Benzene Structure

Benzene is an unusual-looking hydrocarbon. In terms of its structure, it has six carbon atoms but just six hydrogen atoms. However, it is extraordinarily common and extremely important in our everyday lives. F

Benzene Structure Benzene Structure

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    You’ll find benzene rings in many of the compounds that make food taste so great - for example, in vanillin, the molecule responsible for the sweet taste of vanilla. Ibuprofen is also a benzene derivative. And if you’ve ever painted a wall, you’ll know that characteristic wet paint smell. It is caused by toluene - another molecule with a benzene ring, \(C_6H_6\) .

    Benzene is an aromatic compound with the molecular formula \(C_6H_6\) .

    As you learnt in Aromatic Chemistry, aromatic compounds all contain a ring of delocalised pi electrons. Don’t worry - we’ll explore that again next, as we explore Benzene's structure.

    Benzene Structure, benzene, StudySmarterFig. 1 - Benzene

    How is benzene structured?

    Benzene has a unique structure. There are some important structural aspects you need to know about, such as its formula, bond lengths, and electron arrangement.

    Structural and displayed formulae

    As we mentioned above, benzene has the molecular formula \(C_6H_6\) . It forms a hexagonal molecule, which we often represent as a hexagon with a circle inside.

    Benzene Structure, benzene, StudySmarterFig. 2 - The conventional representation of a benzene molecule

    Bond length

    We explored some potential structures for benzene in Aromatic Chemistry, each containing three C=C double bonds. But in actual fact, we know that benzene doesn’t contain any double bonds at all. Instead, all of its carbon-carbon bonds are identical intermediates - halfway between a single and a double bond in length. We’ll explore why in just a second.

    Bond angle

    Each carbon atom in benzene is bonded to two other carbon atoms and just one hydrogen atom, and the bond angle between each bond is 120°. This makes benzene a trigonal planar molecule.

    Benzene Structure, benzene bonds, StudySmarterFig. 3 - Benzene has a bond angle of 120°

    However, we know that carbon has four valence electrons. Only three electrons have formed bonds - what has happened to the last one? To answer that, let’s look at electron orbitals and something called the delocalised pi system.

    The delocalised pi system

    Carbon’s fourth outer-shell electron is found in a pi orbital, whereas the bonded three are found in sigma orbitals. Sigma orbitals stretch between atoms whilst pi orbitals extend above and below the atom. In benzene, all the pi orbitals of the carbon atoms overlap, producing a connected region that stretches above and below the molecule.

    Benzene Structure, benzene structure, StudySmarterFig. 4 - Benzene’s six pi orbitals overlap and delocalise

    The electrons can move anywhere within this overlapping region. We say that they are delocalised. The overall structure is called the delocalised pi system.

    Because of these delocalised electrons, benzene doesn’t need to form any double bonds. As we mentioned above, all of its C-C bonds are instead identical intermediates.

    To summarise, benzene has the following structure:

    • It has a planar hexagonal shape.
    • Each carbon atom is bonded to two other carbon atoms and one hydrogen atom using three of its valence electrons.
    • Each of its C-C bonds is an intermediate, halfway between a single and double bond in length.
    • The angle between bonds is 120°.
    • Each carbon atom’s fourth valence electron is delocalised in a region above and below the molecule.

    Kekulé's structure of benzene, and other theories

    In 1865, the German organic chemist Friedrich August Kekulé published a paper on the structure of benzene. This was a mystery that had puzzled scientists for years. He claimed that he dreamt about a snake biting its own tail, which led him to conclude benzene’s cyclic nature. Kekulé proposed that benzene contained alternating C-C single and C=C double bonds, as shown below. This molecule is systematically known as cyclohexa-1,3,5-triene.

    Benzene Structure, benzene structure, StudySmarterFig. 5 - Kekulé’s proposed structure for benzene: cyclohexa-1,3,5-triene

    However, there were a number of pieces of evidence that didn’t quite support this structure.

