4.2 Versatile Carbon: Homologous Series and Nomenclature
Why does carbon form more compounds than any other element? Meet catenation and tetravalency, learn to draw chains, branches and rings, group compounds into homologous series by their functional groups, and name them with the IUPAC rules — from methane to propanone and hexanoic acid.
Chemists have discovered over ten million carbon compounds, from the methane that burns in your kitchen to the proteins that build your muscles. If you tried to learn each one separately, you would never finish. Luckily carbon compounds fall into families, and the members of a family look alike and behave alike. Once you know the family and a few simple naming rules, you can write the name and the structure of hundreds of compounds from their formulas.
In this section you will see the two properties of carbon that cause this variety, learn to draw chains, branched chains and rings, and meet the idea of a functional group that decides how a compound behaves. You will then group compounds into homologous series, see how their properties change step by step, and finally learn the IUPAC rules that give every compound a name that tells you its structure.
Why Carbon Forms So Many Compounds
Two properties of carbon, working together, explain its variety. First, carbon has valency four (Section 4.1): each carbon atom forms four bonds. Second, a carbon atom can bond to other carbon atoms, again and again.
📘 Definition 4.4 — Catenation
Catenation is the ability of the atoms of an element to join to one another to form long chains, branched chains and rings. Carbon shows it to an extraordinary extent because the carbon–carbon bond is very strong: the carbon atom is small, so the shared pair is held close to both nuclei.
📐 Principle 4.5 — The versatility of carbon
Carbon forms a very large number of compounds because of (1) catenation: the strong C–C bond lets it form chains of many atoms, branches and rings; (2) tetravalency: with four bonds each carbon can join other carbon atoms, hydrogen, oxygen, nitrogen, sulphur, chlorine and many other atoms; (3) it can form single, double and triple bonds between carbon atoms. Silicon, the element below carbon, also forms hydrogen compounds with chains, but only up to about seven or eight atoms, and these compounds are very reactive — silicon–silicon bonds are much weaker.
📘 Definition 4.5 — Saturated and unsaturated compounds
A hydrocarbon contains only carbon and hydrogen. Hydrocarbons whose carbon atoms are joined only by single bonds are saturated (the alkanes, general formula $\mathrm{C_{n}H_{2n+2}}$). Hydrocarbons with at least one double bond (alkenes, $\mathrm{C_{n}H_{2n}}$) or triple bond (alkynes, $\mathrm{C_{n}H_{2n-2}}$) between carbon atoms are unsaturated. The formulas hold for open-chain compounds with one multiple bond.
The names of the first compounds come from the number of carbons: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). The ending tells the bonding: -ane for single bonds, -ene for a double bond, -yne for a triple bond.
Carbons
Alkane (single bonds)
Alkene (double bond)
Alkyne (triple bond)
1
methane $\mathrm{CH_{4}}$
—
—
2
ethane $\mathrm{C_{2}H_{6}}$
ethene $\mathrm{C_{2}H_{4}}$
ethyne $\mathrm{C_{2}H_{2}}$
3
propane $\mathrm{C_{3}H_{8}}$
propene $\mathrm{C_{3}H_{6}}$
propyne $\mathrm{C_{3}H_{4}}$
4
butane $\mathrm{C_{4}H_{10}}$
butene $\mathrm{C_{4}H_{8}}$
butyne $\mathrm{C_{4}H_{6}}$
5
pentane $\mathrm{C_{5}H_{12}}$
pentene $\mathrm{C_{5}H_{10}}$
pentyne $\mathrm{C_{5}H_{8}}$
6
hexane $\mathrm{C_{6}H_{14}}$
hexene $\mathrm{C_{6}H_{12}}$
hexyne $\mathrm{C_{6}H_{10}}$
✏️ Example 7 — Is it saturated?
(a) Write the formulas of the alkane, alkene and alkyne with five carbon atoms. (b) Classify $\mathrm{C_{4}H_{10}}$, $\mathrm{C_{4}H_{8}}$ and $\mathrm{C_{4}H_{6}}$ as saturated or unsaturated. (c) How many hydrogen atoms has an alkane with 10 carbons?
