Source: Lecture notes, The Ohio State University
Tags: functional groups, alkanes, cyclic alkanes, haloalkanes, alkynes, aldehydes, ketones, amines, organic chemistry, substitution, elimination, synthesis, Wöhler
Difficulty: Introductory Prerequisites: Basic understanding of atomic structure and chemical bonds.
This section is the opening inventory of organic chemistry: what carbon-based molecules look like, what pieces (functional groups) give them their reactivity, and how chemists think about building complex molecules from simple ones. If general chemistry was about atoms and ions across the whole periodic table, organic chemistry narrows the focus to carbon and a small supporting cast (H, N, O, B, Si, P, F, Cl, Br, I). Everything that follows in the course depends on recognising functional groups on sight and understanding that structure determines reactivity.
Organic chemistry is the chemistry of carbon and its compounds. The carbon framework provides the skeleton, and functional groups attached to that skeleton determine how a molecule reacts. Knowing your functional groups, and the two main reaction types (substitution and elimination), is the foundation for the rest of the course.
Organic chemistry
The chemistry of carbon and its compounds. It involves only a handful of other elements: H, N, O, B, Si, P, F, Cl, Br, and I.
Functional group
The subunits and bonds that determine the chemical reactivity of organic molecules. Think of it as the "business end" of a molecule, the part where reactions happen.
Alkane
A hydrocarbon composed of only carbon and hydrogen atoms connected by single bonds. These are the simplest organic molecules and form the basic scaffold of organic chemistry. They lack any functional groups.
Cyclic alkane
An alkane in which the carbon atoms form a ring rather than a straight or branched chain (e.g. cyclohexane).
Stereoisomerism
The phenomenon where compounds share the same connectivity (same atoms bonded to the same atoms) but differ in the three-dimensional arrangement of those atoms in space. In simple terms, same connections, different shapes.
Haloalkane (alkyl halide)
A chemical compound derived from an alkane by replacing one or more hydrogen atoms with halogen atoms (F, Cl, Br, I). These participate in two important reaction types: substitution and elimination.
Substitution reaction
A reaction in which one atom or group on a molecule is replaced by another. For example, CH₃–Cl + K⁺I⁻ → CH₃–I + K⁺Cl⁻.
Elimination reaction
A reaction in which adjacent atoms are removed from a molecule, generating a double bond. Think of it as "removing pieces to create a new bond."
Alkyne
A hydrocarbon whose functional group is a carbon–carbon triple bond. Acetylene (HC≡CH) is the smallest alkyne.
Aldehyde
A compound whose functional group is a carbon–oxygen double bond (C=O) at the end of a carbon chain. Formaldehyde (H₂C=O) is the simplest example.
Ketone
A compound whose functional group is a carbon–oxygen double bond (C=O) flanked by carbon atoms on both sides. Acetone is the classic example.
Amine
A compound whose functional group contains nitrogen. Methylamine (H₃C–NH₂) is a simple amine.
Synthesis
The making of molecules, typically constructing complex organic chemicals from simpler, more readily available ones.
The carbon frame of a molecule provides structure; the functional group provides reactivity.
Recognising functional groups lets you predict what reactions a molecule can undergo.
The key principle: the structure of the molecule determines the reactions it can undergo.
Composed entirely of carbon and hydrogen, connected by single bonds (hydrocarbons).
They lack functional groups, which makes them relatively unreactive.
Alkane bonds can be broken by heat, light, or chemical reagents.
Example chlorination reaction: CH₄ + Cl₂ → (heat) → CH₃–Cl + HCl.
Cyclic alkanes contain carbon atoms arranged in a ring.
Compounds with the same connectivity but different spatial arrangements of atoms are called stereoisomers. This concept becomes very important later in the course.
Formed when one or more hydrogens on an alkane are replaced by halogens.
Two key reaction types:
Substitution: one halogen atom is swapped for another (or another group). Example: CH₃–Cl + K⁺I⁻ → CH₃–I + K⁺Cl⁻.
Elimination: adjacent atoms are removed, creating a double bond. Example: CH₃–CH₃ + K⁺OH⁻ → H₂C=CH₂ + HOH + K⁺I⁻.
Alkynes: contain a C≡C triple bond. Acetylene is the smallest.
Aldehydes and ketones: both contain a C=O double bond. In an aldehyde, the C=O is at the end of the chain; in a ketone, it sits between two carbons.
Amines: contain nitrogen as the functional group.
Friedrich Wöhler demonstrated that carbon compounds can be produced by converting inorganic starting materials into urea.
This was historically significant because it disproved the idea that organic compounds could only come from living organisms.
Functional groups are the reason different organic molecules behave so differently, even if their carbon skeletons are similar. Pharmaceutical chemistry, for example, relies heavily on swapping and modifying functional groups to turn an inactive carbon scaffold into a drug that binds a specific target.
Students often assume alkanes are completely inert. They are unreactive compared to other organic molecules, but they do react under the right conditions (heat, light, or reagents).
Substitution and elimination are sometimes confused. Substitution swaps one group for another; elimination removes atoms to create a new bond (usually a double bond).
Aldehydes and ketones both have C=O, so students mix them up. The difference is position: aldehydes have the C=O at the end of the chain, ketones have it in the middle.
⚠️ You will be expected to identify functional groups by sight. Memorise the structures for alkanes, alkenes, alkynes, haloalkanes, aldehydes, ketones, and amines.
⚠️ Know the difference between substitution and elimination reactions, and be able to identify which type is occurring from a reaction equation.
⚠️ Wöhler's synthesis of urea is a classic exam question for its historical significance.
True or false: Alkanes contain at least one functional group.
Fill in the blank: In a ______ reaction, one atom or group is replaced by another.
True or false: Aldehydes and ketones share the same functional group (C=O) but differ in its position on the carbon chain.
Fill in the blank: The smallest alkyne is ______.
True or false: Wöhler's synthesis proved that organic compounds can only be made by living organisms.
Answers: 1. False (alkanes lack functional groups). 2. Substitution. 3. True. 4. Acetylene (HC≡CH). 5. False (it proved the opposite).
Q: What is the defining structural feature of an alkyne?
A: A carbon–carbon triple bond (C≡C).
Q: In the reaction CH₃–Cl + K⁺I⁻ → CH₃–I + K⁺Cl⁻, what type of reaction is occurring?
A: A substitution reaction, because the chlorine atom is being replaced by an iodine atom.
Q: What distinguishes an aldehyde from a ketone?
A: Both contain a C=O group, but in an aldehyde it is located at the end of the carbon chain, while in a ketone it is located between two carbon atoms.
Q: Why was Wöhler's synthesis of urea historically significant?
A: It demonstrated that organic compounds could be synthesised from inorganic starting materials, disproving the prevailing belief that organic molecules required a living organism to produce them.
Q: Name the two main reaction types that haloalkanes participate in.
A: Substitution and elimination.
This section connects directly to Sections 1.3 and 1.4, which cover ionic and covalent bonds and Lewis structures. Understanding how carbon bonds (Section 1.3) is essential before you can draw and interpret the functional groups introduced here. Later chapters on reaction mechanisms will return repeatedly to substitution and elimination as core reaction pathways.
Functional groups, alkanes, hydrocarbons, cyclic alkanes, cyclohexane, stereoisomers, haloalkanes, alkyl halides, substitution reaction, elimination reaction, alkynes, acetylene, triple bond, aldehydes, formaldehyde, ketones, acetone, amines, methylamine, Wöhler synthesis, urea, organic chemistry Chapter 1, OSU organic chemistry