    Electrophilic addition reactions

    A common test for alkenes is to mix them with bromine water. If a C=C double bond is present, the water will become decolourised as the bromine atoms join on to the hydrocarbon in an electrophilic addition reaction. Kekulé’s structure for benzene contains three C=C double bonds, so we’d expect it to react in this way. However, it doesn’t - when mixed with bromine water, the solution remains red-brown. This suggests that benzene doesn’t have any double bonds.

    Enthalpy of hydrogenation

    Reactions that add hydrogen to a molecule are known as hydrogenation reactions. Let’s look at cyclohexene, shown below. It has an enthalpy of hydrogenation of -120 kJ mol-1, meaning that 120 kJ of energy are released when two hydrogen atoms add on to its single double bond. This produces cyclohexane.

    Structure and reactivity enthalpy hydrogenation, cyclohexene, StudySmarterFig. 6 - The enthalpy of hydrogenation of cyclohexene

    If we now look at Kekulé’s structure ,we can see that it has three C=C double bonds. We’d therefore expect it to have an enthalpy of hydrogenation three times as great as cyclohexene, which has just one double bond:

    3 x -120 = -360 kJ mol-1

    However, experiments show that benzene’s enthalpy of hydrogenation is only -208 kJ mol-1. It is 152 kJ mol-1 more stable than expected. This is known as benzene’s resonance energy. We now know that this stability is due to benzene’s ring of delocalised electrons, which stabilises the molecule by spreading the electrons’ negative charges over a larger area.

    Bond lengths

    X-ray diffraction is a type of technique using X-rays to work out the structure of molecules. Scientists used it in 1981 to get an image of benzene. In Aromatic Chemistry, we learnt that C-C single bonds are longer than C=C double bonds. This would give Kekulé’s cyclohexa-1,3,5-ene a distorted shape.

    Benzene Structure, benzene shape, StudySmarterFig. 7 - The distorted shape of cyclohexa-1,3,5-triene

    However, the image showed that benzene was in fact a regular hexagon. This meant that all of its bonds were equal length. Furthermore, scientists measured the length of these bonds and found them to be halfway between a single and a double bond in length - suggesting that they were neither one nor the other, but something different instead.

    Benzene Structure benzene bond length, StudySmarterFig. 8 - A table showing the lengths of different carbon-carbon bonds

    Isomeric products

    Let’s look at one final piece of evidence against Kekulé’s predicted structure of benzene. Take two of benzene’s adjacent carbon atoms. Imagine swapping the attached hydrogen atoms for bromine, for example. If benzene really was cyclohexa-1,3,5-triene, we’d expect it to form two different isomers: one with a double bond between the two affected carbons, and one with a single bond between them. We can see this below.

    Benzene Structure, two isomeric products of benzene, StudySmarterFig. 9 - Two isomeric products of benzene

    However, scientists only ever observed one isomer. This meant that benzene couldn’t have Kekulé’s predicted structure. It had to have identical bonds.

    Poor Kekulé - he really thought he had cracked the mystery of benzene, but all the evidence was against him! He proposed one final idea - benzene consisted of two structures in equilibrium, rapidly shifting between both. This would result in a hybrid molecule that was neither one nor the other. He called this the resonance model. However, there was no evidence to support this. We instead now believe in the delocalised model described earlier. The delocalisation accounts for benzene’s resonance energy and the identical intermediate C-C bonds explain why benzene is a regular shape.

    Structure and reactivity benzene resonance model structure, StudySmarterFig. 10 - The resonance model. Kekulé believed that benzene was actually two structures that rapidly alternated between each other. Notice how the positions of the double bonds differ between the two molecules

    The properties of benzene

    Because of its unique structure and ring of delocalisation, benzene has some unique properties. Let’s explore them below.

    Combustion

    You should know that cyclic alkanes have a general formula CnH2n. A cyclic hydrocarbon with six carbon atoms would therefore have twelve hydrogen atoms. However, benzene has six carbon atoms but only six hydrogen atoms. This higher ratio of carbon to hydrogen means benzene burns with a characteristically sooty flame.

    Melting and boiling point

    Because benzene is nonpolar, the only forces it experiences between molecules are weak van der Waals forces, also known as London forces. However, benzene is a planar molecule. This means that in a solid state, it can pack together closely in neat layers. In contrast, cyclohexane is based on tetrahedral arrangements of atoms, meaning it has different hydrogen atoms sticking off in all directions! This means the molecules can’t fit together as neatly as a solid.