(b) For $n=4$ the alkane is $\mathrm{C_{4}H_{10}}$ — saturated. $\mathrm{C_{4}H_{8}}$ (alkene) and $\mathrm{C_{4}H_{6}}$ (alkyne) have fewer than $2n+2=10$ hydrogens, so they are unsaturated.
Carbon atoms join in three patterns. They can form a straight chain, as in propane $\mathrm{CH_{3}CH_{2}CH_{3}}$; a branched chain, in which a carbon bears a side group, as in 2-methylpropane; or a ring, as in cyclohexane $\mathrm{C_{6}H_{12}}$ (all single bonds) and benzene $\mathrm{C_{6}H_{6}}$ (alternating single and double bonds). Rings can be saturated or unsaturated like chains.
A straight chain, a branched chain and two rings (the corner of each ring is a carbon atom with enough hydrogens to make four bonds).
📘 Definition 4.6 — Structural isomers
Structural isomers are compounds with the same molecular formula but different structural formulas (the atoms are joined in a different order). They have different properties. Example: $\mathrm{C_{4}H_{10}}$ has two isomers, butane (straight chain, boiling point $273\ \mathrm{K}$) and 2-methylpropane (branched, boiling point $261\ \mathrm{K}$).
✏️ Example 8 — Isomers of pentane
Pentane $\mathrm{C_{5}H_{12}}$ has three structural isomers. Describe them and name each by describing its carbon skeleton.
💡 Show solution▶
1 A straight chain of five carbons: $\mathrm{CH_{3}CH_{2}CH_{2}CH_{2}CH_{3}}$ — pentane.
2 A chain of four carbons with a $\mathrm{CH_{3}}$ branch on the second carbon: $\mathrm{CH_{3}CH(CH_{3})CH_{2}CH_{3}}$ — 2-methylbutane.
3 A chain of three carbons with two $\mathrm{CH_{3}}$ branches on the middle carbon: $\mathrm{CH_{3}C(CH_{3})_{2}CH_{3}}$ — 2,2-dimethylpropane.
Check Each has 5 carbons and 12 hydrogens: $\mathrm{C_{5}H_{12}}$. ✓ (Each carbon has exactly four bonds, so no other arrangement exists.)
Functional Groups and Homologous Series
Hydrocarbons are only the start. Replacing a hydrogen by another atom or group gives compounds with different behaviour. Take two compounds of two carbons: ethane $\mathrm{CH_{3}CH_{3}}$ is a gas that burns, while ethanol $\mathrm{CH_{3}CH_{2}OH}$ is a liquid that you can mix with water and that reacts with sodium. The group $-\mathrm{OH}$ is responsible.
📘 Definition 4.7 — Functional group
A functional group is an atom or group of atoms that replaces a hydrogen in a hydrocarbon chain (or a double/triple bond) and decides the chemical properties of the compound. The carbon chain it is attached to is the rest of the molecule, often written R.
Class
Functional group
Prefix / suffix
Example
halogenoalkane (haloalkane)
$-\mathrm{Cl}$, $-\mathrm{Br}$ (halo)
prefix chloro-, bromo-
$\mathrm{CH_{3}CH_{2}Cl}$ chloroethane
alcohol
$-\mathrm{OH}$ (hydroxyl)
suffix -ol
$\mathrm{CH_{3}CH_{2}OH}$ ethanol
aldehyde
$-\mathrm{CHO}$
suffix -al
$\mathrm{CH_{3}CHO}$ ethanal
ketone
$>\mathrm{C{=}O}$ (inside the chain)
suffix -one
$\mathrm{CH_{3}COCH_{3}}$ propanone
carboxylic acid
$-\mathrm{COOH}$
suffix -oic acid
$\mathrm{CH_{3}COOH}$ ethanoic acid
alkene
$\mathrm{C{=}C}$
suffix -ene
$\mathrm{CH_{2}{=}CH_{2}}$ ethene
alkyne
$\mathrm{C{\equiv}C}$
suffix -yne
$\mathrm{HC{\equiv}CH}$ ethyne
The carbon of the $-\mathrm{CHO}$ and $-\mathrm{COOH}$ groups is part of the chain and is counted when you count carbons: $\mathrm{CH_{3}CHO}$ has two carbons, so it is ethanal. Now arrange compounds with the same functional group in order of size:
Each member differs from the next by one $\mathrm{CH_{2}}$ unit. Such a ladder is a homologous series.
