Higher Chemistry Unit 1 Notes

[Adriana Gowen]

Figure8.2: Aromatic Hydrocarbons. Aromatic hydrocarbons contain the 6-membered benzene ring structure (A) that is characterized by alternating double bonds. Ultradur, PBT is a plastic polymer that contains an aromatic functional group. The repeating monomer of Ultradur is shown in (B). Ultradur can be found in showerheads, toothbrush bristles, plastic housing for fiber-optics cables, and in automobile exterior and interior components. Biologically important molecules, such as deoxyribonucleic acid, DNA (C) also contain an aromatic ring structures.

Thus, they have formulas that can be drawn as cyclic alkenes, making them unsaturated. However, due to the cyclic structure, the properties of aromatic rings are generally quite different, and they do not behave as typical alkenes. Aromatic compounds serve as the basis for many drugs, antiseptics, explosives, solvents, and plastics (e.g., polyesters and polystyrene).

In an alkene, the double bond is shared by the two carbon atoms and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious, ie the condensed formula for ethene is CH2CH2. The double or triple bond nature of a molecule is even more difficult to discern from the molecular formulas. Note that the molecular formula for ethene is C2H4, whereas that for ethyne is C2H2. Thus, until you become more familiar the language of organic chemistry, it is often most useful to draw out line or partially-condensed structures, as shown below:

Figure 8.7 The formation of double bonds requires the use of pi-bonds. For molecules to create double bonds, electrons must share overlapping pi-orbitals between the two atoms. This requires the dumbbell-shaped pi-orbitals (show on the left) to remain in a fixed conformation during the double bond formation. This allows for the formation of electron orbitals that can be shared by both atoms (shown on the right). Rotation around the double bond would cause the pi orbitals to be misaligned, breaking the double bond.

The fixed and rigid nature of the double bond creates the possibility of an additional chiral center, and thus, the potential for stereoisomers. New stereoisomers form if each of the carbons involved in the double bond has two different atoms or groups attached to it. For example, look at the two chlorinated hydrocarbons in Figure 8.8. In the upper figure, the halogenated alkane is shown. Rotation around this carbon-carbon bond is possible and does not result in different isomer conformations. In the lower diagram, the halogenated alkene has restricted rotation around the double bond. Note also that each carbon involved in the double bond is also attached to two different atoms (a hydrogen and a chlorine). Thus, this molecules can form two stereoisomers: one that has the two chlorine atoms on the same side of the double bond, and the other where the chlorines reside on opposite sides of the double bond.

Figure 8.8 Alkene Double Bonds Can Form Geometric Isomers. (a) Shows the free rotation around a carbon-carbon single bond in the alkane structure. (b) Shows the fixed position of the carbon-carbon double bond that leads to geometic (spatial) isomers.

For this section, we are not concerned with the naming that is also included in this video tutorial.(Note: All Khan Academy content is available for free using CC-BY-NC-SA licensing at www.khanacademy.org )

The cis-trans naming system can be used to distinguish simple isomers, where each carbon of the double bond has a set of identical groups attached to it. For example, in Figure 8.8b, each carbon involved in the double bond, has a chlorine attached to it, and also has hydrogen attached to it. The cis and trans system, identifies whether identical groups are on the same side (cis) of the double bond or if they are on the opposite side (trans) of the double bond. For example, if the hydrogen atoms are on the opposite side of the double bond, the bond is said to be in the trans conformation. When the hydrogen groups are on the same side of the double bond, the bond is said to be in the cis conformation. Notice that you could also say that if both of the chlorine groups are on the opposite side of the double bond, that the molecule is in the trans conformation or if they are on the same side of the double bond, that the molecule is in the cis conformation.

To determine whether a molecule is cis or trans, it is helpful to draw a dashed line down the center of the double bond and then circle the identical groups, as shown in figure 8.9. Both of the molecules shown in Figure 8.9, are named 1,2-dichloroethene. Thus, the cis and trans designation, only defines the stereochemistry around the double bond, it does not change the overall identity of the molecule. However, cis and trans isomers often have different physical and chemical properties, due to the fixed nature of the bonds in space.