    Closely-packed molecules experience stronger intermolecular forces than spaced-out molecules, so benzene has a higher melting point than cyclohexane. However, in liquid form this neat arrangement is destroyed. Both molecules, therefore, have similar boiling points.

    Structure and reactivity benzene melting boiling point, StudySmarterFig. 11 - The structure and melting and boiling points of benzene and cyclohexane

    Solubility

    Like other nonpolar hydrocarbons, benzene is insoluble in water but soluble in other organic solvents.

    Reactivity of benzene

    As we’ve explored earlier, benzene doesn’t like taking part in addition reactions. This would involve disrupting the strong ring of delocalisation, which is very stable because it spreads the electrons’ negative charges over a larger area.

    However, benzene does take part in substitution reactions. These involve swapping one atom or group for another. In this case, we can swap hydrogen atoms for other species like halogens or hydroxyl groups.

    Benzene’s ring of delocalisation is an area of electron density. This makes it very attractive to electrophiles.

    What are electrophiles?

    Electrophiles are electron pair acceptors. As the term -phile comes from the Latin philos, meaning love, we can say that they really just love electrons! Electrophiles have a positive or partial positive charge and a vacant orbital. Some common examples are H+ and NO2.

    Electrophilic substitution reactions

    We now know that benzene is susceptible to attack by electrophiles, and that it commonly reacts in substitution reactions. We can therefore conclude that most of the reactions involving benzene are electrophilic substitution reactions. We’ll cover these in Reactions of Benzene. These include:

    • Nitration reactions, swapping a hydrogen atom for the -NO2 group. This produces nitrobenzene which is used in dyes and pharmaceuticals.
    • Friedel-Crafts acylation reactions, where benzene reacts with an acid derivative in the presence of an aluminium chloride catalyst. The product is used for plastics and detergents.

    Benzene Structure - Key takeaways

    • Benzene has the molecular formula C6H6. It is a planar hexagonal molecule. Each carbon atom is bonded to two other carbon atoms and one hydrogen atom. The remaining electrons form an area of delocalisation above and below its carbon ring.

    • The organic chemist Kekulé proposed a structure for benzene of alternating C-C single and C=C double bonds. However, evidence such as enthalpy of hydrogenation, bond lengths, isomeric products, and behaviour in addition reactions disproved his theory.

    • Benzene burns with a sooty flame due to its high ratio of hydrogen to carbon atoms. It is relatively stable and insoluble in water.

    • Benzene has a higher melting point than similar cycloalkanes because it is a planar molecule and can be closely packed together.

    • Benzene rarely takes part in addition reactions, because this would involve disrupting its strong ring of delocalisation. Instead it readily undergoes electrophilic substitution reactions, including nitration and Friedel-Crafts acylation.

    Frequently Asked Questions about Benzene Structure

    How many resonance structures does benzene have?

    Benzene has two different resonance structures.

    How do you draw the resonance structures of benzene?

    To draw the resonance structures of benzene, draw a hexagon shape connected by alternating single and double bonds.

    What is the structure of benzene?

    Benzene is a planar hexagonal molecule made from six carbon atoms and six hydrogen atoms. Each C-C bond is an intermediate, halfway between a single and a double bond in length. One electron from each carbon atom is delocalised, and is found delocalised in overlapping pi orbitals above and below the planar carbon ring.

    Why is benzene so reactive?

    Benzene is actually relatively unreactive. Its delocalised pi electrons distribute the charge evenly around the molecule which stabilises it, making it less likely to react.

    What is structure in organic chemistry?

    Structure in organic chemistry is the arrangement of atoms, electrons and bonds in organic molecules, which are molecules based on carbon and hydrogen.

    Test your knowledge with multiple choice flashcards

    Complete the following sentence: The carbon-carbon bonds in benzene are _______.

     How many hydrogen atoms does each carbon atom in benzene bond to?

     Complete the following sentence: Benzene is _____ in water and ______ in organic solvents.

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