971, 75 (22), 3466–3475.
- ^ Jiang, X.; Shen, M.; Tang, Y.; Li, C. Chemoselective Monobromination of Alkanes Promoted by Unactivated MnO2. Tetrahedron Lett. 2005, 46 (3), 487–489.
- ^ Emerson, W. S.; Lucas, V. E.; Heimsch, R. A. The Preparation of Aromatic Acids by Liquid Phase Oxidation. J. Am. Chem. Soc. 1949, 71 (5), 1742–1743.
- ^ Muzart, J.; Aït-Mohand, S. Chromium-Mediated Benzylic Oxidations by Sodium Percarbonate in the Presence of a Phase Transfer Catalyst. Tetrahedron Lett. 1995, 36 (32), 5735–5736.
- ^ Seddon, K. R.; Stark, A. Selective Catalytic Oxidation of Benzyl Alcohol and Alkylbenzenes in Ionic Liquids. Green Chem. 2002, 4 (2), 119–123.
- ^ Saiyed, A. S.; Bedekar, A. V. One-Pot Synthesis of Stilbenes by Dehydrohalogenation-Heck Olefination and Multicomponent Wittig-Heck Reaction. Tetrahedron Lett. 2010, 51 (48), 6227–6231.
- ^ Saiyed, A. S.; Bedekar, A. V. One-Pot Synthesis of Stilbenes by Dehydrohalogenation-Heck Olefination and Multicomponent Wittig-Heck Reaction. Tetrahedron Lett. 2010, 51 (48), 6227–6231.
Literature
[edit]- Allinger, Cava, de Jongh, Johnson, Lebel, Stevens: Organische Chemie, 1. Auflage, Walter de Gruyter, Berlin 1980, ISBN 3-11-004594-X, pp. 367–368, 560–562.
- Streitwieser / Heathcock: Organische Chemie, 1. Auflage, Verlag Chemie, Weinheim 1980, ISBN 3-527-25810-8, pp. 1051, 1073–1080.
- Beyer / Walter: Lehrbuch der Organischen Chemie, 19. Auflage, S. Hirzel Verlag, Stuttgart 1981, ISBN 3-7776-0356-2, pp. 442–444.
- Morrison / Boyd: Lehrbuch der Organischen Chemie, 3. Auflage, Verlag Chemie, Weinheim 1986, ISBN 3-527-26067-6, pp. 707–728.