Definition, Structure, and Properties of Benzene

Benzene is a chemical that is widely used in the chemical industry, especially as a precursor in the synthesis of various other chemicals. Benzene is an aromatic hydrocarbon. The term “aromatic” is given because when it was first discovered many compounds in this group had a distinctive aroma. However, nowadays the term “aromatic” is associated with certain structures and characteristics, apart from the aroma. This is because it was recently discovered that there are aromatic compounds that are odorless.

Benzene Structure

Benzene has the chemical formula C.6H6. Comparison of the number of C and H atoms shows that benzene is highly unsaturated. Initially, experts proposed that benzene has an aliphatic structure in the presence of double or triple bonds. However, the fact is that benzene does not exhibit the unsaturation properties of such structures. The experimental results show the properties of benzene such as:

  • Benzene is very stable (not reactive).
    Benzene does not react with Br2, except with the aid of a catalyst. This is incompatible with the unsaturation properties of alkenes or alkenes which are readily added to bromine.
  • Monosubstitution of the halogen atom (X) in benzene only produces one type of compound, namely C6H5
    This shows that there is no geometrical isomeran as in alkenes.

In 1865, August Kekulé proposed the structure of benzene as a ring consisting of 6 C atoms with alternating single and double bonds. However, this structure again cannot explain the properties of benzene such as:

  • Benzene even though it has a double bond tends to undergo a substitution reaction, not an addition reaction like most alkenes.
  • Based on measurements by X-ray diffraction, benzene has only one bond length between the C atoms, namely 0.139 nm, which indicates that all the bonds in benzene are equal.

benzene structure



Kekulé’s model of the structure of benzene.

Based on these facts, in 1931, Linus Pauling then formulated the benzene structure as a resonance hybrid structure, which is a structure that is between the two Kekulé structures. This structure forms an electron delocalization system which stabilizes the structure.the resonant structure of the linus pauling



The resonance structure of benzene according to Linus Pauling.
The ↔ sign indicates resonating, not equilibrium.resonant hybrid



The structure of a benzene resonance hybrid with a ring in the ring indicates a delocalisation of electrons.

Benzene Derivatives Nomenclature

A benzene derivative can be considered to be derived from benzene in which one or more of its H atoms are replaced by other substituents in the form of atoms such as −Br or atomic groups such as −COOH. Following naming rules for compounds benzene derivatives.

  • If one H atom in benzene is replaced by a substituent such as −Cl, −CH3, −NO2, −NH2, −OH, −CHO, and others, then the structure and naming are as follows.
  • If there is more than one substituent, they are numbered clockwise or counterclockwise so that the substituents get the lowest number possible. For similar substituents, the prefixes di-, tri-, tetra-, penta-, and hexa- are used. The order of priority numbering for some common substituents is as follows.

−COOH, −SO3H, −COOR, −CN, −CHO, −CO, −OH, −NH2, −OR, −R, −X (F, Cl, Br, I), −NO2

If there are two substituents, apart from numbering, the prefix o- (ortho) can also be used for the position atom carbon numbers 1 and 2, m- (meta) for positions 1 and 3, or p- (para) for positions 1 and 4.

Example:

  • If the benzene ring is considered a substituent, not a parent, then the benzene group is missing one H (C) atom6H5-) is called a phenyl group. Meanwhile, the methylbenzene (toluene) group loses one H (C) atom6H5CH2-) is called a benzyl group.

Example:

Properties of Benzene

Benzene including poisonous compounds and is carcinogenic (can cause cancer). Benzene is liquid at room temperature, colorless, and volatile. The melting point is 6 ° C and the boiling point is 80 ° C. Its symmetrical and planar structure results in a denser arrangement of crystals and consequently a higher melting point than hexane (−95 ° C). Like other hydrocarbons, benzene is also nonpolar. Therefore, benzene dissolves in less polar or nonpolar solvents such as ether and tetrachloromethane, but insoluble in polar solvents such as water.

Benzene tends to be less reactive, but it burns easily. Due to the existence of an electron delocalization system that stabilizes benzene, benzene tends to be more prone to substitution reactions than addition reactions. The substitution reaction in benzene involves the attack of a partially or completely positively charged reagent that likes electrons (electrophiles) on the benzene ring. Therefore, the substitution reaction in benzene is often referred to as an electrophilic substitution reaction. Substitution reactions in benzene include:

Benzene reacts with halogens such as Cl2 and Br2 with the help of an iron (III) halide (FeCl) catalyst3 or FeBr3) form halobenzene compounds.

Benzene reacts with concentrated nitric acid with concentrated sulfuric acid as catalyst to form nitrobenzene.

Benzene reacts with SO3 in concentrated sulfuric acid when heated to form benzenasulfonic acid.

Benzene reacts with alkyl halides with the help of an aluminum chloride (AlCl3) to form alkylbenzene.

Uses of Benzene and Its Derivatives

  • Benzene is used as a solvent and is also the basis for the synthesis of various benzene derivatives such as styrene, and others.
  • Phenol is a weak acid and can be used as a raw material for making plastics and medicines. Phenol can also be used as an antiseptic and disinfectant because of its denaturing properties of proteins.
  • Toluene is used as a solvent and raw material for the manufacture of benzoic acid, TNT explosives, and others.
  • Benzoic acid is used as a preservative for food and beverages.
  • Aniline is used to make diazo dyes.
  • Salicylic acid is used to make aspirin, flavorings, perfumes, powders, anti-fungal ointments, and shampoos.

Examples of Benzene Problems and Discussions

Example Problem 1

List the names of the following benzene derivatives.

Answers:

a. methyl benzoate
b. isopropyl benzene
c. p-aminobenzoic acid (4-aminobenzoic acid)
d. 1,3,5-trinitrobenzene

Example Problem 2

Draw the structures of the following benzene derivatives.

a. salicylic acid (2-hydroxybenzoic acid)
b. o-xylene
c. benzyl methyl ether
d. 3,5-dimethoxybenzaldehyde

Answer:

Reference

Brown, Theodore L. et al. 2015. Chemistry: The Central Science (13th edition). New Jersey: Pearson Education, Inc.
Johari, JMC & Rachmawati, M. 2008. High School Chemistry and MA for Class XII Volume 3. Jakarta: Esis
Okuyama, Tadashi & Maskill, Howard. 2014. Organic Chemistry: a mechanistic approach. Oxford: Oxford University Press
Purba, Michael. 2007. Chemistry 3B for Class XII Senior High School. Jakarta: Erlangga
Wade, LG & Simek, JW 2016. Organic Chemistry (9th edition). Harlow: Pearson Education Limited

Contributor: Nirwan Susianto, S.Si.
Alumni of Chemistry, Faculty of Mathematics and Natural Sciences UI

Sumber

Previous Post Next Post

Tinggalkan Balasan

Alamat email Anda tidak akan dipublikasikan. Ruas yang wajib ditandai *