Names, Formulas, and Structures

Why Names, Formulas, and Structures Matter in Organic Chemistry

Organic chemistry deals with an enormous number of different compounds. To talk about them precisely, chemists need:

• A name (what we call the substance)
• A formula (which and how many atoms it contains)
• A structure (how the atoms are connected in space)

These are three different ways of describing the same molecule, with different levels of detail.

In this chapter, you will learn:

• What different types of formulas exist in organic chemistry
• The basic ideas behind systematic naming (nomenclature)
• How names, formulas, and structural drawings are connected
• How to draw and read simple structural formulas used in organic chemistry

Details of full IUPAC rules, isomerism, reaction types, and electronic effects are covered in later chapters; here we focus only on the basic “language” for describing organic molecules.

Types of Formulas in Organic Chemistry

Each organic compound can be represented by several kinds of formulas, depending on how much information you need.

Empirical (Simplest) Formula

The empirical formula shows the simplest whole-number ratio of the atoms of each element in a compound, but not the actual number of atoms in one molecule.

Examples:

• Glucose has empirical formula:
  $$\mathrm{CH_2O}$$
  but one molecule of glucose actually has:
  $$\mathrm{C_6H_{12}O_6}$$
• Benzene has empirical formula:
  $$\mathrm{CH}$$
  but its molecular formula is:
  $$\mathrm{C_6H_6}$$

Empirical formulas are often more useful in inorganic chemistry. In organic chemistry, we usually need more detail.

Molecular Formula

The molecular formula shows the actual number of each type of atom in a single molecule.

Examples:

• Methane: $\mathrm{CH_4}$
• Ethanol: $\mathrm{C_2H_6O}$
• Acetic acid: $\mathrm{C_2H_4O_2}$

The molecular formula tells you the composition, but not how atoms are connected. Different compounds can have the same molecular formula (this is one of the reasons isomerism is so important and gets its own chapter).

General Formula for Homologous Series

For whole families of related organic compounds, we often use a general formula that describes all members of a series.

Examples:

• Saturated (single-bonded) chain hydrocarbons (alkanes):
  $$\mathrm{C_nH_{2n+2}} \quad (n \ge 1)$$
• Monocarboxylic acids (one $-\mathrm{COOH}$ group):
  $$\mathrm{C_nH_{2n}O_2} \quad (n \ge 1)$$

Here, $n$ is the number of carbon atoms in the chain. Each increase of $n$ by 1 adds a repeating unit (for example, $\mathrm{CH_2}$) to the structure.

Condensed (Semi-Developed) Formula

The condensed formula (sometimes called semi-developed formula) still shows the sequence of atoms, but without drawing all bonds.

Examples:

• Methane: CH4
• Ethane: CH3–CH3
• Propane: CH3–CH2–CH3
• Ethanol: CH3–CH2–OH or C2H5OH
• Acetic acid: CH3–COOH

Sometimes parentheses are used to show repeating groups:

• Butane: CH3–CH2–CH2–CH3 or CH3–(CH2)2–CH3
• Hexane: CH3–(CH2)4–CH3

Condensed formulas already contain information about connectivity (who is bonded to whom), but they still do not show bond angles or 3D shape.

Structural Formulas: Showing Connectivity and Geometry

A structural formula shows how atoms are bonded to each other. Organic chemists almost always use some type of structural formula, because the structure often determines the properties.

Fully Developed (Expanded) Structural Formula

In a fully developed structural formula, all atoms and all bonds are drawn explicitly.

Example: ethanol

• Molecular formula: $\mathrm{C_2H_6O}$
• Expanded structural formula (in text form):

H H
| |
H–C–C–O–H
| |
H H

This clearly shows:

• Which atoms are attached to which
• Presence of the functional group (here: $-\mathrm{OH}$)

However, for larger molecules, fully expanded formulas become cluttered. This is why chemists often prefer simplified drawing styles.

Simplified Structural and Skeletal Formulas

Line (Skeletal) Formulas

In organic chemistry, the most common drawing style is the skeletal formula (line-angle formula):

Basic rules:

• Carbon atoms at the ends of lines and at line corners are usually not written; they are implied.
• Hydrogen atoms bonded to carbon are usually not written; they are implied.
• Heteroatoms (non-carbon atoms like O, N, Cl, S, etc.) are always written, and hydrogens attached to them are typically shown.
• Each line represents a bond; double and triple bonds are drawn with two or three parallel lines.