The situation becomes more complex when there are 4 different groups attached to the carbon atoms involved in the formation of the double bond. The cis-trans naming system cannot be used in this case, because there is no reference to which groups are being described by the nomenclature. For example, in the molecule below, you could say that the chlorine is trans to the bromine group, or you could say the chlorine is cis to the methyl (CH3) group. Thus, simply writing cis or trans in this case does not clearly delineate the spatial orientation of the groups in relation to the double bond.

As we saw in Chapter 7, small alkanes can be formed by the process of thermal cracking. This process also produces alkenes and alkynes. In comparison to alkanes, alkenes and alkynes are much more reactive. In fact, alkenes serve as the starting point for the synthesis of many drugs, explosives, paints, plastics and pesticides. Alkanes can undergo five major types of reactions: (1) Combustion Reactions, (2) Addition Reactions, (3) Elimination Reactions, (4) Substitution Reactions, and (5) Rearrangement Reactions. Since combustion reactions were covered heavily in Chapter 7, and combustion reactions with alkenes are not significantly different than combustion reactions with alkanes, this section will focus on the later four reaction types.

Most reactions that occur with alkenes are addition reactions. As the name implies, during an addition reaction a compound is added to the molecule across the double bond. The result is loss of the double bond (or alkene structure), and the formation of the alkane structure. The reaction mechanism of a reaction describes how the electrons move between molecules to create the chemical reaction. Note that in reaction mechanism diagrams, as shown in Figure 8.15, curved arrows are used to show where electrons are moving. The reaction mechanism for a generic alkene addition equation using the molecule X-Y is shown below:

Figure 8.15. Reaction mechanism of a generic addition reaction. In this reaction, an electron from the carbon-carbon double bond of the alkene attacks an incoming molecule (XY) causing the breakage of the carbon-carbon double bond (lefthand diagram) and formation of a new bond between one of the alkene carbons and molecule X. The original electron from X that was participating in the shared bond with Y, is donated to Y causing the breakage of the X-Y bond. In the intermediate state (middle diagram), the alkene is carrying a positively charged carbon ion, called a carbocation, and Y is in a negatively charged anion state. The negative anion is attracted to the positively charged carbocation and donates the two electrons to form the C-Y bond and complete the product of the addition reaction (righthand diagram).

In a Hydrogenation reaction, hydrogen (H2) is added across the double bond, converting an unsaturated molecule into a saturated molecule. Note that the word hydrogen is found in this reaction name, making it easier to remember and recognize: Hydrogen-ation. In a hydrogenation reaction, the final product is the saturated alkane.

Just like when your are feeling thirsty, the terms hydration and dehydration refer to water. Hydration means the addition of water to a molecule, just like when you feel fully hydrated or full of water, while dehydration means the removal or elimination of water, just as when you are feeling dehydrated and need some water to drink. Similar to the hydrohalogenation reaction above, water is also a polar molecule. In this case, the water is split into two groups to be added across the double bond of the alkene. It is split into the H- and the -OH components. Similar to the hydrohalogenation reaction, the hydrogen adds first, as it carries the partial positive charge. the OH group forms the negative anion intermediate and is then added to the carbocation to form the final product, which is an alcohol.

In more complex molecules, hydrohalogenation and hydration reactions can lead the formation of more than one possible product. For example, if 2-methylpropene [(CH3)2CCH2] reacts with water to form the alcohol, two possible products can form, as shown below. However, the addition reaction is not random. One of the products is the major product (being produced in higher abundance) while the other product is the minor product.

A rearrangement reaction is a specific organic reaction that causes the alteration of the structure to form an isomer. With alkene structures, rearrangement reactions often result in the conversion of a cis-isomer into the trans conformation.

Due to the high reactivity of alkenes, they usually undergo addition reactions rather than substitutions reactions. The exception is the benzene ring. The double-bonded structure of the benzene ring gives this molecule a resonance structure such that all of the carbon atoms in the ring share a continually rotating partial bond structure.

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