Source: Purdue University General Chemistry, Ch. 24.2 Organic Chemistry Worksheet
Difficulty: Introductory | Prerequisites: Basic understanding of atomic bonding and Lewis structures (General Chemistry Ch. 8–10 or equivalent).
Organic chemistry is the study of carbon-containing compounds, and aliphatic hydrocarbons are the simplest starting point. This topic sits right at the boundary between general chemistry and organic chemistry, covering how carbon chains are named, drawn, and classified. If you can draw Lewis structures and understand covalent bonding, you have what you need. The payoff is a framework for recognising and naming any straight-chain or branched hydrocarbon you will meet in later chapters.
Aliphatic hydrocarbons are open-chain molecules made of only carbon and hydrogen. They come in three flavours: alkanes (all single bonds), alkenes (at least one C=C double bond), and alkynes (at least one C≡C triple bond). For exams, you need to name them, convert between condensed, skeletal, and ball-and-stick representations, and spot isomers.
Hydrocarbon
A molecule containing only carbon and hydrogen atoms. In simple terms, these are the backbone molecules of organic chemistry, built from just two elements.
Aliphatic hydrocarbon
A hydrocarbon with an open (non-ring) carbon chain. Think of it as the straight or branched chains, as opposed to ring-shaped (aromatic) structures.
Alkane
A hydrocarbon containing only carbon–carbon single bonds. General formula: C_nH_(2n+2). In simple terms, these are "saturated" because every carbon holds as many hydrogens as it can.
Alkene
A hydrocarbon containing at least one carbon–carbon double bond (C=C). General formula: C_nH_(2n). Think of it as an alkane that has lost two hydrogens to form a double bond.
Alkyne
A hydrocarbon containing at least one carbon–carbon triple bond (C≡C). General formula: C_nH_(2n−2). In simple terms, two bonds' worth of hydrogen have been removed compared to the alkane.
Saturated
A molecule with only single bonds between carbon atoms, so it holds the maximum number of hydrogen atoms. Think of it as "full up" with hydrogen.
Unsaturated
A molecule containing at least one double or triple bond between carbon atoms. It could accept more hydrogen atoms if those multiple bonds were broken.
Condensed structure (condensed formula)
A way of writing an organic molecule showing each carbon and its attached hydrogens in sequence, e.g. CH3CH2CH3. In simple terms, it spells out the chain left to right without drawing bonds.
Skeletal structure (line structure)
A shorthand where carbon–carbon bonds are drawn as a zigzag line, carbon atoms are implied at each vertex and endpoint, and hydrogens on carbon are omitted. Think of it as the bare bones of the molecule.
Isomer
Molecules that share the same molecular formula but differ in the arrangement of their atoms. In simple terms, same ingredients, different recipe.
The name is built from a prefix (number of carbons) + a suffix that tells you the bond type.
Prefixes: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), hept- (7C), oct- (8C), non- (9C), dec- (10C).
Suffixes: -ane (all single bonds), -ene (contains a C=C double bond), -yne (contains a C≡C triple bond).
For alkenes and alkynes with five or more carbons, a number indicates the position of the multiple bond. The chain is numbered so the multiple bond gets the lowest possible number. Example: 1-pentyne means a five-carbon chain with a triple bond starting at carbon 1. 3-hexene means a six-carbon chain with a double bond starting at carbon 3.
Condensed to skeletal: Write out the carbon chain as a zigzag. Each vertex represents a carbon. Drop all C–H bonds and carbon labels. Keep heteroatoms (O, N, etc.) visible.
Example: CH3CH2CH2CH2CH3 (pentane) becomes a four-segment zigzag with five vertices/endpoints.
Skeletal to condensed: Count the carbons at each vertex and endpoint. Add enough hydrogens to give each carbon four bonds total. Write them out in sequence.
Example: A zigzag with three segments = four carbons = CH3CH2CH2CH3 (butane).
Ball-and-stick to skeletal: Identify each ball as an atom. Trace the carbon backbone and draw it as a zigzag. Omit the hydrogen atoms bonded to carbon.
Isomers share the same molecular formula but have different structural arrangements.
Structural isomers (also called constitutional isomers) differ in how the atoms are connected. Example: both 2-methylpropanol (CH3CH(CH3)CH2OH) and 2-methyl-2-propanol (CH3C(CH3)(OH)CH3) have the formula C4H10O, but the OH group sits in a different position.
To check whether two molecules are isomers, count their atoms. If the molecular formula is the same but the connectivity differs, they are isomers.
Molecules with the same molecular formula AND the same connectivity are not isomers; they are the same compound, possibly drawn differently.
Hydrocarbons are found primarily in petroleum (crude oil) and natural gas deposits, formed over millions of years from decomposed organic matter.
