Chemistry · Lesson
Structural formulas, skeletal drawings and organic names
Read an organic drawing as a record of atoms and connections. Translate between explicit, condensed and skeletal formulas, then use a simple alcohol to connect the graph with its name.
Back to the Chemistry mapBy the end, you can
- Recover atoms, hydrogens and charge from a stated drawing convention.
- Keep molecular formula, connectivity and configuration distinct.
- Select and number the parent chain in one simple branched alcohol.
Formula, connectivity and configuration answer different questions
A molecular formula records the number of atoms of each element. C₂H₆O alone does not tell you whether the oxygen connects a carbon and a hydrogen, as in ethanol, or two carbons, as in dimethyl ether. A structural formula adds the atom-to-atom connections and bond orders. Stereochemical marks can then add a specified spatial arrangement.
A fully expanded drawing shows the individual atoms and bonds. A condensed formula groups attached hydrogens, as CH₃CH₂OH, and uses parentheses to locate branches. A skeletal drawing usually leaves carbon symbols and their attached hydrogens implicit, while showing heteroatoms, needed heteroatom hydrogens, charges and stereochemical information. These are different representations of a molecular identity, rather than different substances simply because the page looks different.
Read a bond before supplying a hydrogen
In the usual neutral closed-shell carbon examples here, supply enough attached hydrogen for a carbon bond-order sum of four. A double bond contributes two to that sum and a triple bond three. Count every intended endpoint and corner of a skeletal carbon path; do not invent an atom at an unmarked crossing of unrelated lines. Follow which atoms the branch actually joins.
That ordinary-valence shortcut has a declared scope. CH₃⁺, CH₃· and CH₃⁻ each has three hydrogens and a different electron count; none silently becomes CH₄. Explicit charge and radical marks take precedence. For nitrogen and oxygen, inspect bonds, hydrogens, nonbonding electrons and formal charge together. A drawing that omits a required charge or hydrogen can become a different chemical claim.
Follow one branched alcohol through two drawing styles
For CH₃CH(OH)CH(CH₃)CH₂CH₃, the five-carbon path a1–a2–a3–a4–a5 has an OH group at a2 and an extra methyl carbon a6 at a3. The oxygen is a7 and retains its own hydrogen. The two original panels below preserve exactly those six heavy-atom bonds and attached hydrogen counts. Orange atom identifiers are annotations for comparison, not additional atoms.
The drawing coordinates are chosen for legibility. They are not measured bond angles or distances. Plain connections here specify the constitution only: the two potentially stereogenic carbon centers a2 and a3 have no assigned configurations. Turning this page or redrawing the branches does not itself identify an R or S form.
Text equivalent: a1(CH₃)–a2(CH)(OH at a7)–a3(CH)(CH₃ at a6)–a4(CH₂)–a5(CH₃). Heavy bonds: a1–a2, a2–a3, a3–a4, a4–a5, a3–a6, a2–a7. Each bond is single; a7 is OH.
Underlying atom and bond record
| Atom identifier | Element | Attached H | Formal charge | Heavy-atom neighbours |
|---|---|---|---|---|
| a1 | C | 3 | 0 | a2 |
| a2 | C | 1 | 0 | a1, a3, a7 |
| a3 | C | 1 | 0 | a2, a4, a6 |
| a4 | C | 2 | 0 | a3, a5 |
| a5 | C | 3 | 0 | a4 |
| a6 | C | 3 | 0 | a3 |
| a7 | O | 1 | 0 | a2 |
Original condensed-group and skeletal drawings of CH₃CH(OH)CH(CH₃)CH₂CH₃, C₆H₁₄O, net charge zero. Both panels keep a1–a2–a3–a4–a5, a3–a6 and a2–a7, all single bonds. Orange identifiers label the same atoms in each panel. Oxygen a7 bears one hydrogen. Pixel positions carry no measured geometry or assigned stereochemistry.
Choose the parent for the functional group being named
For this selected saturated alcohol, choose a five-carbon parent chain containing the OH-bearing carbon. Number from the end that gives the OH suffix the lower locant: 2 rather than 4. The methyl branch is at 3, giving 3-methylpentan-2-ol. The parent has five carbons, while the whole molecule has six.
A name encodes information according to a nomenclature system. Here the suffix locates the alcohol and a prefix locates a branch. Other functional-group priorities, rings, multiple bonds and retained names need their own rules. This example is an entry point rather than a universal naming algorithm; an unspecified stereocenter remains unspecified in this plain name.
Worked examples
Count the branch and the oxygen hydrogen
Find the formula of the neutral structure CH₃CH(OH)CH(CH₃)CH₂CH₃.
- Count five carbons along the written path and one carbon in the methyl branch: six.
- The attached carbon hydrogens are 3+1+1+2+3+3=13.
- The OH supplies one further hydrogen.
- There is one oxygen, so the full count is C₆H₁₄O.
C₆H₁₄O. The parent-chain length and total carbon count differ.
Choose the alcohol suffix locant first
For the same graph, compare numbering from its two chain ends.
- The selected parent path contains five carbons and the OH-bearing carbon.
- From a1, OH is at 2 and methyl at 3.
- From a5, OH is at 4 and methyl at 3.
- Choose the lower OH suffix locant, giving 3-methylpentan-2-ol.
3-methylpentan-2-ol, with spatial configurations still unspecified.
Check a nitrogen charge without adding an unseen lone pair
Compare neutral CH₃NH₂ with CH₃NH₃⁺ in their ordinary Lewis representations.
- The neutral amine has five hydrogens in total; N has three single bonds and one lone pair.
- The ammonium ion has six hydrogens in total; N has four single bonds and no lone pair.
- Formal charge on that N is 5−0−4=+1, counting one electron per bond for charge bookkeeping.
The additional N–H bond accompanies a composition and charge change. It is not another drawing of the neutral amine.
Try it yourself
In a neutral saturated skeletal path with three segments and no branches or heteroatoms, how many carbons are present?
Show the answer and reasoning
Four.
The two ends and two intervening corners each represent one carbon.
For an ordinary neutral carbon already drawn with one double bond and one single bond, how many attached H complete its usual valence?
Show the answer and reasoning
One.
The visible bond-order sum is three; one C–H bond makes four.
Should CH₃· be silently read as CH₄?
Show the answer and reasoning
No.
The explicit radical specifies an unpaired electron; its stated hydrogen count is three.
Does the plain name 3-methylpentan-2-ol assign either of its two potentially stereogenic centers?
Show the answer and reasoning
No.
The constitution is specified, but the relevant stereodescriptors or an unambiguous stereochemical drawing are absent.
What to carry forward
- Count atoms and connections before interpreting the picture.
- Retain hydrogens, charges and radical marks when converting styles.
- A parent chain can be shorter than the molecule’s total carbon count.
- A plain constitution does not silently assign configuration.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.
- Graphical representation standards for chemical structure diagramsJonathan Brecher (prepared for publication); task group listed in original front matterReference guide
IUPAC; Queen Mary University of London HTML host3 sources
- Blue Book P-2: Parent hydridesHenri A. Favre, Warren H. Powell (prepared for publication), G. P. Moss (HTML preparation)Reference guide
- Blue Book P-3: Characteristic groups and suffixesHenri A. Favre, Warren H. Powell (prepared for publication), G. P. Moss (HTML preparation)Reference guide
- Blue Book P-4: Parent selection and name constructionHenri A. Favre, Warren H. Powell (prepared for publication), G. P. Moss (HTML preparation)Reference guide
OpenStax, Rice University2 sources