📘 Definition 4.8 — Homologous series
A homologous series is a family of compounds that have the same functional group, can be represented by one general formula and in which successive members differ by a $-\mathrm{CH_{2}}-$ unit (a mass difference of $14\ \mathrm{u}$). Examples: the alkanes $\mathrm{C_{n}H_{2n+2}}$; the alcohols $\mathrm{C_{n}H_{2n+1}OH}$; the carboxylic acids $\mathrm{C_{n}H_{2n+1}COOH}$.
📐 Principle 4.6 — Gradation in a homologous series
In a homologous series, (1) successive members differ by $\mathrm{CH_{2}}$, i.e. $14\ \mathrm{u}$ ($12+2\times1$); (2) members have similar chemical properties, because they have the same functional group; (3) their physical properties such as melting point, boiling point and density change gradually with the number of carbon atoms: the boiling point rises steadily as the molecules become larger.
✏️ Example 9 — A homologous series in numbers
(a) Find the molecular masses ($\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16\ \mathrm{u}$) of $\mathrm{CH_{4}}$, $\mathrm{C_{2}H_{6}}$, $\mathrm{C_{3}H_{8}}$ and $\mathrm{C_{4}H_{10}}$ and show that they differ by $14\ \mathrm{u}$. (b) The first four alcohols are $\mathrm{CH_{3}OH}$, $\mathrm{C_{2}H_{5}OH}$, $\mathrm{C_{3}H_{7}OH}$, $\mathrm{C_{4}H_{9}OH}$. Give the next member and the general formula. (c) Which of $\mathrm{C_{2}H_{6}}$, $\mathrm{C_{3}H_{6}}$, $\mathrm{C_{3}H_{8}}$, $\mathrm{C_{5}H_{12}}$ are in the same series as methane?
(b) The next alcohol is $\mathrm{C_{5}H_{11}OH}$ (pentanol, $88\ \mathrm{u}$); general formula $\mathrm{C_{n}H_{2n+1}OH}$.
(c) Methane is an alkane, $\mathrm{C_{n}H_{2n+2}}$. $\mathrm{C_{2}H_{6}}$ ✓, $\mathrm{C_{3}H_{8}}$ ✓, $\mathrm{C_{5}H_{12}}$ ✓; $\mathrm{C_{3}H_{6}}$ is $\mathrm{C_{n}H_{2n}}$, an alkene — a different series.
Interactive 1 — Homologous Series Explorer
Choose a family and press ▶ Play: the chain grows one carbon at a time, and the graph shows the boiling points of all five families (the chosen family is highlighted). The dashed line is room temperature, 25 °C. Watch what each extra CH₂ does to the mass and to the boiling point.
name · formula—
molecular mass—
boiling point—
state at 25 °C—
Naming Carbon Compounds: IUPAC Nomenclature
Old names such as acetic acid and formaldehyde (trivial names) tell you nothing about the structure. The IUPAC system (International Union of Pure and Applied Chemistry) gives every compound a name from which the structure can be drawn.
🧭 Problem-solving strategy — naming a compound
Count the carbons in the chain (including the carbon of $-\mathrm{CHO}$ and $-\mathrm{COOH}$) and write the root: meth, eth, prop, but, pent, hex.
Single bonds only: add -ane to the root (propane). A double bond changes the ending to -ene and a triple bond to -yne.
Functional group: a halogen is a prefix (chloro-, bromo-) put in front: chloropropane. Other groups are suffixes that replace the last letter: -ol, -al, -one, -oic acid.
