Substitution, Addition, Elimination, and Combustion

In Topics 1-7, you learned about all of the organic families that comprise organic compounds. In the Topic 8 and 9, we will focus on the reactions that result in the formation of these compounds and their practical applications.

Upon completion of Topic 8 and 9, you will be able to:

• • describe different types of organic reactions

  • predict and correctly name the products of organic reactions

Millions of reactions take place everyday in the trillions of cells in your body. Since living organisms are composed of organic compounds, many of the reactions we will discuss in this lesson also occur in the cells of your body.

Reactions of Organic Compounds (Part 1)

When a hydrocarbon reacts with oxygen gas it will undergo a combustion reaction. The products of a combustion reaction are always water and carbon dioxide.

hydrocarbon + oxygen gas → water + carbon dioxide

C₅H₁₂ + 8 O₂ → 6 H₂O + 5 CO₂

2 C₃H₇OH + 9 O₂ → 8 H₂O + 6CO₂

• In a substitution reaction, a hydrogen atom or functional group is replaced by a different functional group.

• Alcohols, haloalkanes, and aromatic hydrocarbons typically undergo substitution reactions.

Figure 1. When an alkane reacts with a halogen, the halogen will "substitute" itself for one of the hydrogens on the parent chain. Note, chlorine is diatomic, and thus, only one of the chlorine atoms are "trading places" with a hydrogen from the alkane. The hydrogen removed from the alkane bonds with the lone chlorine atom. The resulting products are a haloalkane and hydrochloric acid.

Figure 2. When an aromatic compound, such as benzene, reacts with a halogen, such as chlorine, the halogen will "substitute" itself for one of the hydrogens in the parent structure. Note, chlorine is diatomic. Only one the chlorine atoms substitute a hydrogen from the aromatic compound. The hydrogen removed from the cyclic structure bonds to the lone chlorine to form hydrochloric acid.

Figure 3. Here, we have benzene reacting with nitric acid, HNO₃. The compound nitrobenzene and water will be the resulting products. A nitro alkyl group, NO₂, from the nitric acid, substitutes a hydrogen on the benzene structure. The hydrogen removed from benzene bonds with the lone hydrogen and oxygen that remain from the nitric acid to form water.

Figure 4. In this example ethanol is reacting with hydrogen iodide. The iodine substitutes the hydroxy group at the terminal end of the parent chain. The hydroxy group that is replaced bonds to the lone hydrogen to form HOH which is also known as water.

• Results in the loss of a small molecule from a larger molecule.

• This reaction can be catalyzed by concentrated sulfuric acid.

• Can also be referred to as a dehydration reaction because it can result in the removal of water.

• Can also be considered the reverse of an addition reaction.

Zaitsev's Rule

• • This rule states, "the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the carbon having the fewest hydrogen substituents".

So, what does this mean?

• • When predicting the products of elimination reactions, remove the hydrogen from the carbon that has the fewest number of hydrogens bonded to it. Terminal carbons have 3 hydrogen bonded to them (usually). The carbons within a parent chain typically have fewer hydrogen bonded to them (usually 1 or 2).

Figure 5. In both examples above (left and right) we remove hydrochloric acid from the haloalkane to form an alkene. Thus, we can remember that haloalkanes that undergo elimination follow the pattern: haloalkane → alkene. The hydrogen is removed from carbon 3 in the example on the left, and carbon 1 in the example on the right. The example on the left follows Zaitsev's rule, and thus, is considered to be the major product of this elimination reaction. The example on the right is possible, however, would be considered a minor product.

Follows the general pattern: alcohol → alkene + water

Figure 6. When an alcohol undergoes an elimination reaction, water is removed from the compound. The resulting products are water and an alkene. The -OH group and a -H are removed. The removal of the -H from the parent chain results in a double bond forming to assure that the carbon it was removed from can form 4 bonds, and therefore, becomes an alkene.

Figure 7. Here we have an alcohol undergoing a substitution reaction catalyzed by sulfuric acid. Water is removed from the compound to form water. The double bond forms to make sure each carbon can form 4 bonds.

• Alkenes and alkynes characteristically undergo addition reactions in which atoms are added to a double or triple bond. The general formula for an addition reaction is:

• Typically, a small molecule such as: H₂O, H₂, HX, or X₂ (where X = F, Br, Cl, or I) is added to an alkene or alkyne. Some examples of addition to an alkene are:

• When a molecule consisting of identical atoms (e.g. H₂, X₂) is added to a double or triple bond, one possible product is formed.

• When a molecule consisting of non-identical atoms (e.g. H₂O, HX) is added to a double or triple bond, tow different products are theoretically possible. To predict which product will be more abundant (i.e. the ‘major’ product), use Markovnikov’s rule.

Figure 8. When this alkene reactions with hydrogen, the molecule becomes "hydrogenated". The addition of the hydrogen to the alkene breaks the double bonds and allows single bonds to form from hydrogen to carbon and carbon to carbon.

Thus, alkene + H₂ → alkane.

Figure 9. When an alkene undergoes a reaction with a halogen, such a chlorine, the atoms will bond to the two carbons on either side of the double bond. The double bond breaks and single bonds form hydrogen to carbon and carbon to carbon.

Thus, alkene + halogen → haloalkane.

Figure 10. When an alkene undergoes a reaction with a water, we view the water molecule as HOH. The -H will bond to a carbon on one side of the double bond, and the -OH will bond to the carbon on the other side of the double bond. The resulting product is an alcohol.

Thus, alkene + water → alcohol.

Figure 11. When an alkene and a hydrohalogen react, an addition reaction takes place. The hydrogen from the hydrohalogen bonds to a carbon on one side of the double bond, and the halogen bonds to the carbon on the other side of the double bond. Markovnikov's rule applies, which states that the hydrogen is added to the carbon of the alkene that has the greatest number of hydrogen substituents. Both products are possible, however, the major product is more frequent.

Thus, alkene + hydrohalogen → haloalkane.

Aromatic compounds do not undergo addition reactions. Aromatic compounds only undergo substitution reactions. The bonds in aromatic compounds are too stable and unreactive. You will simply write N.R. as a product when aromatic compounds are involved in an addition reaction.