Hydrocarbon Reactions Organic Chemistry Lesson # 2

Combustion VIDEO Although alkanes are quite unreactive with acids and bases, they can react with oxygen. Alkanes are used as fuels because their complete combustion releases a lot of energy in the form of heat (thermal energy), along with carbon dioxide and water. Not only alkanes, but all hydrocarbons can undergo combustion. Smaller hydrocarbons are more flammable than larger ones, so the amount of energy released may vary. Example – butane + oxygen → carbon dioxide + water 2 C4H10 (g) + 13 O2 (g) → 8 CO2 (g) + 10 H2O (g)

Reactions of Alkanes

Halogenation Halogenation occurs when a halogen replaces a hydrogen atom in an alkane or alkyl halide. Halogens can replace the hydrogens in an alkane over and over again until all hydrogens are halogens. It is a type of substitution reaction. In these reactions, the halogen acts as a nucleophile – a nucleus loving atom or group of atoms that can be used in substitution. Other nucleophiles include CN-, I-, Br-, F-, Cl-, OR- (R stands for a carbon chain), and OH- (listed in order of increasing reactivity). These reactions are photochemical – they only happen in the presence of light. We use the symbol hv to indicate this over our reaction arrow.

Alkyl Halides Alkyl halides may have more than one halogen, such as CFCs (which have both chlorine and fluorine), and are known as freons – coolants used in refrigeration. They also play a role in ozone depletion. Halogens are very electronegative, and can alkyl halides to be polar molecules, and so have strong intermolecular forces, meaning they have higher melting and boiling points than their corresponding alkanes.

Halogenation Examples Methane + Chlorine → Trichloromethane + Chlorine →

Dehydrogenation When an alkene is formed from an alkane by the removal of hydrogen. The mechanism of this reaction is beyond the scope of the course. Heat, along with a platinum catalyst are required to make this reaction occur. Ethane →

Reactions of Alkenes and Alkynes

Addition A reaction is which the atoms of one molecule are added to another molecule to form a single molecule. These reactions occur across the double or triple bond as they provide a “reaction centre”. When a hydrogen molecule is added, it is known as hydrogenation (requires a catalyst), and when halogen molecule is added, it is again known as halogenation. If both happened, it is called hydrohalogenation. Lastly, if water is added, it is called hydration.

Hydrogenation Ethene + Hydrogen → Propyne + Hydrogen →

Halogenation & Hydrohalogenation Propene + Bromine → But-1-ene + Hydrogen Bromide →

Markovnikov’s Rule Chemical analysis shows that in the reaction above, 99% of the product is 2-bromobutane. This leads to Markovnikov’s rule that states that when a hydrogen halide or water reacts with an alkene, the hydrogen atom will generally bond to the carbon atom in the multiple bond that has the most hydrogen atoms bonded to it. The rich get richer. If the number of hydrogen atoms is equal on both carbons, then equal amounts of products will form.

Hydration Propene + Water →

Reactions of Alkyl Halides

Halogenation Reminder that if a nucleophile is in the presence of an alkyl halide and is more reactive, that the nucleophile can replace the halide (for example – hydroxide ions will replace any halide). Fluoroethane + Sodium Hydroxide

Elimination Elimination is the opposite of addition, where two atoms are removed to form a double bond. The mechanism of this reaction is beyond the scope of our course. When an alkyl halide is in the presence of a strong base, the halogen and an adjacent hydrogen can be eliminated, forming a double bond in their place. 2-bromopropane + Strong Base →

Reactions of Aromatics

Substitution Although aromatics have double bonds, they are less reactive, and are more like alkanes in terms of reactivity. Therefore, they undergo similar reactions to alkanes. Benzene can undergo three types of substitution: halogenation, nitration, and alkylation.

Halogenation Benzene + Chlorine →

Nitration Benzene + Nitric Acid →

Alkylation Benzene + Chloromethane →