Drop the final “e” of the chain name if the suffix begins with a vowel (a, e, i, o, u): propane + ol $=$ propanol; propane + one $=$ propanone; propane + al $=$ propanal; ethane + oic acid $=$ ethanoic acid. (The endings -ene and -yne simply replace -ane.)
📐 Principle 4.7 — Names of the three-carbon family
Starting from propane $\mathrm{CH_{3}CH_{2}CH_{3}}$: chloropropane $\mathrm{CH_{3}CH_{2}CH_{2}Cl}$ · propanol $\mathrm{CH_{3}CH_{2}CH_{2}OH}$ · propanal $\mathrm{CH_{3}CH_{2}CHO}$ · propanone $\mathrm{CH_{3}COCH_{3}}$ · propanoic acid $\mathrm{CH_{3}CH_{2}COOH}$ · propene $\mathrm{CH_{3}CH{=}CH_{2}}$ · propyne $\mathrm{CH_{3}C{\equiv}CH}$. Note that propanal and propanone have the same molecular formula, $\mathrm{C_{3}H_{6}O}$, but different functional groups.
✏️ Example 10 — Naming from the formula
Give the IUPAC name and the series of: (a) $\mathrm{CH_{3}CH_{2}CH_{2}CH_{2}OH}$; (b) $\mathrm{CH_{3}CH_{2}CHO}$; (c) $\mathrm{CH_{3}COOH}$; (d) $\mathrm{CH_{3}CH_{2}Cl}$; (e) $\mathrm{CH_{3}COCH_{3}}$; (f) $\mathrm{CH_{2}{=}CHCH_{3}}$.
A compound has the molecular formula $\mathrm{C_{3}H_{6}O}$. Write two structures, name them, and say how they differ. How many hydrogens does propane, the saturated hydrocarbon with the same three carbons, have?
💡 Show solution▶
1 $\mathrm{CH_{3}CH_{2}CHO}$: propanal (aldehyde). Count: C 3; H $3+2+1=6$; O 1 ✓.
2 $\mathrm{CH_{3}COCH_{3}}$: propanone (ketone). Count: C 3; H $3+3=6$; O 1 ✓.
3 They are isomers with different functional groups, so different properties (propanal is an aldehyde, propanone a ketone, the common solvent in nail-polish remover).
4 Propane is $\mathrm{C_{3}H_{8}}$: 8 hydrogens. In $\mathrm{C_{3}H_{6}O}$ the $\mathrm{C{=}O}$ takes the place of two hydrogens, leaving 6.
Interactive 2 — Functional-Group Switcher
The same three-carbon chain, with a different functional group each time. Press ▶ Play for a tour, or choose a group yourself: the functional group is highlighted in red, and the formula, class and IUPAC name update. Count the carbons — and check that every carbon still has four bonds.
name—
molecular formula—
class of compound—
name pattern—
⚠️ Common mistakes with naming and series
1. Forgetting to count the functional-group carbon. $\mathrm{CH_{3}COOH}$ has two carbons (ethanoic acid), not one.
2. Writing “propaneol”. Drop the final $e$ before a vowel suffix: propanol, propanone, propanal.
3. Using the wrong suffix for a halogen. Chlorine is a prefix: chloropropane, never “propanechloride”.
4. Confusing propanal and propanone. Both are $\mathrm{C_{3}H_{6}O}$; the aldehyde has $-\mathrm{CHO}$ at the end of the chain, the ketone has $>\mathrm{C{=}O}$ inside it.
5. Saying members of a homologous series have identical properties. Their chemical properties are similar, but physical properties change gradually.
6. Counting bonds wrongly. Every carbon in a structural formula must have exactly four bonds; a double bond counts as two.
Exercises
I
Catenation, saturation and isomers.
1 What is catenation? Why does carbon show it more than other elements?
2 Name two properties of carbon that explain the large number of its compounds.
3 Write the formulas of the alkane, alkene and alkyne with 4 carbons. Which are unsaturated?
4 How many hydrogen atoms has an alkane with (a) 7 carbons; (b) 12 carbons?
5 What are structural isomers? Draw (describe) the two isomers of $\mathrm{C_{4}H_{10}}$.
6 Cyclohexane has the formula $\mathrm{C_{6}H_{12}}$ and benzene $\mathrm{C_{6}H_{6}}$. Which is saturated, and how can you tell from the formulas?