Methane (CH4) is the most plentiful hydrocarbon on Earth. It is the main component of natural gas.
General formulas for aliphatic hydrocarbons:
Alkane: C_nH_(2n+2). Example: pentane, C5H12.
Alkene: C_nH_(2n). Example: 3-hexene, C6H12.
Alkyne: C_nH_(2n−2). Example: 1-pentyne, C5H8.
These formulas are your quickest check: if someone hands you a molecular formula, you can immediately tell whether the molecule is an alkane, alkene, or alkyne by seeing which formula it fits.
Degree of unsaturation shortcut: Compare the hydrogen count to what the alkane formula predicts (2n+2). Every pair of "missing" hydrogens means one degree of unsaturation (one double bond or one ring). Two pairs missing could mean one triple bond, two double bonds, or other combinations.
Methane is burned as natural gas to heat homes and generate electricity. Propane and butane are used as portable fuel (camping stoves, lighters). Longer-chain alkanes make up petrol (gasoline), diesel, and lubricating oils. Ethylene (ethene) is the starting material for polyethylene, the world's most common plastic. Acetylene (ethyne) burns hot enough to cut through steel in oxyacetylene welding torches.
Students often assume that any two molecules with the same number of carbons are isomers. They are only isomers if they also have the same molecular formula, meaning the same number of every type of atom.
Drawing a skeletal structure and forgetting that each vertex and each endpoint represents a carbon. This leads to miscounting the number of carbons when converting back to a condensed formula.
Confusing the suffix: "-ene" means a double bond, "-yne" means a triple bond. Mixing these up changes the entire identity of the molecule.
Thinking that "saturated" means the molecule is large or complex. Saturated simply means all C–C bonds are single bonds, regardless of chain length.
⚠️ Expect to be given a condensed structure and asked to draw the skeletal structure (or vice versa). Practise both directions until the conversion is automatic.
⚠️ Naming questions will almost certainly appear. Know the first ten prefixes (meth- through dec-) cold, and remember to include the position number for alkenes and alkynes of five carbons or more.
⚠️ Isomer identification is a classic exam question. You may be shown several structures and asked which are isomers of each other. Count atoms carefully.
⚠️ Know that methane is the most abundant hydrocarbon on Earth and that hydrocarbons come from petroleum and natural gas. This is a straightforward recall question that appears on worksheets and exams alike.
True or False: An alkyne has the general formula C_nH_(2n).
Fill in the blank: The IUPAC name for CH3CH2CH2CH2CH3 is __________.
True or False: Two molecules with the formula C4H10 arranged differently are called isomers.
Fill in the blank: In a skeletal structure, each vertex and endpoint represents a __________ atom.
True or False: 3-hexene contains a triple bond.
Answers: 1. False (that is the alkene formula; alkynes are C_nH_(2n−2)). 2. Pentane. 3. True. 4. Carbon. 5. False (the "-ene" suffix means a double bond).
Q: Given the condensed structure CH3CH2CH2CH2CH3, provide the molecular formula, IUPAC name, and draw the skeletal structure.
A: Molecular formula: C5H12. Name: pentane. Skeletal structure: a zigzag line with four segments (five carbon positions: two endpoints and three vertices).
Q: 1-pentyne has the molecular formula C5H8. Write its condensed structure.
A: HC≡CCH2CH2CH3. The triple bond sits between the first and second carbons.
Q: Two molecules both have the formula C4H10O. One is CH3CH(CH3)CH2OH and the other is CH3C(CH3)(OH)CH3. Are they isomers? Explain.
A: Yes. They share the same molecular formula (C4H10O) but differ in the arrangement of atoms, so they are structural isomers.
Q: What is the most plentiful hydrocarbon on Earth, and where do hydrocarbons primarily come from?
A: Methane (CH4) is the most plentiful hydrocarbon. Hydrocarbons come primarily from petroleum (crude oil) and natural gas deposits.
Q: A molecule has the formula C6H12. Could it be an alkane, alkene, or alkyne?
A: It fits the alkene general formula C_nH_(2n) where n = 6, so it is an alkene (or a cycloalkane, which shares the same general formula).
This connects directly to Ch. 24.4 (Functional Groups), where you will see what happens when atoms other than C and H are attached to the hydrocarbon backbone. Understanding the base hydrocarbon structure makes it much easier to spot and name functional groups.
Isomers come back in a big way in organic chemistry courses, especially stereoisomers (geometric and optical isomers), which build on the structural isomer concept introduced here.
The degree-of-unsaturation calculation connects to spectroscopy topics later, where it helps you narrow down possible structures from a molecular formula before you even look at IR or NMR data.
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