Unit 7: Introducing Organic Molecules
An introduction to the classification and description of carbon-based compounds.
7.1 Overview of Organic Chemistry
Organic chemistry is the study of carbon compounds. Carbon is a unique element because it can form strong covalent bonds with itself and many other elements, creating a vast array of molecules. This ability, known as catenation, allows carbon to form long chains, branched chains, and complex rings.
An organic compound contains carbon atoms, usually bonded to hydrogen atoms, and often to other elements like oxygen, nitrogen, and halogens. A compound that is not classified as organic is called inorganic (e.g., carbon dioxide, carbonates, simple metallic carbides).
In all organic molecules, carbon atoms always form four covalent bonds. This tetravalency allows for the complex and diverse structures that are the basis of all life.
Solved Examples:
- What is the defining element of organic chemistry?
Solution: Carbon. - What property of carbon allows it to form long chains?
Solution: Catenation - the ability to form strong covalent bonds with itself. - Is sodium carbonate ($Na_2CO_3$) an organic compound?
Solution: No, it is considered an inorganic carbonate. - How many covalent bonds does a carbon atom always form in a stable molecule?
Solution: Four. - Name an element commonly found in organic compounds besides carbon and hydrogen.
Solution: Oxygen or Nitrogen. - What is the difference between organic and inorganic chemistry?
Solution: Organic chemistry is the study of carbon compounds (especially hydrocarbons and their derivatives), while inorganic chemistry is the study of all other elements and compounds. - Is methane ($CH_4$) an organic compound?
Solution: Yes, it is the simplest organic compound. - What branch of chemistry studies the organic molecules found in living organisms?
Solution: Biochemistry. - Why can carbon form so many different compounds?
Solution: Because of its ability to form strong single, double, and triple bonds with itself and other elements, leading to chains, branches, and rings. - How many bonds does a hydrogen atom form in an organic molecule?
Solution: One.
7.2 Describing & Classifying Organic Molecules
Organic molecules are classified based on their carbon skeleton structure and the functional groups they contain.
Classification by Carbon Skeleton:
- Aliphatic: Compounds with carbon atoms joined in straight chains, branched chains, or non-aromatic rings.
- Unbranched (Straight-chain): Carbon atoms are linked one after another in a single chain.
- Branched: The molecule has a main carbon chain with one or more smaller carbon chains (branches) attached.
- Alicyclic (or cyclic): Carbon atoms are joined together in a ring structure.
Classification by Functional Group:
A functional group is an atom or group of atoms within a molecule that is responsible for its characteristic chemical reactions. Molecules with the same functional group react in similar ways.
| Type of Compound | Functional Group | Example |
|---|---|---|
| Alkane | Only C-C and C-H single bonds | Ethane, $CH_3CH_3$ |
| Alkene | C=C double bond | Ethene, $CH_2=CH_2$ |
| Alkyne | C≡C triple bond | Ethyne, $CH≡CH$ |
| Haloalkane | A halogen atom (F, Cl, Br, I) | Chloroethane, $CH_3CH_2Cl$ |
| Alcohol (Alkanol) | -OH (hydroxyl) group | Ethanol, $CH_3CH_2OH$ |
A hydrocarbon is an organic compound that consists entirely of hydrogen and carbon atoms. Alkanes, alkenes, and alkynes are all hydrocarbons.
Solved Examples:
- What is a functional group?
Solution: An atom or group of atoms that determines the chemical properties of a molecule. - Identify the functional group in propene ($CH_3CH=CH_2$).
Solution: The C=C double bond, which makes it an alkene. - Is ethanol ($CH_3CH_2OH$) a hydrocarbon? Explain.
Solution: No, because it contains an oxygen atom in addition to carbon and hydrogen. - Classify cyclohexane (a 6-carbon ring with only single bonds) by its carbon skeleton.
Solution: It is an alicyclic compound. - What is the functional group in an alcohol?
Solution: The hydroxyl group (-OH). - What is the difference between a branched and an unbranched alkane?
Solution: An unbranched alkane has all its carbon atoms in a single continuous chain. A branched alkane has a main chain with smaller carbon chains attached as branches. - What class of compound is 2-bromopropane?
Solution: It is a haloalkane. - Are alkanes saturated or unsaturated?
Solution: Saturated, because they only contain single C-C bonds and cannot undergo addition reactions. - What do all hydrocarbons have in common?
Solution: They are composed only of carbon and hydrogen atoms. - Identify the functional group in propyne ($CH_3C≡CH$).
Solution: The C≡C triple bond, which makes it an alkyne.