Examples (described verbally):

• Ethane (CH3–CH3): drawn as a simple short zigzag line: /\ or just a single line between two invisible carbon atoms.
• Propane (CH3–CH2–CH3): drawn as a slightly longer zigzag line with two corners.
• Ethanol (CH3–CH2–OH): a short zigzag line for the carbon chain, with an OH group attached to the end.

Reading skeletal formulas requires practice: you must mentally “fill in” the invisible carbon and hydrogen atoms.

Displaying Double and Triple Bonds

• A double bond is drawn as two parallel lines (=).
• A triple bond is drawn as three parallel lines.

For example:

• Ethene: $\mathrm{C_2H_4}$
  Structural: H2C=CH2
• Ethyne (acetylene): $\mathrm{C_2H_2}$
  Structural: HC≡CH

The exact naming of such compounds (alkenes, alkynes) is treated in another chapter.

Basic Ideas of Systematic Naming (Nomenclature)

A systematic name is designed so that:

• From the name, you can reconstruct the structure.
• From the structure, you can write the name.

The full set of rules is extensive and treated in later chapters (e.g. for aliphatic hydrocarbons and functional groups). Here are the fundamental ideas that keep appearing.

Parent Chain and Substituents

For most organic compounds:

1. Choose a parent structure (often the longest continuous carbon chain or a principal ring system).
   Example: a 4-carbon chain → “but-”.
2. Identify functional groups (like $-\mathrm{OH}$, $-\mathrm{NH_2}$, $-\mathrm{COOH}$, halogens, etc.).
3. Number the carbon atoms in the parent chain so that important features (functional groups, double bonds, substituents) get the lowest possible numbers.
4. Name substituents (groups attached to the parent chain) and indicate their positions with numbers.

You will see this concept applied systematically when naming different families (alkanes, alkenes, alcohols, etc.) in their dedicated chapters.

Prefixes for Number of Carbons (Roots)

For simple chains, the base name (root) indicates the number of carbon atoms in the main chain:

• 1 C: meth-
• 2 C: eth-
• 3 C: prop-
• 4 C: but-
• 5 C: pent-
• 6 C: hex-
• 7 C: hept-
• 8 C: oct-
• 9 C: non-
• 10 C: dec-

These roots are combined with suffixes and other prefixes to form full names.

Suffixes Indicating Main Type of Compound

The suffix indicates the main functional group or family. For example (simplified overview):

• -ane → saturated hydrocarbon (single C–C bonds)
• -ene → hydrocarbon with at least one C=C double bond
• -yne → hydrocarbon with at least one C≡C triple bond
• -ol → alcohol (contains an $-\mathrm{OH}$ group)
• -al → aldehyde (contains a $-\mathrm{CHO}$ group)
• -one → ketone (contains a $>\mathrm{C=O}$ group in the chain)
• -oic acid → carboxylic acid (contains $-\mathrm{COOH}$)

Functional groups, their detailed naming rules, and their chemistry are covered more fully in a later chapter on functional groups.

Prefixes and Locants

To specify positions and the number of identical groups:

• Numbers (locants) show the position of substituents or multiple bonds.
• Multiplying prefixes show how many of the same kind: di- (2), tri- (3), tetra- (4), etc.
• Substituents are often named with -yl endings.

Examples (conceptual):

• 2-methylpropane: a three-carbon chain (prop-) with a CH3 substituent on carbon 2.
• 1,2-dichloroethane: a two-carbon chain with chlorine atoms on carbons 1 and 2.

The exact order and combination of these elements follow more detailed rules that you will learn step by step.

Relating Names, Formulas, and Structures

It is important to be able to move between name, formula, and structure.

From Molecular Formula to Possible Structures

Given $\mathrm{C_4H_{10}}$, for instance, several steps are possible:

1. Count the carbons: 4 → base root “but-”.
2. Use the general formula for alkanes: $\mathrm{C_nH_{2n+2}}$
   For $n = 4$, $\mathrm{C_4H_{10}}$ fits; it is an alkane.
3. Draw possible chains that contain 4 carbons and satisfy single-bond valences:
• Straight chain: CH3–CH2–CH2–CH3
• Branched chain: CH3–CH(CH3)–CH3
4. These correspond to different compounds (structural isomers), which get different names.