7 Why do silicon compounds not rival those of carbon in number?
8Challenging. $\mathrm{C_{6}H_{14}}$ has five structural isomers. Describe their carbon skeletons.
💡 Show solutions▶
1 The ability of atoms to join to one another and form chains, branches and rings. Carbon's small size makes the C–C bond very strong.
2 Catenation and tetravalency (also multiple bonds).
3 $\mathrm{C_{4}H_{10}}$ (alkane), $\mathrm{C_{4}H_{8}}$ (alkene), $\mathrm{C_{4}H_{6}}$ (alkyne); the last two are unsaturated.
5 Compounds with the same molecular formula but different structures. Butane $\mathrm{CH_{3}CH_{2}CH_{2}CH_{3}}$ and 2-methylpropane $\mathrm{CH_{3}CH(CH_{3})CH_{3}}$.
6 Cyclohexane is saturated: a ring of single bonds with two hydrogens on each carbon. Benzene, with only 6 hydrogens on 6 carbons, has alternating double bonds, so it is unsaturated. (A ring has two hydrogens fewer than the open-chain alkane $\mathrm{C_{6}H_{14}}$.)
7 Silicon forms chains of only seven or eight atoms, and the Si–Si bond is weak and reactive, so it shows very little catenation.
1 What is a functional group? Name the functional group of an alcohol, an aldehyde, a ketone and a carboxylic acid.
2 Define a homologous series and give two characteristics.
3 Find the molecular mass of the first five alcohols ($\mathrm{C}=12$, $\mathrm{H}=1$, $\mathrm{O}=16\ \mathrm{u}$) and show that they differ by $14\ \mathrm{u}$.
4 The first two members of a series are $\mathrm{C_{2}H_{4}}$ and $\mathrm{C_{3}H_{6}}$. Give the next two and the general formula.
5 Which of the following belong to the same homologous series: $\mathrm{CH_{3}OH}$, $\mathrm{C_{3}H_{8}}$, $\mathrm{C_{2}H_{5}OH}$, $\mathrm{C_{4}H_{10}}$, $\mathrm{C_{3}H_{7}OH}$?
6 Why do members of a homologous series have similar chemical properties but different boiling points?
7 Ethane boils at $185\ \mathrm{K}$ and propane at $231\ \mathrm{K}$. Predict whether butane will boil above or below $231\ \mathrm{K}$, and why.
8 Give the general formula of the carboxylic acids $\mathrm{C_{n}H_{2n+1}COOH}$ and the molecular formula of the member with $n=3$.
💡 Show solutions▶
1 An atom or group that determines the chemical properties of a compound. Alcohol $-\mathrm{OH}$; aldehyde $-\mathrm{CHO}$; ketone $>\mathrm{C{=}O}$; carboxylic acid $-\mathrm{COOH}$.
2 A family with the same functional group and general formula in which successive members differ by $-\mathrm{CH_{2}}-$ ($14\ \mathrm{u}$); similar chemical properties and gradual change in physical properties.
3 $\mathrm{CH_{3}OH}$ 32; $\mathrm{C_{2}H_{5}OH}$ 46; $\mathrm{C_{3}H_{7}OH}$ 60; $\mathrm{C_{4}H_{9}OH}$ 74; $\mathrm{C_{5}H_{11}OH}$ 88 u; each step is $14\ \mathrm{u}$.
4 $\mathrm{C_{4}H_{8}}$ and $\mathrm{C_{5}H_{10}}$; general formula $\mathrm{C_{n}H_{2n}}$ (alkenes).
6 The functional group (which causes the reactions) is the same; the longer chain makes bigger molecules with stronger attraction between them, so a higher boiling point.
