Hydrocarbons
Easy Overview
Hydrocarbons are the simplest organic compounds β just carbon and hydrogen. But don't let the simplicity fool you. They're the basis of fuels (petrol, diesel), plastics, and even the structure of your DNA. This chapter covers alkanes (all single bonds), alkenes (double bonds), alkynes (triple bonds), and aromatic compounds like benzene.
Alkanes β The Boring but Useful Ones
Alkanes have only single bonds β C-C and C-H. They're called saturated because every carbon has four bonds and there's no room for more. Methane (CHβ) is the simplest β it's natural gas. Alkanes are generally unreactive, which makes them great as fuels. They burn in oxygen to give COβ, HβO, and a lot of energy. That's why your car runs on gasoline (a mix of alkanes).
Alkenes β The Double Bond Changes Everything
Alkenes have at least one C=C double bond. That double bond is a region of high electron density β it's reactive. Ethene (CβHβ) is used to make polyethylene plastic. Alkenes undergo addition reactions where the double bond breaks and new atoms attach. It's like a crowded train β two people on a seat meant for one; they separate so others can sit.
Alkynes β The Triple Bond Warriors
Alkynes have a Cβ‘C triple bond β even more reactive than a double bond. Ethyne (CβHβ, also called acetylene) is used in welding torches because it burns with an extremely hot flame. Triple bonds are shorter and stronger than double bonds. They also undergo addition reactions, sometimes in two steps.
Aromatic Hydrocarbons β The Benzene Mystery
Benzene (CβHβ) looks like it should have alternating single and double bonds, but all C-C bonds are actually identical. It's a ring with delocalized electrons β like a donut of electron density. This makes benzene unusually stable. It undergoes substitution reactions rather than addition β it'd rather swap a hydrogen than break its stable ring.
Preparation of Hydrocarbons
Alkanes come from petroleum or from the Wurtz reaction (2R-X + 2Na β R-R + 2NaX). Alkenes come from dehydration of alcohols or cracking of alkanes. Alkynes come from dehydrohalogenation of vicinal dihalides. Benzene comes from cyclic compounds or from petroleum. Each method is like a different recipe for the same dish.
Key Points
- β’Alkanes: C-C single bonds, spΒ³ hybridized, general formula CβHββββ
- β’Alkenes: C=C double bond, spΒ² hybridized, general formula CβHββ
- β’Alkynes: Cβ‘C triple bond, sp hybridized, general formula CβHββββ
- β’Alkanes undergo substitution (free radical halogenation); alkenes and alkynes undergo addition
- β’Markovnikov's rule: H adds to the carbon with more H already (for unsymmetrical alkenes)
- β’Benzene is aromatic β 6 Ο electrons delocalized in a ring, undergoes electrophilic substitution
- β’Benzene resists addition because it would break aromaticity (stability loss)
- β’Cracking: breaking large alkanes into smaller, more useful ones
Practice Questions
- Explain Markovnikov's rule with an example of addition of HBr to propene.
- How is benzene prepared from acetylene? Describe the mechanism of nitration of benzene.
- Distinguish between alkanes, alkenes, and alkynes based on their hybridization and bond type.
- Write the Wurtz reaction. What are its limitations?
- Why does benzene undergo substitution rather than addition reactions?