The systematic study of such possibilities belongs to the chapter on isomerism; here, the key point is that one molecular formula can correspond to multiple structures and names.

From Name to Structure (Basic Strategy)

As a beginner, when you see a systematic name, you can decode it in steps:

1. Identify the suffix (main family / functional group).
2. Identify the root (number of carbons in the parent chain).
3. Look at any locants (numbers) telling you where groups or bonds are.
4. Add substituents according to their names and positions.

Example (conceptual only):

• Name: 2-butanol
• Root + suffix: but- (4 carbons), -ol (alcohol)
• Number: 2- → OH at carbon 2
• Draw a 4-carbon chain and place an OH on the second carbon.

Again, the detailed rules and practice come in later, but the basic logic remains the same.

Common Organic Notation and Abbreviations

To keep formulas compact, several conventions and abbreviations are frequently used.

Group Abbreviations

Some common groups are often written as abbreviations:

• Me = methyl = $\mathrm{CH_3-}$
• Et = ethyl = $\mathrm{CH_3CH_2-}$
• Ph = phenyl (benzene ring attached as a substituent)
• Ac = acetyl = $\mathrm{CH_3CO-}$
• Ar = generic aryl group (any aromatic ring system)

You may encounter them especially in organic reaction schemes.

R, R′, R″ as “Any Organic Group”

R, R′, R″ are “placeholders” for unspecified organic groups (often alkyl groups).

Examples:

• A simple alcohol may be written generically as R–OH.
• An ester may be written as R–COOR′.

Here, R and R′ can be any organic substituents, not necessarily the same.

Reading and Drawing Simple Organic Structures

Being comfortable with organic “pictures” is essential. Here are minimal practical guidelines tailored to beginners.

Step-by-Step: Drawing a Simple Organic Molecule

Consider drawing propanoic acid (just as an illustration of using names; detailed naming of acids is addressed later):

1. Identify the root: prop- → 3 carbons in parent chain.
2. Suffix: -oic acid → carboxylic acid group ($-\mathrm{COOH}$) at the end of the chain.
3. Draw three carbon atoms in a row: C–C–C.
4. Replace one end carbon with the carboxyl group: CH3–CH2–COOH.

As a skeletal formula, this would be a 3-carbon zigzag with a COOH at the end.

Counting Hydrogens by Valence

Carbon almost always forms 4 covalent bonds. When drawing or reading structures:

• Each line (bond) from a carbon counts as 1 bond.
• The sum of bonds including implicit hydrogens must equal 4.

Example:
In the fragment –CH2–:

• The carbon is bonded to two neighbors (left and right): 2 bonds.
• It also has two hydrogens (2 more bonds).
• Total 4 bonds → valence satisfied.

This rule helps you check whether a structural drawing is sensible.

Practice Suggestions (Without Solutions)

To get used to this “language”, it helps to practice:

1. Write the molecular formula and a condensed structural formula for:
• Methane
• Ethane
• Propane
• Ethene
• Ethanol
2. For the molecular formulas below, draw at least one possible structural formula:
• $\mathrm{C_3H_8O}$
• $\mathrm{C_4H_{10}}$
3. For each general formula, write the first three specific members:
• $\mathrm{C_nH_{2n+2}}$ for $n = 1, 2, 3$
• $\mathrm{C_nH_{2n}}$ for $n = 2, 3, 4$
4. Look up the skeletal formulas of:
• Ethanol
• Acetic acid
• Benzene
  And try to identify which carbons and hydrogens are “hidden” in the line drawings.

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Summary

• Empirical, molecular, and general formulas describe composition with different levels of detail.
• Condensed and structural formulas show how atoms are connected; structural formulas can be fully expanded or simplified (skeletal).
• Systematic names are constructed from:
• A root (number of carbons),
• A suffix (main family / functional group),
• Prefixes and numbers (substituents and positions).
• One molecular formula can correspond to multiple structures and names, which is why structure and naming are tightly linked.
• Organic chemists use standardized abbreviations and skeletal drawings to keep structures readable.

These basic ideas form the foundation for the more advanced topics: detailed nomenclature, functional groups, and isomerism, which are treated in the following chapters.