7 Above $231\ \mathrm{K}$ (it boils at $273\ \mathrm{K}$): the boiling point rises as the molecules grow.
8 General formula as given; $n=3$: $\mathrm{C_{3}H_{7}COOH}=\mathrm{C_{4}H_{8}O_{2}}$ (butanoic acid).
6 The final $e$ of propane must be dropped before a suffix beginning with a vowel: propanol.
7 (a) 2; (b) 4; (c) 6.
8 $\mathrm{HCOOH}$; carboxylic acids (the first member).
IV
Mixed problems.
1 A compound has the formula $\mathrm{C_{4}H_{8}O_{2}}$ and the $-\mathrm{COOH}$ group. Write its structure and name.
2 Name the functional group in each: $\mathrm{CH_{3}CH_{2}Cl}$, $\mathrm{CH_{3}CH_{2}OH}$, $\mathrm{CH_{3}CH_{2}CHO}$, $\mathrm{CH_{3}COCH_{3}}$, $\mathrm{CH_{3}CH_{2}COOH}$.
3 Is $\mathrm{C_{5}H_{10}}$ saturated? Give a possible name if it has a double bond.
4 Write two structures for $\mathrm{C_{2}H_{6}O}$ and name them. (Hint: one is an alcohol.)
5 The boiling points ($^{\circ}\mathrm{C}$) of the first four alcohols are $64.7$, $78.4$, $97.2$ and $117.7$. Find the average rise in boiling point per $\mathrm{CH_{2}}$ added.
6 Which has the higher molecular mass, hexane or hexanoic acid? Calculate both.
7Applied. Cooking gas (LPG) is mainly butane. Write its molecular formula, its isomers, and its molecular mass.
8Applied. Nail-polish remover is mostly propanone. Give its condensed and molecular formulas, name its functional group, and name an aldehyde with the same molecular formula.
💡 Show solutions▶
1 $\mathrm{C_{3}H_{7}COOH}$, i.e. $\mathrm{CH_{3}CH_{2}CH_{2}COOH}$: butanoic acid (4 carbons).
3 The saturated hydrocarbon would be $\mathrm{C_{5}H_{12}}$, so $\mathrm{C_{5}H_{10}}$ is unsaturated; with one double bond it is a pentene.
4 $\mathrm{CH_{3}CH_{2}OH}$ (ethanol, an alcohol) and $\mathrm{CH_{3}OCH_{3}}$ (an ether, methoxymethane); both are $\mathrm{C_{2}H_{6}O}$.
5 Rise from first to fourth: $117.7-64.7=53.0\ ^{\circ}\mathrm{C}$ over 3 steps: about $17.7\ ^{\circ}\mathrm{C}$ per $\mathrm{CH_{2}}$ (the actual rise falls slowly with size).
7 $\mathrm{C_{4}H_{10}}$; butane and 2-methylpropane; molecular mass $4\times12+10=58\ \mathrm{u}$.
8 $\mathrm{CH_{3}COCH_{3}}$, $\mathrm{C_{3}H_{6}O}$; the ketone group $>\mathrm{C{=}O}$; the aldehyde propanal $\mathrm{CH_{3}CH_{2}CHO}$ has the same formula.
E1.Board · 1 mark The IUPAC name of $\mathrm{CH_{3}COCH_{3}}$ is (A) propanal (B) propanone (C) propanol (D) propene
💡 Show solution▶
Idea Three carbons with the $>\mathrm{C{=}O}$ group inside the chain: ketone, suffix -one.
Answer (B).
E2.Board · A–RAssertion (A): Methane, ethane and propane belong to the same homologous series. Reason (R): They have the same functional group and successive members differ by a $-\mathrm{CH_{2}}-$ unit. Choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
💡 Show solution▶
Idea $\mathrm{CH_{4}}$, $\mathrm{C_{2}H_{6}}$, $\mathrm{C_{3}H_{8}}$ fit $\mathrm{C_{n}H_{2n+2}}$ and differ by $\mathrm{CH_{2}}$ (14 u). R defines a homologous series and so explains A.
Answer (A).
E3.Board · 3 marks (a) What is a homologous series? (b) Write the molecular formula of the alkane with 8 carbon atoms. (c) The first three members of a series have the molecular masses 46, 60 and 74. What is the difference, and what does it show?
💡 Show solution▶
(a) A family of compounds with the same functional group and general formula, in which successive members differ by $-\mathrm{CH_{2}}-$.
(b) $\mathrm{C_{8}H_{18}}$ ($2\times8+2=18$).
(c) $60-46=14$ and $74-60=14\ \mathrm{u}$: each member differs by $\mathrm{CH_{2}}$, so they belong to one homologous series.
E4.Board · Case-based Four compounds are given: P $\mathrm{CH_{3}CH_{2}CH_{2}OH}$; Q $\mathrm{CH_{3}CH_{2}CHO}$; R $\mathrm{CH_{3}CH_{2}COOH}$; S $\mathrm{CH_{3}COCH_{3}}$. (i) Name P and R. (1 mark) (ii) Which two are isomers of each other? Give the molecular formula. (1 mark) (iii) Name the functional group of Q and say which compound P, Q, R or S is a ketone. (2 marks)
💡 Show solution▶
(i) P is propanol; R is propanoic acid.
(ii) Q (propanal) and S (propanone), both $\mathrm{C_{3}H_{6}O}$.
(iii) Q has the aldehyde group $-\mathrm{CHO}$; S is the ketone.
E5.NEETFoundation. The number of structural isomers of $\mathrm{C_{5}H_{12}}$ is (A) 2 (B) 3 (C) 4 (D) 5
💡 Show solution▶
Idea Pentane (5-chain), 2-methylbutane (4-chain, one methyl) and 2,2-dimethylpropane (3-chain, two methyls on the middle carbon).
Answer (B).
E6.Board · 2 marks Write the IUPAC names of (a) $\mathrm{CH_{3}CH_{2}CH_{2}CH_{2}COOH}$ and (b) $\mathrm{CH_{3}CH_{2}CH_{2}Cl}$.
💡 Show solution▶
(a) 5 carbons including the $-\mathrm{COOH}$ carbon: pentanoic acid.
(b) 3 carbons, Cl as a prefix: chloropropane.
🔬 Research–Teaching NexusResearch-tutored · optional, two evenings
Research question. How many different structures can you build for $\mathrm{C_{6}H_{14}}$, and does branching change the boiling point in the way chemists observe?
Why it matters. Petrol companies tune the structure of fuel molecules: branched isomers burn more smoothly in engines than straight chains of the same formula (this is the idea behind octane ratings), and drug designers choose among isomers because one can heal while its isomer does nothing. Counting and recognising isomers is a basic skill of organic chemistry.
What to do.
Build molecular models with sweets and toothpicks (or clay and sticks): four sticks on each carbon, one stick for each hydrogen.
Find every structure for $\mathrm{C_{4}H_{10}}$ and $\mathrm{C_{5}H_{12}}$ and check against 2 and 3 isomers; then try $\mathrm{C_{6}H_{14}}$.
Draw each as a skeleton and name it by its longest chain and side groups.
Look up the boiling point of each isomer (a chemistry handbook or an online database) and compare straight-chain and branched isomers.
Tools: sweets or clay, toothpicks, a handbook or database of boiling points, a notebook.
Hand in: Photographs or drawings of all isomers found, their names, their boiling points and one sentence on the trend.
Going further. Which functional isomers can you make for $\mathrm{C_{3}H_{6}O}$ and $\mathrm{C_{2}H_{6}O}$?
Instructor note: expect 2, 3 and 5 isomers; hexane boils at about $69\ ^{\circ}\mathrm{C}$ and the most compact isomer, 2,2-dimethylbutane, at about $50\ ^{\circ}\mathrm{C}$: more branching, lower boiling point. Common errors: counting the same structure twice by flipping or rotating the chain, or drawing a carbon with five bonds. Check every carbon has four sticks.
🧪 Self-check quiz
0/5 correct
Q1. The general formula of the alkenes is
Q2. The IUPAC name of CH₃CH₂CHO is
Q3. Successive members of a homologous series differ in molecular mass by
Q4. Which pair are structural isomers?
Q5. The functional group present in ethanoic acid is